GB2267408A - Colour television signal decoding using a comb filter. - Google Patents

Abstract

A decoder for decoding a PAL colour television signal comprises a comb fail detection circuit including delay means 13 for providing signal samples from consecutive lines of the television signal, summing means 14 and rectifying means 15 for providing an output signal the magnitude of which is proportional to the lack of vertical correlation in the picture content of the signal, threshold detecting means 16 and an OP gate 18 one input of which is connected directly to the output of the threshold detecting means 16 and the other of which is connected to the same output via a frame delay 17. The output of the OR gate 18 provides a comb fail detection signal. A pulse stretching circuit 19 serves to remove high frequency fluctuations in the comb fail signal. The comb fail signal is then applied to the decoder circuit (Fig.1 not shown). <IMAGE>

Description

IMPROVEMENTS RELATING TO COLOUR TELEVISION SIGNAL DECODING This invention concerns improvements relating to colour television signal decoding and more particularly to a comb filter decoder with an adaption circuit.

A comb filter is a means of separating the luminance (Y) and chrominance (C) signals which compose a PAL composite video signal. The Y and C share the high frequency (HF) part of the video signal frequency band so cannot be separated by a simple filter. However, by combining contributions from the same horizontal position on different lines the Y and C can be separated.

The Y/C split is very good if the picture content is the same on all the lines of the combing aperture, but the Y and C mix if there are line to line changes in picture content giving spurious patterning on the decoded picture.

To give a full bandwidth decoded picture without this patterning, line to line changes are looked for and when they are found, the filter is switched to 'simple' mode in which the C is separated by a band pass filter, and a similar band is stopped in the Y channel. This process of switching to simple filtering when the input signal would cause the comb filtering to fail is called 'adaption'.

The interaction of the PAL ident (the reversal of phase of the V subcarrier every line) with the odd number (six hundred and twenty five) of lines in a frame means for some comb fail detection schemes, that detail changing with a vertical period of less than about four lines will give different outputs from a comb fail detector based on a single frame, on each of the two ident frame types - a 12 1/2 Hz flicker on a stationary picture. (These high vertical frequencies may typically be encountered in graphics, captions and test signals, rather than pictures from cameras).

It has hitherto been proposed to overcome the problem of the 12 1/2 Hz flicker by subjecting the output signal of the comb fail detector to a one frame delay before applying the signal to cause adaption of the comb filter.

Furthermore the delayed signal may be applied by a nonadditive mixer that is also arranged to receive the undelayed comb fail signal and to pass whichever of the two signals is of greater magnitude.

In the known arrangement the construction of the comb filter is such that the comb fail signal applied to effect adaption of the comb filter is required to be a proportional signal, and thus the circuitry needed to effect a one frame delay and mixing of a proportional signal is relatively complex and expensive, even when implemented in digital form.

It is accordingly an object of the present invention to provide an improved colour television decoder that is of simplified construction.

The invention accordingly provides a decoder for decoding a PAL colour television signal, comprising filter means for receiving a PAL composite signal and separating said signal into luminance and chrominance signals, said filter means being capable of being switched from a comb filtering mode to a simple filtering mode in response to a comb fail detection signal provided at a switching input of said filter means; a comb fail detection circuit comprising means for detecting lack of vertical correlation in picture content of the television signal and for providing an output signal of corresponding magnitude; threshold detecting means coupled to an output of said comb fail detection circuit and arranged to provide an output signal to indicate whether the signal from the comb fail detection circuit is above or below a given magnitude; and an OR logic gate having a first input coupled directly to an output of said threshold detecting circuit, a second input coupled to said output of said threshold detecting circuit via a one frame delay means, and an output coupled to said switching input of said filter means.

Further preferred features and advantages of the invention will become apparent from the following description of one example of a decoder according to the invention and the drawings, in which: Figure 1 is a block circuit diagram of a comb filter decoder, and Figure 2 is a block circuit diagram of a comb fail detection circuit for use with the decoder of Fig. 1.

THE COMB FILTER (Fig. 1) The PAL signal entering the decoder is filtered in a band pass filter 1 centred on the PAL subcarrier frequency with a bandwidth sufficient to contain the chrominance signal which is to be separated. The PAL signal is also passed to a two line delay 2 for use later in making the Y output signal.

The band pass signal is delayed in a three line delay 3 with taps at one line intervals giving, at its outputs, access to the same horizontal position on three spatially adjacent lines.

Signals delayed by one line and three lines are added 4 with weighting factors of a half. The non-integral phase difference between the two points is half a cycle so the subcarrier components cancel and the output consists only of HF luminance passed by the band pass filter, with an average timing two lines delayed from the input to the decoder. Call this signal the combed HF Y.

