United States Patent (19) (11) 4,065,784 Rossi 45) Dec

United States Patent (19) (11) 4,065,784 Rossi 45) Dec. 27, 1977

54 METHOD AND APPARATUS FOR Assistant Examiner-Aristotelis M. Psitos PCM-ENCOONG NTSC COLOR Attorney, Agent, or Firm-Spencer E. Olson TELEVISION ATSUB-NYQUISTRATE 57 - ABSTRACT 75 Inventor: John P. Rossi, New York, N.Y. NTSC television signals are digitally encoded at sub (73) Assignee: CBS Inc., New York, N.Y. Nyquist rates by placing the alias components into those (21) Appl. No.: 727,819 parts of the spectrum not normally occupied by the luminance or chrominance components of the video 22 Filed: Sept. 29, 1976 signal. In a system described, the sampling frequency, f, 5ll Int. Cl’...... H04N 9/32 is exactly 2f + for 2f -f, where fis the NTSC 52 U.S. C...... 358/13 color subcarrier frequency and f is the line-scan fre 58) Field of Search ...... 358/13 quency. Most of the alias signals in the thus encoded (56) References Cited signal are removed from the baseband video by comb filtering between f-f and f, where fis the baseband U.S. PATENT DOCUMENTS video bandwidth. 3,858,240 12/1974 Golding et al...... 358/13 Primary Examiner-John C. Martin 9 Claims, 13 Drawing Figures

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4,065,784 1. 2 minance components of the video signal. This allows METHOD AND APPARATUS FOR most of the alias signals in the encoded signal to be PCM-ENCODNG NTSC COLOR TELEVISION AT removed from the baseband video by comb filtering SUB-NYQUISTRATE between f-f and f, wheref, is the baseband video bandwidth. BACKGROUND OF THE INVENTION Specific features of the invention, and the construc This invention relates generally to television appara tion and operation of apparatus in which it is embodied, tus, and more particularly to a method and apparatus will be described in conjunction with the accompany for digitally encoding an NTSC color television signal. drawings. Where a transmission channel has a limited data rate 10 or a digital store has a limited capacity, it sometimes BRIEF DESCRIPTION OF THE DRAWINGS becomes necessary to reduce the data rate of pulse FIG. 1 is a diagram showing the foldover of the code-modulated (PCM) color television signals. One lower encoded video sideband onto the baseband video direct way of doing this is to lower the encoding fre when fis less than 2f; quency, f. However, the Nyquist sampling limit is 15 FIG. 2 is a diagram showing the spectral characteris reached when f = 2f, wheref, is the highest video tic of the main Y and Cenergy components of an NTSC frequency being encoded. Reducing f. further causes color television signal within the chrominance side beating distortions due to the lower sidebands off over band; lapping the baseband video frequencies. For NTSC FIG. 3 is a diagram showing main spectral lines of the television, f, = 4.2 MHz and the Nyquist sampling limit 20 baseband luminance and chrominance signals and the is 8.4 MHz. alias signals for an encode rate of 2f +f With a monochromevideo signal, if f is chosen to be FIG. 4 is a diagram showing the frequency response an odd integral multiple of one-half the line scan fre characteristic of one form of comb filter; quency, f, it is possible to remove most of the aliasing FIG. 5 is a diagram showing the frequency response distortion by comb filtering after the codec; a system for 25 of another form of comb filter; doing this is described in an article by L. S. Golding FIGS. 6, 7 and 8 are block diagrams of three possible entitled, "A Digital Communications System for Satel comb filters useful in the practice of the invention; lite Links', Proceedings of Second International Confer FIG. 9 is a set of phasor diagrams depicting the effect ence on Digital Satellite Communications, Paris, Novem of sub-Nyquist encoding on the television signal; ber 1972. This is true because the alias components are 30 FIG. 10 is a block diagram of an encoding system largely confined to bursts of energy centered on (n + ) employing sub-Nyquist sampling according to the in f, where n is an integer, and the luminance component vention; (Y) of the video signal is confined to bursts centered on FIG. 11 is a set of phasor diagrams useful to the un in f. Only those frequencies lying between f-f andf, derstanding of the operation of the encoder of FIG. 10; need be comb filtered. 