Title of Invention	VSB ENCODER AND RF MODULATOR FOR DIGITAL TELEVISION RECEIVER
Abstract	This invention relates to an encoder-modulator for coupling a digital baseband television signal to a VSB digital receiver that includes filters, equalization circuitry and forward error correction circuitry for correcting signal impairments that are below a given threshold. The encoder modulator processes the baseband signal for low power transmission on an RF channel with less-than- nominal bandpass characteristics. A coaxial cable or other noise communication link directly connects the RF signal to the RF input of the television receiver. Any errors or signal impairments in the transmitted signal that are below the predetermined threshold are corrected by the filters and equalization circuitry built into the VSB digital television receiver.

Title of Invention

VSB ENCODER AND RF MODULATOR FOR DIGITAL TELEVISION RECEIVER

Abstract

This invention relates to an encoder-modulator for coupling a digital baseband television signal to a VSB digital receiver that includes filters, equalization circuitry and forward error correction circuitry for correcting signal impairments that are below a given threshold. The encoder modulator processes the baseband signal for low power transmission on an RF channel with less-than- nominal bandpass characteristics. A coaxial cable or other noise communication link directly connects the RF signal to the RF input of the television receiver. Any errors or signal impairments in the transmitted signal that are below the predetermined threshold are corrected by the filters and equalization circuitry built into the VSB digital television receiver.

