U.S. patent application number 09/748551 was filed with the patent office on 2001-07-19 for transmitting device, receiving device, and receiving method.
Invention is credited to Yuasa, Naoki.
Application Number | 20010008391 09/748551 |
Document ID | / |
Family ID | 26582561 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008391 |
Kind Code |
A1 |
Yuasa, Naoki |
July 19, 2001 |
Transmitting device, receiving device, and receiving method
Abstract
In a power line transmission/reception system, when a receiving
device receives data via a power line transmission network which
transmits the same data with the same timing in parallel via a
plurality of communication channels by means of superposition on AC
power, the reception of the data is performed using a communication
channel selected on the basis of the mean channel usage periods
measured for the respective communication channels, thereby
assuring high-quality communication. Transmission data is generated
by converting Input data into packets and outputting each same
packet repeatedly a plurality of times. The transmission data is
transmitted via the plurality of communication channels at the same
time by means of superposition on AC power thereby assuring
high-quality data communication.
Inventors: |
Yuasa, Naoki; (Chiba,
JP) |
Correspondence
Address: |
Jay H. Maioli
Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
26582561 |
Appl. No.: |
09/748551 |
Filed: |
December 26, 2000 |
Current U.S.
Class: |
370/489 ; 381/77;
725/79 |
Current CPC
Class: |
H04L 5/0005 20130101;
H04B 2203/545 20130101; H04B 1/207 20130101; Y02B 70/30 20130101;
H04L 12/2838 20130101; H04L 2012/2843 20130101; Y04S 20/20
20130101; H04L 12/2803 20130101; H04L 5/0044 20130101; H04L 5/006
20130101; H04B 3/54 20130101 |
Class at
Publication: |
340/310.01 ;
381/77 |
International
Class: |
H04M 011/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
JP |
P11-374186 |
Nov 14, 2000 |
JP |
P2000-352532 |
Claims
What is claimed is:
1. A receiving device for receiving data via a power line
transmission network which transmits the same data with the same
timing in parallel via a plurality of communication channels by
means of superposition on AC power, said receiving device
comprising: receiving means for extracting data superimposed on AC
power from a particular channel of said network thereby receiving
said data; timer means for measuring a channel usage period during
which a communication channel is used by said receiving means to
receive data; storage means for storing the channel usage period
measured by said timer means, for each communication channel;
calculation means for calculating the mean channel usage period of
each communication channel from the channel usage periods stored in
said storage means; and control means which controls said storage
means so as to store the channel usage period measured by said
timer means for each communication channel, and which controls said
receiving means so as to select a communication channel used to
receive data on the basis of the mean channel usage periods of the
respective communication channels calculated by said calculation
means and so as to receive said data using the selected
channel.
2. A receiving device according to claim 1, wherein said selection
of the communication channel is performed when a communication
failure occurs in a communication channel being used.
3. A receiving device according to claim 2, wherein it is
determined that said communication failure occurs when an error
rate is detected as being equal to or greater than a predetermined
level.
4. A receiving device according to claim 1, wherein: the
transmission of said data via the network is performed in such a
manner that said data is first converted into packets each having a
predetermined amount of data, and then each same packet is
transmitted a plurality of times; and said receiving device selects
one packet from each succession of the same packets and uses the
selected packets.
5. A receiving device according to claim 1, further comprising
storage control means for controlling said storage means so as to
store the mean channel usage period measured by said calculation
means for each communication channel.
6. A receiving device according to claim 1, wherein said selection
of the communication channel is performed such that the mean
channel usage periods stored in said storage means are evaluated
and a communication channel having a greatest mean channel usage
period is selected.
7. A receiving device according to claim 1, wherein: said data
transmitted via the network is compressed data; said receiving
device further comprises decoding mean for decompressing said
compressed data; and said receiving device decompresses, using said
decoding means, the compressed data received by said receiving
means.
8. A receiving device according to claim 1, wherein before starting
the reception of data via the network, said receiving device
measures the channel usage period from a time at which a
communication channel is selected to a time at which a
communication failure occurs for each communication channel, and
said receiving device selects a communication channel having a
greatest measured channel usage period.
9. A receiving device according to claim 1, wherein said data
transmitted via the network is continuous in terms of time.
10. A transmitting device for transmitting input data via a power
line transmission network which has a plurality of communication
channels and which transmits data by means of superimposing data
upon AC power, said transmitting device comprising: compression
means for compressing said input data; packet conversion means for
converting the data compressed by said compression means into
packets each having a predetermined length of data; and
transmitting means for transmitting the data converted into packets
by said packet conversion means via the plurality of communication
channels at the same time.
11. A transmitting device according to claim 10, wherein said
packet conversion means outputs each same packet repeatedly a
plurality of times to said transmitting means such that said
transmitting means receives a succession of a plurality of packets
having the same content.
12. A transmitting device according to claim 10, wherein said
plurality of packets having the same content are transmitted
successively from said transmitting means.
13. A transmitting device according to claim 11, wherein the number
of times that each same packet is output repeatedly is determined
on the basis of the compression ratio of the data compressed by
said compression means.
14. A transmitting device according to claim 10, wherein said
plurality of communication channels transmit data using different
carrier frequencies.
15. A receiving method for receiving data via a power line
transmission network which transmits the same data with the same
timing in parallel via a plurality of communication channels by
means of superposition on AC power, said receiving method
comprising the steps of: extracting data superimposed on AC power
from a particular channel of said network thereby receiving said
data, and measuring a channel usage period during which a
communication channel is used to receive said data; when a
communication failure is detected during the reception of said
data, calculating the mean channel usage period of said particular
channel from the channel usage period measured during the current
receiving operation and from the channel usage periods of said
particular channel measured in the past; and selecting a
communication channel to be used, on the basis of the calculated
mean channel usage period and the mean channel usage periods
calculated for the other respective communication channels.
16. A receiving method according to claim 15, wherein it is
determined that said communication failure occurs when an error
rate is detected as being equal to or greater than a predetermined
level.
17. A receiving method according to claim 15, wherein the
transmission of said data via the network is performed in such a
manner that said data is first converted into packets each having a
predetermined amount of data, and then each packet is transmitted a
plurality of times; said receiving method further comprising the
step of selecting one packet from each succession of the same
packets.
18. A receiving method according to claim 15, wherein in said
communication channel selection step, a communication channel
having a greatest calculated mean channel usage period is
selected.