35 FIG. 12 is a block diagram of one system for generat The unique characteristics of PAL color television ing sampling signals for use in the system of FIG. 10; signals permit their being encoded at sub-Nyquist rates and and comb filtered to remove aliasing, in the manner FIG. 13 is a block diagram of an alternative system described by V. G. Devereux and G. J. Philips in an for generating sampling signals for use in the system of article entitled, "Bit-Rate Reduction of Digital Video FIG. O. Signals Using Differential PCM Techniques', pp. 83-89, IEEE Conference Publication No. 119. This is DESCRIPTION OF THE PREFERRED possible due to the chrominance (C) signal energy being EMBODIMENT centered on bursts of frequencies at (n-:)f. When the When it is attempted to reduce the encoding fre PAL signal is encoded at a sub-Nyguist frequency equal quency, f, in the PCM-encoding of NTSC television to an odd integral multiple of f, the Y alias compo signals below the Nyquist limit wheref=2f, where fis nents are restricted to frequencies centered on (n--)f the highest video frequency being encoded, beating or which can then be placed in the nulls of the comb filter "aliasing' distortion occurs due to the lower sidebands for removal. off overlapping the baseband video signals, as shown in NTSC television signals could also be digitally en 50 FIG. 1. (The set of different possible signals with the coded at sub-Nyquist rates if means were available to same set of sample values are called "aliases' or "alias place the alias components into those parts of the spec components' of the band limited signal.) For NTSC trum not occupied by Y or C components of the video television, f = 4.2MHz and the Nyquist sampling limit signal. It would then be possible, by suitable comb filter is 8.4MHz. ing, to remove most of the alias signals from the base 55 Before proceeding to the description of the sub band video. It is a primary object of the present inven Nyquist encoding system of the invention, it will be tion to provide a method and apparatus for satisfying useful to review the significant spectral characteristics this need, thereby to reduce the data rate of PCM of the NTSC color television signal. The spectral en encoded NTSC television signals. ergy of the luminance (Y) signal is essentially centered at harmonics of the line scanning frequency f, i.e., nf, SUMMARY OF THE INVENTION where n + integer. The chrominance (C) signal spectral Briefly, the data rate of PCM-encoded NTSC televi energy peaks at odd harmonics of f, i.e., (n -- )f sion is accomplished by encoding at sub-Nyquist rates Thus, the Y and the C energy bundles are frequency by using a sampling frequency, f, that is exactly 2f-h interleaved as shown in FIG. 2. or 2f-h, where f is the NTSC color subcarrier fre 65 To encode the PCMNTSC color television signal at quency and f is the line-scan frequency, thereby to sub-Nyquist rates and be able to later remove the alias place the alias components into those parts of the spec components, it is necessary to choose an encoding fre trum not normally occupied by the luminance or chro quency f. that frequency interlaces the alias components 4,065,784 3. 4. between the desired Y and C components. This interlac The system of FIG. 7 contains the same elements as ing is accomplished in the system to be described by the system of FIG. 6, but are differently connected; making f = (n + i)f (or alternatively, (n - )f corresponding elements are identified by the reference In addition, as the chrominance signal from approxi numerals used in FIG. 6 except that they are primed. In mately 3MHz to 4.2MHz is a double sideband modu this embodiment, the low frequencies appearing at the lated signal centered at f (approximately 3.58MHz), output terminal 22' are delayed by two scan lines, alias components likewise symmetrical to f would whereas in the FIG. 6 system the low frequencies are cause the least color difference signal crosstalk by af. undelayed. fecting the chrominance sidebands equally. This sug Each of the described comb filters has an output for gests that f should be equal or nearly equal to 2f Com 10 frequencies between f-f that is equal to the sum of the bining these requirements has led to the conclusion by present input signal and the input signal delayed by applicant that satisfactory sub-Nyquist encode rates exactly twice the line-scan period (2H). For frequencies would be 2? + for 2f - if, where f is 455/2f. below f-f the output of the FIG, 6 filter is equal to Encoding at 2? + Af, results in the frequency spec the input signal, and the output of the FIG. 7 filter is trum shown in FIG. 3, wherein are shown the peak 15 equal to the input signal delayed by 2H. frequency components of the baseband luminance Yp In the comb filter of FIG. 8, which utilizes three and chrominance CB, and the alias luminance compo television lines, the present line lat input terminal 