MXPA99009105A - Vsb encoder and rf modulator for digital television receiver - Google Patents

Abstract

An encoder-modulator (12, 13, 15) for coupling a digital baseband television signal to a VSB digital television receiver (24) that includes filters (26), equalization circuitry (30) and forward error correction circuitry (31) 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 (23) or other low noise communication link directly connects the RF signal to the RF input of the television receiver (24). 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 (24).

Description

- LOW COST AND RF ODOR ENCODER FOR SUPPLYING BASE BAND SIGNALS VSB TO DIGITAL TELEVISION RECEIVER RF POWER BACKGROUND BACKGROUND OF THE INVENTION AND PRIOR ART ~ This invention relates generally to vestigial side-band television receivers (VSB = vestigial) side band) digitally and specifically to a simple low cost system for coupling a digital VSB signal from any of a plurality of digital MPEG signal sources (MPEG = Motion Picture Experts Group) to a digital VSB television receiver. While the description is directed to a digital VSB format, it will 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 sub-system that has a terrestrial broadcast mode (8VSB) and a high-speed data mode (16BSV). There are other VSB modes available, ie 2VSB, 4VSB and 8VSB (non-terrestrial). The 8VSB terrestrial mode has the 4VSB data transport capacity. (The Trellis coding that is added for the terrestrial environment creates the additional modulation levels). The digital data signal is processed by MPEG and Dolby AC-3 and must be decompressed before application to conventional video and audio circuits. For REF diffusion: 30827 terrestrial, the data signal is: randomized; subjected to Reed-Solomon type coding (R / S) for error correction; interspersed coded Trellis; Grouped with segment synchronization and field synchronization; supplied with a DC pilot; submitted to pre-compensation filtering; Modulated and converted uplink RF for transmission. The digital television receiver includes a VSB tuner and demodulator to reveal the baseband signal, which is in compressed form. The demodulated signal is applied to a compensator for the received signal. The baseband signal is applied to a transport demultiplexer which routes the data to an appropriate MPEG decoder and a Dolby decoder to retrieve the video and audio in a form suitable for application to video and audio processing circuits.

Digital signals from other sources, such as DVD players (DVD = digital disc video), VCRs (video cassette recorder), PCs (personal computers), digital cable decoders, satellite receivers and the like, will be supplied to digital television receivers, as is the case for the current analogue television receivers. In an analog system, it is well known that coupling the signal of a VCR or other video source to a baseband power of a television receiver (if one is available) produces superior results than those achieved by modulating the signal to channel 3 or RF channel 4 and supply it to the tuner power of the television receiver. This however is not 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. According to the invention, a compressed baseband digital signal is encoded in a particular format for digital transmission and modulated in an RF carrier for direct application via a cable network or wireless link to the RF power of a television receiver digital. The RF signal that 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 the interference introduced during transmission, which are below a certain threshold, are corrected by the correction circuits at the front end of the digital television receiver. The low signal strength, the corrective circuits for the digital signal and the benign environment of the communications link, allow components and circuits of much less tolerance (and cost) to be used in the coding and modulation process. The result is an encoder-modulator that is very low cost and very efficient in coupling a digital baseband signal to a digital television receiver over an RF channel. OBJECTIVES OF THE INVENTION A primary objective of the invention is to provide a novel digital signal translation system. Another object of the invention is to provide a digital encoder and modulator for coupling a digital baseband signal to a digital television receiver VSB. A further objective of the invention is to provide a low cost digital signal coupling system for a digital television receiver VSB. 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 drawings, wherein: Figure 1 is a simplified block diagram of a digital signal encoder-modulator VSB constructed according to the invention; Figure 2 is a simplified block diagram of another form of a digital signal encoder-modulator constructed in accordance with the invention; and Figure 3 are curves illustrating the bandpass characteristics of the modulator-encoder of the invention. Description of the Preferred Modality With reference to Figure 1, a digital MPEG encoded signal source 10 derived from any of a number of different types of sources is illustrated. For example, the source may comprise a satellite receiver, a VCR, a DVD, a digital cable box, a PC, etc. The output of the source 10 is a digital signal encoded MPEG (and Dolby AC-3 at the baseband frequency.) This baseband signal can be applied directly to a baseband power of a digital television receiver 24. However , according to the invention, the baseband signal is applied to an ATSC encoder 12, where the signal is subjected to interleaving, randomization, R / S error coding and Trellis coding. Field and segment synchronizers are added, and while not indicated, a