19. A receiving method according to claim 15, further comprising
the steps of: before starting the reception of data via the
network, measuring the channel usage period from a time at which a
communication channel is selected to a time at which a
communication failure occurs for each communication channel; and
selecting a communication channel having a greatest channel usage
period of the channel usage periods measured for the respective
communication channels.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a receiving device and a
receiving method for receiving data via a power line transmission
network which transmits the same data with the same timing in
parallel via a plurality of communication channels by means of
superposition on AC power, in which a communication channel used to
receive the data is selected on the basis of channel usage periods
measured for the respective communication channels.
[0003] The present invention also relates to a transmitting method
for transmitting data such that transmission data is generated by
converting input data into packets and outputting each same packet
repeatedly a plurality of times, and the transmission data is
transmitted via a plurality of communication channels at the same
time by means of superposition on AC power.
[0004] 2. Description of the Related Art
[0005] In recent years, a power line transmission/reception system
has been proposed and realized, for transmitting/receiving
information such as an audio or video signal via a power line used
to distribute commercial AC power to rooms in a home. An example of
a power line transmission technique is disclosed in U.S. patent
application Ser. No. 09/247,943 field on Feb. 11, 1999, which is
incorporated herein by reference. In such a power line system, for
example, a transmission signal is generated by modulating an audio
signal or a video signal, and the resultant transmission signal is
superimposed on commercial AC power distributed via a power line.
In a receiving device, the transmission signal component
superimposed on the commercial AC power is extracted and then
demodulated thereby reproducing the original audio signal or video
signal.
[0006] In general, not only a power line transmission/reception
system but also other devices such as an electric lamp and various
types of electronic devices are connected to a power line. In the
power line transmission/reception systems, therefore, there is a
rather high possibility that noise generated from electronic
devices connected to the power line interferes with
reception/reception.
[0007] One known technique of avoiding the above problem in the
power line transmission/reception systems is to transmit the same
information via a plurality of channels. In this technique, the
same audio or video signal is modulated using a plurality of
carriers having different frequencies and transmitted via different
channels corresponding to the carrier frequencies. That is, the
same information is transmitted from a transmitting device via a
plurality of channels which are obtained in the above-described
manner.
[0008] In a receiving device, a channel which provides a best
reception, that is, which has a highest channel quality, is
selected from the plurality of channels and is used for
transmission of the information.
[0009] In this technique in which the same information is
transmitted via the plurality of channels, even when a channel with
a certain frequency is influenced by noise generated by another
device, the receiving device can receive the information under a
good condition by selecting a channel with another frequency which
is not influenced by the noise.
[0010] The channel selection may be performed manually by a user.
It is also known in the art to construct a receiving device such
that the channel is automatically switched without needing a manual
operation when degradation in the condition of the current channel
is detected.
[0011] FIG. 1 illustrates an example of a manner in which the
channel is switched in a receiving device having such an automatic
channel switching capability.
[0012] In this example shown in FIG. 1, transmission is performed
using three channels #1, #2, and #3.
[0013] When the channel #1 is used for reception, if a reception
error is detected, that is, if the reception condition of the
channel #1 becomes worse than an allowable level, the receiving
device automatically switches the reception channel to the channel
#2. If a reception error is detected during the reception using the
channel #2, the reception channel is switched to the channel
#3.
[0014] If a further reception error is detected during the
reception using the channel #3, the reception channel is switched
to channel #1 which was used first.
[0015] That is, the channel is switched in a predetermined fixed
order, such as #1.fwdarw.#2.fwdarw.#3.fwdarw.#1 . . . , each time a
reception error occurs.
[0016] However, the conventional channel switching technique
described above with reference to FIG. 1 has the following
problems.
[0017] When a channel is switched to another channel in response to
detection of a reception error, it is not assured that a good
reception condition is obtained in the new channel. If the new
channel does not provide good reception, the channel is further
switched to another channel until a good reception condition is
obtained. Thus, in some cases, it takes a long time to reach a good
channel. That is, a communication error or a bad communication
condition can often occur over a rather long period of time.
[0018] Herein, let us assume that a channel which provides the best
reception quality is now being used and all the other channels are
under steady bad reception conditions.
[0019] The current channel having the best reception quality can
temporarily fall into a bad reception state due to sudden noise.
However, in such a case, the channel should not be switched to
another channel, and the current channel should be maintained.
[0020] However, in the channel switching technique shown in FIG. 1,
detection of a reception error always causes switching from the
current channel to a predetermined next channel, and thus, in this
specific example, the current channel which provides the best
reception is switched to the next channel which is steadily in the
bad state and further to another bad channel. Thus, it takes a long
time to return to the best channel.
[0021] As described above, the conventional technique has the
problem that the simple channel switching in the fixed order does
not allow the channel to be properly switched depending upon the
actual reception conditions of the respective channels.
SUMMARY OF THE INVENTION
[0022] According to an aspect of the present invention, there is
provided a receiving device for receiving data via a power line
transmission network which transmits the same data with the same
timing in parallel via a plurality of communication channels by
means of superposition on AC power, the receiving device
comprising: receiving means for extracting data superimposed on AC
power from a particular channel of the network thereby receiving
said data; timer means for measuring a channel usage period during
which a communication channel is used by the receiving means to
receive data; storage means for storing the channel usage period
measured by the timer means, for each communication channel;
calculation means for calculating the mean channel usage period of
each communication channel from the channel usage periods stored in
the storage means; and control means which controls the storage
means so as to store the channel usage period measured by said
timer means for each communication channel, and which controls the
receiving means so as to select a communication channel used to
receive data on the basis of the mean channel usage periods of the
respective communication channels calculated by the calculation
means and so as to receive the data using the selected channel.
[0023] According to another aspect of the present invention, there
is provided a transmitting device for transmitting input data via a
power line transmission network which has a plurality of
communication channels and which transmits data by means of
superimposing data upon AC power, the transmitting device
comprising: compression means for compressing the input data;
packet conversion means for converting the data compressed by the
compression means into packets each having a predetermined length
of data; transmitting means for transmitting the data converted
into packets by the packet conversion means via the plurality of
communication channels at the same time.
