De-emphasis for digitized composite color television signals

In a standard NTSC composite video signal, the peak excursion of the luminance plus chrominance signal for certain saturated colors exceeds the peak excursion of the white-representative luminance alone. When digitized, the signal is subject to quantizing noise. The effective spectial distribution of the quantizing noise is improved by reducing the amplitude of the chrominance signal so that the peak excursion of luminance plus chrominance for saturated colors does not substantially exceed the peak amplitude of white-representative luminance.

This invention relates to the transmission of composite color television 
signals in which the color signal components are de-emphasized or 
attenuated relative to the luminance signals. 
As standardized, monochrome television signals are signals having an 
amplitude which is representative of the brightness in the actual scene 
being televised. With the advent of compatible color television, the color 
standard includes a high-frequency color subcarrier modulated with the 
color information which is superimposed upon the analog luminance signal. 
The color information is modulated onto the color subcarrier in a 
well-known fashion in which in the absence of a color signal (as for a 
white region of the picture) the color subcarrier is suppressed, so that 
only the luminance signal exists. When the picture contains color 
information, the amplitude of the modulated signal increases with 
increasing saturation and also increases with increasing brightness of the 
picture. Consequently, the maximum amplitude of the color-representative 
signal occurs when the brightness or luminance component is also a 
maximum. The magnitude or amplitude of the chrominance-representative 
signal is thus determined by the saturation of the color being represented 
and by the magnitude of the luminance signal. A predetermined amplitude 
ratio has been established for each color which represents the maximum 
amplitude ratio. For example, for a fully saturated red the amplitude of 
the chrominance signal is 2.1 times the magnitude of the luminance signal, 
and for yellow the ratio is 0.5 times the luminance. 
The predetermined ratios were established based upon requirements of 
monochrome compatibility, the characteristic color response of the eye and 
the noise characteristics of analog transmission systems. Analog 
transmission systems generally include a "triangular" noise component 
which within a given bandwidth increases in amplitude with increasing 
frequency. Consequently, high-frequency signal components are subject to 
more noise than are low-frequency signal components. Thus, analog signal 
transmission systems often include means for "pre-emphasis" by which the 
amplitude of the high frequency components of the signal are increased 
relative to the amplitudes of the low-frequency components prior to 
passing signals through the transmission system. A corresponding 
de-emphasis at the receiving end of the transmission system restores the 
original amplitude-frequency response and at the same time attenuates the 
high-frequency noise introduced by the transmission system. Such 
pre-emphasis and de-emphasis is described for example at pages 306 and 307 
of the text "Information Transmission, Modulation and Noise" by Mischa 
Schwartz published by McGraw Hill Book Company in 1959. 
Digital transmission systems are essentially noise-free because 
regeneration of the transmitted pulses can be accomplished for all except 
the worst transmission conditions. If the basic information being 
transmitted is analog, it must be quantized before it can be passed 
through the digital system. The quantization or digitizing causes an error 
which gives rise to a wide-band noise. It is known to reduce the effects 
of quantizing error on an audio signal by pre-emphasis of the signal prior 
to quantizing and by de-emphasis prior to the following decoding, as 
described at page 269 of "Digitals in Broadcasting" by Harold E. Ennis 
published in 1977 Howard W. Sams And Company. 
It is known that the presence of a color signal in the composite television 
signal being digitized acts as a dither which tends to subjectively 
improve the television picture, as described in the article "PCM Encoded 
NTSC Color Television Subjective Tests" by A. A. Goldberg which begins at 
page 21 of the book "Digital Video" published in March 1977 by the 
S.M.P.T.E. The dither reduces the apparent effect of quantizing errors on 
the signal. 
Decoding of color information requires information derived from at least an 
entire subcarrier cycle. Each subcarrier cycle is ordinarily represented 
by two or more digital words, depending upon the sampling frequency. 
Consequently, the effects of quantizing noise on the chrominance signal 
are reduced by an averaging effect. Thus, the subjective signal-to-signal 
noise (S/N) of the color signal is better than might be expected based 
upon consideration only of the magnitude of the quantizing step. It is 
desirable to improve the apparent signal-to-noise ratio at the output of a 
digital system. 