pilot is inserted and compensation filtering is used (See ATSC Standard A / 53, ATSC Digital Television standard for a detailed description of these operations.) The signal is then supplied to a digital converter to analog (D / A) 14. The D / A 14 supplies the analog signal to a pair of multipliers 13 and 15 which in turn supply signals to a pair of surface acoustic wave (SAW) filters 16 and 18. Each one of the multipliers 13 and 15 is supplied with a frequency of Fl and F2, respectively to produce two different outputs. The SAW filters are selectively controlled by a switch 20, with one of the SAWs used for a select channel RF output and the other used for another select channel RF output. The particular RF channel selected depends on the environment and where the encoder-modulator is used, in particular the availability and types of RF channels in the receiving area. SAW filters are relatively low cost devices and are widely used in television receivers. Those skilled in the art will recognize that highly precise modulators generate I and Q signals with signals that combine to cancel one of the sidebands and generate a vestigial sideband signal. These modulators are quite complex and very expensive. In one form of the invention, that structure is replaced with a multiplier (13, 15) and a SAW filter (16, 18). The output of the multiplier is a suppressed carrier signal of double sideband. The following SAW filter removes most of a sideband (preferably but not necessarily the lower sideband) to produce a VSB signal and also approaches the nominal Nyquist slope of the channel bandpass. As a result of the bandpass characteristic less than nominal, the transmitted signal will be characterized by a degree of deterioration in the form of intersymbol interference (ISI). The SAW filters configure the edges of the signal bandpass to approximate the Nyquist slopes, nominally required at the band edges in the ATSC standard. The? Yquist slopes can be seen by reference to the curve of solid lines in Figure 3). The SAW filter characteristics also allow adjacent channel rejection. The filtered signal of the selected SAW is applied to an RF uplink converter 22, and converts the signal to any desired RF channel, channels 3 or 4 are normally used. The encoded and modulated VSB signal is supplied on a cable network 23, which may comprise a simple coaxial cable or a relatively complex domestic cable network, to an RF channel input of a tuner 26 of the digital television receiver VSB 24. The signal can also be extended by a low power RF amplifier 25 (shown in dotted lines in Figure 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 28 demodulator, where the baseband digital signal is recovered. The demodulated signal is applied to a compensator 30 which operates to adjust the response of the receiver to correspond very closely to the Nyquist response of solid lines (Figure 3) thereby correcting any signal impairments, including any symbol interference introduced in the - transmitter and any linear distortions introduced into the communication link (i.e. the cable network or wireless link) that collectively are below a certain threshold level, which depends on the nature of the compensator. The signal from the compensator 30 is processed by a progressive error code correction sub-system to correct any residual errors due to intersymbol interference and errors due to interference, both white and pulse, introduced by the cable or wireless link network. The performance of the compensator 30 and the error correction sub-system 31 adjust the limit on the amount of intersymbol interference and linear dispersion that 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 having separate outputs for the video and audio portions of the signal. The video portion is applied to an MPEG decoder 34 for decoding and decoding and the audio portion is applied to a Dolby AC-3 decoder 36 for complementary processing. The resulting 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 sensitive to distance as a baseband signal and the coaxial cable environment also minimizes errors or deterioration introduced in the transmission. However, even if some signal impairments are introduced, they can be compensated by the corrective circuits constructed in the digital television receiver VSB, in particular the compensation circuits. The result is a signal translation structure that results in excellent signal transfer from the source 10 to the digital television receiver VSB 24, since the signal is encoded VSB and any errors or deteriorations of signal below a certain threshold, correct in the television receiver. In contrast, any baseband digital signals that are supplied to the television receiver 24 are introduced after the compensator 30 and its error correction system 31 and consequently are not subject to the corrective effects of the circuits in the digital television receiver VSB. These signals are also limited by distance and subjected to non-correctable errors or deterioration which make the method of the invention considerably superior. In additional accordance with the invention, wherein the digital television receiver has supply to receive both a digital and an analog signal NTSC, to prioritize the RF channel output of the uplink converter 22, the first selection is to supply the signal in a vacant RF channel in the service area of the television receiver and the second selection is to supply it in the digital of a pair of analog and digital NTSC RF channels in the service area of the television receiver. In this way, any NTSC interference in the digital channel can be minimized by the circuits in the digital channel of the television receiver.

