U.S. patent application number 11/499126 was filed with the patent office on 2007-02-08 for system and methods for aligning capture and playback clocks in a wireless digital audio distribution system.
Invention is credited to Michael D. Hudson, Kelly M. McArthur.
Application Number | 20070030986 11/499126 |
Document ID | / |
Family ID | 37728012 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070030986 |
Kind Code |
A1 |
McArthur; Kelly M. ; et
al. |
February 8, 2007 |
System and methods for aligning capture and playback clocks in a
wireless digital audio distribution system
Abstract
A wireless digital audio distribution system includes a
transmitter that performs parallel acquisition of audio data from
plural audio data channels. The acquired data is stored to a first
buffer during intervals defined by a first clock unit. A radio
transmitter provides for the packetization of the parallel
collected data and the inclusion of a timing marker in a
predetermined packet of data transmitted during an interval. A
receiver unit delivers audio output data corresponding to one of
the audio data channels. The receiver unit includes a radio
receiver for receiving packetized data and storing second parallel
collected data to said second buffer. A demultiplexer is coupled to
the second buffer to select the audio output data corresponding to
the selected one of the plural audio data channels, whereby the
audio output data is available for the selective reproduction of
the chosen audio data channel.
Inventors: |
McArthur; Kelly M.;
(Hillsboro, OR) ; Hudson; Michael D.; (Portland,
OR) |
Correspondence
Address: |
GERALD B ROSENBERG;NEW TECH LAW
260 SHERIDAN AVENUE
SUITE 208
PALO ALTO
CA
94306-2009
US
|
Family ID: |
37728012 |
Appl. No.: |
11/499126 |
Filed: |
August 4, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60705723 |
Aug 4, 2005 |
|
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|
60705724 |
Aug 4, 2005 |
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Current U.S.
Class: |
381/311 |
Current CPC
Class: |
H04L 2012/2849 20130101;
H04L 2012/2841 20130101; H04H 20/28 20130101; H04H 20/61 20130101;
H04H 20/89 20130101; H04L 12/40058 20130101; H04S 3/008
20130101 |
Class at
Publication: |
381/311 |
International
Class: |
H04R 5/02 20060101
H04R005/02 |
Claims
1. A wireless digital audio distribution system comprising: a) a
transmitter unit providing for the parallel acquisition of audio
data from a plurality of audio data channels, said transmitter unit
including a first buffer providing for the storage of data
collected over a predetermined interval, a multiplexer providing
for the parallel collection of data from a plurality of data
channels, said multiplexer providing first parallel collected data
to said first buffer, a first clock unit providing a clock signal
defining said predetermined interval, and a radio transmitter
providing for the packetization of said parallel collected data,
said radio transmitter including a timing marker in a predetermined
packet of data transmitted during said predetermined interval; and
b) a receiver unit providing for the delivery of audio output data
corresponding to a predetermined one of said plurality of audio
data channels, said receiver unit including a radio receiver for
receiving said predetermined packet of data, a second buffer
coupled to said radio receiver, said radio receiver providing
second parallel collected data to said second buffer, a
demultiplexer coupled to said second buffer to select said audio
output data corresponding to said predetermined one of said
plurality of audio data channels from said second parallel
collected data, whereby said audio output data is provided for the
selective reproduction of the audio content corresponding to said
predetermined one of said plurality of audio data channels.
2. The wireless digital audio distribution system of claim 1
wherein said radio receiver includes a timer unit responsive to
receipt of said timing marker and wherein said timer unit controls
the sequential transfer of said audio output data from said second
buffer through said demuiltiplexer.
3. The wireless digital audio distribution system of claim 2
wherein receipt of said timing marker synchronizes an audio data
output interval within which said demultiplexer transfers said
audio output data with said predetermined interval.
4. The wireless digital audio distribution system of claim 3
wherein said radio transmitter provides for the redundant
retransmission of said predetermined packet of data during said
predetermined interval.
5. The wireless digital audio distribution system of claim 4
wherein said radio receiver receives multiple copies of said
predetermined packet and wherein said radio receiver selects a
predetermine one said multiple copies for use in providing said
second parallel collected data to said second buffer.
6. The wireless digital audio distribution system of claim 5
further comprising: a) a first error correction unit coupled to
said radio transmitter, said first error correction unit providing
for the inclusion of error correction data in said predetermine
packet of data; and b) a second error correction unit coupled to
said radio receiver, said second error correction unit enabling
selection of said predetermined one said multiple copies.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No(s). 60/705,723 and 60/705,724, all filed Aug. 4,
2005.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] The present application is related to High Quality.
