GB2078054A - PAL decoding - Google Patents

Abstract

A Weston PAL decoder based on one or more delay devices suffers from chrominance-luminance cross-talk etc. when there is movement in the picture. Circuitry (50-58) is provided to detect the presence of movement in the PAL signal. In a simple mode (movement detected) a switch (58) selects the output of the first delay, while in the 'Weston' mode (no movement) the switch selects the average of the first and second chrominance paths. The delays may be one-picture delays. A first chrominance path averages (16,18) the undelayed and delayed signals and subtracts (20) this from the mid-point of the delay. A second chrominance path subtracts (22) the input and the output of the second delay, halves (24) and band pass filters (26) the resultant which is applied to a PAL modifier (28) with associated band pass filter (30). A control signal is generated from differences between these chrominance signals detected in a subtractor (50) which are demodulated (52,54) and used to control the selector switch (58). Linear interpolators may alternatively replace a switch; the chrominance output is selected between the 'Weston' signal and a signal derived by averaging the output of the first delay (12) with a signal obtained from two one-line delays via processing similar to the second chrominance path; and used to switch from the one-field delays (12,14) to one-line delays instead. <IMAGE>

Description

SPECIFICATION PAL decoding This invention relates to a decoder for decoding PAL colour television signals, of the type comprising one or more delay devices and means for combining the delayed and undelayed signals, such as to decode the PAL input signal.

One method of separating chrominance and luminance in a PAL decoder, known as the Weston System, is described in our UK Patent Specifications 1534269 and 2044577, and has a number of advantages over simpler methods. One circuit arrangement of such a type is shown in Figure 1 of the drawings accompanying this application and is particularly favourable, as it is suitable for both analogue and digital implementation and a wide range of sampling frequencies.

The circuit shown in Figure 1 has an input 10 connected to two similar delay devices 12, 14 connected in series. The input signal and the output of the second delay are averaged in an adder 16 and divide-by-two circuits 18 to produce a signal which contains essentially the luminance component of the PAL signal. This is subtracted in a subtractor 20 from the PAL output of the first delay 12 to provide a first chrominance signal.

The input and the output of the second delay 14 are also subtracted in a subtractor 22 and halved in a divide-by-two circuit 24 to cancel the luminance components leaving the chrominance components.

This is applied to a PAL modifier circuit comprising a chrominance band pass filter 26, a multiplier 28 where its signal is multiplied by a signal at twice the colour subcarrier frequency, and a chrominance band pass filter 30. The PAL modifier phase shifts the U and V phase subcarrier signals to match those of the first path. The chrominance band pass filter 30 is included to remove unwanted chrominance components produced by the modifier at three-times subcarrier frequency.

The chrominance signals from these two paths, namely from the subtractor 20 and filter 30, are averaged by an adder 32 and divide-by-two circuit 34 and band pass filtered in a chrominance band pass filter 36. This chrominance signal is subtracted from the PAL output from delay 12 in a subtractor 38 to provide the luminance output of the decoder, and is demodulated in conventional fashion in demodulation circuits 40 to provide U and V output signals.

The delays 12, 14 used in the circuit can be both 1-line, 313 lines or 625 lines, or even combinations of these. The version with 625-line delays is capable of "perfect" decoding of still pictures, completely free of the cross-colour and cross-luminance effects normally associated with PAL encoded pictures.

Rapidly moving areas of picture, however, are affected by chrominance-luminance cross effects and, in addition, U-V crosstalk and luminance aliasing. These effects are even more disturbing when the decoded signals are recoded with a new subcarrier phase or PAL switch sense and subsequently decoded again. This would occur for PAL signals entering studio equipment operating on YUV signals, which were subsequently recoded to PAL at the studio output. As a result, the remnants of the first subcarrier would be decoded according to the reference phase of the second subcarrierto produce hue errors on the edges of moving coloured objects.

Although most pictures do not contain highly saturated coloured objects moving sufficiently fast to show up the impairment, some picture material, such as that from sporting events, produces unacceptable impairment. In the moving areas simpler methods of decoding would give superior performance.