The invention resides in the concept of introducing a determined level of deterioration in the encoded digital baseband signal and taking advantage of the signal correction circuits that are constructed at the front end of the digital television receiver, to compensate for said deteriorations. The cost is kept minimal since in the application of the invention, the transmission power is very low, the distance is limited, the coaxial cable is a low interference environment and the channel shape (Nyquist slope) does not need to be so rigorously defined for adjacent channel rejection, signal radiation, etc. This results in low cost filtering to create a less-than-nominal Nyquist slope. While this will clearly cause some intersymbol interference, as mentioned above, all these signal impairments that are below a predetermined threshold can be compensated by the digital receiver by the complementary filtering cooperative action, the compensation circuits and the circuits- of progressive error correction. The encoder-modulator of Figure 2 is a different version of the invention. Here the low-cost SAW filters of Figure 1 have been replaced by digital filters 50, indicated as finite impulse response filters (FIR = Finite Impulse Response). A digital modulator 52 is supplied with the output of a first local oscillator 56 and converts the filtered feed signal to a first IF signal having a frequency, for example about 12 MHz. This signal is applied to D / A 14 and therein an RF uplink converter 54 which is supplied with the output of a second local oscillator 58. The local oscillator 58 is controlled by a switch 60 to develop an RF output from the uplink converter 54 in any of a pair of RF channel frequencies, such as the tuner of Figure 1. The output signal is transmitted to the television receiver over the coaxial cable 23. This version of the invention appears as a more stable system. Since both systems will involve integrated circuits, it remains to be seen whether the cost of the SAWs in the version of Figure 1 will be less than the cost of the additional chip required in the implementation of the integrated circuit of the version of Figure 2. In the Figure 3, the curve of solid lines indicates the channel bandpass response characteristic of the ideal Nyquist slope of the transmitted signal. The dashed line curve indicates the less-than-nominal Nyquist slope, which results from using the low-cost SAWs 16 and 18_in Figure 1, and the dotted-area curve represents the response for the implementation of Figure 2 using the filters FIR 50. Curves should be recognized as representative only and their current forms depend on the accuracy and number of filtering elements used. As mentioned, due to the less-than-nominal bandpass characteristic produced by either the FIR or SAW filter mode of the invention, a certain level of intersymbol interference will be introduced into the transmitted signal. However, as discussed above, the amount of any of this intersymbol interference along with any interference introduced into the signal on the communication link, can be substantially corrected in the television receiver to provide almost perfect signal translation and reception. In this way, the invention allows a low-cost, high-performance coder-remodulator, by deliberately selecting components of lower tolerance and applying the signal to a digital receiver that has the ability to compensate for signal deteriorations below a certain threshold. - What has been described is a novel method and apparatus for translating a digital television signal from a digital source to a digital television receiver VSB. It is recognized that numerous changes to the described modality of the invention will be apparent to those skilled in the art, without departing from its spirit and actual scope. The invention will be limited only as defined in the claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (20)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. - Method for operating a digital signal system to supply a digital television receiver having correction circuits capable of correcting deteriorations in a received signal , which are below a predetermined threshold, characterized in that it comprises: developing a compressed baseband digital signal; encoding the compressed baseband signal in a particular format for digital transmission; and modulating the compressed baseband digital signal encoded in an RF carrier for direct application to the RF power in the digital television receiver; the modulation step results in a less-than-nominal bandpass, which produces deteriorations in the transmitted signal that are less than the predetermined threshold; with which the correction circuits are able to correct the deteriorations introduced in the modulation stage.