Full Text	LOW COST VGB ENCODER AND RF MODULATOR FOR SUPPLYING VSB BASEBAND SIGNAL TO RF INPUT OF DIGITAL TELEVISION RECEIVER Background of the Invention and Prior Art This invention relates generally to digital VSB (vestigial side band) television receivers and specifically to a simple low cost system for coupling a digital VSB signal from any of a plurality of sources of MPEG (Motion Picture Experts Group) digital signal to a digital VSB television receiver. While the description is directed to a VSB digital format, it should be understood that the invention in its broader aspects is not limited to a particular digital format. The recently adopted Digital Television Standard specifies a VSB subsystem having a terrestrial broadcast mode (8VSB) and a high data rate mode (16 VSB). There are other VSB modes available, namely 2 VSB, 4 VSB and 8 VSB (non terrestrial). The 8 VSB terrestrial mode has the data carrying capability of 4 VSB. (The Trellis coding that is added for the terrestrial environment creates the additional modulation levels.) The digital data signal is MPEG and Dolby AC-3 processed and must be subjected to decompression before application to conventional video and audio circuitry. For terrestrial broadcasting, the data signal is: randomized; subjected to Reed-Solomon (R/S) type encoding for error correction; interleaved; Trellis encoded; multiplexed with segment sync and field sync; supplied with a DC pilot; subject to pre-equalization filtering; modulated; and RF upconverted for transmission. The digital television receiver includes a tuner and a VSB demodulator for developing the baseband signal, which is in compressed form. The demodulated signal is applied to an equalizer for equalizing the received signal. The baseband signal is applied to a transport demultiplexer which directs the data to an appropriate MPEG decoder and a Dolby decoder for recovering the video and audio in proper form for application to video and audio processing circuitry. Digital signals from other sources, such as DVD (digital video disk) players, VCRs (video cassette recorders), PCs (personal computers), digital cable boxes, satellite receivers and the like will be supplied to digital television receivers, as is the case for present analog television receivers. In an analog system, it is well known that coupling the signal from a VCR or other video source to a baseband input of a television receiver (if one is available) yields superior results than those achieved by modulating the signal to RF channel 3 or channel 4 and supplying it to the tuner input of the television receiver. This is not, however, necessarily true in a digital environment in which case the distance between the baseband signal source and the television receiver must be kept relatively short. In accordance with the invention, a compressed baseband digital signal is encoded in a given format for digital transmission and modulated on an RF carrier for direct application via a cable network or wireless link to the RF input of a digital television receiver. The RF' signal, which is at a low power level and has a less-than- nominal bandpass, takes advantage of the front-end signal processing that is built into the digital television receiver. Signal impairments, due to the less-than-nominal bandpass characteristic and noise introduced during transmission, that are below a given threshold are corrected by the correction circuitry in the digital television receiver front end. The low signal power, the corrective circuitry for the digital signal and the benign environment of the communications link, enable components and circuits of much lower tolerance (and cost) to be used in the encoding and modulating process. The result is an encoder-modulator that is very low cost and very effective in coupling a digital baseband signal to a digital television receiver over an RF channel. Objects of the Invention A principal object of the invention is to provide a novel digital signal translation system. Another object of the invention is to provide a digital encoder-modulator for coupling a baseband digital signal to a VSB digital television receiver. A further object of the invention is to provide a low cost digital signal coupling system for a VSB digital television receiver. Brief Description of the Drawings These and other objects and advantages of the invention will be apparent upon reading the following description in conjunction with the drawing, in which: FIG 1 is a simplified block diagram of a VSB digital signal encoder-modulator constructed in accordance with the invention; FIG 2 is a simplified block diagram of another form of a digital signal encoder- modulator constructed in accordance with the invention; and FIG 3 are curves illustrating the bandpass characteristics of the encoder modulator of the invention. Description of the Preferred Embodiment Referring to FIG 1, a source of digital MPEG encoded signal 10 derived from any of a number of different source types, is shown. For example, the source may comprise a satellite receiver, a VCR, a DVD, a digital cable box, a PC, etc. The output of source 10 is an MPEG (and Dolby AC-3) encoded digital signal at baseband frequency. This baseband signal may be applied directly to a baseband input of a digital television receiver 24. However, in accordance with the invention, the baseband signal is applied to an ATSC encoder 12 where the signal is subjected to interleaving, randomizing, R/S error encoding and Trellis encoding. Field and segment syncs are added, and while not indicated, a pilot is inserted and equalization filtering is employed. (See ATSC Standard A/53, ATSC Digital Television Standard for a detailed description of these operations.) The signal is then supplied to a digital to analog (D/A) converter 14. D/A 14 supplies the analog signal to a pair of multipliers 13 and 15 that in turn supply signals to a pair of SAW (surface acoustic wave) filters 16 and 18. Multipliers 13 and 15 are each supplied with a frequency Fl and F2, respectively for producing two different outputs. The SAW filters are selectively controlled by a switch 20, with one of the SAWs being used for one selected channel RF output and the other being used for another selected channel RF output. The particular RF channel selected is dependent upon the environment in which the encoder-modulator is used, in particular the availability and type of RF channels in the receiving area. The SAW filters are relatively low cost devices and are extensively used in television receivers. Those skilled in the art will recognize that highly accurate modulators generate I and Q signals with the signals being combined to cancel one of the sidebands and generate a vestigial sideband signal. Such modulators are quite complex and very expensive. In one form of the invention, that arrangement is replaced with a multiplier (13, 15)and a SAW filter (16, 18). The output of the multiplier is a double sideband suppressed carrier signal. The following SAW filter removes most of one sideband (preferably, although not necessarily, the lower sideband) to produce a VSB signal and also approximates the nominal Nyquist slope of the channel bandpass. As a result of the less than nominal bandpass characteristic, the transmitted signal