SUMMARY OF THE INVENTION 
An improved arrangement for digitizing signals from a source of composite 
analog video signals representing color image includes a digitizer for 
digitizing composite analog signals into a predetermined number of bits. 
The composite analog signals are composed of a luminance component which 
has in the case of white-representative signal a predetermined maximum 
level. A chrominance component modulated onto a subcarrier is superimposed 
upon the luminance component. The chrominance-modulated subcarrier has a 
value or amplitude for certain colors such that the composite signal 
exceeds the predetermined level. The digitizing operation introduces 
broadhand quantizing noise which depends in part upon the amplitude of the 
signal being digitized. When the amplitude of the composite signal is 
selected to use all the available quantizing levels, the amplitude of the 
white-representative luminance component is represented by less than the 
predetermined number of bits and is therefore subject to increased 
broadband noise. According to the improvement, the composite signals are 
coupled to the digitizer through an attenuator for attenuating the 
chrominance component relative to the luminance component for decreasing 
the broadband noise affecting the luminance component.

DESCRIPTION OF THE INVENTION 
FIG. 1A represents a voltage-time plot of a multiburst signal selected to 
provide steps of successively decreasing luminance value. The luminance 
value of the signal in the case of FIG. 1A is a stairstep signal having a 
peak amplitude of 1.0 for a white-representative signal and extending 
successively to a value of 0.11 for a blue-representative signal. At the 
left of FIG. 1A, the luminance component has a maximum value of 1.0, 
representing one hundred IRE units. This portion of the signal represents 
the brightest possible white signal. The next adjacent portion includes a 
luminance component of 0.89 (89 IRE units) upon which is superimposed a 
yellow-representative chrominance component having a peak amplitude of 
0.44. In order to simplify FIG. 1, only the envelope of the high-frequency 
chrominance signal is illustrated. The peak value of the composite signal 
is 1/3 larger than the peak value of the white-representative luminance 
signal. Consequently, the peak value of the composite signal in the case 
of a maximum saturation yellow signal extends to 1.33. Similarly, for the 
case of a full-saturation cyan signal the luminance or brightness 
component has a magnitude of 0.7 and the chrominance component has a 
peak-to-peak (P-P) value of 0.63, so the peak value of the composite 
signal is 1.33, as in the case of the yellow signal. A green 
representative signal as illustrated has a luminance component of 0.59 and 
the peak excursions of the chrominance component which extend to 1.18. 
Thus, the green-representative signal has a peak-value 18% greater than 
the peak value of a white-representative signal. Magenta is also 18% 
greater but the peak excursion is in the opposite direction. For the red 
and blue-representative signals the composite signal has peak values 
extending to -0.33, and thus the peak value is 1/3 larger than the value 
of a white-representative signal. 
When a composite color signal is to be digitized, the signal is applied to 
an analog-to-digital converter (ADC). In the ADC, the applied composite 
signal is periodically compared with a predetermined number of reference 
voltages representing the quantizing levels and a digital word is 
generated representing the quantizing level most closely approximately the 
applied signal. It will be clear that if the peak value of the applied 
signal is too small, some of the predetermined quantizing levels will 
never be utilized as digital words. Also, if the peaks of the applied 
composite signal exceed the value of the highest quantizing level the 
digital word representing that highest quantizing level will necessarily 
be used to represent all higher values of the applied signals. This 
results in gross distortion, while failure to use all available quantizing 
levels results in increased levels of quantizing noise. 
The P-P value of the signal of FIG. 1A is 1.66. If a predetermined number 
of quantizing levels is used (for example, 256 levels representing 8-bit 
quantization) and the signal is manually or automatically adjusted to just 
fill the available number of quantizing levels, then the luminance 
component of the signal will be represented by approximately 154 
quantizing levels rather than by 256 quantizing levels. Consequently, the 
luminance component of the signal is represented by fewer quantizing steps 
and will be subject to a broadband noise. This broadband noise gives rise 
to a graininess of the image and in severe cases can result in 
"contouring". 