This invention provides in a first aspect a PAL decoder of the type comprising one or more delay devices, and means for combining the delayed and undelayed signals such as to decode the PAL input signal, characterised by means for varying the effective signal combinations used to form the decoder output signals in dependence upon the sensed presence of movement in the PAL signal.

The detection of movement in a monochrome signal is a simple matter consisting of looking for differences from picture to picture by subtracting across a picture delay. For coded PAL signals, the problem is greater because the subcarrier is itself a moving pattern for a stationary coloured area.

Further, if the coloured area is moving, then the subcarriercan become a stationary pattern. If the subcarrier is removed by lowpass filtering before the movement detector, then moving areas of chrominance cannot be detected.

In a preferred aspect, this invention provides a PAL decoder based upon the above-described Weston decoder which is particularly adapted to take account of moving areas of chrominance. The invention thus further provides a PAL decoder of the type defined in claim 2 below, to which reference should now be made.

The invention will be described in more detail, by way of example, with reference to the drawings, in which Figure 1 (referred to above) is a block diagram of a PAL decoder based upon our afore-mentioned UK Patent Specifications; Figure 2 illustrates the temporal frequency characteristics for a subtractor added across the averager in Figure 1; Figure 3 is a block diagram of a first PAL decoder embodying the invention; Figure 4 illustrates the principle filtering characteristics of the circuit of Figure 3; Figure 5 is a block diagram of a modification of the circuit of Figure 3; and Figure 6 illustrates some modified characteristics of the circuit of Figure 5.

Referring again to Figure 1, the construction of which is described above, if the signal content, that is, the luminance and baseband chrominance components represented by the coded PAL signal, is the same at the middle and ends of the delays 12, 14, then the modulated chrominance signals at the output of the two paths are equal. So, if the picture is stationary, a 625-line delay decoder will have identical signals reaching the inputs of the final averager (32). As the picture moves, the signals at the ends of the delays become different and, because the two paths have different filtering charnoteristics, the signals entering the averager also become different.

Therefore, a subtractor placed in parallel with the final averager would produce zero output for stationary areas of picture and will give an output for moving objects. The temporal frequency characteristics for U and V phased signals to the output of such a subtractor are shown in Figure 2. The nulls at 0 and 183/4 Hz in the U response, and 0 and 6104 Hz in the V response, ensure that there is no outputforstation- ary chrominance and luminance.

Since much of the output of the subtractor is modulated chrominance, this must be demodulated to produce a satisfactory control signal. This can be achieved by full-wave rectification of the subtractor output, which in a digital circuit is conveniently carried out by a read-only memory, followed by lowpass filtering to suppress the residual carrier components. This simple amplitude demodulator is superior to a product demodulator in this instance because it allows low frequency luminance compo nexts to remain at low frequencies and so contribute to the movement detection.

The demodulation process produces a smooth waveform of one polarity indicating the magnitude of the difference between pictures. This could be used with a slicing circuit with an appropriately chosen threshold to switch between the 625-line Weston circuit and an alternative with better movement performance. Unfortunately, such simple systems are prone to disturbing effects caused by signals near the threshold repeatedly crossing it due to the effects of noise. Although this problem can be alleviated by including hysteresis, a better solution is to use the control signal, perhaps through an appropriately chosen non-linear law, to combine proportions of the two alternative signals. The implementation would then consist of a linear interpoiator used to cross-fade between the two alternative decoding methods according to the control signal amplitude.

The choice of the auxiliary decoding method is determined by its performance under the conditions which cause the 625-line Weston decoder to fail. The use of simple demodulation of chrominance and lowpass filtering of luminance as the auxiliary method would be particularly easy to implement and would give adequate performance for many signals.