will be characterized by a degree of impairment in the form of intersymbol interference (ISI). The SAW filters shape the edges of the signal bandpass to approximate the Nyquist slopes, nominally called for at the band edges in the ATSC standard. (The Nyquist slopes may be seen by reference to the solid line curve of FIG 3.) The SAW filter characteristics also provides for adjacent channel rejection. The filtered signal from the selected one of the SAWs is applied to an RF upconverter 22, which upconverts the signal to any desired RF channel, channels 3 or 4 being the ones normally used. The encoded and modulated VSB signal is supplied over a cable network 2,3, which may comprise a single coaxial cable or a relatively complex home cable network, to an RF channel input of a tuner 26 of a VSB digital television receiver 24. The signal may also be amplified by a low power RF amplifier 25 (shown in dotted line in FIG. 1) and transmitted wirelessly to the digital television receiver in an assigned RF broadcast television channel. The received signal is supplied through a companion SAW filter to a VSB demodulator 28 where the baseband digital signal is recovered. The demodulated signal is applied to an equalizer 30 that operates to adjust the response of the receiver to very closely match the solid line Nyquist response (FIG 3) thereby correcting any signal impairments, including any intersymbol interference introduced in the transmitter and any linear distortions introduced in the communications link (i.e. the cable network or wireless link) that collectively are below a given threshold level that is dependent upon the nature of the equalizer. The signal from equalizer 30 is processed by a forward error correction subsystem 31 to correct any residual errors due to intersymbol interference and errors due to noise, both white and impulse, introduced by the cable network or wireless link. The performance of equalizer 30 and error correction subsystem 31 sets the limit on the amount of intersymbol interference and linear distortion which can be introduced by the transmitter and communications channel before the system suffers serious degradation. The corrected signal (in MPEG encoded form) is supplied to a transport demultiplexer 32 that has separate outputs for the video and audio portions of the signal. The video portion is applied to an MPEG decoder 34 for decompression and decoding and the audio portion is applied to a Dolby AC-3 decoder 36 for complementary processing. The resultant video and audio signals are applied to a video processor 38 and an audio processor 40, respectively. It will be appreciated by those skilled in the art that the RF signal is not as distance sensitive as a baseband signal and the coaxial cable environment further minimizes errors or impairments being introduced in transmission. However, even if some signal impairments are introduced, they may be compensated for by the corrective circuitry built into the VSB digital television receiver, in particular the equalization circuitry. The result is a signal translation arrangement that results in excellent transfer of signal from source 10 to VSB digital television receiver 24, since the signal is VSB encoded and any errors or signal impairments below a given threshold are correctable in the television receiver. In contrast, any baseband digital signals that are supplied to television receiver 24 are introduced after equalizer 30 and error correction subsystem 31 and are consequently not subject to the corrective effects of the circuitry in the VSB digital television receiver. Such signals are also distance limited and subject to uncorrectable errors or impairments, which makes the method of the invention far superior. In further accordance with the invention, where the digital television receiver has provision for receiving both a digital and an analog NTSC signal, in establishing priority of the RF channel output of upconverter 22, the first choice is to supply the signal on a vacant RF channel in the service area of the television receiver and the second choice is to supply it on the digital one of a pair of digital and analog NTSC RF channels in the service area of the television receiver. In this way, any NTSC interference into the digital channel can be minimized by the circuitry in the digital channel of the television receiver. The invention resides in the concept of introducing a given level of impairments in the encoded digital baseband signal and taking advantage of the signal correction circuitry that is built into the front end of the digital television receiver to compensate for such impairments. The cost is kept minimal since in the application of the invention, transmission power is very low, distance is limited, the coaxial cable is a low noise environment and the channel shape (Nyquist slope) need not be as rigorously defined for adjacent channel rejection, signal radiation, etc. This translates into low cost filtering to create a less-than-nominal Nyquist slope. While this will clearly cause some intersymbol interference, a mentioned above, all such signal impairments that are below a predetermined threshold, may be compensated for in the digital receiver by the cooperative action of complementary filtering, the equalization circuitry, and the forward error correction circuitry. The encoder-modulator of FIG 2 is a different version of the invention. Here the low cost SAW filters of FIG 1 have been replaced by digital filters 50, indicated as being FIR (finite impulse response) filters. A digital modulator 52 is supplied with the output of a first local oscillator 56 and converts the filtered input signal to a first IF signal having a frequency, for example, of about 12 MHz. This signal is applied to D/A 14 and thence to an RF upconverter 54 that is supplied with the output of a second local oscillator 58. Local oscillator 58 is controlled by a switch 60 to develop an RF output from upconverter 54 at either of a pair of RF channel frequencies, as in the tuner of FIG 1. The output signal is transmitted to the television receiver over coaxial cable 23. This version of the invention appears to be a more stable system.. Since both systems will involve integrated circuitry, it remains to be seen whether the cost of the SAWs in the FIG 1 version will be less than the cost of the additional chip area required in the integrated circuit implementation of the FIG 2 version. In FIG 3 the solid line curve indicates the ideal Nyquist slope channel bandpass response characteristic of the transmitted signal. The dashed line curve indicates the less-than-nominal Nyquist slope that results from using the low cost SAWs 16 and 18 in FIG 1 and the dotted line curve represents the response for the FIG 2 implementation using the FIR filters 50. The curves should be recognized as being representative only and their actual shapes are dependent upon the precision and the number of the filtering elements used. As mentioned, because of the less-than- nominal bandpass characteristic produced by either the SAW or the FIR filter embodiment of the invention, a certain level of intersymbol interference will be introduced into the transmitted signal. However, as discussed above, the amount of any such intersymbol