[0024] According to still another aspect of the present invention,
there is provided a receiving method for receiving data via a power
line transmission network which transmits the same data with the
same timing in parallel via a plurality of communication channels
by means of superposition on AC power, the receiving method
comprising the steps of: extracting data superimposed on AC power
from a particular channel of the network thereby receiving the
data, and measuring a channel usage period during which a
communication channel is used to receive the data; when a
communication failure is detected during the reception of the data,
calculating the mean channel usage period of the particular channel
from the channel usage period measured during the current receiving
operation and from the channel usage periods of the particular
channel measured in the past; selecting a communication channel to
be used, on the basis of the calculated mean channel usage period
and the mean channel usage periods calculated for the other
respective communication channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic diagram illustrating an automatic
channel switching operation according to a conventional
technique;
[0026] FIG. 2 is a schematic diagram illustrating an example of the
general configuration of a transmitting and receiving system
according to an embodiment of the present invention;
[0027] FIG. 3 is a block diagram illustrating an example of the
internal configuration of a server;
[0028] FIG. 4 is a block diagram illustrating an example of the
internal configuration of a client;
[0029] FIG. 5 is a schematic diagram illustrating the format of
encoded data generated by an encoding process performed by the
server and also illustrating the format of decoded data generated
by a decoding process performed by the client;
[0030] FIGS. 6A and 6B are time charts illustrating a specific
example of the channel switching operation according to the
embodiment of the present invention;
[0031] FIG. 7 is a schematic diagram illustrating changes in the
content of a history table, corresponding to the automatic channel
switching operation shown in FIG. 6;
[0032] FIG. 8 is a flow chart illustrating the automatic channel
switching operation according to the embodiment of the present
invention; and
[0033] FIG. 9 is a flow chart illustrating the operation of
determining an initial channel.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] The preferred embodiments of the present invention are
described below in the following order:
[0035] 1. System Configuration
[0036] 1.1 General Configuration
[0037] 1.2 Server
[0038] 1.3 Client
[0039] 2. Channel Switching
[0040] 2.1 Specific Examples
[0041] 2.2 Process
[0042] 1. System Configuration
[0043] 1.1 General Configuration
[0044] A receiving device according to a present embodiment serves
as a client in a transmission/reception system for
transmitting/receiving, via a power line, an audio or video signal
output from an AV (audio visual) device. The configuration of the
transmission/reception system of the present embodiment is
described first.
[0045] FIG. 2 illustrates an example of the general configuration
of the system of the present embodiment.
[0046] As shown in FIG. 2, the system of the present embodiment
includes at least two electronic devices, that is, a server 100
serving as a transmitting device, and a client 200 serving as a
receiving device. A CD (Compact Disc (TM)) player 300 serving as an
AV device is connected to the server 100. An audio signal recorded
on a CD is read by the CD player 300 and input to the server
100.
[0047] The server 100 is connected to a power line 2 via an outlet
3 so that commercial AC power 1 required for the operation of the
server 100 is supplied to the server 100. The server 100 performs
signal processing such as modulation upon the audio signal received
from the CD player 300, as described later, and superimposes the
resultant signal upon the power line thereby transmitting the
signal to the client 200.
[0048] The client 200 is also connected to the power line 2 via an
outlet 3 so that commercial AC power 1 required for the operation
of the client 200 is supplied to the client 200.
[0049] The client 200 is capable of receiving and demodulating the
audio signal which is transmitted from the server 100 via the power
line 2. The resultant audio signal is output from the client 200
and supplied to a speaker 400 connected to the client 200. Thus,
the audio signal recorded on the CD is transmitted from the server
100 and a corresponding audio sound is output from the speaker
400.
[0050] When such a system is used in a home, the server 100 and the
client 200 may be placed in different rooms. For example, an audio
set serving as the server 100 is placed in a living room, while the
client 200 may be placed in a bedroom so that the audio sound
played back by the audio set serving as the server can be listened
to via the client 200 in the bedroom.
[0051] 1.2 Server
[0052] FIG. 3 illustrates an example of the internal configuration
of the server 100.
[0053] In this example, the server 100 has an external audio input
terminal 101 which is designed, for the purpose of general
versatility, to accept an analog audio signal. The analog audio
signal output terminal of the CD player 300 is connected to the
external audio input terminal 101 so that the audio signal output
from the CD player 300 is input to the server 100.
[0054] The analog audio signal, which is read from the CD and input
via the external audio input terminal 101, is first applied to an
A/D converter 102.
[0055] A digital audio signal (digital audio data) is output from
the A/D converter 102 and applied to a buffer memory 103. The A/D
converter 102 also outputs a clock CLK synchronized with the
digital audio data converted from the analog audio signal. The
clock CLK is input to a timing generator 108. On the basis of the
clock CLK, the timing generator 108 generates a clock used to
control the timing of the operation of the buffer memory 103 and a
compression circuit 104, which will be described later. The
generated clock is supplied to the buffer memory 103 and the
compression circuit 104.
[0056] The buffer memory 103 temporarily stores the input digital
audio data. The digital audio data is then read from the buffer
memory 103 and supplied to the compression circuit 104. The
compression circuit 104 compresses the received audio data
according to a predetermined scheme and outputs the resultant
compressed data to an encoder 105. In accordance with a
predetermined scheme, the encoder 105 adds an error detection code
and a synchronization pattern to the compressed audio data and
encodes the audio data into a form suitable for transmission over a
power line.
[0057] An feature of the present embodiment is in that the encoder
105 performs rearrangement of the audio data in terms of time as
shown in FIG. 5A.
[0058] More specifically, as shown in FIG. 5A, the audio data is
divided into a plurality of packets, and the packets are arranged
in the order D1.fwdarw.D1.fwdarw.D2.fwdarw.D2.fwdarw.D3.fwdarw.D3
and so on. Packet data D1, D2, and D3 each include a fixed length
of audio data corresponding to a predetermined length of playback
time. If the packets are concatenated in the order
D1.fwdarw.D2.fwdarw.D3, the obtained audio data becomes correctly
continuous in terms of time.
[0059] The encoder 105 transmits each same packet of the audio data
twice successively such that that a succession of two packets
having the same content is transmitted.
[0060] The reason which the encoder 105 can transmit each same
packet of the audio data twice successively is that the compression
of the audio data performed by the encoder 105 results in a
reduction in the time required for transmission. The number of
packets including the same content is determined depending upon the
compression ratio of the compression process performed by the
encoder 105. For example, if the data is compressed by the encoder
105 to a data size one-fifth the original data size, the compressed
data can be transmitted, via each channel, with a transmission
efficiency improved by a factor of five. This means that each
channel is not used for 4/5of unit time. Therefore, the same packet
of the compressed data can be transmitted successively a plurality
of times, as long as the number of repetition times that each same
packet is transmitted is equal to or less than the reciprocal of
the compression ratio, that is, equal to or less than 5 in this
specific example. That is, the number of repetition times that the
same packet is transmitted is determined depending upon the
compression ratio of the data compressed by the encoder 105.
