GB2169772A - Detecting horizontal colour changes in a video signal - Google Patents

Abstract

Delays 3,4 provide three constructive lines of video chrominance signal CH0, CH1, CH2. These are combined 24,25 to provide to signals which are multiplied 26 to provide an output indicating a change in the colour subcarrier phase i.e. a horizontal colour change. The amplitude of CH0, CH2 are also compared, by adder 29, to indicate a horizontal transition. Either output signal provides a transition detected output, which is used to alter the configuration of a PAL decoder (Fig. 3). When a change in chrominance is detected the decoder uses only one line of video signals in the decoding of the chrominance and luminance and in the separation of the colour difference signals. If no chrominance change is detected three adjacent lines are used in the decoding. <IMAGE>

Description

SPECIFICATION Improvements in or relating to pal decoders This invention relates to improvements in video signal decoders particularly those for signals in a PAL format.

It is well known that in the PAL colour system the colour difference signals U and V are modulated in quadrature onto a sub-carrier with the phase of the V signal being reversed on successive lines. The video signal which is transmitted includes this signal, called chrominance, a luminance signal and reference information.

At the television receiver the chrominance and luminance signals must be separated and the chrominance signal must be further separated into the two colour difference signals in order for the image to appear on the screen.

A known type of decoder separates the two parts of the combined signal by means of a notch filter in the luminance path and a band pass filter in the chrominance path. The separation of the two components however, is not usually complete and this has an effect on the resulting picture. The incomplete removal of the chrominance component from the luminance signal and the luminance component from the chrominance signal cause deteriorated picture quality. One aspect of television production where it is particularly noticeable is in the use of still stores. To overcome these problems it is possible to use a comb filter circuit to decode the video signals. An example of a comb filter can be seen in Fig.

1. Lines of video signals are delayed for one or two lines so that three adjacent lines are available for decoding and these are then added or subtracted so that the chrominance and luminance signals are separated. In this way the chrominance is separated by cancellation, rather than passive loss and this reduces the problems encountered with the other decoder mentioned, however, the colour difference signals produced are now averages of the three lines of video signals and this may result in smearing of colour at horizontal or near horizontal colour boundaries.

It has therefore been proposed to change the method of decoding video signals when horizontal colour boundaries are encountered.

For this purpose the video signals to be decoded are passed through a detector circuit which detects changes in colour. Most detectors rely on determing the amplitudes of the video signals which are to be decoded and comparing these to try to detect a change in colour. This will give a signal when there is a change in saturation, where saturation is related to amplitude, but if saturation remains the same and only the phase of the chrominance signal changes, even though this indicates a change in colour the detector will not give an output. An example of this type of arrangement is given in GB 2066615. Further, even when a colour boundary has been detected prior art systems still use chrominance which is the average of two or more lines in the colour difference separator and some smearing still appears.The deterioration of picture quality caused by the vertical smearing at colour boundaries may be very noticeable.

An object of the present invention is to provide a simple detector circuit for detecting colour boundaries in a group of video signals which will detect both changes in phase and amplitude of the chrominance signal.

A further object of the invention is to provide a decoder for video signals which will operate in one mode when there is a colour boundary present, thus reducing smearing, and a second mode when no colour boundary is present.

According to the present invention there is provided a detector circuit for detecting a horizontal colour boundary comprising; input means for colour video signals, delay means for delaying said colour video signals, said delay means being arranged to produce at least three consecutive lines of video signals for processing, means for producing different combinations of said lines of video signals and means for multiplying said combinations to produce a change signal when there is a change in the relative position of the phase.

Preferably there is also provided means for comparing the amplitude of two lines of video signals so that a change signal is produced when there is a change in amplitude.

Preferably the change signal from the phase comparing means and the change signal from the amplitude comparing means can be combined to form the output from the detector circuit.

According to a further aspect of this invention there is provided a decoder for video signals comprising: input means for video signals, delay means for delaying lines of video signals, said delay means being arranged to provide at least three adjacent lines of video signals, means for separating the chrominance signal from the composite video signal, said means being arranged to use video signals from only one line when a colour boundary is detected in the incoming video signals, and means for separating the chrominance signal into the two colour difference signals, said means also being arranged to use video signals from only one line when a colour boundary is detected in the incoming video signals.