interference, together with any noise introduced into the signal over the communications link, can be substantially corrected in the television receiver to provide near perfect signal translation and reception. In this manner the invention permits a low cost, high performance encoder-remodulator by deliberately selecting lesser tolerance components and applying the signal to a digital receiver that has the capability of compensating for signal impairments below a given threshold. What has been described is a novel method and apparatus for translating a digital television signal from a digital source to a VSB digital television receiver. It is recognized that numerous changes to the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims We Claim: 1. A method of operating a digital signal system for supplying a digital television receiver having correction circuitry capable of correcting impairments in a received signal that are below a predetermined threshold comprising: - developing a compressed baseband digital signal; - encoding the compressed baseband signal in a given format for digital transmission; and - modulating the encoded compressed baseband digital signal on an RF carrier for direct application to the RF input of the digital television receiver, the modulating step resulting in a less-than- nominal bandpass which produces impairments in the transmitted signal that are less than the predetermined threshold, whereby the correction circuitry is capable of correcting the impairments introduced in the modulating step. 2. The method as claimed in claim 1, wherein when the encoding step comprises data interleaving, data randomizing, error protection and the addition of VSB field and segment sync, the method comprising: - performing the modulating step at a low power level. 3. The method as claimed in claim 1, comprising: - supplying the encoded compressed baseband digital signal through a first filter having a response designed to cooperate with a second filter in the digital television receiver for producing a less than nominal Nyquist slope. 4. The method as claimed in claim 1 comprising: - supplying the encoded compressed baseband digital signal through a first filter having a response designed to cooperate with the equalization circuitry and a second filter in the digital television receiver for generating a nominal Nyquist slope and providing adjacent channel rejection at each end of the signal bandpass. 5. The method as claimed in claim 1, when the television receiver comprises a digital signal processing system and an analog NTSC signal processing system, the method comprising generating the modulated RF digital signal for application to the digital processing system of the television receiver. 6. The method as claimed in claim 1, wherein the RF carrier is applied to the RF input of the digital television receiver by a coaxial cable network. 7. The method as claimed in claim 1, wherein said modulating step comprises: - multiplying the base band signal by a predetermined carrier frequency to generate a double side band suppressed carrier signal; and - using a filter to remove one of the sidebands to develop a VSB signal and provide the less-than-nominal band pass. 8. The method as claimed in claim 7, wherein the filter is a SAW and wherein the bandpass has a less-than-nominal Nyquist slope. 9. The method as claimed in claim 1, wherein the correction circuitry comprises equalization and forward error correction circuitry. 10. The method as claimed in claim 1, wherein the RF carrier is applied wirelessly to the RF input of the digital television receiver over a broadcast television channel. 11. A system for supplying a compressed digital baseband signal to a digital television receiver (24) that comprises circuitry (30, 31) capable of correcting impairments in a received signal that are below a predetermined threshold, the system comprising: - a source (10) of compressed base band signal; - an encoder (12) for encoding said baseband signal in a given digital format; - a modulator (13-25) having a less than nominal bandpass that produces impairments in the translated signal that are below said predetermined threshold; and - means (23) for directly applying said modulated signal to an RF input of said digital television receiver (24), whereby said correction circuitry (30, 31) is capable of correcting said impairments introduced by said modulator (13-25). 12.The system as claimed in claim 11, wherein said encoder (12) and said digital television receiver (24) each comprises a filter, (16, 18, 26) and wherein said filters cooperate to generate a less than nominal Nyquist slope. 13. The system as claimed in claim 12, wherein said filters (16, 18, 26) and equalization circuitry (30) cooperate to generate a nominal Nyquist slope and provide adjacent channel rejection at each end of the signal band pass. 14.The system as claimed in claim 11, wherein said given digital format is VSB, said compressed digital signal is in MPEG form, said encoder (12) comprises a data randomizer, a data interleavor, error protection circuitry and means for adding VSB frame and field syncs and wherein said modulator (13-25) operates at low power. 15. The system as claimed in claim 11, wherein said RF carrier is applied to the RF input of the television receiver (24) by a coaxial cable network (23). 16.The system as claimed in claim 11, wherein said digital television receiver (24) comprises a digital signal processing system and an analog NTSC signal processing system and wherein said modulated RF digital signal is generated for application to the digital processing system of the television receiver (24). 17. The system as claimed in claim 11 comprising: - multiplying means (13, 15) in said modulator (13-25) for developing a double sideband suppressed carrier signal from said baseband signal; and - filter means (16, 18) coupled to said multiplying means for substantially removing one of said side bands to develop a VSB signal and for producing said less than nominal bandpass. 18.The system as claimed in claim 17, wherein said filter means (16, 18) comprises a SAW filter and wherein said bandpass comprises a less than nominal Nyquist slope. 19.The system as claimed in claim 11, wherein the correction circuitry (30, 31) comprises equalization (30) and forward error correction (31) circuitry. 20. The system as claimed in claim 11, wherein the RF carrier is applied wirelessly to the RF input of the digital television receiver (24) over a broadcast television channel. This invention relates to an encoder-modulator for coupling a digital baseband television signal to a VSB digital receiver that includes filters, equalization circuitry and forward error correction circuitry for correcting signal impairments that are below a given threshold. The encoder modulator processes the baseband signal for low power transmission on an RF channel with less-than- nominal bandpass characteristics. A coaxial cable or other noise communication link directly connects the RF signal to the RF input of the television receiver. Any errors or signal impairments in the transmitted signal that are below the predetermined threshold are corrected by the filters and equalization circuitry built into the VSB digital television receiver.

