U.S. patent application number 11/079031 was filed with the patent office on 2005-07-21 for universal modulator.
This patent application is currently assigned to General Instrument Corporation. Invention is credited to Patel, Dipakkumar R., Waight, Matthew G..
Application Number | 20050160475 11/079031 |
Document ID | / |
Family ID | 34752644 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050160475 |
Kind Code |
A1 |
Waight, Matthew G. ; et
al. |
July 21, 2005 |
Universal modulator
Abstract
A modulator generates a combined signal consisting of audio and
video signals, and converts the combined signal to one of a
plurality of frequencies in dependence upon a desired output
frequency and broadcast standard. The modulator includes a summing
amplifier, a first frequency synthesizer and a second frequency
synthesizer. The summing amplifier has a first input for receiving
a video signal, a second input for receiving a first audio signal,
a third input for receiving a second audio signal, and an output
for outputting a modulated summed signal. The first frequency
synthesizer generates a first frequency for mixing with the
modulated summed signal to generate a high intermediate frequency
(HI-IF) signal. The second frequency synthesizer generates a second
frequency for mixing with the HI-IF signal to generate a desired RF
output signal.
Inventors: |
Waight, Matthew G.;
(Pipersville, PA) ; Patel, Dipakkumar R.;
(Hatboro, PA) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.
DEPT. MOT
UNITED PLAZA, SUITE 1600
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
General Instrument
Corporation
Horsham
PA
|
Family ID: |
34752644 |
Appl. No.: |
11/079031 |
Filed: |
March 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11079031 |
Mar 14, 2005 |
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09857010 |
May 29, 2001 |
|
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09857010 |
May 29, 2001 |
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PCT/US99/28232 |
Nov 30, 1999 |
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60110254 |
Nov 30, 1998 |
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Current U.S.
Class: |
725/151 |
Current CPC
Class: |
H03D 7/161 20130101 |
Class at
Publication: |
725/151 |
International
Class: |
H04N 005/50 |
Claims
What is claimed is:
1. A method for combining a plurality of received signals and
outputting a desired output signal, the method comprising: (a)
receiving a video signal; (b) receiving a first audio signal; (c)
receiving a second audio signal; (d) generating a first frequency;
(e) mixing the first frequency with the first audio signal to
produce a modulated signal; and (f) combining the video signal, the
modulated signal and the second audio signal to generate a summed
signal.
2. The method of claim 1 further comprising: (g) generating a
second frequency; (h) mixing the second frequency with the summed
signal to generate a high intermediate frequency (HI-IF) signal;
(i) generating a third frequency; and (j) mixing the third
frequency with the HI-IF signal to generate the desired output
signal.
3. The method of claim 1 further comprising: (g) limiting the
amplitude of the video signal.
4. The method of claim 1 further comprising: (g) adjusting the gain
of the video signal.
5. The method of claim 1 further comprising: (g) clipping signal
peaks of the video signal.
6. The method of claim 1 further comprising: (g) adjusting the gain
of the first audio signal.
7. A modulator for combining a plurality of received signals and
outputting a desired output signal, the modulator comprising: (a) a
first frequency synthesizer for generating a first frequency; (b)
an audio mixer in communication with the first frequency
synthesizer, the audio mixer for mixing an audio signal with the
first frequency to generate a mixed audio signal; (c) a summing
amplifier having a first input for receiving a video signal, a
second input for receiving the mixed audio signal, and an output
for outputting a modulated summed signal; (d) a second frequency
synthesizer for generating a second frequency for mixing with the
modulated summed signal to generate a high intermediate frequency
(HI-IF) signal; and (e) a third frequency synthesizer for
generating a third frequency for mixing with the HI-IF signal to
generate the desired output signal.
8. The modulator of claim 7 wherein the audio mixer is configured
to be selectively activated or deactivated.
9. The modulator of claim 7 further comprising: (f) an
up-conversion mixer having an input electrically coupled to the
output of the summing amplifier for receiving the modulated summed
signal and outputting the HI-IF signal.
10. The modulator of claim 9 wherein the up-conversion mixer is
electrically coupled to the second synthesizer for receiving the
first frequency for mixing with the modulated summed signal to
generate the HI-IF signal.
11. The modulator of claim 7 further comprising: (f) a
down-conversion mixer for receiving the HI-IF signal and outputting
the desired output signal.
12. The modulator of claim 11 wherein the down-conversion mixer is
electrically coupled to the third frequency synthesizer for
receiving the third frequency for mixing with the HI-IF signal to
generate the desired output signal.