Preferably when no change is detected signals from three or more adjacent lines are used to produce the separate colour difference signals.

One embodiment of the invention will now be described with reference to the following drawings: Fig. 1 shows a prior art comb filter for decoding PAL video signals.

Fig. 2 shows the detector circuit for detecting changes in chrominance between lines of video signals.

Fig. 3 shows the decoder incorporating the detector circuit.

The circuit shown in Fig. 1 acts on lines of video signals in the PAL format that is signals including a luminance signal Y and a chrominance signal composed of two colour difference signals U and V. Between each line of video signals there is a quarter sub-carrier period off set of the chrominance sub-carrier signal and the phase of the V signal is reversed.

Looking at Fig. 1 the signals on line 0 are the incoming video signals, the signals on line 1 are video signals delayed by a line period by delay 3 and on line 2 the video signals are those delayed for two line periods by delays 3 and 4. It can be seen that this arrangement provides three adjacent lines of video signals to be decoded as is required. Considering the phases of the colour difference signals as shown in Fig. f it can be seen that the phases on lines 0 and 2 are opposing and the phase on line 1 is a quarter sub-carrier period off-set from line 0 but with the phase of V reversed. These phase representations are only correct if there is no change in colour between the lines. It can be readily seen that by adding the signals on line 0 and 2 the chrominance signals may be made to cancel and leave only the luminance component.This addition occurs in adder 8 and produces luminance Y on line 9. Before passing to the adder the lines 0, 1 and 2 have been band pass filtered so that only the high frequency luminance and the chrominance signals remain.

Thus half the output of the band pass filter 5 acts as an input to adder and the other half the input to adder 16.

The video signals on line 1 pass through band pass filter 6 and are subtracted from the luminance signal on line 9 to produce a chrominance output on 12 which has the opposite phase to the chrominance in line 1.

Again it is obvious that if this is added to the signals from line 1 the chrominance will cancel and this occurs in adder 13. The output from line 13 is the luminance signal. The chrominance signal on line 12 is also used for decoding into the colour difference signals. The second input to the colour difference decoding circuit is the output 15 from adder 16 which output is the signals from line 2 subtracted from the signals from line 0 which gives a chrominance output with a phase as shown.

The chrominance signal from line 12 passes through a quarter sub-carrier period delay 14 which produces the phase which is the equivalent to the phase on line 15 but with V reversed.

To separate the chrominance signal into the colour difference signals U and V the two inputs are added together in adder 17 and subtracted in adder 18. The output of adder 17 will be the U signal and that of adder 18 the V signal. The reason for this can be seen by looking at the two phases of the incoming signals which have the V signals opposing and the U signals at the same phase. The separated U and V signals are then demodulated by multiplying by sin wt and cos wt respectively to produce B-Y and R-Y signals, where R and B are the red and blue colour signals and Y is luminance.

The circuit shown in Fig. 1 is the prior art system and it can be seen why the errors occur so that a different mode of decoding is required, when there is a chrominance change between the lines. The improved detecting circuit according to the invention which detects such a change in chrominance can be seen in fig. 2. The detector again works on three adjacent lines of video signals and these can be obtained from the same delays as used in the comb filter and these lines are passed through band pass filters 21, 22 and 23. If there has been no change in hue, which is related to the phase of the chrominance signal, then the phase on lines 0 and 2 should be equal and opposite and the phase on line 1 should be a quarter line off-set from line 0.When the differences between the line 0 and line 1 and line 2 and line 1 are obtained then these two values should be then all at right angles and when multiplied together should produce no output. This is the operation which is performed by the adders 24 and 25 and the multipler 26. The output of adder 24 is line 1-line 2 which gives the relative phase between lines 2 and 1 and the output of adder 25 is line 1-line 0 which gives the relative phase between these two lines. If the phase of the chrominance has changed in one of the lines then the relative phase between it and the previous line will have changed also and the two phases will not be in quadrature.

Thus output of the multipler 26 will be zero unless the relative phases are not at right angles to each other in which case there has been a change in the phase of the chrominance signal on one of the lines.