Full Text

LOW COST VGB ENCODER AND RF MODULATOR FOR SUPPLYING VSB
BASEBAND SIGNAL TO RF INPUT OF DIGITAL TELEVISION RECEIVER
Background of the Invention and Prior Art
This invention relates generally to digital VSB (vestigial side band) television
receivers and specifically to a simple low cost system for coupling a digital VSB
signal from any of a plurality of sources of MPEG (Motion Picture Experts Group)
digital signal to a digital VSB television receiver. While the description is directed to
a VSB digital format, it should be understood that the invention in its broader aspects
is not limited to a particular digital format.
The recently adopted Digital Television Standard specifies a VSB subsystem
having a terrestrial broadcast mode (8VSB) and a high data rate mode (16 VSB).
There are other VSB modes available, namely 2 VSB, 4 VSB and 8 VSB (non
terrestrial). The 8 VSB terrestrial mode has the data carrying capability of 4 VSB.
(The Trellis coding that is added for the terrestrial environment creates the additional
modulation levels.) The digital data signal is MPEG and Dolby AC-3 processed and
must be subjected to decompression before application to conventional video and
audio circuitry. For terrestrial broadcasting, the data signal is: randomized; subjected
to Reed-Solomon (R/S) type encoding for error correction; interleaved; Trellis
encoded; multiplexed with segment sync and field sync; supplied with a DC pilot;
subject to pre-equalization filtering; modulated; and RF upconverted for transmission.
The digital television receiver includes a tuner and a VSB demodulator for developing
the baseband signal, which is in compressed form. The demodulated signal is applied
to an equalizer for equalizing the received signal. The baseband signal is applied to a
transport demultiplexer which directs the data to an appropriate MPEG decoder and a
Dolby decoder for recovering the video and audio in proper form for application to
video and audio processing circuitry. Digital signals from other sources, such as DVD
(digital video disk) players, VCRs (video cassette recorders), PCs (personal
computers), digital cable boxes, satellite receivers and the like will be supplied to
digital television receivers, as is the case for present analog television receivers.
In an analog system, it is well known that coupling the signal from a VCR or
other video source to a baseband input of a television receiver (if one is available)
yields superior results than those achieved by modulating the signal to RF channel 3
or channel 4 and supplying it to the tuner input of the television receiver. This is not,
however, necessarily true in a digital environment in which case the distance between
the baseband signal source and the television receiver must be kept relatively short.
In accordance with the invention, a compressed baseband digital signal is
encoded in a given format for digital transmission and modulated on an RF carrier for
direct application via a cable network or wireless link to the RF input of a digital
television receiver. The RF' signal, which is at a low power level and has a less-than-
nominal bandpass, takes advantage of the front-end signal processing that is built into
the digital television receiver. Signal impairments, due to the less-than-nominal
bandpass characteristic and noise introduced during transmission, that are below a
given threshold are corrected by the correction circuitry in the digital television
receiver front end. The low signal power, the corrective circuitry for the digital signal
and the benign environment of the communications link, enable components and
circuits of much lower tolerance (and cost) to be used in the encoding and modulating
process. The result is an encoder-modulator that is very low cost and very effective in
coupling a digital baseband signal to a digital television receiver over an RF channel.
Objects of the Invention
A principal object of the invention is to provide a novel digital signal
translation system.
Another object of the invention is to provide a digital encoder-modulator for
coupling a baseband digital signal to a VSB digital television receiver.
A further object of the invention is to provide a low cost digital signal coupling
system for a VSB digital television receiver.
Brief Description of the Drawings
These and other objects and advantages of the invention will be apparent upon
reading the following description in conjunction with the drawing, in which:
FIG 1 is a simplified block diagram of a VSB digital signal encoder-modulator
constructed in accordance with the invention;
FIG 2 is a simplified block diagram of another form of a digital signal encoder-
modulator constructed in accordance with the invention; and
FIG 3 are curves illustrating the bandpass characteristics of the encoder
modulator of the invention.
Description of the Preferred Embodiment
Referring to FIG 1, a source of digital MPEG encoded signal 10 derived from
any of a number of different source types, is shown. For example, the source may
comprise a satellite receiver, a VCR, a DVD, a digital cable box, a PC, etc. The
output of source 10 is an MPEG (and Dolby AC-3) encoded digital signal at baseband
frequency. This baseband signal may be applied directly to a baseband input of a
digital television receiver 24. However, in accordance with the invention, the
baseband signal is applied to an ATSC encoder 12 where the signal is subjected to
interleaving, randomizing, R/S error encoding and Trellis encoding. Field and
segment syncs are added, and while not indicated, a pilot is inserted and equalization
filtering is employed. (See ATSC Standard A/53, ATSC Digital Television Standard