Controlled Latency Multi-Channel Wireless Digital Audio
Distribution System and Methods, Ser. No. ______, filed Aug. 4,
2006 and assigned to the Assignee of the present Application.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention is generally related to the wireless
distribution of high-quality audio signals and, in particular to a
system and methods of distributing high bit rate, multi channel,
audio wirelessly while maintaining a constant, low, playback to
source latency and channel to channel phase coherency.
[0005] 2. Description of the Related Art
[0006] In the audio space there are many places that latency, high
quality, and more than two channels are critical to the quality of
the experience. It is also difficult to retrofit standard spaces
with cables to the support multiple channels of audio. Today's
definition of high end audio in the Home Theater space is 7
channels of audio samples at 48,000 samples per second with 24 bits
of data per sample. Further, the marketplace is rapidly maturing
from 5.1 (6 channel) to 11.1 (12 channel) sound system
requirements.
[0007] Conventional wireless solutions rely on simple, low-cost
radio technologies, such as frequency modulation (FM) and basic
spread spectrum modulation schemes. The consequence of this is a
reduction in the number of bits used for each audio sample, with a
corresponding reduction in dynamic range and audio quality.
[0008] A critical requirement exists in both spaces to minimize and
establish a constant or fixed latency in the system and to keep all
channels aligned in time. Latency refers to time delays measured
from audio source-to-output and from channel-to-channel.
Source-to-output delays are a problem for all sound venues
including, in particular, Home Theater and other video/audio
systems, where the audio program material is synchronized to a
video screen ("lip-sync"). Acoustics engineers generally consider
source-to-output delays greater than 10 milliseconds to be
noticeable. As for latency from channel-to-channel, the human ear
is extremely sensitive to these phase delays and experts describe
audio delivered with channel-to-channel delays greater than 1
millisecond as sounding "disjointed" or "blurry".
[0009] The same data and sampling rate are in use in recording and
sound reinforcement, only the desired number of channels is
generally between 8 and 32. In conferencing use, the latency and
wireless requirement are compounded by a need for accurate routing
of audio paths with intelligent addition of signals and echo
cancellation.
[0010] Consequently, there is a clear need to solve all of these
problems in a wireless audio distribution system.
SUMMARY OF THE INVENTION
[0011] Thus, a general purpose of the present invention is to
provide an efficient wireless, high bit rate, multi channel, audio
system capable of maintaining constant, low, playback to source
latency while further maintaining channel to channel phase
coherency.
[0012] This is achieved in the present invention by providing a
wireless digital audio distribution system that includes a
transmitter that performs parallel acquisition of audio data from
plural audio data channels. The acquired data is stored to a first
buffer during intervals defined by a first clock unit. A radio
transmitter provides for the packetization of the parallel
collected data and the inclusion of a timing marker in a
predetermined packet of data transmitted during an interval. A
receiver unit delivers audio output data corresponding to one of
the audio data channels. The receiver unit includes a radio
receiver for receiving packetized data and storing second parallel
collected data to said second buffer. A demultiplexer is coupled to
the second buffer to select the audio output data corresponding to
the selected one of the plural audio data channels, whereby the
audio output data is available for the selective reproduction of
the chosen audio data channel.
[0013] An advantage of the present invention is base configurations
are immediately capable of distributing 16 channels of audio with a
full 24 bits per sample and 48,000 samples per second.
[0014] Another advantage of the present invention is the initial
preferred embodiments are capable of achieving a fixed, repeatable
inter-channel differential latency of less than 0.001 millisecond
and a fixed, repeatable source to speaker latency of less than 2
milliseconds.
[0015] A further advantage of the present invention is that it
enables multi-channel audio sources to be placed "out-of-view",
while supporting a full complement of audio speakers to be
installed throughout a room without wires. Costly physical rewiring
is not required.
[0016] Still another advantage of the present invention is that the
audio playback delays can be precisely adjusted and maintained in
fixed relation to "tune" audio phasing for specific
listener/speaker positions and room acoustics.