For luminance, there would be a loss of detail on moving objects, but this is unlikely to be serious because moving detail tends to be reduced by the characteristics of the camera anyway. The lowpass filter would ensure that the subcarrier was adequately suppressed on moving coloured edges and that no luminance aliasing would be present. The simple chrominance circuit would avoid the U-V crosstalk effects produced by the Weston circuit. The amount of cross-colour produced by the simple circuit would sometimes be greater than and sometimes less than that produced by the Weston circuit, depending on the rate of movement.

The disadvantage of the simple circuit is its treatment of phase distorted waveforms which would give rise to Hanover bars. Although the Weston circuit suppresses Hanover bars, these would be detected as a moving pattern and the system would adapt to the auxiliary method, which would give no suppression. Also, for small amplitudes of Hanover bars, the changeover would not be complete, so although the Weston chrominance circuit would partially suppress the Hanover bar pattern, the Weston luminance circuit would carry the signal as luminance which, after subsequent recoding and decoding, could result in a static hue error. For suppression of Hanover bars, therefore, a - non-complementary luminance and chrominance system is necessary.

The most suitable auxiliary method appears to be one which uses lowpass filtered luminance, for the reasons given above, and two-line delay line chrominance to suppress Hanover bars. The only disadvantage of this chrominance circuit is a slight loss of vertical chrominance resolution which would occur on moving objects. However, this is more than offset by the accompanying reduction of cross-colour, which would be half that of the simple circuit for moving line-repetitive luminance. The circuit would be considerably more complicated, though, because separate cross-fade channels would be required for luminance and chrominance.

AfurthercircumstanceunderwhichtheWeston circuit using 625-line delays does not give adequate performance is when the normal mathematical relationship between the line and subcarrier frequencies is broken. The filtering characteristics of the Weston circuit with 625-line delays are related to the picture frequency of the signal. Therefore, if the subcarrierfrequency is non-mathematical and shifted by even a few Hertz, the subcarrier in plain coloured areas will tend to change to the auxiliary system in coloured areas, depending on the saturation.

With a line-locked demodulator of the type described in our UK Patent Specification 2059711 this situation can be detected simply and rapidly by integrating the frequency control signals in the demodulator loop. For signals with the correct relationship the result will be zero, but nonmathematical signals will cause the integrator to overflow. This overflow condition can be used to switch to an alternative form of decoder, such as 1-line delay Weston, which performs well for nonmathematical signals. The 1-line delay Weston circuit could stili operate adaptively for nonmathematical signals, in this case to remove impair menus to vertical chrominance detail.

Clearly, a whoie range of adaptive decoding methods can be developed from the Weston circuit with the subtractor used to provide adaptive control.

To illustrate this, two preferred circuits will be described, the first representing one of the simpler implementations which is capable of good performance for most signals, and a second more complicated circuit which avoids the shortcomings of the simpler circuit.

Figure 3 shows a 625-line Weston circuit based on Figure 1 with the addition of an alternative simple decoder for moving areas of picture. Much of the circuit is the same as in Figure 1 and carries the same reference numerals and will not therefore be described again. However the circuit additionally includes a subtractor 50 connected to the outputs of subtractor 20 and filter 30 respectively, in parallel with adder 32. The output of subtractor 50 is applied to a full-wave rectifier 52, a low-pass filter 54 and a slicer 56. The output of the slicer 56 is used as a control signal to a selector switch 58 to apply either the output of averager 32/34 or the output of delay 12 to the filter 36 for subsequent demodulation.

Thus, the adaptive control signal is obtained by demodulating the output of the extra subtractor 50 and slicing the resulting signal. Crossing of the initial slicing level in each direction would reset the slicing level to give hysteresis, and thus avoid repeated switching between the two modes due to noise.

The main filtering characteristics of this simple adaptive circuit are shown in Figure 4. The changeover point between the Weston and simple circuits is shown as an abrupt transition, although the precise changeover point would vary according to signal amplitude. Apart from the slight disadvantages of removing high frequency luminance and increasing cross-colour, the simple mode virtually eliminates cross-luminance, luminance aliasing and U-V crosstalk, and chrominance blurring. However, non-mathematical input signals would cause the decoder to switch to the simple mode even for still pictures. Similarly, parts of stationary signals containing substantial amplitudes of Hanover bars would use the simple mode, so that the Hanover bars would not be suppressed. In this case, though, full luminance resolution, in 'luminance only' areas of picture, would be retained.