[0061] When the client 200 receives the data arranged as shown in
FIG. 5A, the client 200 performs decoding by selecting one of the
two successive packets including the same content. The selected
packets are then concatenated as shown in FIG. 5B.
[0062] In this specific example, data is transmitted in the order
D1.fwdarw.D1.fwdarw.D2.fwdarw.D2.fwdarw.D3.fwdarw.D3 as shown in
FIG. 5A. A packet D1 located at the first position of the
succession of packets D1 and D1 is first selected. A packet D2 at
the second position of the succession of packets D2 and D2 is then
selected. Furthermore, a packet D3 at the first position of the
succession of packets D3 and D3 is selected. The selected packets
D1, D2, and D3 are then combined in this order so as to obtain a
series of packets placed at correct positions, in terms of time,
corresponding to the original audio data.
[0063] The determination as to which of the two successive packets
having the same content should be selected is made depending upon
the reception condition under which the packets are received. More
specifically, for example, the selection is made depending upon the
quality of the received signal. If the first packet of two
successive packets having the same content is better in signal
quality, the first packet is employed. Conversely, if the second
packet is better, the second packet is employed.
[0064] The above-described manner in which the data is transmitted
and received makes it possible that data is maintained in a correct
time series even when the reception channel is switched during the
normal operation, as will be described in detail later.
[0065] A modulator 106 modulates the encoded data output from the
encoder 105.
[0066] In the present embodiment, three carrier generators 107a,
107b, and 107c are connected to the modulator 106. The carrier
generators 107a, 107b, and 107c generates carries with different
frequencies f1, f2, and f3.
[0067] The modulator 106 modulates the carries generated by these
carrier generators 107a, 107b, and 107c, in accordance with the
encoded data output from the encoder 105.
[0068] Because the modulating signal is a digital signal, FSK
(Frequency Shift Keying), which is one of digital modulation
schemes, is employed as the modulation method for the modulation
performed by the modulator 106. The three data modulated according
to the FSK method using carrier frequencies f1, f2, and f3 are
transmitted in a multiplexed fashion.
[0069] That is, in the present embodiment, the same data is
transmitted via three channels. More specifically, the same data
encoded as shown in FIG. 5A is transmitted at the same time via the
three channels.
[0070] The modulation performed by the modulator 106 may be
performed according to a method other than the FSK. For example,
PSK (Phase Shift Keying, spread spectrum modulation, or other
digital modulation techniques may also be employed. Furthermore,
the modulation signal may be an analog signal. In this case,
frequency modulation or amplitude modulation may be employed.
[0071] The data signal modulated by the modulator 106 is
superimposed on the power line 2 such that the data signal is
transmitted as a power line signal.
[0072] A controller 112, including a microcomputer, ROM, and RAM,
controls the operation of various functional circuits of the
server.
[0073] A control command input unit 113 includes various command
buttons for inputting various commands to the server 100. A command
output from the control command input unit 113 is applied to the
controller 112. In response to the received command, the controller
112 performs a control operation.
[0074] A display 114 displays, under the control the controller
112, information corresponding to the current operation status.
[0075] 1.3 Client
[0076] FIG. 4 illustrates an example of the internal configuration
of the client 200.
[0077] The power line signal received via the power line 2 is
applied to a channel selector 214. In the present embodiment, as
described above, the same audio data is received in parallel via
the thee channels. The channel selector 214 selects, under the
control of a controller 210, one of the three channels.
[0078] For the above purpose, the channel selector 214 includes a
bandpass filter (not shown) whose passband can be switched among
the thee carrier frequencies f1, f2, and f3. Under the control of
the controller 210, the passband of the bandpass filter is switched
so as to pass only one of carrier frequencies f1, f2, and f3
thereby selecting a desired channel.
[0079] The received signal of the channel selected by the channel
selector 214 is then applied to an RF amplifier 201. The RF
amplifier 201 extracts a signal component superimposed on the power
signal. The resultant signal is then detected by a detector 202 and
thus a data signal is extracted. In the present embodiment, the
data signal transmitted from the server 100 to the client is audio
data. However, in practice, various command signals and control
information are also transmitted. The audio data is applied to a
decoder 203, while the command signal is applied to the controller
210.
[0080] The audio data output from the detector is also supplied to
a timing generator 208. The timing generator 208 detects, for
example, the synchronization pattern added to the audio data and
generates a clock on the basis of the detected synchronization
pattern. The generated clock is output to a decoder 203, a buffer
memory 204, and a decompression circuit 205, which will be
described later, thereby controlling the timing of the operation of
the decoder 203 the buffer memory 204, and the decompression
circuit 205.
[0081] The audio data supplied to the decoder 203 is first
subjected to error detection performed by an error detection
circuit 203a. FSK-decoding process is then performed. The decoded
data is temporarily stored in the buffer memory 204 and then output
to the decompression circuit 205. The decoded data output from the
decoder has the form described above with reference to FIG. 5B.
That is, in the case of audio data, compressed audio data in the
form of a correct time series is obtained.
[0082] If the error rate detected during the error detection
process performed by the error detection circuit 203a is greater
than a predetermined level, the error detection circuit 203a
determines that an error occurs and outputs an error notification
signal Ser indicating the occurrence of the error to the controller
210. In response to the error notification signal Ser, the
controller 210 performs an automatic channel switching operation as
will be described in detail later.
[0083] The data supplied to the decompression circuit 205 is
decompressed, and the resultant data is applied to a D/A converter
206.
[0084] The D/A converter 206 converts the applied audio data into
an analog audio signal and outputs the resultant signal to an
amplifier 207. The amplifier 207 amplifies the received audio
signal and outputs the amplified audio signal to a speaker.
[0085] The controller 210 includes, for example, a microcomputer, a
ROM, and a RAM, and controls the operation of various parts of the
client.
[0086] In the present embodiment, there is provided a history table
211 used by the controller 210. As will be described in detail
later, channel quality information representing the history about
the reception condition of channels is recorded in the history
table 211. In response to switching of the reception channel during
the normal operation, the controller 210 updates the channel
quality information described in the history table 211. When an
error occurs, the controller 210 selects a channel on the basis of
the content of the history table 211 and switched the reception
channel to the selected channel.
