GB2035751A - Reducing disturbances due to colour crosstalk in a PAL television signal - Google Patents

Abstract

A method is proposed for the reduction of disturbances due to colour crosstalk in a PAL colour television signal. For this purpose, at the PAL coder those components of the luminance signal are suppressed which lie in the direction of the U modulation axis of a colour carrier signal modulated in accordance with the PAL standard. In the PAL coder shown, the suppression is performed by demodulating the luminance signal Y in the direction of the U modulation axis in a demodulator 2, effectively subtracting the demodulated signal from the video frequency colour difference signal U in an adder 3, low pass filtering the combined signal at 4, modulating the combined signal onto the colour carrier in a modulator 5, and adding the resultant colour carrier frequency signal to the original luminance signal in the adder 8. <IMAGE>

Description

SPECIFICATION Method of reducing disturbances due to colour crosstalk in a PAL television signal This invention relates to a method of reducing disturbances due to colour crosstalk in a PAL television signal.

In a method described in the book "Farbfernsehen 2" by H. Schönfelder, published by Justus von Liebig-Verlag, 1966, pages 118 - intheluminancechannel of of a PAL coder.

However the presence of the notch filter causes a loss of a part of the definition information so that the picture resolution is reduced. Furthermore, as a result of group delay errors of the notch filter, overshoot effects occur in the luminance signal which impair the quality of the picture. Apart from this it has been shown that the selective action of the notch filter is not adequate for certain picture presentations.

An improvement is achieved by the method described in British Patent Specification 849 81. In this method a comb filter is used to reduce components of the luminance signal at those places in the frequency spectrum which are occupied by the components of the modulated chrominance signal subsequently added during coding. However, because these comb filters are very expensive and resolution losses are nevertheless detectable in the picture reproduction, PAL coders have in the past not been provided with a comb filter in the luminance channel.

According to the present invention there is provided a method of reducing disturbance due to colour crosstalk in a television signal coded in accordance with the PAL standard, wherein at the PAL coder certain spectral components of the luminance signal are reduced which lie within the frequency range of a colour carrier frequency chrominance signal which is added to the luminance signal such spectral components being substantially only those which give rise to colour crosstalk disturbance and which lie in the direction of the U modulation axis of the chrominance signal.

In contrast to the prior art, the invention possesses the advantage that in the frequency spectrum of a luminance signal only those disturbing components are suppressed which cause the relevant colour crosstalk (cross-colour).

The invention is based upon the recognition that in the luminance signal those colour crosstalk disturbance components which lie in the direction of the V modulation axis of a PAL modulated colour carrier are of necessity reduced by the nature of the PAL system, since in consequence of the line alternation switching of the V colour difference signals as well as the integrating action of the PAL delay line the disturbing components in the V direction are to a large extent eliminated in the received signal. The disturbing colour fringes visible in a colour television picture due to crosstalk may therefore be attributed primarily to disturbing components which lie in the direction of the U modulation axis of a colour carrier modulated according to the PAL Standard.Thus, those spectral components in the luminance signal which lie in the direction of V modulation axis and which evoke no significant colour crosstalk may be transmitted without modification. The luminance signal can therefore be reproduced without suffering any material resolution losses.

In an embodiment of the invention it is of particular advantage that by performing a simple synchronous demodulation of the luminance signal in the direction of the U modulation axis of the PAL colour carrier it becomes possible to suppress the above described disturbance components in the direction of the U modulation axis. After frequency converting these unwanted disturbance components into the original baseband range, and subsequent subtraction from the original luminance signal, disturbances due to colour crosstalk are reduced in a simple manner.

Afurther simplification of the method may be achieved in that, for the purpose of frequency conversion of the disturbance components into the baseband range, it is possible to use certain stages already available in a conventional PAL coder, such as the modulator for the U colour difference signal with a series connected low-pass filter. It is only necessary that the unwanted disturbance components be combined with reversed polarity to the U colour difference signal which is to be modulated.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a block schematic diagram of a PAL coder embodying the invention, Figure 2 are amplitude-frequency characteristics for explaining the operation of the block schematic diagram of Figure 1, and Figure 3 are vector diagrams also for explaining the operation of the block schematic diagram.

In the PAL colour television system a colour carrier is quadrature modulated with the colour difference signals U and V. The V modulation axis is switched through 1800 at line frequency for reducing the effect of phase errors. A PAL coder receives in the usual way a luminance signal and a chrominance signal channel, which are combined in an adding stage. In the chrominance channel the colour difference signal V is modulated with a V modulator and the colour difference signal U is modulated with a U modulator. To the U modulator there is delivered a colour carrier with the phase position 0 and to the V modulator there is delivered the colour carrier with the line alternating phase position of + 90 .

In Figure 1 there is shown a portion of a PAL coder, which has been supplemented by certain stages for performing the invention. In the block schematic diagram there are indicated only those stages which are necessary for an understanding of the invention. As will be described, the stages 11 and 12 are optional and for the present may be ignored. At a terminal 1 a luminance signal Y is delivered to a synchronous demodulator 2, following which there are connected an adding stage 3, a low-pass filter 4 and a modulator 5.

A colour carrier signal fscu available at a terminal 6 is delivered to the synchronous demodulator 2 and to the modulator 5, the phase position of this colour carrier signal coinciding with the U modulation axis of the PAL standard, whilst fine phase adjustment may be effected with the phase rotation member 13. To a further input (terminal 7) of the adding stage there is applied the video frequency colour difference signal U. To the inputs of a further adding stage 8 there are applied the original luminance signal Y after a compensation delay adjusted by the delay stage 14, the signal derived from the modulator 5, and at the terminal 9 further signals including the colour carrier frequency colour difference signal V. At the output terminal 10 of the adding stage 8 there is available a coded PAL colour television signal for transmission.