2. 2. - The method according to claim 1, characterized in that the coding step includes data interleaving, data scrambling, error protection and the addition of segment and field VSB synchronizations and further comprises: performing the modulation step a a low power level.
3. 3. The method according to claim 1, characterized in that it further comprises supplying the coded compressed baseband digital signal through a first filter having a response designed to cooperate with a second filter in the digital television receiver, to produce a Nyquist slope less-than-nominal.
4. 4. - The method according to claim 1, characterized in that it further comprises: supplying the encoded compressed baseband digital signal through a first filter having a response designed to cooperate with the compensation circuits and a second filter in the digital television receiver, to generate a nominal Nyquist slope and provide adjacent channel rejection at each end of the signal bandpass.
5. 5. - The method according to claim 1, characterized in that the television receiver comprises a digital signal processing system and an analog NTSC signal processing system and further comprises generating the digital RF signal modulated for application to the processing system digital television receiver.
6. 6. - The method according to claim 1, characterized in that the RF carrier is applied to the RF power of the digital television receiver by a coaxial cable network.
7. 7. - The method according to claim 1, characterized in that the modulation step comprises: multiplying the baseband signal by a predetermined carrier frequency to generate a suppressed carrier signal of double sideband, - and using a filter to remove a of the sidebands to develop a VSB signal and provide the less-than-nominal bandpass.
8. 8. - The method according to claim 7, characterized in that the filter is a SAW and where the bandpass has a less-than-nominal Nyquist slope.
9. 9. - The method - according to claim 1, characterized in that the correction circuits comprise progressive error correction and compensation circuits.
10. 10. The method according to claim 1, characterized in that the RF carrier is applied wirelessly to the RF power of the digital television receiver on a broadcast television channel.
11. 11. System for supplying a compressed digital baseband signal to a digital television receiver that includes correction circuits capable of correcting deteriorations in a received signal, which are below a predetermined threshold, characterized in that it comprises: a signal source of compressed baseband; an encoder for the baseband signal in a given digital format; a modulator for the baseband signal encoded in an RF carrier; the modulator has a less-than-nominal bandpass, which produces impairments in the translated signal, which are below the predetermined threshold; and means for directly applying the modulated signal to an RF power of the digital television receiver, whereby the correction circuits are able to correct the impairments introduced by the modulator.
12. 12. - The system according to claim 11, characterized in that each of the digital television encoder and receiver includes a filter, and wherein the filters cooperate to generate a less-than-nominal Nyquist slope.
13. 13. The system according to claim 12, characterized in that the filters and compensation circuits cooperate to generate a nominal Nyquist slope and provide adjacent channel rejection at each end of the signal bandpass.
14. 14. - The system according to claim 11, characterized in that the given digital format is VSB, the compressed digital signal is in MPEG form, the encoder includes a data scrambler, a data interleaver, error protection circuits and means to add VSB field and frame synchronizers and where the modulator operates at low power.
15. 15. - The system according to claim 11, characterized in that the RF carrier is applied to the RF power of the digital television receiver by a coaxial e network.
16. 16. - The system according to claim 11, characterized in that the digital television receiver comprises a digital signal processing system and an analog NTSC signal processing system, and wherein the modulated RF digital signal is generated for application to the digital processing system of the television receiver.
17. 17. The system "according to claim 11, further comprising: multiplying means in the modulator, to develop a suppressed carrier signal of double sideband from the baseband signal, and filter media coupled to the media multipliers to substantially remove one of the sidebands to develop a VSB signal and to produce a less-than-nominal bandpass
18. 18. The system according to claim 17, characterized in that the filter means comprise a SAW filter and wherein the bandpass includes a less-than-nominal Nyquist slope
19. 19. The system according to claim 11, characterized in that the correction circuits comprise progressive error compensation and compensation circuits. according to claim 11, characterized in that the RF carrier is applied wirelessly to the RF power of the television receiver digital n over a broadcast television channel.