30 is nents YA and chrominance components C4. Unless the combined in a summing circuit 32 with the input signal alias components can be acceptably removed, obviously delayed by exactly 4H by a delay device 34. The ampli the utilization of the aforementioned sub-Nyquist en 20 tude of the sum signal from the summing circuit is re code rates would not serve the intended purpose of duced by one-half by a suitable attenuating device 36 satisfactorily reducing the data rate of PCM television and combined in a second summing circuit 38 with the signals. input signal delayed by exactly 2H by a delay device 40. Another feature of the invention is applicant's recog The sum signal from summing circuit is transmitted nition that most of the alias signals can be repeated by 25 through a high pass filter 42 and combined in a summing means of suitable transversal comb filtering. As will be circuit 44 with the 2H-delayed signal after filtering by a seen in FIG. 3, the centers of the alias energy bursts are low pass filter 46. Thus, this filter has an output signal separated by frequency intervals of f; thus, in order to on output line 48 for frequencies between f-f andf, reject the alias signals, the comb filter should have its that is equal to the sum of one-half the input signal plus maximum responses (teeth) or minimum responses 30 the input signal delayed by exactly 2H plus one-half the (nulls), at frequency intervals off. Such a comb filter input signal delayed by exactly 4H. For frequencies can be made by combining video signals from alternate below f-f the output of the filter is equal to the input time-sequential television lines. For example, in a partic signal delayed by 2H. ular field, a given line l would be combined with line Inasmuch as the described comb filters will exhibit a (1-2) or line (l-2). Since a transversal filter that com 35 rise in response within the frequency band of the comb bines television lines can cause objectionable transients filter, it is desirable to introduce a complementary and a loss of vertical resolution in the television picture, equalizer into the system. it is desirable to combine as few lines as possible to The effects of the described sub-Nyquist encoding on obtain the necessary comb filter frequency response. the television signal will be better understood from The alias signal can be removed from a sub-Nyquist 40 examination of the signals in the time domain through encoded NTSC color television signal by using either of the use of phasor diagrams of FIG. 9. Assume that the the following comb filter algorithms: television signal represents a solid color field with con 1. Add television line l to (1-2) line stant Y and C signals, and the C signal is represented by 2. Add television line l to (l--2) a constant amplitude phasor, and choose a two-dimen 3. Add television line l to (1-2) + (l--2) 45 sional array of picture points that are vertically aligned with lines from a single field to avoid the need for inter on five adjacent sequential television lines, l to (l-4). field storage in the decoder. It will be noted that algo Each line of the array consists of three equidistant rithms (1) and (2) combine two television lines, and that points approximately one-third of a television line apart, algorithm (3) combines three lines. The frequency re such that the C phasor has the same phase at these sponse of a comb filter utilizing either of algorithms (1) 50 points. Due to the selected points being invertical align or (2) is shown in FIG. 4, and the frequency response of ment, the C phasor will be in opposite phase on adjacent a comb filter using algorithm (3) is shown in FIG. 5. In lines. all cases the comb filters have nulls at the alias peak Since the television signal is being sampled at f. amplitude frequency components, and the teeth are +(2f - f), it will be seen in FIG. 9 that the -f centered at the peak amplitude frequency components 55 factor will cause the C signal to be sampled at an angle of the baseband signal. that smoothly changes 45 across each line. At the be FIGS. 6 and 7 show in block diagram form two dif ginning of line l, phasor C is 90 from the subcarrier ferent versions of a comb filter that utilizes algorithms reference, and the sampling axis is arbitrarily placed at (1) or (2). In the system of FIG. 6, the present input 45 from the reference. The resultant sampled signal is video signal (l) at the input 10 is added in a suitable 60 the projection of Con the sampling axis (FIG. 9a). Near summing circuit 14 to the input signal delayed by a the center of that television line, the sampling axis is at delay device 