13. The modulator of claim 7 further comprising a common
communication bus, electrically coupled to the first, second and
third synthesizers, for programming the first, second and third
frequencies.
14. The modulator of claim 7 further comprising a clamp for
limiting the amplitude of the video signal.
15. The modulator of claim 14 further comprising an adjustable
amplifier, electrically coupled to the clamp, for adjusting the
gain of the video signal.
16. The modulator of claim 15 further comprising a common
communication bus, electrically coupled to the adjustable
amplifier, for controlling the gain adjustment.
17. The modulator of claim 15 further comprising a limiter,
electrically coupled to the adjustable amplifier, for clipping
signal peaks of the video signal.
18. The modulator of claim 12 further comprising an adjustable
amplifier for adjusting the gain of the audio signal.
19. The modulator of claim 18 further comprising a common
communication bus, electrically coupled to the adjustable
amplifier, for controlling the gain adjustment.
20. The modulator of claim 7 wherein the modulator is incorporated
into a cable television (CATV) settop box.
21. The modulator of claim 20 wherein the desired output signal is
coupled to a television receiver external to the modulator.
22. A method for combining a plurality of received signals and
outputting a desired output signal, the method comprising: (a)
receiving a video signal; (b) receiving an baseband audio signal;
(c) generating a first frequency; (d) mixing the first frequency
with the baseband audio signal to produce a broadcast standard
signal; (e) combining the video signal and the broadcast standard
signal to generate a modulated summed signal; (f) generating a
second frequency; (g) mixing the second frequency with the
modulated summed signal to generate a high intermediate frequency
(HI-IF) signal; and (h) generating a third frequency; and (i)
mixing the third frequency with the HI-IF signal to generate the
desired output signal.
23. A method for combining a plurality of received signals and
outputting a desired output signal, the method comprising: (a)
receiving a video signal; (b) processing the video signal to
produce a processed video signal by: (b1) limiting the amplitude of
the video signal, (b2) adjusting the amplitude of the video signal,
and (b3) clipping signal peaks of the video signal; (c) receiving a
baseband audio signal; (d) processing the baseband audio signal by:
(d1) adjusting the amplitude of the baseband audio signal, (d2)
generating a first frequency, (d3) mixing the first frequency with
the baseband audio signal to produce a broadcast standard signal,
(d4) lowpass filtering the broadcast standard signal, and (d5)
adjusting the amplitude of the broadcast standard signal; and (e)
combining the processed video signal and the broadcast standard
signal to generate a modulated summed signal.
24. The method of claim 23 further comprising: (f) generating a
second frequency; (g) mixing the second frequency with the
modulated summed signal to generate a high intermediate frequency
(HI-IF) signal; (h) generating a third frequency; and (i) mixing
the third frequency with the HI-IF signal to generate the desired
output signal.
25. A modulator for combining a plurality of received signals and
outputting a desired output signal, the modulator comprising: (a) a
first input for receiving a video signal; (b) a second input for
receiving an baseband audio signal; (c) means for generating a
first frequency; (d) means for mixing the first frequency with the
baseband audio signal to produce a broadcast standard signal; (e)
means for combining the video signal and the broadcast standard
signal to generate a modulated summed signal; (f) means for
generating a second frequency; (g) means for mixing the second
frequency with the modulated summed signal to generate a high
intermediate frequency (HI-IF) signal; (h) means for generating a
third frequency; and (i) means for mixing the third frequency with
the HI-IF signal to generate the desired output signal.
26. A modulator for combining a plurality of received signals and
outputting a desired output signal, the modulator comprising: (a) a
video signal processing circuit including: (a1) a clamp for
receiving and limiting the amplitude of a video signal, (a2) a
first adjustable amplifier for receiving the video signal from the
clamp and adjusting the amplitude of the video signal, and (a3) a
limiter for receiving the video signal from the first adjustable
amplifier and clipping signal peaks of the video signal; (b) a
baseband audio signal processing circuit including: (b1) a second
adjustable amplifier for receiving and adjusting the amplitude of a
baseband audio signal, (b2) a phase-locked loop (PLL) synthesizer
for generating a first frequency, (b3) a baseband audio mixer for
receiving the baseband audio signal from the second adjustable
amplifier and mixing the first frequency with the baseband audio
signal to produce a broadcast standard signal, (b4) a lowpass
filter for receiving the broadcast signal from the baseband audio
mixer and filtering the broadcast standard signal, and (b5) a third
adjustable amplifier for receiving the broadcast standard signal
from the lowpass signal and adjusting the amplitude of the
broadcast standard signal; and (c) a summing amplifier having a
first input electrically coupled to an output of the third
adjustable amplifier, and a second input electrically coupled to an
output of the limiter, the summing amplifier for combining the
processed video signal and the broadcast standard signal to
generate a modulated summed signal.
