GB2137845A - Coding and decoding of PAL colour television signals - Google Patents

Abstract

A PAL decoder circuit uses as part of the separator circuitry a delay of an odd number of line periods substantially equal to one picture period. Similar circuitry may be used as a prefilter in a PAL coder. Two series- connected delay devices 36 are employed each of a delay of 625 lines (for a 625 line PAL signal). A subtractor 38 subtracts the input and output of the delays and a divide-by-two circuit 40 halves the resultant. A 90 DEG phase shift circuit 42 is connected to the output of the circuit 40. Digital delay devices are preferred. <IMAGE>

Description

SPECIFICATION Coding and decoding of PAL colour television signals This invention relates to improved methods of encoding and decoding PAL colour television signals.

PAL decoders commonly use a one-line delay in order to separate the U and V components of the chrominance signal. Such a decoder is well known as a delay-line decoder. Proposals have been made to use improved decoders in which two or possibly four one-line delays are used, as shown for example in BBC Engineering, December 1977, No. 108, pages 29 to 33, UK Patent Applications Nos. 2,003,695; 2,018,087 and 7905757; and BBC Research Department Report No. 1975/36.

In accordance with this invention we have appreciated that the delay used in the known decoders can be replaced by delays of an odd number of line periods substantially equal to one field or one picture, or, in principle, any integral number of fields.

Furthermore, prefilters based on these delays can be used to prefilter the signals at a PAL encoder.

In principle, the invention is applicable to analogue circuits, or to digital circuits, either with line-locked sampling or with four times subcarrier locked sampling. The type of filter used in the luminance and chrominance paths of a single encoder or decoder do not necessarily have to be the same. Where possible, the type of decoder selected will have regard to the form of prefilter used at the encoder.

The invention will now be described in more detail with reference to the drawings, in which: Figures 1 to 6 each show a chrominance processing circuit using delays; and Figures 7 to 13 each show a luminance processing circuit or filter incorporating delays.

Figure 1 shows a circuit based on a delay line PAL decoder. The circuit 10 shown receives a composite PAL signal at an input 12 and applies this to a delay line path 14 containing a delay element 1 6 and a 900 phase shift circuit 18, which may in fact be constituted by appropriate choice of the length of the delay line 1 6. An adder 20 coupled with a divide by two circuit 22 averages the signals at the input and output of the delay path 14, whereas a subtractor 24 coupled with a divide by two circuit 26 takes half the difference between the input and output of the delay path.Each of the signals from the divide by two circuits 22, 26 is synchronous demodulated with respect to a respective subcarrier frequency signal in demodulators 28, 30, and the resultant signals are bandpass filtered in filters 32 and 34 to provide U and V outputs respectively. As shown, the reference signals to demodulators 28, 30 are phase displaced by 900 with respect to each other, and that to the demodulator 30 is inverted on alternate lines, to take account of the V axis switch in the PAL signal.

In accordance with this invention, the delay 1 6 is no longer of one line delay, but is of a delay equivalent to an odd number of lines substantially equal to one field or picture period. That is to say for a 625 line picture, the delay 16 has a delay of 313 or 625 line periods. The delay must of course be accurate to a small fraction of a subcarrier period, and therefore digital delays are preferred, though in principle the circuit is capable of analogue implementation.

The circuit of Fig. 1 is particularly useful where chrominance resolution is important and there is not much high frequency luminance. The circuit is the 2-field equivalent of conventional delay line decoding. It suppresses Hanover bars, without any loss of chrominance spatial resolution on still pictures, and provides 3 dB reduction of crosscolour. The circuit provides extremely good movement performance and there is no UV crosstalk.

Figure 2 shows, by way of comparison, a delay line PAL decoder based on a delay of twice that shown in Figure 1. In this case the difference across two N-line delays 36 is taken in a subtractor 38 and halved in a divide by two circuit 40. The signal is then conveniently phased displaced by 900 in an appropriate circuit 42, and applied to the demodulator 28 and filter 32 as in Figure 1. The signal from circuit 42 is also inverted in an inverter 44 before being applied to the demodulator 30 and filter 34. As with the Figure 1 circuit, each of the delay elements 36 is preferably of 313 or 625 line periods delay, rather than the one-line delay used hitherto.

Figure 3 shows another PAL chrominance filter or separator embodying the invention and based on a decoder using two one-line delays. This is in essence something of a combination of the circuits of Figures 1 and 2, as will readily be seen, and corresponding components are given the same reference numerals.

