GB2124052A

GB2124052A - Reducing cross-colour in colour television decoding apparatus

Info

Publication number: GB2124052A
Application number: GB08213093A
Authority: GB
Inventors: Timothy Kevin Gale
Original assignee: Questech Ltd
Current assignee: Questech Ltd
Priority date: 1982-05-06
Filing date: 1982-05-06
Publication date: 1984-02-08
Also published as: DE3316080A1; GB2124052B; DE3316080C2

Abstract

A discriminator circuit to reduce the effects of errors in the separation of luminous from chrominance, in a colour television decoder comprises a bandpass filter (1) which is driven by the separated luminance signal and is centred at the colour subcarrier frequency, an envelope detector (2), filter (3), and a non-linear transfer circuit (4). The low frequency, conditioned luminance signal is applied to one input of each of two multipliers (5 and 6) which are inserted in the colour difference paths. The delay introduced by the elements (1) to (4) is compensated for in the colour difference paths by the delay lines (7 and 8) and in the luminance path by the delay line (9). The effect of the circuit is to attenuate the colour difference signals when a significant amount of high frequency information is present at the colour subcarrier frequency and which is therefore present in the chrominance channels. <IMAGE>

Description

SPECIFICATION Improvements in and relating to colour television decoding apparatus This invention concerns improvements in and relating to colour television decoding apparatus, and relates more especially to a circuit for the reduction of coloured interference on high frequency luminance detail.

As is well known, a PAL or NTSC colour television signal comprises a luminance (Y) signal and two colour difference or chrominance signals (U and V), which are quadrature modulated onto a subcarrier near the upper end of the bandwidth of the video signal.

Separation of the chrominance and luminance signals in such a manner as to retain the luminance bandwidth usually entails employing the well known technique of comb filter decoding, for example see Patent Specifications Nos. 2066 61 5A and 2067 872A.

Although comb filter decoding enables a luminance signal of greater bandwidth to be provided at the video output with improved separation of the luminance signal from the chrominance signals, improper separation of the luminance signal from the chrominance signals can still occur in certain circumstances. Failure of a decoder to correctly separate luminance from chrominance gives rise to two related distortions. The first is residual chrominance in the luminance channel, but the nett effect of this is usually minimal due to the inherent averaging of the dot pattern at the display device, and its common coincidence with luminance level transitions.A far more objectionable distortion is that known as "cross colour" where high frequency luminance detail has transferred into the chrominance channel at the separation stage and has then been demodulated by the chrominance demodulators to give rise to coloured patterning which is substantially unrelated to the original picture content.

It is an object of the present invention to provide a circuit for the reduction of cross colour distortion in a video output signal.

The present invention accordingly provides a colour television decoding apparatus which is arranged to derive from a composite colour television video signal separated luminance and colour difference signals, said apparatus having, at outputs arranged to provide said separated colour difference signals, variable attenuating means for attenuating the amplitude of said signals in accordance with the level of a signal at a control input of said variable attenuating means, means, including a bandpass filter, for deriving a signal containing the high frequency luminance information of the video signal, means for demodulating said high frequency luminance information signal in order to provide a signal of lower frequency of which the amplitude is representative of the presence or absence of high frequency luminance information, and means for connecting the output of said demodulating means to the control input of said variable attenuating means, whereby the amplitude of the decoded colour difference signals at the output of the decoding apparatus is reduced in proportion to the amount of high frequency luminance energy in the video signal.

Although the arrangement in accordance with the invention is of primary benefit when employed in conjunction with, or as part of, a comb filter type decoder, in which case the high frequency luminance information signal can be derived directly by bandpass filtering the separated luminance information signal at the output of the comb filter decoder, it will be appreciated that in principle the arrangement of the invention could be applied to any colour television decoding means wherein a signal containing the appropriate high frequency luminance information is present.

Preferably the means for demodulating the high frequency luminance information comprises an envelope demodulator to the output of which is connected a low pass filter for removing unwanted high frequency products of the envelope detector.

Advantageously, there is provided between the output of the demodulating means and the input to the variable attenuating means a circuit having a non-linear transfer function.

The non-linear circuit may, for example, be a threshold value circuit.

The invention is illustrated by way of example in the accompanying drawing, the single figure of which is a block circuit diagram of a device in accordance with the invention.

Referring to the drawing, it will be seen that the block on the left-hand side of the drawing represents a comb filter decoder of known type such as that described in more detail in Patent Specification No. 2 067 872A. Such a decoder provides at its respective outputs a separated luminance signal containing high frequency luminance information and separated "u" and "v" colour difference signals.

A circuit in accordance with one embodiment of the invention comprises a band pass filter 1 connected to a tapping at the luminance output of the decoder, the centre frequency of the bandpass filter being similar to that of the colour subcarrier used at the input to the decoder. An envelope detector 2 connected to the output of the bandpass filter 1 is arranged to envelope demodulate the separated high frequency luminance information signal provided by the bandpass filter 1, and a low pass filter 3 connected to the output of the detector 2 is arranged to remove unwanted high frequency products of the envelope detector.

The output signal from the filter 3 then passes through a circuit 4 having a non-linear transfer function before being applied to one input of each of two multipliers 5 and 6. The other inputs to the multipliers are connected to receive the decoded colour difference signals (U and V) from the outputs of the comb filter decoder. Thus the amplitudes of the decoded colour difference signals are reduced in proportion to the amount of high frequency energy found in the decoded luminance signal. Amongst the functions of the non-linear circuit 4 is the removal of any effects due to noise in the decoded luminance signal, to prevent the absolute desaturation of the colour difference signals at 5 and 6, and to condition the signal for the multiplier. In order to maintain the correct time relationship between the final output signals, delay lines 7, 8 and 9 are provided in the respective signal channels to compensate for the delay introduced by the circuit elements 1 to 4.

It will be appreciated that by the reference to "high frequency luminance information", in the above description is meant luminance information at a frequency which is sufficient to cause disturbing cross colour distortion.

One of the most common source materials which cause cross colour are check jackets.

The above described arrangement will detect the fine detail (high frequency luminance) of the jacket and reduce the colour difference signals proportionally, and hence the disturbing cross colour signals are suppressed.

Claims

1. A colour television decoding apparatus which is arranged to derive from a composite colour television video signal separated luminance and colour difference signals, said apparatus having at outputs arranged to provide said separated colour difference signals, variable attenuating means for attenuating the amplitude of said signals in accordance with the level of a signal at a control input of said variable attenuating means, means, including a bandpass filter, for deriving a signal containing the high frequency luminance information of the video signal, means for demodulating said high frequency luminance information signal in order to provide a signal of lower frequency of which the amplitude is representative of the presence or absence of high frequency luminance information, and means for connecting the output of said demodulating means to the control input of said variable attenuating means, whereby the amplitude of the decoded colour difference signals at the output of the decoding apparatus is reduced in proportion to the amount of high frequency luminance energy in the video signal.

2. A decoding apparatus as claimed in Claim 1 wherein the means for demodulating the high frequency luminance information comprises an envelope demodulator to the output of which is connected a low pass filter for removing unwanted high frequency products of the envelope detector.

3. A decoding apparatus as claimed in Claim 1 and 2, wherein there is provided between the output of the demodulating means and the input to the variable attenuating means a circuit having a non-linear transfer function.

4. A decoding apparatus as claimed in Claim 3, wherein the non-linear circuit is a threshold value circuit.

5. A colour television decoding apparatus substantially as described herein with reference to the accompanying drawing.