27. The modulator of claim 26 further comprising: (d) a switch for
selectively inputting a second audio signal to a third input to the
summing amplifier, wherein the summing generator combines the
processed video signal, the broadcast standard signal and the
second audio signal to generate the modulated summed signal.
28. The modulator of claim 27 further comprising: (e) a
communication bus in communication with the switch, the first
adjustable amplifier, the second adjustable amplifier, the third
adjustable amplifier, the PLL synthesizer and the baseband audio
mixer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/857,010 filed May 29, 2001, which claims
the benefit of U.S. provisional patent application 60/110,254 filed
on Nov. 30, 1998, which are incorporated by reference as if fully
set forth.
BACKGROUND
[0002] The present invention generally relates to cable television
(CATV) and consumer video communication systems. More particularly,
the invention relates to a dual-conversion universal modulator
having programmable synthesized phase-locked loop oscillators
driving their respective mixers which select a specific HI-IF
frequency depending upon what output frequencies or standards are
desired. Such standards include NTSC, PAL, NICAM, DIN, SECAM and
any other known standard.
[0003] To allow reception of more than the 12 VHF channels on an
older television receiver, most CATV systems require a settop
terminal at a subscriber's location. Today, settop terminals not
only provide a means for accepting a plurality of channels
broadcast with varying bandwidths and guardbands for forward and
reverse frequencies, but they also secure pay television services
from unauthorized viewing. Other functions include decoding digital
video and audio, interactive services, creating personalized viewer
channels and the like.
[0004] In addition to the conversion from a cable transmission to a
standard output frequency, a variety of descrambling techniques are
employed depending upon the techniques used at a system headend.
CATV equipment manufacturers are developing more sophisticated
scrambling techniques using complicated encryption methods and
digital processing to thwart pirating.
[0005] Most settop terminals are tunable. A block diagram for a
prior art settop terminal is shown in FIG. 1. Incoming signals from
a CATV transmission network are coupled to an input bandpass
amplifier and up-converted to a high intermediate frequency
(HI-IF). The up-conversion requires a tunable local oscillator
which selects a desired channel and an associated mixer. The mixer
is coupled to a bandpass filter and down-converted to an IF channel
using a fixed-frequency local oscillator and mixer. The output
channel is filtered and forwarded to a subscriber's television
receiver. Prior art settop terminals use one down-converter mixer
with an oscillator having slight frequency agility to provide an
output at one or two preselected channel frequencies. The output
frequencies and bandwidths depend upon the transmission standard
used.
[0006] In the United States, the NTSC (National Television System
Committee) is the standard for color television. Other countries
have chosen different systems. SECAM (sequentiel couleur avec
mmoire) is used by France and Russia. PAL A and PAL B (phase
alternation line) are used by many European countries such as
Germany and the United Kingdom. Accordingly, television receivers
are typically manufactured for a specific transmission standard.
For worldwide use, a settop terminal must be adapted to the
established broadcast standards.
[0007] Accordingly, there exists a need for an inexpensive method
to adapt the output of a settop terminal to a variety of television
broadcast standards.
SUMMARY OF THE INVENTION
[0008] The present invention is a universal modulator that accepts
baseband audio and video inputs and modulated audio or data and
converts the combined signal to one of a plurality of frequencies
in dependence upon a desired output frequency and broadcast
standard. The universal modulator is located between baseband video
and audio outputs of a settop terminal demodulator/decoder and an
antenna input of a television receiver or other audio/video
component (such as a VCR). The universal modulator includes a dual
conversion architecture using an up-converter mixer and a
down-converter mixer. Each mixer receives an oscillator input from
a corresponding addressable, programmable, PLL (phase-locked loop)
frequency synthesizer. Each PLL frequency is controlled by firmware
in the settop terminal. Configuration is performed via manual input
using settop terminal controls, or interrogation directly by the
CATV headend or by programmed settings. A communication bus coupled
to the firmware distributes addressable instructions to selectively
control each PLL frequency and obviate oscillator difference beat
frequencies (ODBFs) that may be manifested.