[0087] The history table 211 may be stored in a particular memory
area of the RAM in the controller 210.
[0088] A control command input unit 213 includes various command
buttons for inputting various commands to the client 200. A display
213 displays, under the control the controller 210, information
corresponding to the current operation status.
[0089] 2. Channel Switching
[0090] 2.1 Specific Examples
[0091] In the transmission/reception system according to the
present embodiment, as described above, information is transmitted
using a plurality of channels, for example, three channels. The
receiving device serving as the client 200 selects one of the
plurality of channels and receives the information via the selected
channel. By selecting a channel having high channel quality from
the plurality of channels, it becomes possible for the receiving
device to substantially always output a high-quality audio or video
signal.
[0092] In the present embodiment, the receiving device serving as
the client 200 is constructed so as to automatically switch the
channel depending upon the reception condition of the respective
channels thereby making it possible to maintain a better reception
condition than can be achieved by the conventional technique in
which the channel is simply switched in the fixed order.
[0093] FIGS. 6A and 6B are time charts illustrating a specific
example of the channel switching operation performed by the client
200.
[0094] In the present embodiment, the automatic channel switching
operation consists of an operation for determining an initial
channel which is first selected when the client 200 is started up
and a normal operation performed after the determination of the
initial channel.
[0095] FIG. 6A illustrates the operation for determining the
initial channel. In the following description, the respective three
channels are distinguished by channel number and represented such
as channel #1, channel #2, and channel #3.
[0096] At a time t0 in FIG. 6A, the electric power of the client
200 is turned on and thus the operation of the client 200 is
started. The client 200 first selects, for example, the channel #1
for a predetermined period of time, for example a sec, and counts
the number of errors which occur during the period in which the
channel #1 is selected. The decision as to whether an error has
occurred is made by the controller 210 on the basis of the error
notification signal Ser received from the error detection circuit
203a described earlier with reference to FIG. 4.
[0097] At a time t1 after the passage of time of a sec since the
selection of the channel #1 at time t0, the channel #2 is selected
for a period having the same length of time, a sec, from t1 to t2,
and the number of errors which occur during that period is
counted.
[0098] Similarly, at a time t2, the channel #3 is selected for a
period having the same length of time, a sec, from t2 to t3, and
the number of errors which occur during that period is counted.
[0099] Thus, at the time t3, information about the number of errors
for the same length of time, a sec, is obtained for the respective
channels #1, #2, and #3. In the present embodiment, the number of
occurrences of errors is compared among the channels, and a channel
having the smallest number of occurrences of errors is employed as
the initial channel.
[0100] The employment of the channel having the smallest number of
occurrences of errors is equivalent to the selection of a channel
which is currently best in terms of the reception condition, that
is, the selection of a channel having the highest channel
quality.
[0101] The selection of the initial channel according to the
present embodiment provides the following advantages.
[0102] In practice, noise generated by a device connected to a
power line generally has frequency components limited in a
particular fixed frequency band, depending upon the type of the
device. In such a case, only a fixed channel is influenced and
degraded by the noise.
[0103] Therefore, once a channel having the best initial channel
quality is employed as the initial channel, it is not necessary, in
many cases, to switch the channel to another channel during the
operation after the determination of the initial channel.
[0104] In the present embodiment, during the normal operation, the
channel is automatically switched depending upon the condition in
terms of the error occurrence, as will be described later. If the
normal operation is started using a predetermined channel without
performing the initial channel selection described above, it takes
a long time to reach a high-quality channel, and a high-quality
reception is impossible until reaching the high-quality channel. In
contrast, if the initial channel is selected in the manner
described above with reference to FIG. 6A, it is possible to start
the normal operation under the good reception condition.
[0105] Even in the normal operation after selecting the initial
channel, there is a possibility that the reception condition of the
channel, that is, the channel quality changes due to a change in
the environment or a change in the operation condition of the
device itself or due to other factors. In the present embodiment,
in order to handle such a change, the channel is switched during
the normal operation, as will be described in detail later.
[0106] In the channel switching during the normal operation, the
channel to be employed next is determined on the basis of the
content of the history table 211.
[0107] An example of a change in the content of the history table
211 is shown in FIGS. 7A to 7H. An example of the normal operation
is described below with reference to FIG. 6B and FIGS. 7A to
7H.
[0108] Herein, let us assume that the channel #1 is selected as the
initial channel in the operation described above with reference to
FIG. 6A.
[0109] When the normal operation is started at a time t11 in FIG.
6B, the initial channel #1 is also employed as the channel for
reception. The controller starts the measurement of the length of
the channel usage period during which the currently selected
channel (current channel) is continuously used.
[0110] Immediately after the start of the normal operation at time
t11 in FIG. 6B, no history information is described in the history
table 211.
[0111] Let us assume that an error occurs at a time t12 when 5 sec
has elapsed from time t11. When the first error occurs during the
normal operation, the current channel is switched to another
channel.
[0112] When the error occurs, the measured channel usage period is
described in the history table 211 and stored in a memory. The
content of the history table 211 at time t2 is shown in FIG. 7A.
The mapping structure of the history table 211 is described
below.
[0113] In the history table 211, rows 1, 2, and 3 are assigned to
the channel #1, #2, and #3, as shown in FIG. 7A. The length of the
channel usage period is described row by row, in the order of row
number, each time the same channel is employed. In the fourth row,
the mean channel usage period calculated from the values currently
described in the first to third rows for each channel is described.
That is, in the history table 211, the channel usage periods for
the last three usages of each channel and the mean value thereof
are described as the history information representing the history
of the reception condition or the channel quality.
[0114] In the example shown in FIG. 6B, at time t12, the channel
usage period for the channel #1 employed as the initial channel is
measured as 5 sec. Thus, the value of 5 is described in the first
row in the column corresponding to the channel #1, as shown in FIG.
7A. At this point of time, no valid values are described in the
second and third row for the channel #1. In FIG. 7A, the absence of
the valid value is represented by a symbol "-". The same
representation is used also in FIGS. 7B to 7H. At this point of
time, the mean channel usage period of the channel #1 is also 5
sec, and thus the value of 5 is described in the fourth row. At
this point of time, the channels #2 and #3 have not been used at
all, and thus no valid values are described in the history table
211 for the channel #2 and #3.