FIG. 1B represents a composite color signal similar to that of FIG. 1A in 
which the amplitude of the chrominance component has been reduced to 1/4 
of its previous amplitude. The white-representative luminance signal 
remains unchanged at an amplitude of 1.0. The yellow-representative 
chrominance signal, however, has its peak excursion reduced from 0.44 to 
0.44/4, which equals 0.11. Consequently, the peak excursion of the yellow 
representative composite signal is the luminance component 0.89 plus the 
0.11 peak excursion of the chrominance component for a total value of 1.0. 
In a similar fashion the peak excursion of the chrominance component for 
the various colors illustrated in FIG. 1B do not exceed the peak value of 
luminance-representative component. Consequently, in order to digitize the 
signal FIG. 1B, the same 256 quantizing levels are available as were 
available for the signal of FIG. 1A but the magnitude of the signal is 
smaller. Naturally the entire signal of FIG. 1B can be increased in 
magnitude before being digitized so as to have the same P-P value as does 
the signal of FIG. 1A. FIG. 1C represents the signal of FIG. 1B increased 
in amplitude by 1.66:1 so as to equal the P-P value of the signal of FIG. 
1A. When quantized, the white-representative portion in FIG. 1C can be 
quantized by the entire predetermined number of quantizing levels 
available, which in a 8-bit quantizing of the example is 256 levels. 
Consequently, the broadband quantizing noise affecting the lower-frequency 
luminance component of the composite signal is reduced. It should be 
understood that the noise for the luminance components of color 
representative signals is also reduced. The quantizing noise affecting the 
higher-frequency chrominance component is increased. Thus, the effect of 
the change of reapportionment in amplitudes is a shift in the effective 
noise spectrum so as to reduce low-frequency noise and increase 
high-frequency noise. In a television context, this is a desirable 
tradeoff because as mentioned the high-frequency noise is subjectively 
less apparent. 
In FIG. 2 a color television camera illustrated as 210 produces separate 
red (R), green (G) and blue (B) signals which are applied to a matrix 212. 
As is known, matrix 212 produces a broadband luminance (Y) signal 
according to the equation: 
EQU E.sub.y =0.30 E.sub.R +0.59E.sub.G +0.11E.sub.B. 
Matrix 212 also produces I and Q signals from R, G and B signals according 
to the equations: 
EQU E.sub.I =0.60E.sub.R -0.28E.sub.G -0.32E.sub.B and 
EQU E.sub.Q =0.21E.sub.R -0.52E.sub.G +0.31E.sub.B. 
The I signal is applied to a lowpass filter 214 to limit the bandwidth to 
1.5 MHz, and the Q signal is applied to a lowpass filter 260 to limit its 
bandwidth to 0.5 MHz. The band-limited I and Q signals are applied to the 
inputs of modulators 218 and 220, respectively. Also applied to modulators 
218 and 220 are signals from a subcarrier generator 222. The signals from 
modulators 218 and 220 are summed in a summing circuit 224 to produce a 
signal represented by E.sub.I cos (wt+33.degree.)+E.sub.Q sin 
(wt+33.degree.). This signal is applied through a resistor 226 to a 
further summing circuit 228. The Y signal is applied through a delay 
circuit represented as a block 230 and through a resistor 232 to another 
input summing circuit 228. Delay 234 is selected to delay the Y signal by 
an amount equal to the delay experienced by the chrominance signals, which 
is chiefly caused by lowpass filters 214 and 216. In the prior art, 
resistors 226 and 232 have values selected to produce a composite signal 
E.sub.C according to the equation: 
EQU E.sub.C =E.sub.Y +[E.sub.I cos (wt+33.degree.)+E.sub.Q sin (wt+33.degree.)] 
to produce a signal proportioned as shown in FIG. 1A. However, according to 
the invention, the value of resistor 226 is increased by a factor of four 
so as to reduce the magnitude of the chrominance signal components being 
summed with the luminance component to produce a signal proportioned as 
illustrated in FIG. 1B. 