[0009] Accordingly, it is an object of the present invention to
provide a universal modulator within a settop terminal which is
able to couple a CATV transmission network to a customer's
television receiver notwithstanding the broadcast standard used to
transmit the television programs.
[0010] Other objects and advantages will become apparent to those
skilled in this art after reading the detailed description of the
preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more detailed understanding of the invention may be had
from the following description of a preferred example, given by way
of example and to be understood in conjunction with the
accompanying drawing wherein:
[0012] FIG. 1 is a block diagram of a prior art CATV settop
terminal;
[0013] FIG. 2 is a block diagram of a settop terminal incorporating
the present invention;
[0014] FIG. 3 is a block diagram of the preferred embodiment of the
universal modulator of the present invention for use in a settop
terminal;
[0015] FIG. 4 is a block diagram of an addressable, programmable,
phase-locked loop;
[0016] FIG. 5 is a flow chart of the universal modulator
configuring process;
[0017] FIG. 6 is a flow chart of the ODBF translation process;
[0018] FIG. 7 is a block diagram of a prior art headend;
[0019] FIG. 8 is a block diagram of a headend made in accordance
with the present invention; and
[0020] FIGS. 9A and 9B are graphs of oscillator difference beat
frequencies.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] The preferred embodiment will be described with reference to
the drawing figures where like numerals represent like elements
throughout.
[0022] FIG. 2 is a block diagram of a settop terminal 17 with a
universal modulator 19 shown coupled to the outputs 23, 25, 27 of a
demodulator/decoder 21. The demodulator/decoder 21 supplies a
customer's channel selection to the universal modulator 19 as a
baseband audio signal via the output 23 and as a baseband video
signal via the output 25. An alternate (second) audio signal, such
as a NICAM carrier or modulated audio signal which differs from the
baseband audio signal, may also be supplied to the universal
modulator 19 via the output 27 of the demodulator/decoder 21. A
reference clock signal 37 originating from a master oscillator (not
shown) and a common communication bus 39 are also coupled to the
universal modulator 19. The functional description of the
demodulator/decoder 21 is beyond the scope of the present invention
and shall not be described in further detail.
[0023] The higher quality baseband audio and video signals provided
by outputs 23 and 25 of the demodulator/decoder 21 are made
available as settop terminal outputs 31, 33, respectively, and may
be coupled to television receivers that have baseband inputs. The
alternate audio signal provided by output 27 may be made available
as settop terminal output 29. For television receivers that lack
these features, the universal modulator 19 provides an
up-conversion output 35 compatible with the television broadcast
standard used, from baseband to VHF or UHF for coupling to an
antenna input.
[0024] The universal modulator 19 is shown in more detail in FIG.
3. The common communication bus 39 shown is an I.sup.2C interface
from Phillips.RTM. Electronics. Other bus communication protocols
may alternatively be used. The configuration for a settop terminal
17 may be downloaded from the CATV system headend via a dedicated
channel, or inband on the VBI of a channel. One skilled in this art
would appreciate that an advanced cable system can address and
interrogate a specific settop terminal and alter its functionality.
If the settop terminal has all configurations stored in firmware,
the CATV system headend may simply instruct the settop terminal 17
of the standard being used. In this fashion, the settop terminal 17
does not require a technician to configure the unit but can
auto-configure upon initial energization.
[0025] The communication bus protocol permits configuring component
parameters to a particular broadcast standard using a unique
addressing system within the settop terminal 17. As shown in FIG.
3, the I.sup.2C bus 39 communicates with: an addressable
programmable PLL frequency synthesizer 41 for a baseband audio
mixer 69, a solid state switch 43, adjustable amplifiers 45 and 47
for the baseband video input 59 and baseband audio input 67, an
addressable programmable PLL frequency synthesizer 49 for an
up-conversion mixer 91 and an addressable programmable PLL
frequency synthesizer 51 for a down-conversion mixer 101. Although
the addressable programmable PLL frequency synthesizer 51 has been
described as being coupled to a "down-conversion" mixer 101, the
down-conversion mixer 101 may in fact further up-convert a HI-IF
signal 93 to a higher frequency signal. It should be noted that
each PLL frequency synthesizer 41, 49, 51 has an associated
oscillator driver LO1, LO2, LO3 respectively (not shown). Each
respective component has its own address to permit firmware
contained parameters to be loaded for a given broadcast standard
configuration.