for a detailed description of these operations.) The signal is then supplied to a digital
to analog (D/A) converter 14. D/A 14 supplies the analog signal to a pair of
multipliers 13 and 15 that in turn supply signals to a pair of SAW (surface acoustic
wave) filters 16 and 18. Multipliers 13 and 15 are each supplied with a frequency Fl
and F2, respectively for producing two different outputs. The SAW filters are
selectively controlled by a switch 20, with one of the SAWs being used for one
selected channel RF output and the other being used for another selected channel RF
output. The particular RF channel selected is dependent upon the environment in
which the encoder-modulator is used, in particular the availability and type of RF
channels in the receiving area. The SAW filters are relatively low cost devices and are
extensively used in television receivers.
Those skilled in the art will recognize that highly accurate modulators generate
I and Q signals with the signals being combined to cancel one of the sidebands and
generate a vestigial sideband signal. Such modulators are quite complex and very
expensive. In one form of the invention, that arrangement is replaced with a multiplier
(13, 15)and a SAW filter (16, 18). The output of the multiplier is a double sideband
suppressed carrier signal. The following SAW filter removes most of one sideband
(preferably, although not necessarily, the lower sideband) to produce a VSB signal and
also approximates the nominal Nyquist slope of the channel bandpass. As a result of
the less than nominal bandpass characteristic, the transmitted signal will be
characterized by a degree of impairment in the form of intersymbol interference (ISI).
The SAW filters shape the edges of the signal bandpass to approximate the
Nyquist slopes, nominally called for at the band edges in the ATSC standard. (The
Nyquist slopes may be seen by reference to the solid line curve of FIG 3.) The SAW
filter characteristics also provides for adjacent channel rejection. The filtered signal
from the selected one of the SAWs is applied to an RF upconverter 22, which
upconverts the signal to any desired RF channel, channels 3 or 4 being the ones
normally used.
The encoded and modulated VSB signal is supplied over a cable network 2,3,
which may comprise a single coaxial cable or a relatively complex home cable
network, to an RF channel input of a tuner 26 of a VSB digital television receiver 24.
The signal may also be amplified by a low power RF amplifier 25 (shown in dotted
line in FIG. 1) and transmitted wirelessly to the digital television receiver in an
assigned RF broadcast television channel. The received signal is supplied through a
companion SAW filter to a VSB demodulator 28 where the baseband digital signal is
recovered. The demodulated signal is applied to an equalizer 30 that operates to adjust
the response of the receiver to very closely match the solid line Nyquist response (FIG
3) thereby correcting any signal impairments, including any intersymbol interference
introduced in the transmitter and any linear distortions introduced in the
communications link (i.e. the cable network or wireless link) that collectively are
below a given threshold level that is dependent upon the nature of the equalizer. The
signal from equalizer 30 is processed by a forward error correction subsystem 31 to
correct any residual errors due to intersymbol interference and errors due to noise,
both white and impulse, introduced by the cable network or wireless link. The
performance of equalizer 30 and error correction subsystem 31 sets the limit on the
amount of intersymbol interference and linear distortion which can be introduced by
the transmitter and communications channel before the system suffers serious
degradation.
The corrected signal (in MPEG encoded form) is supplied to a transport
demultiplexer 32 that has separate outputs for the video and audio portions of the
signal. The video portion is applied to an MPEG decoder 34 for decompression and
decoding and the audio portion is applied to a Dolby AC-3 decoder 36 for
complementary processing. The resultant video and audio signals are applied to a
video processor 38 and an audio processor 40, respectively.
It will be appreciated by those skilled in the art that the RF signal is not as
distance sensitive as a baseband signal and the coaxial cable environment further
minimizes errors or impairments being introduced in transmission. However, even if
some signal impairments are introduced, they may be compensated for by the
corrective circuitry built into the VSB digital television receiver, in particular the
equalization circuitry. The result is a signal translation arrangement that results in
excellent transfer of signal from source 10 to VSB digital television receiver 24, since
the signal is VSB encoded and any errors or signal impairments below a given
threshold are correctable in the television receiver.
In contrast, any baseband digital signals that are supplied to television receiver
24 are introduced after equalizer 30 and error correction subsystem 31 and are
consequently not subject to the corrective effects of the circuitry in the VSB digital
television receiver. Such signals are also distance limited and subject to uncorrectable
errors or impairments, which makes the method of the invention far superior.
In further accordance with the invention, where the digital television receiver
has provision for receiving both a digital and an analog NTSC signal, in establishing
priority of the RF channel output of upconverter 22, the first choice is to supply the
signal on a vacant RF channel in the service area of the television receiver and the
second choice is to supply it on the digital one of a pair of digital and analog NTSC