The composite chrominance signal produced in summing circuit 228 is applied 
to an ADC 230 to produce digitized video which may be passed through a 
digital circuit as a single multiplexed channel or as a plurality (for 
example, 8) of parallel or simultaneous channels. 
FIG. 3 illustrates a decoder which may be used in conjunction with the 
encoder of FIG. 2. Digital video from an encoder is applied to 
digital-to-analog converter (DAC) 310 which converts the digital signal to 
a composite analog signal. The composite analog signal is applied to a 
comb filter 312 of known type for separating chrominance information from 
luminance information. The luminance information is applied over a 
conductor 314 to a summing circuit 316. The chrominance information, 
together with certain residual components of the luminance information, 
appears on conductor 318. The chrominance information is passed through a 
high pass filter (HPF) 320 and a resistor 322 to one input of a summing 
circuit 324. The low-frequency components of the luminance residue are 
passed through a low pass filter (LPF) 326 and summed in summing circuit 
316 with the luminance signal on conductor 314 to produce an improved 
luminance signal which is applied by way of a resistor 328 to summing 
circuit 324. The amplitude of the chrominance signal at the output of HPF 
320 compared with the amplitude of the luminance signal at the output of 
summing circuit 316 is 12 db lower than the amplitude of the chrominance 
at the output of summing circuit 224 compared with the amplitude of the 
luminance at the output of delay circuit 230 in FIG. 2. In other words, 
the chrominance and luminance signals applied to resistors 322 and 328 are 
proportioned as illustrated in FIGS. 1B and 1C, while the chrominance and 
luminance signals applied to resistors 226 and 232, respectively, of FIG. 
2, are proportioned as in FIG. 1A. Resistor 328 of FIG. 3, however, has a 
resistance value four times that of resistor 322, and as a result, the 
analog video produced at the output of summing circuit 324 has the 
luminance attenuated with respect to the chrominance by a factor of four 
times. This restores the relative amplitudes of luminance and chrominance 
to those shown in FIG. 1A. Thus, the arrangement of the encoder of FIG. 2 
and the decoder of FIG. 3 coupled to the ends of the digital transmission 
path (not shown) allow transmission of composite color-representative 
signals over a digital signal path having a predetermined number of 
digitizing levels with reduced visibility of noise. The reduced visibility 
of the noise results from the redistribution of the noise spectrum, which 
in turn results from the selection of the ratio of the amplitudes of the 
chrominance and luminance so as not to substantially exceed the peak value 
of the white-representative luminance or other peak value of luminance 
alone. 
FIG. 4 illustrates an encoder for signals from a source of NTSC-standard 
composite video. The NTSC composite video is proportioned as in FIG. 1A, 
with the peak amplitude of the composite signal substantially exceeding 
the peak amplitude of the white-representative luminance alone. In order 
to reduce the amplitude of the chrominance signal relative to the 
luminance signal, an arrangement similar to that of the decoder of FIG. 3 
is used in the encoder of FIG. 4. The NTSC video is applied to a comb 
filter 412 for separation into a luminance component which is applied over 
a conductor 414 to a summing circuit 416. The chrominance component and a 
residue of luminance appears on conductor 318, and the low-frequency 
portion of the residue is applied to summing circuit 416 by way of a low 
pass filter 426. The chrominance portion is applied through a high pass 
filter 420 and a resistor 422 to a summing circuit 424 where it is 
combined with luminance applied through a resistor 428. Unlike the 
arrangement of FIG. 3, resistor 422 has an amplitude four times that of 
resistor 428, which attenuates the chrominance component of the composite 
analog video signal appearing on a conductor 428 and applied to ADC 430 
for quantization. In this way, NTSC video is rearranged to the proportions 
shown in FIGS. 1B and 1C, whereby the quantization noise of the digitized 
signal is reduced. It will be apparent to those skilled in the art that 
signals according to the various standards may be digitized with 
reduced chroma levels to achieve the same advantages as in the case of 
NTSC signals.