[0026] An alternate (second) audio carrier 53, provided by the
output 27 of the demodulator/decoder 21, is coupled to the solid
state switch 43. The output of the switch 43 is coupled to a first
input 55 of a summing amplifier 57. The baseband video input 59 is
coupled to a clamp 61 which limits signal amplitude. The output
from the clamp 61 is coupled to the video adjustable amplifier 45
where signal gain is increased or attenuated depending upon the
broadcast standard. The output from the adjustable amplifier 45 is
coupled to a hard limiter 63 which clips signal peaks. The output
from the limiter 63 is coupled to a second input 65 of the summing
amplifier 57. The baseband audio input 67 is coupled to a baseband
audio mixer 69 via an adjustable amplifier 68. The baseband audio
mixer 69 modulates the baseband to the broadcast standard. The
baseband audio mixer 69 may be selectively activated or deactivated
by the I.sup.2C bus as required to support the standard in use. The
output from the baseband audio mixer 69 is coupled to a lowpass
filter 71 to remove RF. A second input to the audio lowpass filter
71 is provided as a modulated audio input 72. The audio lowpass
filter 71 is coupled to an audio adjustable amplifier 47 where
signal gain is increased or attenuated. The audio adjustable
amplifier 47 output is coupled to a third input 73 of the summing
amplifier 57.
[0027] Each of the mixers mix a signal input with the outputs of
the three addressable, programmable PLL frequency synthesizers 41,
49, 51. The PLL output frequencies vary depending on the broadcast
standard and the RF output frequency 105 desired. An addressable,
programmable PLL frequency synthesizer 41, 49, 51 is shown in FIG.
4.
[0028] The PLL 41, 49, 51 includes a phase detector 75, a
voltage-controlled oscillator (VCO) 77 and a loop filter 79. The
programmable PLL uses digital and analog techniques for frequency
synthesis. The phase detector 75 compares two input frequencies
81a, 81b and generates an output 83 that is a measure of their
phase difference. If both inputs 81a, 81b differ in frequency, the
output is periodic at the difference frequency. If the frequency
input does not equal the frequency output of the VCO 77, the
phase-error signal, after being filtered, causes the VCO frequency
to deviate in the direction of the input frequency. When the
frequencies match, the VCO 77 locks to the input frequency
maintaining a fixed phase relationship with the input signal. The
filtered output of the phase detector 75 is a dc signal. A modulo-n
counter 87 is coupled between the VCO 77 output and the second
input 81a to the phase detector 75 to generate a multiple of the
input reference frequency providing frequency synthesis.
[0029] Each PLL synthesizer 41, 49, 51 employed in the present
invention 19 is addressable such that the input frequency can be
adjusted by using an input modulo-n counter 89 or divide-by-n to
adjust output frequency. Both the input frequency divide-by-n 89
and loop frequency divide-by-n 87 are addressable components. Each
of the PLLs 41, 49, 51 are addressed and controlled in accordance
with a predetermined settop terminal 17 configuration. The
configuration determines both the output frequency and operating
bandwidth of the settop terminal 17 and adjusts the up- and
down-converter PLLs 49, 51 accordingly.
[0030] Referring back to FIG. 3, the summer amplifier 57 output is
modulated with the frequency output from the second programmable
PLL 49 to drive the up-conversion mixer 91 and translate the summed
output to a high intermediate frequency (HI-IF) 93. The HI-IF 93 is
higher than the highest expected frequency in the summed amplifier
57 output bandwidth. In the present invention 19, the input to the
up-conversion mixer 91 is not bandwidth limited.
[0031] The summing amplifier 57 output frequencies are translated
to a new bandwidth, starting at a low frequency of the second PLL
49 minus the highest input band frequency, and ending at a high
frequency of the third PLL 51 minus the lowest input band
frequency. The second PLL 49 frequency is selected to translate the
summing amplifier 57 output to correspond to the passband of an
intermediate lowpass filter 95. The output from the lowpass filter
95 is coupled to a buffer amplifier 97 to restore gain losses. The
output from the buffer amplifier 97 is input to a final lowpass
filter 99. The buffer amplifier 97 maintains the system noise
figure by overcoming the losses in the up-conversion mixer 91 and
first HI-IF filter 95. The signal is filtered by a HI-IF filter 99,
with the output coupled to a down-conversion mixer 101. The third
PLL synthesizer 51 is coupled to the down-conversion mixer 101. The
difference between the HI-IF 93 and the third PLL 51 frequency is
the desired output channel in the IF band. It should, however, be
noted that the down-conversion mixer 101 may accept the HI-IF 93
and further up-convert the signal to a higher frequency RF signal.