[0017] Yet another advantage of the present invention is that the
transmitters and receivers, as implemented in the preferred
embodiments, can and will coexist with present wireless networking
systems without introducing interference, without loss of audio
fidelity, and while meeting all FCC and CSA certification
requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a block diagram illustrating a system
implementation of a preferred embodiment of the present
invention;
[0019] FIG. 2 is a flow diagram illustrating the pipeline
processing of data through a wireless transmitter and receiver in
accordance with a preferred embodiment of the present
invention;
[0020] FIG. 3 is a block diagram of a wireless audio packet content
transmitter unit constructed in accordance with a preferred
embodiment of the present invention;
[0021] FIG. 4 is a block diagram of a wireless audio packet content
transmitter unit constructed in accordance with a preferred
embodiment of the present invention;
[0022] FIGS. 5A and 5B are block diagrams illustrating the
preferred flow of data through transmitter and receiver units as
implemented in a preferred embodiment of the present invention;
and
[0023] FIG. 6 is a flow diagram showing the bit parallel packing
and of channel data in data words to build audio data packets for
transition and corresponding unpacking on reception as implemented
in a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention provides for the packet transmission
of audio data from a transmitter, typically coupled to a multiple
channel audio data source, to a set of wireless packet data
receivers. The receivers are programmable to associate operation
with an assigned transmitter. The receivers are further
programmable to select and decode a specified channel or channels
of the transmitted multiple channel content. In preferred
configuration, a separate receiver is provided for each audio
reproduction speaker in a sound system and, dependent on the
speaker type and placement, selects and decodes a corresponding
channel of the audio content. Receivers associated with the center
channel, base, various left and right side and rear effects
speakers each preferably decode respective audio content channels
provided through the transmitter for respective speakers.
[0025] The transmitters and receivers of the present invention
preferably support both digital or analog format inputs and outputs
for audio data. In particular, the receivers of the present
invention provide may be integrated into the speaker enclosures and
closely integrated with the speaker amplification system. That is,
wireless transmission of audio content while maintaining high audio
fidelity enables audio component manufacturers to locate and
isolate speaker amplifiers internal to the speaker enclosures. This
removes the "hot and heavy" power sources and amplifiers from audio
source appliances. Migration of these components out to the
speakers themselves enables manufactures to fully implement modern
digital switching amplifier topologies, including specifically
Class D amplifier designs, in the speakers. This will enable
fundamental improvements in sound reproduction while achieving
reduced size, cost, power consumption, and EMI radiation in all
system components. Users also gain the advantages of flexible
installation and reconfiguration.
[0026] The transmitters and receivers used in the preferred
embodiments are preferably based on the high-volume commodity radio
components used in conventional wireless networking systems, such
as IEEE 802.11g and 802.11n. For purposes of implementation, the
present invention provides for the replacement of the conventional
Media Access Control (MAC) layer with a data processing engine
specifically designed to deliver high bit rate isochronous data,
such as audio and video, with low latency in accordance with the
present invention. Clock capture and alignment by the data
processing engine of the present invention is further described in
the co-pending application, High Quality Controlled Latency
Multi-Channel Wireless Digital Audio Distribution System and
Methods, Ser. No. ______, filed concurrently herewith, assigned to
the assignee of the present invention, which is hereby incorporated
by reference.
[0027] The system and methods of the present invention utilize a
basic architecture that can be configured for operation in multiple
ways. All configurations are generally based on the same elements.
The overall system configuration is shown in FIG. 1.
[0028] A set of audio input sources delivers audio signals, encoded
either as analog or digital audio, to a transmitter unit. The
transmitter unit contains digitization, synchronization, buffering,
MAC (Media Access Controller), and RF transmitter elements. A
receiver unit contains RF receiver, MAC, buffering, synchronization
and audio output elements. The audio output can be decoded for
analog speakers through a digital-to-analog signal conversion or
appropriately transcoder for digital audio uses. The resulting
audio outputsignals are provided as outputs.
[0029] FIG. 2 shows a preferred method of transmission and
reception of audio data packets as implemented by a master
transmitter unit with receipt and playback on a slave receiver
unit. The following describes operation at the corresponding stages
illustrated in FIG. 2. [0030] 1) The samples are collected from the
CODECS or Digital Audio Interfaces into a Sample Block Buffer. The
data in the Sample Block Buffer preferably implements data
redundancy injection for Forward Error Correction (FEC) and is
organized into a Send Buffer. [0031] 2) The Sample Block Buffer
(Send 1) is transmitted over the radio link as a packet. The Block
may be sent more than once (Send 2) to provide data redundancy. The
first Data Block sent in this mode of operation will have its
Sample Block Marker bits set. [0032] 3) When the receiver radio and
MAC decode a valid Sample Block with the Marker bits set the MAC
will trigger a Sample Block Marker at a delay determined during the
initialization of the radio link. The delay will provide a Sample
Block Marker at Sample Block boundaries. [0033] 4) The Sample Block
is played starting at the Sample Block Marker generated in step 3.
The received Data Buffer is processed through a convolutional
decoder and the resulting data is checked and repaired by use of
the FEC methods employed and is returned to being a Sample Block
that can the be sequence for playing. [0034] 5) The entire sample
Block is sequenced out. The three phases of collection, transport,
and playback are pipelined such that every step is running
simultaneously.
[0035] A preferred embodiment of a transmitter unit, constructed in
accordance with the present invention, is shown in FIG. 3. A set of
CODECs (CODer/DECoders) convert the incoming multi-channel analog
signals to serial digital bit streams. If a digital audio input
signal is used, the CODECs can be bypassed.