The circuit of Figure 3 is like two of the circuits of Figure 1 in cascade and is a good compromise circuit useful where there is more high frequency luminance. On still pictures there is again no loss of chrominance spatial resolution and there is 6 dB of cross-colour attenuation. There is no UV cross talk. As against this, movement performance is not so good but it is still adequate.

Figure 4 shows a chrominance circuit based on that of our British Patent Specification 2,01 8,087.

In this circuit the input signal and the output of the second delay are averaged by an adder 46 and divide by two circuit 48 and the resultant subtracted from the output of the first delay element in a subtractor 50. The resultant signal can then be demodulated. Again, the length of the delays 36 is chosen, in accordance with this invention to be an odd number of line periods substantially equal to a multiple of one field period.

The particular virtue of the circuit of Figure 4 is its complete freedom from cross-colour on stationary pictures coupled with no loss of chrominance resolution even on moving pictures.

However, these advantages are obtained at the expense of introducing UV cross-talk on moving pictures, some cross-colour in moving pictures is amplified by 6 dB and there is no attenuation of Hanover bars.

The more complex decoder of Figure 5 is based on our British Patent Application 80 05486 (Serial No. 2044577) and likewise uses two delay elements. This can in some respects be regarded as a combination of the circuits of Figure 2 and 4.

The outputs of the phase shift circuit 42 and the subtractor 50 are averaged by an adder 52 and divide by two circuit 54, while half the difference between these outputs is taken by a subtractor 56 and divide by two circuit 58. The resultant signals are then demodulated. In this case also the delays 36 are of 313 or 625 lines.

Finally Figure 6 shows a chrominance circuit in which the received encoded PAL signal is applied to four delays 60 connected in series. An adder 62 averages the input and output of the delay chain in combination with a divide by two circuit 54, and the difference between the resultant of this averaging and the middle output of the delay chain is determined in a subtractor 66 and halved by a divide by two circuit 68. The resultant signal is then in a form which it can be demodulated and filter as in the previous figures. This circuit also completely suppresses cross-colour on stationary pictures without any loss of spatial resolution and there is no UV cross-talk. However, movement performance is poor and there is no attenuation of Hanover bars.

In all these examples, if the circuits are to be used with a 525 line PAL colour television signal, then the delay elements may be of 263 or 525 lines in length.

Although circuits according to the invention can be utulised for matched coding and decoding, with complementary luminance and chrominance so that only one filter is required at each end, the luminance and chrominance circuits need not be complementary. Thus, the chrominance circuits of Figures 1 to 6 may also be used at the coder for treating the modulated chrominance.

Reference will now be made to the remaining Figures 7 to 12 which describes circuits for use with the luminance signal.

These may either be used to separate out the luminance signal from a PAL signal in a decoder, or may be used to "clean up" a luminance signal before encoding to avoid undue cross colour in the decoded signal.

Figure 7 shows a luminance circuit which is in fact based on that of our British Patent Specification 1,568,741 and which makes use of a PAL modifier circuit. A PAL signal received at an input 102 is applied to a delay element 104 the output of which is phased shifted by 900 by a circuit 106 and bandpass filtered to limit it to the chrominance band by a filter 1 08. The output of filter 108 is applied to a modulator or multiplier 110 which receives a reference signal at twice the colour subcarrier frequency. The output of this is again bandpass filtered in a filter 11 2 and applied to the inverting input of a subtractor 114, the noninverting input of which receives the PAL signal from the input 1 02. The output of subtractor 114 constitutes a separated luminance signal.

In accordance with this invention the delay 104 is no longer a one-line delay, but rather is a 313 or 625 line delay, that is for the 625 line PAL system.

The circuit of Figure 7 gives complete rejection of cross-luminance on stationary pictures but the high frequency luminance is accompanied by an alias spectrum centred on twice the subcarrier frequency. On movement there is no loss of resolution but the cross-luminance reappears rather quickly.

An alternative system is shown in Figure 8 in which the output of the multiplier 110 is averaged with the input signal by an adder 11 6 and divide by two circuit 11 8 before being applied to the bandpass filter 112. In other respects this is similar to Figure 7. Again there is complete rejection of cross-luminance on stationary pictures but the wanted high frequency luminance and the accompanying alias are both attenuated by 6 dB.

On movement, cross-luminance still reappears quickly but is 6 dB lower than before.

Yet another circuit based on Figure 7 is shown in Figure 9. This is based on a two line delay implementation of the Figure 7 circuit. In this instance, two delay elements 1 20 are used each of 313 or 625 line periods in length. The input and output of the series connected delays are subtracted by a subtractor 122 and this difference halved by a divide by two circuit 124. The remaining circuits are similar to those of Figure 7, as indicated by the use of the same reference numerals. The circuit of Figure 9 appears to be preferable to the circuits of Figures 7 and 8 in that it gives complete rejection of cross-luminance on stationary pictures without an accompanying luminance alias.On movement, there is no loss of resolution and the alias reappears more slowly than in the case of the two previous figures (although the reappearing alias may cause the resultant signal to exceed the permitted dynamic range).