The output is then filtered via a low pass filter 103, (or other
appropriate filter if up-converted), and forwarded as an RF output
frequency 105 for reception by a television receiver.
[0032] As discussed above, the second 49 and third 51 programmable
PLLs are controlled by the common communication bus 39. The bus 39
is coupled to a processor in the settop terminal
demodulator/decoder 21 which receives instructions from the system
headend or from the settop terminal's 17 keypad. The configuration
takes place transparently upon initial energization of the settop
terminal 17 if the system headend is equipped to send broadcast
configuration instructions to the settop terminal 17. If the system
headend does not have this capability, the settop terminal 17 is
configured via the keypad and function display (not shown). The
configuration request, whether from the headend or at a consumer
location, outputs the predetermined parameters onto the I.sup.2C
bus 39 for each of the addressable components. The predetermined
parameters are related to the standard that is being employed by
the CATV system on which the settop terminal 17 is located. These
parameters will include the determination of whether a second audio
carrier 53 exists, whether the baseband audio input 67 or the
modulated audio input 72 are to be used and the frequency at which
the RF output frequency 105 is desired. These parameters may also
include any other configurable parameters which are employed by any
of the addressable components coupled to the communication bus 39.
It should also be recognized that since many of the components are
addressable by the communication bus 39, a user may manually input
and address a particular component and selectively configure that
component if desired.
[0033] An undesirable artifact of dual conversion is the generation
of harmonics based on the fundamental oscillator frequencies. The
harmonics of the second and third PLL frequency synthesizers 49, 51
mix with each other, thereby creating ODBFs. To obviate the
intrusive effects of these PLL harmonics, the system and method of
the present invention 19 eliminate this type of interference by
translating the significant ODBFs out of the desired output
channel.
[0034] A flowchart of the preferred method of the present invention
19 is shown in FIG. 5. Upon making the necessary connections to the
CATV cable 15 and subscriber's television receiver, the settop
terminal 17 is energized (step 201) establishing communication with
the system headend. If the cable system headend has forward
communication ability (step 205), the settop terminal is instructed
how to configure itself for the applicable broadcast standard by
downloading the parameters for the regional standards being used
and the channel broadcast maps (step 207). The predetermined PLL
frequencies derived from the channel and broadcast maps in memory
are converted into corresponding "divide-by" numbers for the PLL
modulo-n converters 87, 89 and output to the second 49 and third 51
PLL frequency synthesizers. The settop terminal 17 acknowledges
when configuration is complete. If the cable system does not have
forward communication capability, the user will be prompted to
enter the applicable information via a display and keypad, thereby
manually loading the applicable broadcast configuration (step
209).
[0035] The settop terminal 19 reviews the loaded channel and
broadcast maps. The predetermined frequencies are examined for
potential ODBFs (step 211). If it is determined that ODBF's are
likely (step 213), an ODBF translation is performed (step 215) as
shown in FIG. 6 (which will be explained in greater detail
hereafter). Otherwise, the original frequencies are maintained
(step 217) (FIG. 5). The frequencies are addressed to their
respective PLL synthesizers as words over the I.sup.2C
communication bus (step 219).
[0036] Referring to the flow diagram of FIG. 6, the elimination of
ODBFs is achieved by selectively adjusting the frequencies of the
second 49 and third PLLs 51 to obtain the desired RF output
frequency. For a typical NTSC signal, the up-converter mixer 91
modulates the input video 59 and audio signals 67 with the output
93 of the second PLL 49 to up-convert the input RF signal of the
selected channel to the HI-IF 93 (step 301).
LO1=audio carrier frequency (Equation 1)
LO2=HI-IF (Equation 2)
[0037] The down-converter mixer mixes 101 the HI-IF 93 with the
output of the third PLL synthesizer 51 (step 303) to down-convert,
(or further up-convert if desired), to obtain the desired RF output
frequency 105.
LO3=(HI-IF)+RF output (Equation 3)
[0038] Multiples of the second and third PLL synthesizer 49, 51
fundamental frequencies define the even and odd harmonics,
m(LO2) and m (LO3), for m=1, 2, 3, 4, . . . .infin., (Equation
4)
[0039] which represent all possible harmonics, (step 305). However,
due to the high system frequencies involved, examination of
frequencies beyond the 10th harmonic is unnecessary.