To obtain information on any change in amplitude of the chrominance signals the signals fro! line 0 and 2 are full wave rectified and then the difference taken. If there has been any change in amplitude this differencing will produce a signal. The difference is obtained in adder 29 which also has an input the output of multiplier 26 thus a signal is produced if the phase of the chrominance signal has changed and/or the amplitude of the chrominance signal has changed. The output of adder 29 is passed through low pass filter 30 to remove the twice sub-carrier frequency which is generated by the multiplication in 26.

The decoder is shown in Fig. 3 with the circuit of Fig. 2 shown as ref 31. The output of the detecting circuit is used to control the switches 35, 37 and 38, which determine which mode the decoder circuit is working in.

The circuit contains many components which were in the prior art system of fig. 1 and these are referred to by the same reference numbers. Delays 32, 33 and 34 are provided so that the detector circuit may operate on the video elements before the decoding begins. When there is no output from the detector circuit the decoder operates substantially as in the prior art system and switch 35 is closed. If a colour change is detected then switch 35 is opened and switches 37 and 38 are closed and the colour difference signals are produced from one line of video signals only. In this mode video signals from line 1 are passed through the band pass filter 6 and used in the colour difference signal decoder.

These signals are substantially chrominance but may have some high frequency luminance signals present. As the signals pass through adder 10 they are inverted and so when summed in adder 13 with video signals from line 1 which have not passed through the band pass filter a luminance signal is produced. Signals on line 1 pass through a compensating delay before being summed.

The chrominance on line 12 passes through the quarter sub-carrier period off-set 14 and is then used in the U and V decoder. To obtain the U component the chrominance is inverted at one input of 39 where it is added to the chrominance from adder 15. This output which can be represented by CH1,-CH14 passes via switch 37, which is closed for this mode of action, to adder 41. The second input to adder 41 is produced by adding the chrominance from adder 1 5 (CH,s) to that from the quarter line off-set 14 (CH,4) in adder 17 and inverting this. Thus the output from adder 41 is (CH 5-CH 14)-(CH15 + CH,4) which produces only chrominance-CH14 frorn the quarter line off-set.It will be appreciated that the chrominance present at the quarter line off-set is derived from the middle line 1 alone The U component is demodulated from chrominance from one line of signals only by multiplying by sinwt in multiplier 19.

Similarly an output of chrominance from the middle line alone is obtained at the input of multiplier 20 where the U component is demodulated by multiplying by coswt.

It is clear that there may be factors applied where necessary in the circuit to correct for changes in amplitude produced by addition and subtraction during separation.

The decoder then gives an output of two colour difference signals and a luminance signal from an input of video signals in the PAL format. When there is no colour change in the video signals to be decoded the decoding is achieved using 3 lines of video signals and when there is a colour change using one line of video signals.

It is to be understood that these circuits may be used separately or together for other purposes than those described in this application and can be equally applicable to digital or analogue signals.

Claims (8)

1. A detector circuit for detecting a horizontal colour boundary comprising; input means for colour video signals, delay means for delaying said colour video signals, said delay means being arranged to produce at least three consecutive lines of video signals for processing means for producing different combinations of said lines of video signals, and means for multiplying said combinations to produce a change signal where there is a change in the relative position of the phase.

2. A detector circuit as claimed in claim 1 further including means for comparing the amplitude of two lines of video signals so that a change signal is produced when there is a change in amplitude.

3. A detector circuit as claimed in claim 2 further including, means for combining the change signal from the phase comparing means with the change signal from the amplitude comparing means and output means for outputting the combination to form the output from said detector circuit.

4. A decoder for video signals comprising; input means for video signals, delay means for delaying lines of video signals, said delay means being arranged to provide at least three adjacent lines of video signals, means for separating the chrominance signal from the composite video signal, said means being arranged to use video signals from only one line when a colour boundary is detected in the incoming video signal, and means for separating the chrominance signal into the colour difference signals said means also being arranged to use video signals from only one line when a colour boundary is detected in the incoming video signal.

5. A decoder as claimed in claim 4 wherein said means for separating the composite video signal uses signals on three or more adjacent lines when no change in chrominance is detected.

6. A decoder as claimed in claim 4 wherein said means for separating the chrominance signal into two colour difference signals operates on chrominance signals from three or more adjacent lines when no change in chrominance is detected.

7. A detector circuit substantially as described herein with reference to Fig. 2.

8. A decoder for video signals substantially as described herein with reference to Fig. 3.