GB1580724A

GB1580724A - Colour television

Info

Publication number: GB1580724A
Application number: GB1581177A
Authority: GB
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1974-05-13
Filing date: 1977-04-15
Publication date: 1980-12-03

Description

(54) IMPROVEMENTS RELATING TO COLOUR TELEVISION (71) We, BRITISH BROADCASTING CORPORATION, a British Body Corporate, of Broadcasting House, London, WIA IAA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to analogue-to-digital and digital-to-analogue converters for PAL colour television signals.

Our British Patent Application No. 21118/74 (our earlier application), now Patent No. 1511230, describes an analogue-to-digital converter (a.d.c.) for PAL signals which operates at a sub-Nyquist frequency equal to twice the colour subcarrier frequency. As shown in Figs. 3B and 4 of our earlier application such converters include comb filters to reduce or remove alias components in the resultant signal. This invention, which is defined in the appended claims, is concerned with improved converters in which the comb filters operate in the digital mode.

The invention will be described by way of example with reference to the drawing accompanying the Provisional Specification, in which: Fig. 1 shows an improved analogue-to-digital converter embodying the invention; and Fig. 2 shows an improved digital-to-analogue converter embodying the invention.

The converter of Fig. 1 has an analogue PAL input 10 connected to a 5.5 MHz low-pass filter 12. This filter ensures there are no signals present above the nominal maximum video frequency. To the output of filter 12 is connected an analogue-to-digital converter circuit 14 of conventional construction which samples the analogue signal at four times the colour subcarrier frequency (4 fsc).

The circuit 14 is driven by a voltage-controlled oscillator 16 which is controlled by the output of a burst comparator 18 which receives the output of converter 14.

The output of the converter 14 is also applied to a clocked switch 20 which is clocked at twice the colour subcarrier frequency. The switch is shown diagrammatically as in practice it may be possible to dispense with it, the necessary switching then arising by virtue of the clocking of the subsequent circuitry. One output of the switch is applied to a one-line delay 22 and thence through a compensating delay 24 to the non-inverting input of a subtractor 26, the output 28 of which constitutes the output of the circuit.

The other output of the switch 20 is applied to the inverting input of a halving subtractor 30, the non-inverting input of which receives the output of the one-line delay 22. The output of the subtractor 30 is applied to a band-pass filter 32 the passband of which covers the chrominance band of the PAL signal, i.e. about 3.36 to 5.5 MHz. The output of the filter 32 is applied to the inverting input of the subtractor 26.

Naturally all the circuits after the converter 14 are digital circuits. These are all clocked at 2 fsc. The operation of the converter will be seen from the following.

The PAL signal P consists of two chrominance signals, U and V, modulated onto quadrature subcarriers and added to the luminance signal Y, i.e.

Pn=Yn+Un . cos(2nf,,t)+(-1)"V, . sin(2nf,,t) where f,,=frequency of the colour subcarrier#2833/4fL n=line number in the 4 field sequence (1--1250) so that (-1)" represents the phase reversal of the V component on alternate lines.

The a.d.c. 14 at the input of the digital comb filter samples the incoming PAL at 4 fs at 45" and 135 after the positive and negative peaks of the U component, i.e. when (2fsct)=7t/8, 3'r/8, 578, 77r/8.

The values of the 4 5C samples on three successive lines are thus as follows:

The samples taken 450 after the Ú peaks are indicated by double brackets.

These samples pass through the line delay 22. The other samples (taken 135 after the U peaks) are fed direct to the subtractor 30. They arrive at the same time as the delayed samples of the previous line from the output of delay 22.

Subtracting 135 samples (single brackets) of one line from 45" samples ((double brackets)) of the previous line gives zero output, so long as the values of Y, U and V remain the same. In this case the delayed ((45 )) samples pass unchanged to the output through the equalising delay 24 and subtractor 26.

The output 2 5C PAL samples Ps are thus of the form:

If the values of Y, U or V change from one line to the next the difference will appear at the subtractor 30 output and may modify the output samples to prevent the problems that occur when a 2 5C system operates without an input comb filter.

It is important for the correct samples to be applied to the delay 22 and subtractor 30 respectively, otherwise the PAL signal will not be properly generated at the output.