[0040] The existence of an interfering ODBF is determined by
serially calculating the differences between two harmonics of the
second 49 and 51 third PLL synthesizers that are separated by at
least one degree until the absolute value of an ODBFm,n is within a
given bandwidth or a predetermined number of ODBFm,n values are
calculated. When an ODBFm,n absolute value is found within the RF
channel bandwidth, it is designated as an interfering oscillator
difference beat frequency (ODBF). The general equation for
calculating ODBFs is:
ODBF.sub.m,n=(m+n)(LO2)-(m)(LO3), form m=1, 2, 3, 4, . . . 10,
(Equation 5)
[0041] with n=1 for a first series, n=2 for a second series, n=3
for a third series, and so on up to n=8 for all previously
calculated harmonics (step 305). The ODBFm,n calculated from the
differing degrees of the second 49 and third PLL 51 harmonics are
then examined (step 307). For example, if the ODBF lies outside of
the desired RF output channel bandwidth, no adjustment of the
second 49 and third 51 PLL frequency synthesizers is required.
[0042] For an ODBF which falls inband, the following equations can
be used to determine which direction the second 49 and third 51 PLL
frequencies should be adjusted to translate the ODBF out of band.
In these equations, CLB is the channel low-band; CMB is the channel
mid-band; and CHB is the channel high-band.
If -CHB.ltoreq.ODBF<-CMB; then HI-IF is moved downward.
(Equation 6A)
[0043] (If the result of Equation 5 is negative and the magnitude
is greater than the mid-band of the desired RF output channel (step
309), the HI-IF is moved downward (step 311)).
If -CMB.ltoreq.ODBF.ltoreq.-CLB; then HI-IF is moved upward.
(Equation 6B)
[0044] (If the result of Equation 5 is negative and the magnitude
is less than or equal to the mid-band of the desired RF output
channel (step 313), the HI-IF is moved upward (step 315)).
If CLB.ltoreq.ODBF.ltoreq.CMB; then HI-IF is moved downward.
(Equation 6C)
[0045] (If the result of Equation 5 is positive and the magnitude
is less or equal to than the mid-band of the desired RF output
channel (step 317), the HI-IF is moved downward (step 319)).
If CMB<ODBF.ltoreq.CHB; then HI-IF is moved upward. (Equation
6D)
[0046] (If the result of Equation 5 is positive and the magnitude
is greater than the mid-band of the desired RF output channel (step
321), the HI-IF is moved upward (step 323)).
[0047] The second 49 and third 51 PLLs are then adjusted (step 327)
in accordance with the following: To translate the oscillator
difference beats below or above the desired RF output channel, the
following equation is used to determine the .DELTA. in frequency
for the second 49 and third 51 PLL frequency synthesizers. 1 = CMB
- [ ( m + n ) ( LO2 ) - m ( LO3 ) ] ( m + n ) - m ( Equation 7
)
[0048] The new second 49 and third 51 PLL frequencies (LO2' and
LO3' respectively) are derived as shown in LO2' is calculated as
shown in FIG. 6.
[0049] The new PLL frequencies LO2' and LO3' translate the ODBFs
above or below the desired RF output channel. The new PLL frequency
values are used to program the second 49 and third 51 PLL frequency
synthesizers (step 327).
[0050] In an alternative embodiment, a fixed value for .DELTA. can
be used to simplify the calculations and the operation of the
system. For example, a value of 4 MHz for .DELTA. will suffice for
NTSC and PAL systems.
[0051] The present invention will now be explained with reference
to several examples. In the first example, if the HI-IF is 960 MHz
and the desired RF output channel has a picture carrier frequency
of 319.25 MHz, we have the following: 2 LO2 = HI - IF = 960 MHz ;
and ( from Equation 2 ) LO3 = HI - IF + RF output = 960 + 319.25 =
1279.25 MHz . ( from Equation 3 )
[0052] The graph for ODBFs versus the RF output frequencies for m=2
and n=1 is shown in FIG. 9A. If m=2 and n=1, then: 3 ODBF 2 , 1 (
960 ) = ( m + n ) ( LO2 ) - m ( LO3 ) = 3 ( 960 ) - 2 ( 1279.25 ) =
2880 - 2558.5 = 321.5 MHz ( from Equation 5 )
[0053] Since the desired RF output channel has a picture carrier
frequency of 319.25 MHz (and assuming the bandwidth is 6 MHz for an
NTSC channel), the ODBF is in-band for the desired RF output
channel. From Equation 6D, since the ODBF is above the mid-band of
the desired RF output channel, the HI-IF is moved upward. Assuming
that .DELTA. will be a fixed value of 4 MHz, LO2' will be 964 MHz
and L03' will be 1283.25 MHz. Accordingly, 4 ODBF 2 , 1 ( 964 ) = 3
( 964 ) - 2 ( 964 + 319.25 ) = 2892 - 2566.5 = 325.5 MHz . ( from
Equation 5 )
[0054] The ODBF is now out of band.