Figure 10 will be seen to represent a combination of the circuits of Figures 8 and 9.

Again corresponding elements are designated by the same reference numerals, and further description is not necessary here. However, it is noted that the advantages of Figure 9 again apply and the wanted luminance and the alias are both attenuated by 6 dB as in the circuit of Figure 8.

Accordingly the permitted dynamic range is not exceeded.

Figure 11 shows, for purposes of comparison, a luminance separation circuit which is based on British Patent Specification 201 8087 above mentioned. This will be seen to have certain similarities to the chrominance circuit of Figure 4.

Unlike the circuits of Figures 7 to 10, this does not use a PAL modifier. it does however have two series connected delay elements 120, the input and output of which is averaged by an adder 126 and halving circuit 128. The resultant is subtracted in subtractor 130 from the output of the first delay element. The subtractor output is bandpass filtered to pass chrominance band signals, and these are subtracted in subtractor 114 from the output of the first delay to produce the luminance output signal Y.

Figure 1 2 shows a luminance separation circuit or filter which in principle corresponds to the chrominance circuit of Figure 5. It will be seen that in principle it represents a combination of the circuits of Figures 10 and 11. The output of the subtractor 1 30 of Figure 11 is added in an adder 132 to the output of the PAL modifier modulator 110, and the resultant halved in a halving circuit 134 before applicaation to the chrominance band filter 112.

Finally, Figure 13 shows a luminance processing circuit which is in principle an extension of the circuit of Figure 11, and corresponds to the chrominance circuit of Figure 6. Here, four delay elements 1 40 are used, and the outputs of which are processed in substantially the same way as in Figure 11. It is now necessary to introduce an additional halving circuit 140 between the subtractor 1 30 and filter 112. The circuit of Figure 1 3 gives complete rejection of cross-luminance on stationary pictures without introducing an alias, even on moving pictures.

Nevertheless, its movement performance is not good and cross-luminance reappears rather quickly.

For any given application, the choice of the optimum decoding system depends on the type of coder used. It is not however essential for the decoder to incorporate the same type of filtering system as is used in the coder prefilter. Nor is it necessary for the chrominance and luminance paths to be based on the same filter type, although obviously economies of circuit construction can be obtained if they are. The particular circuit employed will also depend upon the characteristics of the signals to be treated and the trade-off between advantages and disadvantages of each circuit. Nevertheless, the circuits of Figures 1, 3, 9 and 10 appear to be the most advantageous circuits.

It should be noted that the chrominance circuits can be implemented as a modification of the corresponding one of the luminance circuits, in which the output of the chrominance bandpass filter 112 is demodulated in a simple PAL demodulator, i.e., a pair of synchronous demodulators. Also, the 90 degrees phase shift circuit 106 used in the modifier prefilters can be incorporated into the modifier by an appropriate choice of the phase of the reference signal applied to the modifier multiplier 110.

Claims (6)

1. A PAL decoder or coder circuit including delay means as part of the separating or prefiltering circuitry respectively, in which the delay means comprises first and second seriesconnected delay devices, each providing a delay substantially of an odd number of lines substantially equal to one picture period, combining means for subtracting the input of the first delay device and the output of the second delay device and for providing half the resultant, and a 90 phase shift circuit connected to the output of the combining means.

2. A PAL circuit according to claim 1, including an adder and a subtractor both connected to the output of the phase shift circuit and also receiving a signal dependent upon the output of the first delay device.

3. A PAL circuit according to claim 2, in which the last-mentioned signal is derived as the average of the input to the first delay device and the output of the second delay device subtracted from the output of the first delay device.

4. A PAL circuit according to claim 1, including a chrominance bandpass filter connected to the output of the phase shift circuit, a multiplier connected to multiply the filter output by a signal of twice colour subcarrier frequency, and means comprising a second chrominance bandpass filter for applying a signal dependent upon the multiplier output to a subtractor so as to be subtracted from the output of the first delay device.

5. A PAL circuit according to claim 4, in which the last-mentioned means also comprises averaging means for averaging the output of the multiplier and a signal dependent upon the output of the first delay device, the second chrominance bandpass filter being connected to the output of the averaging means.

6. A PAL circuit according to claim 5, in which the last-mentioned signal is derived as the average of the input to the first delay device and the output of the second delay device subtracted from the output of the first delay device.