12 by exactly twice the line scan period 67.5 from C (FIG. 9b) resulting in the projection (2H). The sum signal is transmitted through a high-pass Ccos67.5' on the sampling axis. By the end of the line, filter 16 having a pass band between f -f, and f, and the sampling axis is in phase with the reference, result the filtered signal is added in a summing circuit 18 to the 65 ing in zero projection of C on this axis (FIG. 9c). present signal after filtering by a low pass filter 20 hav At the beginning of line (l-1), the sampling axis is ing a cutoff frequency off -f, to produce the NTSC still on the reference. By the end of (1-1), the sampling video signal at an output terminal 22. axis will be at - 45° from the reference (FIG.9f). This 4,065,784 5 6 continues in the same manner where for every televi NTSC color signal prior to encoding to remove any (n sion line the sampling axis undergoes a 45 phase shift -- )f energy components. with respect to a fixed color subcarrier reference. The use of pre-encoding comb filtering, however, It will now be apparent by looking at the sampling results in an additional loss of vertical resolution, partic axes for different television lines and at different points ularly on colored horizontal boundaries and diagonal along each line that the original Csignal can be regener boundaries in the luminance image. And if a three-line ated by adding sampled chrominance components sepa comb filter, as shown in FIG. 8, is used both pre- and rated by exactly two television lines. It should also be post-encoding, the resultant signal on line l will contain clear that one can also add (l--2) to il + (l-4) to a weighted average of high frequency information from obtain the C signal. In fact, as the chrominance sam 10 lines (l-2) and (l-4). This will cause a marked reduc pling axes separated by two television lines are in quad tion in the vertical resolution of image components that rature, it is possible to regenerate the original chromi are subjected to comb filtering. The alternative is to use nance by adding its quadrature components. the two-line comb filters shown in FIGS. 6 or 7, using In the decoder of the system utilizing the described the comb filter of FIG. 6 for pre-encoding and the comb encoding method, the sampled C signal is reconstructed 15 filter of FIG. 7 after decoding, or vice versa. after the digital-to-analog converter by a conventional . Transversal comb filters are known to cause some boxcar hold circuit, whose output is a square wave with objectionable transients, the most disturbing of which is a peak amplitude determined by the projection of the the generation of one or two extra lines of chrominance chrominance of the sampling axis. Then, by passing the at sharp vertical color transitions. Using the described square wave through a low-pass filter, the fundamental 20 sub-Nyquist encoding system, those chrominance tran sinusoidal component, having an amplitude 4/T times sients generated by the comb filters would have a 7.5Hz the peak amplitude of the square wave, is recovered. flicker rate. Such flicker can be removed by using the This can be compensated by lowering the C signal by a adaptive comb filter described in applicant's copending factor of 7/4 in order to scale it to its original ampli application Ser. No. 705,204, filed July 14, 1976, that tude. 25 can detect color transients and automatically switch off the comb filter. Sub-Nyquist encoding of a PCMNTSC color televi FIG. 10 shows in block diagram form a system for sion signal in the described system is achieved with f = PCM-encoding a NTSC color television signal at the 2f + f (or 2f - f). Withf.close to, but not equal sub-Nyquist rate discussed earlier. An input NTSC to, 2f all signal processing must be performed with 30 video signal in analog form, which has preferably been extreme accuracy in order to avoid generating extrane subjected to comb filtering for the reasons, and in the ous color beats. In particular, it can be seen from FIG. manner, discussed above, is received on a line 50 and 9 that when a comb filter, of the kinds described, is used coupled to the input of an analog-to-digital converter to remove the alias signal, complementary quadrature 52, to a horizontal sync separator 54 and to an encode components of the original chrominance are added 35 command generator 56. The ADC, which in the present every other line. For example, if a constant chromi embodiment, is a PCM modulator, samples the analog nance signal is sampled at +4.5' on line l, it will be video signal under control of a sampling signal, or en sampled at -45 (l--2), at 0 on (l-1) and 90' on (1-3). code clock, online 58, which is the output of a commu Then, if the processing system introduces any differen tator or switch 60, schematically shown as a single-pole tial phase and gain error, which is inevitable with quan double throw switch having input terminals 60a and 60b tizing errors in the analog-to-digital converter, the re and then converts the sampled television signal into sultant reconstructed NTSC color signal will exhibit digital form. The sampling signal generated by encode noticeable hue and saturation changes on alternate lines. command generator 56 is coupled to terminal 60a, and A partial solution to this problem is constant vertical after being shifted in phase by 90' by a phase-shifter 62, quadrature phase encoding. It involves shifting the 45 is coupled to terminal 60b of the switch. These two phase off by 90' on alternate television lines, which sampling signals (of the same frequency but differing in results in a 45" phase shift in the sampling axis with phase by 90') are alternately utilized as sampling signals respect to the color subcarrier. Upon comb filtering, the for successive horizontal lines of the television field to vertically aligned chrominance signal on every line is provide the constant vertical quadrature phase encod reconstructed using exactly the same quadrature com 50 ing described above. This is achieved by using the hori ponents. FIG. 11 shows the relationship between a zontal sync signal, obtained by sync separator 54, to constant chrominance signal and its sampling axes on control the switch 60. adjacent lines when constant vertical quadrature phase As discussed above, the frequency fof the sampling sampling is used. Comparison of this set of phasor dia signal may be either (2f -- f) or (2f - if); in the grams with those of FIG. 9 will show the effect of 55 NTSC system of television 2f = 7159090Hz and f = shifting the encoding phase. An encoder for achieving 3934Hz, so that fmaybe 7163024Hz or 7155156Hz. For constant vertical quadrature phase encoding will be purposes of illustration, the encode command generator described presently, following a brief discussion of pre 56 produces a sampling signal of the latter frequency. cautions to be observed as regards the NTSC signal to Whichever fis used, it is essential for the reasons dis be applied to the encoder. cussed earlier, that harmonics off, be kept out off. It The input NTSC color television baseband signal has been found that the PCM encoding frequency fis spectrum should be free of energy components at fre not critical; it can be as much as 50 or 100Hz off lock quencies (n + )f at which frequencies the alias energy and still not produce objectionable color beats. will be concentrated. This is especially true within the The encode command generator 56 may take a vari range of frequencies from f -f, to f. Otherwise, the 65 ety of forms, two different circuits for generating f alias components generated by encoding at f = (2f. -- being shown in FIGS. 12 and 13. The circuit of FIG. 12 f) will overlap the baseband video signal and will be utilizes a digital single-sideband mixer of the type de inseparable. This can be avoided by comb filtering the scribed by Counselman and Hinteregger in an article 4,065,784 7 8 entitled, "Digital Single-Sideband Mixer” appearing at The effect of the sub-Nyquist system on chrominance pp. 478-479 of Proceedings of IEEE, April 1973, in was judged imperceptible with most broadcast signals. which only "digital' logic devices and no reactive or Only highly saturated colors generate a noticeable 7.5Hz flicker at sharp vertical color transitions. This frequency-selective "analog' circuit elements are used. problem is particularly noticeable with 100% saturated Exclusive-or logic gates 70, 72, 74 and 76 act as the 5 split field color bars. Other stationary pictures including mixers in the digital circuit. Two signals of frequencies scenes from slides No. 1 to 15 of the SMPTE Television 4f and f, derived elsewhere in the system are applied Color Reference slide set were judged not objection to frequency divider circuits 78 and 80, respectively, in ably impaired. which the input signal frequency is divided by two. The I claim: signals at the outputs of dividers 78 and 80 are applied to 10 1. A method of digitally encoding an NTSC color the input terminals of exclusive-orgate 70, and also as television signal having a color subcarrier frequency f. one input of gates 72 and 74, respectively; the other and a line-scan frequency f. comprising the steps of: input to gates 72 and 74 are 4f and f, respectively. generating a first