[0115] The channel to be employed for the operation after time t12
cannot be determined from the content of the history table 211
because the channels other than channel #1 have not been used and
no information about the channels #2 and #3 is described in the
history table 211. For the above reason, at this point of time, the
channel is switched to channel #2 having a channel number
immediately following that of the current channel #1.
[0116] Herein, let us assume that at time t13 after the passage of
time of 6 sec from t12, an error occurs in the channel #2 selected
at time t12. In this case, in the history table 211, a value of 6
is described in the first row for the channel #2 as shown in FIG.
7B, to indicate that the measured channel usage period of the
channel #2 is 6 sec, and a value corresponding to 6 sec is
described as the mean value for the channel #2.
[0117] At this point of time, the channel #3 having a channel
number following that of the channel #2 remains unused. Thus, the
channel is switched to the channel #3. Let us further assume that
an error occurs at time t14 after the passage of time of 4 sec from
t13 at which the channel #3 was selected. In response to the
occurrence of the error, a value of 4 is described in the first row
for the channel #3 in the history table 211 as shown in FIG. 7C, to
indicate that the measured channel usage period of the channel #3
is 4 sec, and a value corresponding to 4 sec is described as the
mean value for the channel #3.
[0118] At this point of time t14, information about the mean
channel usage period of all channels becomes available from the
history table 211, as shown in FIG. 7C.
[0119] Therefore, in the operation after this point of time, the
channel to be employed for the next usage is determined on the
basis of the mean channel usage periods described in the history
table 211, as described below.
[0120] A greater value of the mean channel usage period indicates a
better reception condition or better channel quality. Conversely, a
small value of the mean channel usage period indicates that the
channel quality has been bad. Therefore, in the present embodiment,
a channel having the greatest mean channel usage period in the
history table 211 is selected for the next usage.
[0121] In the example shown in FIG. 7C, the history table 211
indicates that the channel #2 has the greatest mean channel usage
period at this point of time. Thus, the channel #2 is employed in
the operation after time t14.
[0122] In FIG. 6B, an error occurs at time t15 after the passage of
time of 2 sec from t14 at which the channel #2 was selected. In
response to the occurrence of the error at time t15, the content of
the history table 211 is updated as shown in FIG. 7D. That is, in
addition to the channel usage periods of the respective channels
described in the first row, a value of 2 corresponding to the
channel usage period of 2 sec from t14 to t15 for the channel #2 is
described in the second row.
[0123] At this point of time, the channel usage periods are
described in both first and second rows for the channel #2. Thus,
the mean value calculated from the values of these two channel
usage periods is described in the fourth row. More specifically,
the mean value is calculated as (6+2)/2=4, and thus the mean value
for the channel #2 is replaced with 4.
[0124] In FIG. 7D, the history table 211 indicates that, among all
channels, the channel #1 has the largest mean channel usage period.
Thus, the channel #1 is employed in the following operation after
time t15.
[0125] In FIG. 6B, an error occurs at time t16 after the passage of
time of 5 sec from t15 at which the channel #1 was selected.
[0126] In response to the occurrence of the error at time t16, a
value of 5 indicating the channel usage period of 5 sec is
described in the second row for the channel #1. The mean channel
usage period of the channel #1 is calculated as (5+ 5)/2=5, and
thus a value of 5 is described in the fourth row for the channel
#1.
[0127] The history table 211 shown in FIG. 7E indicates that the
channel #1 has the greatest mean channel usage period at this point
of time.
[0128] That is, the content of the history table shown in FIG., 7E
indicates that the channel #1, which was used in the operation
before time t16, is still best in channel quality. Therefore, the
channel #1 is maintained without being switched to another channel.
Thus, in FIG. 6B, the channel #1 is further used in the operation
after time t16.
[0129] In the present embodiment, as described above, when an error
occurs in the currently used channel, if the history information
indicates that the current channel is still best in channel
quality, the use of the current channel is maintained. The reason
is described in further detail below.
[0130] For example, when an error due to momentary noise occurs in
the current channel which have had a good reception condition for a
long continuous period of time, further maintaining the use of the
current channel without selecting another channel having a worse
reception condition can result in achievement in a better reception
condition. Thus, the above-described operation according to the
present embodiment can provide a better reception condition.
[0131] Let us assume that, at time 17 after the passage of time of
2 sec since t16, an error occurs in the channel #1 which was not
switched to another channel at time t16 but has been further
used.
[0132] In response to the occurrence of the error, a value of 2
indicating the channel usage period of 2 sec is described in the
history table 211, in the third row in the column corresponding to
the channel #1, as shown in FIG. 7F. The mean channel usage period
is calculated as (5+5+2)/3=4, and thus the value of the mean
channel usage period in the history table 211 is replaced with a
value of 4.
[0133] At this pint of time, the content of the history table
indicates, as can be seen from FIG. 7F, that all channels have the
same mean channel usage period, that is, 4 sec. In this case, there
are a plurality of channels having the maximum mean channel usage
period, and the current channel is one of such channels.
[0134] In this case, the current channel is switched to another
channel. When there is only one channel, in addition to the current
channel, that has the maximum mean channel usage period, the
current channel is switched to that channel. However, when there
are two or more such channels in addition to the current channel,
the current channel is switched to one of such channels in
accordance with a predetermined rule. For example, the switching
may be performed in the ascending order of channel numbers.
[0135] Thus, in this specific example, the channel #1 which has
been used till time t17 is switched to the channel #2.
[0136] In the example shown in FIG. 6B, an error occurs in the
channel #2 at time t18 after the passage of time of 2 sec from t17
at which the use of the channel #2 was started.
[0137] In response to the occurrence of the error, a value
corresponding to the channel usage period of 2 sec is described in
the third row in the column corresponding to the channel #2, as
shown in FIG. 7G. The mean channel usage period of the channel #2
is calculated as (6+2+2)/3.apprxeq. 3.3, and thus the values of the
mean channel usage period of the channel #2 is replaced with
3.3.
[0138] At this point of time, as shown in FIG. 7G, the history
table 211 indicates that the channels #1 and #3 both have a mean
channel usage period of 4 and the channel #2 has a mean channel
usage period of 3.3.
[0139] In this case, the channel #2 is switched to the channel #1
or #3. In the present embodiment, a channel having a smaller
channel number, that is, the channel #1 is selected. Thus, the
channel #1 is used in the operation after time t18. Note that the
channel #3 may be selected instead of the channel #1.