Similarly timed Y with the subcarrier band removed is made by subtracting at 5 the two line delayed band pass filtered signal from the two line delayed full bandwidth signal. In comb decoding mode the combed HF Y is added to the band stopped Y at 6 to form a full bandwidth spatially correct Y signal; but if a line to line difference greater than a preset value has been detected by the comb fail detector described below, the amount of HF Y added is reduced to zero by a multiplier 7 gradually, by a time determined by the filtering of the comb fail detect signal in a low pass filter 8 , to stop the fast edges of the comb fail detect signal producing spurious high frequency components in the output Y signal.

Comb chrominance separation is achieved by adding, in 10, the two line delayed band pass filtered signal to either the one line or the three line'delayed band pass filtered signal alternated by a switch 9 at twice subcarrier frequency and the result passed to the U and V demodulators 12. This separation method only works for a system running at four times subcarrier frequency sampled at 45 degrees from the U and V axes; in analogue or line locked systems, the matrixing would be rather more complex. When comb failure is detected, the input to the demodulators is selected by a hard switch 11 to be the two line delayed band passed signal.

THE COMB FAIL DETECTOR (FIG. 2) The result from the band pass filter 1 is delayed in a four line delay 13 with taps at one line intervals, samples are added 14 in the following proportions: undelayed +1/2 1 line delayed -1 2 lines delayed +1 3 lines delayed -1 4 lines delayed +1/2 (Proportions such as these are easily realised in the digital domain by inversion or significance shifting of data bits).

The theory is that if the picture content of all the points in the aperture is the same, the result of this summation will be zero, otherwise its magnitude should be non-zero.

The zero result can be visualised as follows bearing in mind that the subcarrier phase shift between points two lines apart is 180 degrees (exact inversion) though the Y content of all five points is the same: (-1 X 1 line delayed + (-1 X 2 lines delayed ) = -2Y (Where Y is the luminance content of any of the lines - the chrominance components have cancelled as they are equal and opposite for the same picture content and the luminance has added) (1/2 X undelayed) + (1 line delayed) + (1/2 X 4 lines delayed = +2Y (The chrominance components are the same, reversed twice, on the undelayed and four lines delayed signal, which are opposite to those on the two lines delayed signal).

and: -2Y + 2Y = 0 Unlike the chrominance separation which simplifies for four times subcarrier frequency sample, this adapt detection scheme will also work in analogue and line locked systems as it is insensitive to subcarrier phase. Burst to picture phase errors also do not cause spurious comb fail detections as is the case with three line apertures.

This non-correlation signal is rectified at 15 to give its magnitude and compared to a preset level at 16 to give a continuous (point by point in a digital system) comb fail detected signal.

The result of the threshold detector is input to a 1 bit frame delay 17 and 'or'ed at 18 with the output of the frame delay. The result of this is stretched by a fixed time 19, so that high frequency comb fail detections run into each other, and is used to operate the adaption.

Claims (7)

1. A decoder for decoding a PAL colour television signal, comprising filter means for receiving a PAL composite signal and separating said signal into luminance and chrominance signals, said filter means being capable of being switched from a comb filtering mode to a simple filtering mode in response to a comb fail detection signal provided at a switching input of said filter means; a comb fail detection circuit comprising means for detecting lack of vertical correlation in picture content of the television signal and for providing an output signal of corresponding magnitude; threshold detecting means coupled to an output of said comb fail detection circuit and arranged to provide an output signal to indicate whether the signal from the comb fail detection circuit is above or below a given magnitude; and an OR logic gate having a first input coupled directly to an output of said threshold detecting circuit, a second input coupled to said output of said threshold detecting circuit via a one frame delay means, and an output coupled to said switching input of said filter means.

2. A decoder according to Claim 1, wherein the output of the OR gate is coupled to said switching input of the filter means via a pulse stretching circuit.

3. A decoder according to Claim 1 or 2, wherein said means for detecting lack of vertical correlation comprises delay means for providing signals simultaneously from five consecutive lines of the television picture, and means for summing said signals in predetermined proportions to provide an output signal the magnitude of which is related to the degree of non-correlation in the original picture.

4. A decoder according to Claim 3 including rectifying means coupled between the output of said lack of vertical correlation detecting means and said threshold detecting means.

5. A decoder according to any one of Claims 1-4, wherein said filter means comprises subtracting means for subtracting from the television signal, signal components within the chrominance frequency band; adding means for returning to the television signal combed high frequency luminance signal components derived from within the chrominance frequency band; and means coupled to said switching input for disabling said adding means.

6. A decoder according to Claim 5, wherein said disabling means includes a soft edge generator arranged to provide a gradual reduction of the value of the signal added by said adding means.

7. A decoder according to Claim 1 substantially as described herein with reference to the accompanying drawings.