RF channels in the service area of the television receiver. In this way, any NTSC
interference into the digital channel can be minimized by the circuitry in the digital
channel of the television receiver.
The invention resides in the concept of introducing a given level of
impairments in the encoded digital baseband signal and taking advantage of the signal
correction circuitry that is built into the front end of the digital television receiver to
compensate for such impairments. The cost is kept minimal since in the application of
the invention, transmission power is very low, distance is limited, the coaxial cable is
a low noise environment and the channel shape (Nyquist slope) need not be as
rigorously defined for adjacent channel rejection, signal radiation, etc. This translates
into low cost filtering to create a less-than-nominal Nyquist slope. While this will
clearly cause some intersymbol interference, a mentioned above, all such signal
impairments that are below a predetermined threshold, may be compensated for in the
digital receiver by the cooperative action of complementary filtering, the equalization
circuitry, and the forward error correction circuitry.
The encoder-modulator of FIG 2 is a different version of the invention. Here
the low cost SAW filters of FIG 1 have been replaced by digital filters 50, indicated as
being FIR (finite impulse response) filters. A digital modulator 52 is supplied with the
output of a first local oscillator 56 and converts the filtered input signal to a first IF
signal having a frequency, for example, of about 12 MHz. This signal is applied to
D/A 14 and thence to an RF upconverter 54 that is supplied with the output of a
second local oscillator 58. Local oscillator 58 is controlled by a switch 60 to develop
an RF output from upconverter 54 at either of a pair of RF channel frequencies, as in
the tuner of FIG 1. The output signal is transmitted to the television receiver over
coaxial cable 23. This version of the invention appears to be a more stable system..
Since both systems will involve integrated circuitry, it remains to be seen whether the
cost of the SAWs in the FIG 1 version will be less than the cost of the additional chip
area required in the integrated circuit implementation of the FIG 2 version.
In FIG 3 the solid line curve indicates the ideal Nyquist slope channel
bandpass response characteristic of the transmitted signal. The dashed line curve
indicates the less-than-nominal Nyquist slope that results from using the low cost
SAWs 16 and 18 in FIG 1 and the dotted line curve represents the response for the
FIG 2 implementation using the FIR filters 50. The curves should be recognized as
being representative only and their actual shapes are dependent upon the precision and
the number of the filtering elements used. As mentioned, because of the less-than-
nominal bandpass characteristic produced by either the SAW or the FIR filter
embodiment of the invention, a certain level of intersymbol interference will be
introduced into the transmitted signal. However, as discussed above, the amount of
any such intersymbol interference, together with any noise introduced into the signal
over the communications link, can be substantially corrected in the television receiver
to provide near perfect signal translation and reception. In this manner the invention
permits a low cost, high performance encoder-remodulator by deliberately selecting
lesser tolerance components and applying the signal to a digital receiver that has the
capability of compensating for signal impairments below a given threshold.
What has been described is a novel method and apparatus for translating a
digital television signal from a digital source to a VSB digital television receiver. It is
recognized that numerous changes to the described embodiment of the invention will
be apparent to those skilled in the art without departing from its true spirit and scope.
The invention is to be limited only as defined in the claims
We Claim:
1. A method of operating a digital signal system for supplying a digital
television receiver having correction circuitry capable of correcting
impairments in a received signal that are below a predetermined threshold
comprising:
- developing a compressed baseband digital signal;
- encoding the compressed baseband signal in a given format for
digital transmission; and
- modulating the encoded compressed baseband digital signal on an
RF carrier for direct application to the RF input of the digital
television receiver, the modulating step resulting in a less-than-
nominal bandpass which produces impairments in the transmitted
signal that are less than the predetermined threshold, whereby the
correction circuitry is capable of correcting the impairments
introduced in the modulating step.
2. The method as claimed in claim 1, wherein when the encoding step
comprises data interleaving, data randomizing, error protection and the
addition of VSB field and segment sync, the method comprising:
- performing the modulating step at a low power level.
3. The method as claimed in claim 1, comprising:
- supplying the encoded compressed baseband digital signal through
a first filter having a response designed to cooperate with a second
filter in the digital television receiver for producing a less than
nominal Nyquist slope.
4. The method as claimed in claim 1 comprising:
- supplying the encoded compressed baseband digital signal through
a first filter having a response designed to cooperate with the
equalization circuitry and a second filter in the digital television
receiver for generating a nominal Nyquist slope and providing
adjacent channel rejection at each end of the signal bandpass.