It will be appreciated that if the inverting and non-inverting inputs of the subtractor 30 are interchanged, the subtractor 26 can be replaced by an adder. The halving function provided by the subtractor 30 could be provided anywhere between this subtractor and circuit 26. Other derivatives of the circuit will be apparent to those skilled in the art.

Fig. 2 shows a digital-to-analogue converter which can be used to regenerate an analogue PAL signal from the sub-Nyquist digital signal produced by the circuit of Fig. 1. The circuit receives the digital PAL signal at an input 50 and also receives the 2 fsc clock signal indicated at 52. The digital PAL signal is applied to a bandpass filter 54 similar to the filter 32 and to a compensating delay 56. A subtractor 58 has its non-inverting input connected to the output of the delay 56 and its inverting input to the output of the filter 54 (which is halved). The output of the filter 54 is also applied to a one-line delay 60, and a clocked switch 62 selects the outputs of the subtractor 58 and delay 60 for application to a digital-to-analogue converter 64.

The converter is clocked at 4 5c The converter output is applied through a 5.5 MHz low-pass filter 66 to form the reconstituted PAL output 68.

In this way the desired effects of the comb filters of our earlier application are achieved with simple circuitry which, apart from the converters 14 and 64, need be clocked only at 2 5c The numerical values given in this specification are those appropriate to a 625/50 signal such as is standard in the United Kingdom, for which 5c=4 43 MHz, and the maximum video frequency f,=5.5 MHz. This system is defined as 1/PAL (C.C.I.R. Report No. 624, Geneva 1975); other systems called B/PAL and G/PAL use 5c=4.43 MHz and fv=5 MHz, and M/PAL uses fsc=3.58 MHz and fv=4.2 MHz, this being a 525/60 system. The values of fsc are given to their conventional degree of approximation, they are in fact defined theoretically to an accuracy of onehundredth of one hertz (see the above report). By appropriate alteration of the values given, the circuits illustrated can be adapted for these other systems.

The invention relies on the fact that in the PAL system the line frequency f, and the colour subcarrier frequency 5C are related, in that the colour subcarrier frequency is approximately equal to an odd integral multiple of one-quarter of the line frequency. In fact there is a slight offset equal to the picture frequency (e.g. 25 Hz) but this is insignificant or can be allowed for.

The circuits can be modified along the lines of our Application No. 27265/76 (Patent No. 1524749), by using a delay line which instead of having a delay of one line period provides a delay equal to an odd integral number of line periods which is equal to one field period plus or minus half of a line period. In a 625 line system, a 313 line delay can be used as the delay 22 or 60.

WHAT WE CLAIM IS: 1. A method of generating a digital PAL colour television signal from an analogue composite PAL colour television signal in which the colour subcarrier frequency 5C is substantially an odd integral multiple of one-quarter of the line frequency f,, comprising sampling the analogue PAL signal at 4 fsc while maintaining the sampling instants spaced substantially by one-eighth of the period of the subcarrier from the zero crossing points of the U-component of the chrominance component of the PAL signal, subjecting a first set of alternate ones of the samples to a delay equal to one line period or to an odd integral number of line periods which is equal to one field period plus or minus half a line period to produce delayed samples, subtracting the delayed samples and the other set of alternate ones of the 4 fsc samples, bandpass filtering and halving the samples resulting from the subtraction, the filter having a pass-band from 2 fscfv to where f is the frequency of the highest substantial frequency component of the analogue PAL signal, and combining the filtered and halved samples with the delayed samples to provide an output signal sampled at 2 fsc 2. A method of generating a digital PAL colour television signal, substantially as herein described with reference to Fig. 1 of the drawing accompanying the Provisional Specification.

3. Apparatus for generating a digital PAL colour television signal from an analogue composite PAL colour television signal in which the colour subcarrier frequency fSc is substantially an odd integral multiple of one-quarter of the line frequency f,, comprising an input terminal sampling means connected to the input terminal for sampling an analogue PAL signal at 4 fsc while maintaining the sampling instants spaced substantially by one-eighth of the period of said subcarrier from the zero crossing points of the U-component of the chrominance component of the PAL signal, a first circuit path connected to the output of the sampling means for subjecting a first set of alternate ones of the samples to a delay equal to one line period or to an odd integral number of line periods which is equal to one field period plus or minus half a line period to produce delayed samples, a second circuit path connected to the output of the sampling means and to the first circuit path for subtracting the delayed samples and the other set of alternate ones of the 4 fSc samples and for bandpass filtering and halving the samples resulting from the subtraction, the second circuit path including a filter for the bandpass filtering having a pass-band from 2 5Cfv to fv where fv is the frequency of the highest substantial frequency component of the analogue PAL signal, and means for combining the outputs of the first and second circuit paths to provide an output signal sampled at 2 fisc.