The method is equally applicable to the 313-line and 1-line delay versions of the Weston circuit, although the process then detects vertical detail in addition to, or rather than, movement.

The decoder of Figure 5 shows a substantially more complicated arrangement which appears to overcome the main drawbacks of the circuit of Figure 3. The circuit has three extra features any of which could be added separately as an improvement to the simpler circuit.

First, when the control signal indicates that 'simple' decoding is required, the output of delay 12 is used, as before, for the luminance signal, but for the chrominance signal the output of an auxiliary chrominance signal path is taken. The auxiliary path is based on the use of two one-line delays 12A, 14A, and comprises a subtractor 22A, halver 24A, band pass filter 26A, and PAL modifier 28A, and an adder 66 and divide-by-two circuit 68 connected to average the outputs of modifier 28A and delay 12A.The delays 12, 14 are split into one-line delays 12A, 14A and 624-line delays 12B, 14B. The use of a two-line delay system for the auxiliary chrominance path reduces the amount of cross-colour on linerepetitive luminance to one-half, so that the HYU and HYV characteristics shown in Figure 4 are altered to those of Figure 6. At vertical luminance frequencies corresponding to those of the chrominance subcarriers, however, the amount of cross-colour would be unchanged.

Secondly linear interpolators 62,64 are used to take proportions of the signals from the two alternative decoding methods according to the control signal amplitude. Separate interpolators are required for the chrominance and luminance paths because the auxiliary decoding method is noncomplementary. The slicer 56 of Figure 3 is replaced by a non-linear law circuit (e.g. a ROM) 60.

Finally, means are included to detect nonmathematical PAL. This means comprises a burstlocked oscillator 70 connected to the U and V outputs of the decoder to detect situations in which it would be inappropriate to use signals from different fields in combination. In such circumstances, the oscillator 70 actuates switches 72,74 to use effectively only the one-line delays 12A, 14A, by switching out the 624-line delays 12B, 14B.

Claims (8)

1. A PAL decoder of the type comprising one or more delay devices, and means for combining the delayed and undelayed signals such as to decode the PAL input signal, characterized by means for varying the effective signal combinations used to form the decoder output signals in dependence upon the sensed presence of movement in the PAL signal.

2. A PAL decoder comprising an input, two delay devices connected to the input and each of one line or of a number of lines delay substantially equal to one or more field periods, first chrominance providing means comprising means for averaging the input signal and the output of the second delay and subtracting the resultant from the output of the first delay, second chrominance providing means comprising means for subtracting the input signal and the output of the second delay and halving the resultant and a PAL modifier circuit connected to the change of phase of the chrominance components of the resultant signal, control signal providing means comprising a subtractor connected to the outputs of the first and second chrominance providing means and means for demodulating the subtractor output to provide a control signal, and means responsive to the control signal for varying the effective signal combinations used to form the decoder output signals.

3. A decoder according to claim 2, in which the demodulating means comprises a full-wave rectifier and a low-pass filter.

4. A decoder according to claim 2 or 3, in which the control signal operates to select between two signals one of which is the average of the outputs of the first and second chrominance providing means.

5. A decoder according to claim 5, in which the other signal comprises the output of the first delay device.

6. A decoder according to claim 5, in which the other signal is the average of the outputs of the first delay device and of an auxiliary chrominance providing means which is substantially the same as the second chrominance providing means and receives PAL input signals from the input and output of a two-line delay.

7. A decoder according to any of claims 2 to 6, in which the delay devices provide a total of two fields of delay, and comprising means responsive to the decoder chrominance outputs for substituting two lines of delay for the two fields in the presence of non-mathematical PAL input signal.

8. A PAL decoder substantially as herein de scribed with reference to Figure 3 of the drawings with orwithoutthe modifications of Figure 5.