[0140] In the example shown in FIG. 6B, an error occurs at time t19
after the passage of time of 3 sec from t18 at which the channel #1
was selected. In response to the occurrence of the error, the
history table 211 is updated as shown in FIG. 7H.
[0141] At time t18, the values of the channel usage period of the
channel #1 in the past three usages were already described in the
first to third rows of the history table 211. In such a case, the
value is described again from the first row, and the value for the
past usage is replaced with a new value each time the channel is
used.
[0142] In the example shown in FIG. 7H, the value of the channel
usage period in the first row in the column corresponding to the
channel #1 is replaced with a new value of 3 sec. In response, the
mean channel usage period of the channel #1 is recalculated. In
this specific example, the mean channel usage period is calculated
as (3+5+2)/3= 10/3.apprxeq.3.3, and thus the mean channel usage
period of the channel #1 is replaced with 3.3.
[0143] At this point of time, the content of the history table 211
shown in FIG. 7H indicates that the channel #3 has the greatest
mean channel usage period. As a result, the channel #3 is selected
for the use after time t19.
[0144] In the present embodiment, the channel switching is
automatically performed in the manner described above.
[0145] In the present embodiment, as described above, the automatic
channel switching is performed on the basis of the values of
channel usage periods which indicate the channel quality. This
makes it possible to switch the channel on the basis of the actual
channel quality of the respective channels so as to obtain a better
reception condition than can be achieved by the conventional
technique in which the channel is simply switched in a
predetermined order.
[0146] Furthermore, in the present embodiment, the values of the
channel usage periods in a predetermined number of usages (three
usages in the example shown in FIG. 7) in the past are stored
thereby making it possible to select a still better channel on the
basis of the mean channel usage values indicating the channel
quality of the respective channels.
[0147] Although in the specific example described above, the values
of the channel usage periods in the last three usages for each
channel are stored in the history table, and the mean value of the
channel usage periods in the last three usages is calculated, the
number of values used to calculate the mean channel usage period is
not limited to three.
[0148] However, if the number of samples of the channel usage
periods is too small, a momentary reduction in the channel usage
period due to a rare error can cause the mean channel usage period
to deviate from a value representing the real channel quality.
[0149] Conversely, if the number of samples of the channel usage
periods is too large, when the channel quality of the respective
channels changes with the passage of time, it takes a long time to
obtain a mean channel usage period which corresponds correctly to
the real channel quality.
[0150] Therefore, the number of samples of channel usage periods
should be properly determined taking into account the factors
described above.
[0151] 2.2 Process
[0152] The channel switching operation according to the present
embodiment described above with reference to FIGS. 6A and 6B and
FIGS. 7A to 7H is described in further detail with reference to the
flow charts shown in FIGS. 8 and 9. The operation shown in FIGS. 8
and 9 is performed by the controller 210 of the client 200.
[0153] If the power of the client 200 is turned on, the operation
of the controller 210 is started in step S101 in FIG. 8. In the
next step S102, the initial channel is determined in the manner
described above with reference to FIG. 6A.
[0154] The process of step 102 for determining the initial channel
is described in further detail below with reference to FIG. 9.
[0155] In step S201, a variable n representing the channel number
is set such that n=1. In the next step S202, a variable m
representing the number of occurrences of errors is set such that
m=0.
[0156] In step S203, the channel selector 214 is controlled such
that a channel #n corresponding to the current value of the
variable n is selected.
[0157] In the next step S204, it is determined whether a
predetermined length of time (a sec) defined as the initial channel
usage period for each channel has elapsed. If the decision in step
S204 is negative, the process goes to step S205.
[0158] In step S205, it is determined whether an error has occurred
on the basis of an error notification signal Ser output from the
error detection circuit 203a in the decoder 203.
[0159] If it is determined in step S205 that there is no error, the
process returns to step S204. However, if an error is detected in
step S205, the process goes to step S206. In step S206, the
variable m is incremented such that m=m+1, and then the process
returns to step S204.
[0160] In steps S204 to S206 the number of occurrences of errors
during the predetermined channel usage period with a length of a
sec is counted for each channel.
[0161] If it is determined in step S204 that the predetermined
length of time has elapsed, the process goes to step S207.
[0162] In step S207, the current value of the variable m is stored
as the number of occurrences of errors for the currently used
channel #n. More specifically, the number of occurrences of errors,
m, is stored in a predetermined memory area of an internal RAM. In
the next step S208, it is determined whether the current value of
the variable n is equal to the maximum allowable value. In this
specific embodiment, it is determined whether n=3. If the decision
in step S208 is negative, there is a channel which is to be
evaluated in terms of the number of occurrences of errors, and thus
the process goes to step S209. In step S209, the variable n is
incremented such that n=n+1, and then the process returns to step
S202.
[0163] On the other hand, if the decision in step S208 is
affirmative, the process goes to step S210.
[0164] In step S210, the value of m representing the number of
occurrences of errors, which has been measured and stored in the
internal RAM, is compared among the channels #1 to #3, and the
channel selector 214 is controlled such that a channel having the
smallest value is selected. Thus, in step S210, the channel is
switched to the initial channel.
[0165] After determining the initial channel in the process shown
in FIG. 9, the process goes to step S103 in FIG. 8 to start a
normal receiving operation in which the channel is switched as
required. That is, the process, an example of which is shown in
FIG. 6B, is started.
[0166] In step S103, a timer disposed in the controller 210 is
reset, that is, the timer time T is reset to the initial value such
that T=0. The timer is then started.
[0167] The process then waits in step S104 until an error occurs.
If an error is detected in step S104, the process goes to step
S105.
[0168] In step S105, the measured timer time T is employed as the
channel usage period of the current channel and is written in a
particular field of the history table 211 wherein the field is
determined in the manner described above with reference to FIG.
7.
[0169] In the next step S106, the mean channel usage period of the
current channel is calculated from the values of channel usage
periods described in the history table 211. In the next step S107,
the calculated mean value is written in the history table 211, in
the field corresponding to the current channel.
[0170] In step S108, the current values of the mean channel usage
periods of the respective channels #1 to #3, described in the
history table 211, are compared with each other. If the comparison
indicates that there is no channel having a mean channel usage
period greater than that of the current channel, that is, if only
the current channel has the maximum mean channel usage period, the
process returns to step 103. In this case, the currently used
channel is further used without being switched to another channel,
as is the case at time t1 in FIG. 6B.