4. Apparatus for generating a digital PAL colour television signal, substantially as herein described with reference to Fig. 1 of the drawing accompanying the Provisional Specification.

5. A method of regenerating an analogue PAL colour television signal from a sampled signal generated by the method of claim 1 of our Patent No. 1511230, comprising bandpass filtering and halving the received samples, the filter having a pass-band from fsfv to fv where f, is the sampling frequency of the received samples and fv is the frequency of the highest substantial frequency component of the analogue PAL signal, subtracting the filtered samples from the received samples, and subjecting the filtered samples to a delay equal to one line period or to an odd integral number of line periods which is equal to one field period plus or minus half a line period, combining the samples resulting from the subtraction with the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. this being a 525/60 system. The values of fsc are given to their conventional degree of approximation, they are in fact defined theoretically to an accuracy of onehundredth of one hertz (see the above report). By appropriate alteration of the values given, the circuits illustrated can be adapted for these other systems. The invention relies on the fact that in the PAL system the line frequency f, and the colour subcarrier frequency 5C are related, in that the colour subcarrier frequency is approximately equal to an odd integral multiple of one-quarter of the line frequency. In fact there is a slight offset equal to the picture frequency (e.g. 25 Hz) but this is insignificant or can be allowed for. The circuits can be modified along the lines of our Application No. 27265/76 (Patent No. 1524749), by using a delay line which instead of having a delay of one line period provides a delay equal to an odd integral number of line periods which is equal to one field period plus or minus half of a line period. In a 625 line system, a 313 line delay can be used as the delay 22 or 60. WHAT WE CLAIM IS:

1. A method of generating a digital PAL colour television signal from an analogue composite PAL colour television signal in which the colour subcarrier frequency 5C is substantially an odd integral multiple of one-quarter of the line frequency f,, comprising sampling the analogue PAL signal at 4 fsc while maintaining the sampling instants spaced substantially by one-eighth of the period of the subcarrier from the zero crossing points of the U-component of the chrominance component of the PAL signal, subjecting a first set of alternate ones of the samples to a delay equal to one line period or to an odd integral number of line periods which is equal to one field period plus or minus half a line period to produce delayed samples, subtracting the delayed samples and the other set of alternate ones of the 4 fsc samples, bandpass filtering and halving the samples resulting from the subtraction, the filter having a pass-band from 2 fscfv to where f is the frequency of the highest substantial frequency component of the analogue PAL signal, and combining the filtered and halved samples with the delayed samples to provide an output signal sampled at 2 fsc

2. A method of generating a digital PAL colour television signal, substantially as herein described with reference to Fig. 1 of the drawing accompanying the Provisional Specification.

delayed samples to provide samples at a 2 fc sample rate, and converting these samples into analogue form.

6. A method of regenerating an analogue PAL colour television signal, substantially as herein described with reference to Fig. 2 of the drawing accompanying the Provisional Specification.

7. Apparatus for regenerating an analogue PAL colour television signal from a sampled signal generated by the method of claim 1 of our Patent No. 1511230, the apparatus comprising an input terminal, filtering means connected to the input terminal for bandpass filtering and halving the samples applied to the input terminal and including a filter for the bandpass filtering having a pass-band from fsfv to fv where fs is the sampling frequency of the received signal and is twice the PAL signal colour subcarrier frequency of approximately 4.43 MHz or 3.58 MHz and fv is the frequency of the higest substantial frequency component of the analogue PAL signal, a subtractor connected to the input terminal and to the output of the filtering means for subtracting the filtered samples from the received samples, a delay for subjecting the filtered samples to a delay equal to one line period or to an odd integral number of line periods which is equal to one field period plus or minus half a line period, combining means connected to the subtractor and the delay for combining the samples from the subtractor with the delayed samples to provide samples at a 2 15 sample rate, and a converter connected to the combining means for converting these samples into analogue form.

8. Apparatus for regenerating an analogue PAL colour television signal, substantially as herein described with reference to Fig. 2 of the drawing accompanying the Provisional Specification.