5. The method as claimed in claim 1, when the television receiver comprises
a digital signal processing system and an analog NTSC signal processing
system, the method comprising generating the modulated RF digital signal
for application to the digital processing system of the television receiver.
6. The method as claimed in claim 1, wherein the RF carrier is applied to the
RF input of the digital television receiver by a coaxial cable network.
7. The method as claimed in claim 1, wherein said modulating step
comprises:
- multiplying the base band signal by a predetermined carrier
frequency to generate a double side band suppressed carrier
signal; and
- using a filter to remove one of the sidebands to develop a VSB
signal and provide the less-than-nominal band pass.
8. The method as claimed in claim 7, wherein the filter is a SAW and wherein
the bandpass has a less-than-nominal Nyquist slope.
9. The method as claimed in claim 1, wherein the correction circuitry
comprises equalization and forward error correction circuitry.
10. The method as claimed in claim 1, wherein the RF carrier is applied
wirelessly to the RF input of the digital television receiver over a broadcast
television channel.
11. A system for supplying a compressed digital baseband signal to a digital
television receiver (24) that comprises circuitry (30, 31) capable of
correcting impairments in a received signal that are below a
predetermined threshold, the system comprising:
- a source (10) of compressed base band signal;
- an encoder (12) for encoding said baseband signal in a given digital
format;
- a modulator (13-25) having a less than nominal bandpass that
produces impairments in the translated signal that are below said
predetermined threshold; and
- means (23) for directly applying said modulated signal to an RF
input of said digital television receiver (24), whereby said correction
circuitry (30, 31) is capable of correcting said impairments
introduced by said modulator (13-25).
12.The system as claimed in claim 11, wherein said encoder (12) and said
digital television receiver (24) each comprises a filter, (16, 18, 26) and
wherein said filters cooperate to generate a less than nominal Nyquist
slope.
13. The system as claimed in claim 12, wherein said filters (16, 18, 26) and
equalization circuitry (30) cooperate to generate a nominal Nyquist slope
and provide adjacent channel rejection at each end of the signal band
pass.
14.The system as claimed in claim 11, wherein said given digital format is
VSB, said compressed digital signal is in MPEG form, said encoder (12)
comprises a data randomizer, a data interleavor, error protection circuitry
and means for adding VSB frame and field syncs and wherein said
modulator (13-25) operates at low power.
15. The system as claimed in claim 11, wherein said RF carrier is applied to
the RF input of the television receiver (24) by a coaxial cable network
(23).
16.The system as claimed in claim 11, wherein said digital television receiver
(24) comprises a digital signal processing system and an analog NTSC
signal processing system and wherein said modulated RF digital signal is
generated for application to the digital processing system of the television
receiver (24).
17. The system as claimed in claim 11 comprising:
- multiplying means (13, 15) in said modulator (13-25) for developing
a double sideband suppressed carrier signal from said baseband
signal; and
- filter means (16, 18) coupled to said multiplying means for
substantially removing one of said side bands to develop a VSB
signal and for producing said less than nominal bandpass.
18.The system as claimed in claim 17, wherein said filter means (16, 18)
comprises a SAW filter and wherein said bandpass comprises a less than
nominal Nyquist slope.
19.The system as claimed in claim 11, wherein the correction circuitry (30,
31) comprises equalization (30) and forward error correction (31) circuitry.
20. The system as claimed in claim 11, wherein the RF carrier is applied
wirelessly to the RF input of the digital television receiver (24) over a
broadcast television channel.
This invention relates to an encoder-modulator for coupling a digital baseband
television signal to a VSB digital receiver that includes filters, equalization
circuitry and forward error correction circuitry for correcting signal impairments
that are below a given threshold. The encoder modulator processes the
baseband signal for low power transmission on an RF channel with less-than-
nominal bandpass characteristics. A coaxial cable or other noise communication
link directly connects the RF signal to the RF input of the television receiver. Any
errors or signal impairments in the transmitted signal that are below the
predetermined threshold are corrected by the filters and equalization circuitry
built into the VSB digital television receiver.

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Patent Number

225448

Indian Patent Application Number

IN/PCT/1999/00002/KOL

PG Journal Number

46/2008

Publication Date

14-Nov-2008

Grant Date

12-Nov-2008

Date of Filing

21-Jul-1999

Name of Patentee

ZENITH ELECTRONICS CORPORATION

Applicant Address

1000 MILWAUKEE AVENUE, CLENVIEW, ILLINOIS

Inventors:

#	Inventor's Name	Inventor's Address
1	HAUGE RAYMOND C.	9011 GARDENERS ROAD, FOX RIVER GROVE, ILLINOIS 60071
2	SNOPKO PAUL ADAM	360 EAST RANDOLPH DRIVE NO. 3001, CHICAGO, ILLINOIS 60601
3	CITTA RICHARD W	739 NORTH COLUMBIAN, OAK PARK, ILLINOIS 60302

PCT International Classification Number

H04N 5/21, 7/04

PCT International Application Number

PCT/US1999/00130

PCT International Filing date

1999-01-06

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	09/020,274	1998-02-06	U.S.A.