[0036] In the particular embodiment of this invention shown in FIG.
3, a 48 kHz clock, derived from an 18.432 MHz crystal oscillator,
is used for the sampling of analog audio data. On each sample clock
interval, the data present on each of the CODEC serial outputs is
captured at the input of the MUX & FIFO. This block multiplexes
the multiple codec inputs, and places them into a FIFO buffer for
transmission to the radio transmitter MAC interface. Preferably, as
data is added to the FIFO buffer, a forward error correction value
is calculated and associated with the buffered data. In this
particular implementation, the 18.432 MHz crystal is used to
provide a bus interface clock to the MAC interface section.
[0037] A 1 kHz Sample Block Marker output is also derived from the
crystal oscillator. It is used to indicate the start of a block of
samples 1 msec in duration. All data sampled from the CODECs during
a Sample Block interval is presented to the Radio MAC during the
duration of the current interval. Thus, the 1 msec duration of data
collected in one interval is provided to, packetized, and
transmitted by the MAC during the following interval. The preamble
of the initial packet sent during an interval is marked with the
Sample Block Marker. Copies of this packet may be re-transmitted,
though without the Sample Block Marker, to provide data redundancy
for the receivers. The number of copies re-transmitted may be
programmatically defined, preferably provided retransmission can be
completed within the balance of the current interval.
[0038] A preferred embodiment of a receiver unit, constructed in
accordance with the present invention, is shown in FIG. 4. The
Radio MAC interface in the receiver delivers a Sample Block Marker
that indicates the start of a block of sampled data. In the
particular embodiment of this invention described by the
illustration, this Sample Block Marker is a 1 kHz signal.
[0039] When the Clock Divider Block receives the Sample Block
Marker, the counter/divider from which the Sample Clock is derived
is asynchronously reset to zero. In this particular example, it
insures a phase alignment for the sample clock of .+-.54.3 nsec, or
one period of the 18.432 MHz clock.
[0040] The Sample Block Marker also indicates the start of a block
of sampled audio data to the FIFO & DEMUX section. In this
particular implementation, the 18.432 MHz crystal is used to
provide a bus interface clock to this section.
[0041] The radio receiver MAC will load the FIFO buffer with new
data. Forward error correction will be applied. If the error
correction fails to correct the data packet contents, a redundant
packet is corrected and loaded into the FIFO instead. If a packet
and redundant copies are entirely lost or beyond correct,
interpolation is used to cover the data error duration. The data
delivered to the FIFO will be demultiplexed for synchronous
transmission to the CODECs. All data received from the Radio MAC
during the duration of the current Sample Block Interval will be
sent to the CODECs during the duration of the current interval.
CODECs for only the number of channels supported by a receiver unit
need be physically populated in the receiver unit. Where a receiver
unit supports only a single channel, as is expected for the typical
case, that one channel is demultiplexed from the FIFO and provided
to a single CODEC.
[0042] The operational data transfer sequence, as described for the
preferred embodiments of the present invention, thus ensures a
maximum latency of two intervals. In the particular embodiment of
this system described in the illustrations, this limits the overall
latency to 2 msec with a 54.3 nsec accuracy for all receiver
channels.
[0043] The preferred data sequencing operation of the transmitter
unit is shown in FIG. 5A. The corresponding preferred data
sequencing operation of the receiver is shown in FIG. 5B. For the
preferred embodiments of the present invention, the channel data is
processed through the multiplexer interface by packing data bits
from each channel in parallel into data words that are then
collected into a data packet for transmission, as generally shown
in FIG. 6. In a simple case, sixteen audio input channels are
input. The channel data is separately serialized with a bit per
channel being stored as a respective bit in parallel in a data
word. If the per data channel sample resolution is 12 bits, then
twelve data words are used to transmit a single sample from each of
the sixteen channels.
[0044] On the receiver side, the channel demultiplexer reconstructs
the channel samples. Where, for typical implementations, the
receiver unit is intended to support only a single or reduced set
of channels, only those selected channels are output from the
demultiplexer and processed, as appropriate, through transcoders
for production of digital audio data streams, or through CODECs to
produce analog audio data signals.
[0045] Thus, a system and methods for providing for the
distribution of high bit rate, multi channel, audio wirelessly
while maintaining a constant, low, playback to source latency and
channel to channel phase coherency operable in multiple
configurations has been described.
[0046] In view of the above description of the preferred
embodiments of the present invention, many modifications and
variations of the disclosed embodiments will be readily appreciated
by those of skill in the art. It is therefore to be understood
that, within the scope of the appended claims, the invention may be
practiced otherwise than as specifically described above.
* * * * *