[0055] In the second example, if the HI-IF is 960 MHz and the
desired RF output channel has a picture carrier frequency of 481.25
MHz, we then have the following: 5 LO2 = HI - IF = 960 MHz ; and (
from Equation 2 ) LO3 = HI - IF + RF output = 960 + 481.25 =
1441.25 MHz . ( from Equation 3 )
[0056] The graph for ODBFs versus the RF output frequencies for m=3
and n=1 is shown in FIG. 9B. If m=3 and n=1, the ODBF can be
calculated as: 6 ODBF 3 , 1 ( 960 ) = 4 ( 960 ) - 3 ( 1441.25 ) =
3840 - 4323.75 = - 483.75 MHz . ( from Equation 5 )
[0057] Since the selected channel is 481.25 MHz, (and assuming an
NTSC channel), the ODBF is in-band and the HI-IF must be relocated.
The result of Equation 5 for this example is negative and the
magnitude is greater than the mid-band of the desired RF output
channel (481.25 MHz). Accordingly, from Equation 6A, the HI-IF is
moved lower. Assuming that .DELTA. will be a fixed value of 4 MHz,
LO2' will be 956 MHz and LO3' will be 1437.25 MHz. Recalculating
the ODBF provides: 7 ODBF 3 , 1 ( 956 ) = 4 ( 956 ) - 3 ( 956 +
481.25 ) = 3824 - 4311.75 = - 487.75 MHz . ( from Equation 5 )
[0058] The ODBF is now out of band.
[0059] Due to the simple design of the present invention and since
there are no shielding requirements to avoid ODBFs, the universal
modulator may be incorporated onto a single integrated circuit.
This was not possible with prior art designs.
[0060] Although the present invention has been described with
reference to a settop terminal, it should be understood by those of
skill in the art that the invention is adaptable to other
applications within the CATV environment, or even other
communication applications which do not pertain to CATV
systems.
[0061] For example, as shown in FIG. 7, a prior art headend 700
generally includes two pieces of equipment; a baseband section 702
and an IF section 704. These two sections 702, 704 are typically
designed to operate as "stand alone" units. Together, the two
sections 702, 704 output a single RF channel. The baseband section
702 generally comprises a video section 706 and an audio section
708. These sections 706, 708 receive audio and video baseband
inputs and combine these inputs to an intermediate frequency for
output to the IF section 704. In the IF section 704, the
intermediate frequency is up-converted to the desired RF output
channel. Since both sections 702, 704 comprise units of equipment
which are designed to work independently, this requires the
duplication of many components between units 702, 704.
[0062] Referring to FIG. 8, a headend 800 made in accordance with
the present invention is shown. The headend 800 includes an audio
pre-processing section 802, a video pre-processing section 804, the
universal modulator 808 of the present invention (which is coupled
to two filters 810, 812), a transmitter 814 (if desired), and a
microprocessor 806, which controls all of the components of the
headend 800. As was previously described hereinbefore, since the
universal modulator 808 can convert a baseband input signal to any
desired RF output signal while avoiding ODBFs, the universal
modulator 808 may be utilized to replace most of the components in
the baseband section 702 and the IF section 704. This significantly
reduces the number of components required for a headend 800.
Accordingly, the cost and complexity are also thereby reduced.
[0063] It should be understood by those of skill in the art, with
reference to FIG. 8, that the universal modulator 808 of the
present invention may also be used to accept a baseband digital VSB
signal and remodulate the signal to a desired RF output signal for
use with broadcast HDTV television receivers. The universal
modulator 808 could also be used to transmit RF signals to devices
which require high frequency RF signals, including wireless
appliances such as a cordless telephone or a wireless LAN receiver.
In such an application, the second mixer up-converts the HI-IF
signal to a higher frequency RF signal, instead of down-converting
the HI-IF as previously described. The desired RF output signal
would be:
RF output=(HI-IF)+LO3 (Equation 8)
[0064] The RF output signal may then be transmitted directly to the
wireless appliance.
* * * * *