sampling signal having a frequency The outputs of gates 72 and 74 are applied as the two f. that differs from 2f by Af inputs to exclusive-or gate 76, the output of which is 15 sampling the television signals in response to the combined with the output of gate 70 in a resistive com sampling signal, and bining network 84 to produce at output terminal 84 a converting the sampled television signal into digital signal of frequency 2f - if Analysis of the circuit form. will show that when this, the lower sideband frequency 2. The method according to claim 1, wherein said is selected, the unwanted upper sideband, as well as the 20 first sampling signal has a frequency f, equal to 2f + fundamental frequencies, are effectively suppressed. f However, considerable phase error is introduced by the 3. The method according to claim 1, wherein said presence of (2f -- if), which is suppressed relative to first sampling signal has a frequency f. equal to 2f - (2f - if) by only 10dB. This undesirable component f can be effectively eliminated by using a crystal oscilla 25 4. The method according to claim 1, including the tor 86 in a phase-locked loop 88, also including a phase further steps of: detector 90, to which the output of the mixer is applied, producing a second sampling signal of the same fre to lock to the desired frequency and no other. quency and in quadrature with said first sampling The circuit of FIG. 13 utilizes a frequency synthesiz signal, ing technique to generatef, which for purposes of the 30 alternately enabling the first and second sampling following description is assumed to be (2f-f). Since signals during successive horizontal scan lines of the television signals, and in the NTSC system sampling the television signals in response to the enabled sampling signal. f = g-f, 35 5. The method according to claim 1, including the further step of: 1 comb filtering the television signals before sampling and 2?. --fi = 455, -f, to remove therefrom any (n + )f energy compo nents, where n is an integer. therefore f = 19. f 6. Apparatus for digitally encoding an NTSC color television signal having a color subcarrier frequency f. The frequency synthesizer includes a voltage-con and a line-scan frequency f. comprising, in combina trolled oscillator 92 having a center frequency of 2f tion, if (or 1819/4f). The output of the oscillator is divided means for generating a first sampling signal having a down in frequency by 1819 in a frequency divider 94, 45 frequency f. differing from 2f by Af resulting in a signal frequency off, which is compared means for sampling the television signals in response in a phase detector 96 to a reference signal of frequency to the sampling signal, and Af. Any deviation generates an error voltage, which is means for converting the sampled television signal applied through a loop filter 98 to the oscillator to con into digital form. trol the same. 50 7. Apparatus according to claim 6, further compris Tests of sub-Nyquist sampling with a variety of 1ng: NTSC color television signals gave promising results. means for producing a second sampling signal of the Its effect on the luminance details is dependent on the same frequency and in quadrature with said first angle between the picture detail and the scanning lines. sampling signal, Vertical luminance transitions are normally not im- 55 switching means responsive to the NTSC color tele paired since their frequency components do not extend vision signal for alternately enabling said first and into the passband of the comb filter. With diagonal second signals during successive horizontal scan transitions, however, the comb filter reduces the ampli lines of said television signal, tude of wanted frequency components above f-f and means responsive to the enabled sampling signal for the corresponding alias components are not completely 60 sampling the television signal; and removed. In general, however, a small deterioration of means for converting the sampled television signal diagonal transitions does not appear subjectively objec into digital form. tionable. The most obvious deterioration appeared on 8. Apparatus according to claim 6, wherein said first small test circles; objectionable moire patterns resulted sampling signal has a frequency f. equal to 2fe + if in this case, but that situation is rarely encountered in a 65 9. Apparatus according to claim 7, wherein said first broadcast program. Horizontal transitions are virtually and second sampling signals have a frequency fequal to unaffected because their energy components do not 2f. - if. normally fall within the nulls of the comb filter.