In the synchronous demodulator 2 the luminance signal Y coming from the terminal 1, the amplitude-frequency characteristic of which is shown in Figure 2a, is demodulated in the direction of the U modulation axis by means of the colour carrier signal fscu available at the terminal 6. The signal obtained at the output of the synchronous demodulator 2 contains those components which, as already described, are the primary cause of colour crosstalk. In the stage 3 these disturbing components are added in the inverted mode (i.e. effectively subtracted) to the video frequency colour difference signal U. In Figure 2b there is shown the corresponding amplitude-frequency characteristic of the signal derived by the addition and subsequent low-pass filtering in stage 4.By a subsequent amplitude modulation with carrier suppression in the modulator 5, the signal available at the output of the low-pass filter 4 is transposed into the frequency range of the colour carrier fscu, whereby the position of the disturbing luminance components is returned to that of the original baseband, Figure 2c. In the present practical example for performing the invention use is made of the stages 4, 5, 8 and 14 which are already available in a conventional PAL coder for the conversion of the video frequency colour difference signal U into a colour carrier frequency colour difference signal, the disturbing luminance components lying in the direction of the U modulation axis being removed by performing a subtraction operation on the colour difference signal U prior to its modulation onto the colour carrier.By this means it is possible to avoid a separate conversion of the disturbing luminance components into the colour carrier range and subsequent subtraction from the luminance signal Y.

The additional capital expenditure for the PAL coder lies solely in the provision of the synchronous demodulator 2 and adder 3. It is thus possible to omit the formerly installed colour carrier filters and ail-pass filters or comb filters for the purpose of reducing colour crosstalk.

Figure 2dshows the resulting frequency characteristic, which is obtained when those components are subtracted which cause the relevant cross-colour distubances.

In Figure 3a there is shown a complex plane. In this plane the V modulation axis is plotted upon the ordinate and the U modulation axis is plotted upon the abscissa. A vector of a selected luminance spectral line shown in the first quadrant of the complex plane is assumed to move at an angular velocity Ato. A projection of the vector onto the U axis corresponds to a demodulation in the U direction.Thus a vector according to the expression A cos (coo + Aw) t 1 is converted by demodulation into a resultant vector A cos Acot in accordance with A cos (coo + Aco)t cos toOt -2cos Acot + A2 COS (2w, + Aco)t The upper sideband is removed in the low-pass filter 4 of Figure 1.The transposition of the remaining vector component 1/2 A cos Acot into the original baseband range is effected in the modulator 5 by multiplication with the colour carrier cosw,t in the Y direction, whence A -2cosAcot cosw0t =A4cos (co0 - Aco)t +A4cos (cho0 + no)t After a gain compensation the following expression applies for amplitude modulation with suppressed carrier A cos (co0 - Aco)t+ A cos (cho0 + Aco)t in which the first term corresponds to the lower sideband and the second term corresponds to the upper sideband. Figure 3b shows the equivalent vector diagram of the expression. According to Figure 3c the resultant instantaneous value of the amplitude modulated signal in vector form lies upon the U axis. On the other hand in a subtraction from the baseband signal those signal components are produced which lie in the direction of the V axis.

A modification in the above method may be achieved by arranging that the luminance signal Y at the terminal 1 is not delivered direct to the synchronous demodulator 2, but is first differentiated in a differentiating stage 11 and subsequently amplitude evaluated in a threshold detector 12. This processing of the luminance signal has the result that the inventive method does not operate at small signal amplitudes in the luminance signal Y. It has been determined that at small signal amplitudes in the luminance signal only small disturbance components are generated, which in turn give rise only to slight colour crosstalk. For reasons of optimization and for the reduction of resolution losses in picture reproduction it is possible to disregard these small disturbance components.

Claims (9)

1. A method of reducing disturbance due to colour crosstalk in a television signal coded in accordance with the PAL standard, wherein at the PAL coder certain spectral components of the luminance signal are reduced which lie within the frequency range of a colour carrier frequency chrominance signal which is added to the luminance signal, such spectral components being substantially only those which give rise to colour crosstalk disturbance and which lie in the direction of the U modulation axis of the chrominance signal.

2. A method according to claim 1, wherein the luminance signal is demodulated in a phase-selective manner with the colour carrier signal for the colour difference signal U, and wherein the disturbance components obtained by the demodulation are subtracted from the original luminance signal after being transposed in frequency to the original baseband range.

3. A method according to claim 2, wherein the disturbance components obtained by the demodulation are added with inverted polarity to the video frequency colour difference signal U, and wherein after amplitude modulation with suppressed colour carrier the signal thus obtained is added to the original luminance signal.

4. A method according to claim 3, wherein the combined disturbance components obtained by the demodulation and the video frequency colour difference signal U are subjected to low pass filtering before amplitude modulation.

5. A method according to claim 2,3 or 4, wherein the luminance signal is synchronously demodulated in the direction of the U modulation axis of the colour carrier.

6. A method according to claim 2,3 or 4, wherein the luminance signal is demodulated in the direction of the U modulation axis of the colour carrier by means of a multiplying mixing device.

7. A method according to any one of claims 2 to 6, wherein prior to demodulation the luminance signal is differentiated and then amplitude evaluated.

8. A method as claimed in claim 1, substantially as described with reference to the accompanying drawings.

9. An apparatus for performing the method claimed in claim 1, substantially as described with reference to the accompanying drawings.