[0171] On the other hand, the process goes to step S109 when the
comparison in step S108 indicates one of the following results: 1)
there is another channel, other than the current channel, which has
the maximum mean channel usage period; 2) the current channel and
another channel have the same maximum mean channel usage period;
and 3) there is a channel which has not been used yet. In this
case, the current channel is switched to another channel depending
upon which of results 1) to 3) is obtained in the comparison in
step S108, as described below.
[0172] If the comparison result is 1), the current channel is
switched to another channel having the maximum mean channel usage
period. When there are a plurality of channels, other than the
current channel, which have the maximum mean channel usage periods,
one of such channels is selected in accordance with a predetermined
rule.
[0173] In the case of 2), if there is only one channel, other than
the current channel, which has the same maximum channel usage
period as that of the current channel, that one channel is
selected. When there are three or more channels are used, as is the
case in the present embodiment, there is a possibility that there
are a plurality of channels which have the same maximum channel
usage period as that of the current channel. Also in this case, one
of such channels is selected in accordance with a predetermined
rule.
[0174] In the case of 3), one of channels which are detected as
having not been used yet is selected in accordance with a
predetermined rule.
[0175] After completion of step S109, the process returns to step
S103.
[0176] Steps S103 to S109 are performed repeatedly during the
normal operation thereby automatically switching the channel in
response to an occurrence of error, as described earlier with
reference to FIG. 6B.
[0177] Although not described above, information indicating which
channel is currently selected may be displayed on the display 114
so as to notify the user of the current status in terms of the
usage of the channel.
[0178] In the present invention, the automatic channel switching
operation is not limited to that described above with reference to
the specific embodiment. For example, only the process of
determining and selecting the initial channel described earlier
with reference to FIG. 6A may be performed, and the automatic
channel switching operation described earlier with reference to
FIG. 6B may not be performed. In usual power lines, because
channels having bad quality are generally fixed, the execution of
only the process of selecting the initial channel can provide great
benefits.
[0179] The details of the automatic channel switching operation
according to the present embodiment may be modified as required.
For example, when an error is detected, if there are a plurality of
channels, including the current channel, which have the same
maximum mean channel usage period, the current channel may be
further used without being switched to another channel, or one of
such the plurality of channels may be selected in a random fashion
and the current channel may be switched to the selected
channel.
[0180] The details of the server and the client may also be
modified. For example, the server may have a receiving capability
and the client may have a transmitting capability so that various
kinds of data such as control data may be transmitted between the
server and the client.
[0181] In the example shown in FIG. 4, an amplifier is disposed in
the inside of the client. Alternatively, an output terminal for
outputting a source signal in an analog or digital form may be
provided on the client and another external amplifier or an audio
device may be coupled to the client via the output terminal so that
an audio/video signal is output from a speaker or a monitor device
connected to the external amplifier or the audio device.
[0182] Furthermore, the source signal transmitted from the server
is not limited to a signal output from a CD player. For example, a
signal output from another type of digital audio device such as an
MD (Mini Disc) player or a DAT (Digital Audio Taperecorder) may
also be transmitted. A signal output from a conventional cassette
tape recorder or a tuner may also be transmitted. The type of the
signal transmitted from the server is not limited to the audio
signal, but another signal such as a video signal may also be
transmitted. In this case, various types of AV devices such as a
VTR (Video Tape Recorder), a DVD player for reproducing a video
signal recorded on a DVD, and a television set may be connected to
the server.
[0183] In the present invention, as described above, the same
information is transmitted via a plurality of channels (a plurality
of different carriers) over a power line, and a receiving device
obtains channel quality information representing the channel
quality of the respective channels and selects a channel having the
best channel quality on the basis of the channel quality
information.
[0184] This capability of preferentially selecting the best channel
depending upon the current reception condition prevents a long-time
reception failure which often occurs in the conventional technique.
In other words, the present invention assures a good reception
condition over a long period of time.
[0185] The channel quality information is represented on the basis
of a period from a time when a channel is selected to a time when
degradation in the channel quality (error) occurs. The
representation of the channel quality information on the basis such
a period of time makes it possible to easily obtain the channel
quality information by simple processing without having to perform
complicated calculations.
[0186] By storing the channel quality information in a memory
(storage means, memory area), it becomes possible to properly
switch, when an error occurs, the channel to a better channel on
the basis of the history of the channel quality information.
[0187] Using the channel quality information stored in the memory,
it is possible to obtain information about the change in channel
quality for each channel. On the basis of the information about the
change in the channel quality, it is possible to properly select
the channel to which the current channel is to be switched. This
makes it possible to select the channel in a more adequate
fashion.
[0188] The channel quality information includes information about
the period from a time at which a channel is selected to a time at
which degradation in channel quality due to an error occurs, and
the mean period is calculated from the values of the periods stored
in the memory.
[0189] The change in the channel quality can be evaluated from the
above mean period. On the basis of the mean periods of the
respective channels, a channel to which the current channel is to
be switched is selected. Thus, it is possible to precisely select a
proper channel by simply calculating the mean period.
[0190] On the basis of information about the channel quality of
each channel in the past, which is obtained from the channel
quality information of each channel stored in the memory, it is
determined whether the current channel should be further used
without being switched to another channel or the current channel
should switched to another channel.
[0191] In the case where a momentary and rare error occurs in the
current channel which is better in channel quality than the other
channels, the use of the current channel can be maintained without
being switched to another channel which worse in channel quality.
Also in this sense, the channel can be selected in a more adequate
fashion, and a better reception condition can be steadily
obtained.
[0192] In the present invention, the starting-up operation is
performed such that channels are selected one by one in fixed
intervals, and the channel quality of each channel is evaluated. On
the basis of the obtained channel quality information, a channel
having the best channel quality is selected as an initial
channel.
[0193] The selection of the initial channel in the above-described
manner makes it unnecessary to select a best channel after starting
the receiving operation and makes it possible to start the
receiving operation using the channel having the best channel
quality.
[0194] In practical power line transmission/reception systems,
noise generally has frequency components limited in a particular
fixed frequency band. Therefore, if the initial channel has good
channel quality, it is seldom necessary to switch the channel to
another channel during the following operation. That is, good
reception can be obtained for a long period of time immediately
after starting the operation.
* * * * *