GB2035745A - Comb-filtering colour television signals - Google Patents

Abstract

Comb-filtering is used to remove alias components in a sub-Nyquist sampled digital PAL or NTSC colour television signal, the comb-filter comprising delay elements giving a delay D2 equal to one picture period accurate to a small fraction of a colour sub-carrier cycle. Stationary picture impairments introduced in systems employing a one-line or one-field delay are thereby avoided. The moving-picture impairments thus introduced can be reduced by an adaptive system which switches between or, better, combines with a weighted adder the effects of a one-line and one-picture delay under the control of a motion detector. The comb-filtering is normally applied to a high frequency band only. Where tandem filter arrangements are used, the picture delay is operative to effect comb-filtering on the h.f. band in both filter arrangements, but to provide a pure delay upon lower frequencies in one filter arrangement only. <IMAGE>

Description

SPECIFICATION Improvements relating to colour television This invention relates to colour television, and in particular to the transmission, processing, storage and recording of colourtelevision signals in digital form, e.g. when pulse-code modulated. The present invention is concerned with improvements in the inventions covered by our existing U.K, patent applications, numbers: 21118/74 (Serial NO 1511230), 56049/74 (Serial NO 1534268), 27265/76 (Serial NO 1524749) and 15811/77.

It is known that in general when a signal is sampled, a certain minimum sampling frequency is required if all the information in the signal is to be preserved. This minimum frequency limit is known as the Nyquist limit and is equal to twice the bandwidth of the signal; for a baseband signal the Nyquist limit corresponds to twice the highest frequency component in the signal.Any frequency component fp in the signal will combine with the sampling frequency fas to generate a number of components including one of frequency f,- f,. As long as the sampling frequency fs is at least twice the highest signal frequency fv, all the components fs - fp will be of frequency higher than the highest signal frequency f,, and can be removed by simple lowpass filtering. However, if the sampling frequency 5 falls below twice fv, then some of the components fs - fp will fall in the signal frequency band. These unwanted frequency components are called "alias" components.

For coding television signals it has previously been proposed to take advantage of the line periodicity of the signal to permit the use of sampling frequencies below the Nyquist limit, by making the sampling frequency equal to an odd integral multiple of half the line frequency, and using a special type of filter at the decoder. This filter is arranged to remove all frequencies above the highest video frequency fv, to pass all frequencies below fs - fv, where fs is the sampling frequency, and to act as a comb filter in the range fs - fv to fv by removing frequencies at or near (n + g) times the line frequency, where n is an integer.In this way, the system substantially removes the alias frequency components of the type fs - fp arising from a frequency fp in the original signal, since fop will tend to concentrate around integral multiples of the line frequency, f5 is defined as (n + 2) times line frequency, and thus the fs - fp components will tend to concentrate about odd integral multiples of half the line frequency, and are removed by the combfiltering.

It has previously been proposed to apply such a sub-Nyquist sampling system to monochrome television signals orto separated luminance and colour difference signals. However, our earlier patent applications describe how sub-Nyquist sampling may be used for: a) a composite PAL-coded colour television signal in which the sampling frequency is equal to precisely twice the colour subcarrier frequency. (Our patent applications numbers 21118/74 (Serial NO 1511230), 27265/76 (Serial NO 1524749) and 15811/77 b) the generation of a predominantly luminance signal component (Y) from a composite PAL-coded colour television signal in a digital Y, U-V/, U+V coding system. In this system the Y signal is sampled at twice the colour subcarrier frequency.

(Our patent application number 56049/74).

In each of these systems comb-filtering may be employed for the generation of sub-Nyquist sampled signals and is always needed in the subsequent re-construction of PAL-coded colour television signals in standard form. The proposals now to be described are concerned specifically with improvements to the form of comb-filtering. These will be described in relation to system (a) by way of example but they may similarly be applied to system (b) to give similar benefits.

Patent application number 2118/74 described preand post-sampling filters which each incorporate a one-line delay to produce the essentail comb response. Such filters introduce the following small impairments into the final decoded signal: (a) reduction of high frequency diagonal luminance resolution (b) reduction of vertical chrominance resolution (c) introduction of cross-talk between U and V chrominance components.

These impairments can be reduced, but not eliminated, by using comb-filters which incorporates a 313-line delay (i.e. a delay close to one television field period) rather than one-line delays. This improvement is described in our patent application number 27265/76 (Serial NO 1524749). Application number 15811/77 (1524749) described a further modification to the instrumentation. The present application is concerned with a further improvement by means of which the above impairments may be eliminated.

The invention is defined in claim 1 below.

The basis of the present invention is therefore that the comb-filters should each effectively employ a delay equal to precisely one picture-period, i.e. 625 lines (for a 625/50 system) accurate to a small fraction of a subcarrier cycle. Such filters will serve both of the necessary functions for which the 1-or 313-line period delay filters were previously proposed, namely the removal of luminance alias components and the reconstruction of standard PAL chrominance information.

The stationary pictures the impairments listed above are thereby eliminated. For moving picturecontent, however, other impairments would be introduced by such picture delay comb-filters, for example, colour and high-frequency luminance information will tend to 'leak' out of moving-picture areas.

A development of the basic proposal therefore is that adaptive comb-filtering may be employed whereby in general picture-delay comb-filters operate for sationary picture-content and line-delay comb-filters for moving picture-content. The adaptive filter may take one of two basic forms.

(a) An essentially bi-modal system in which, as described above, the filtering is switched between picture-delay combing for stationary picture-content and line-delay combing for moving picture-content.

(b) A more sophisticated arrangementment comprising a mixed-picture-delay, line-delay comb-filter system whose output is given by xL + (1 - x)P, where L is the output of a line-delay comb-filter and P is that of a picture-delay comb-filter, x denoting the proportion of L.

Afixed value of x could give a comprise between resolution loss (i.e. when xis large) and movement impairment (when xis small).

An adaptive strategy would vary x according to the degree of movement so that minimum impairment is visible in all picture areas.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 are block diagrams of two non-adaptive comb filters, and Figures 3 and 4 are block diagrams of two adaptive filters.

Figures 1 and 2 give block diagrams of two possible arrangements of one form of comb-filter employing a fixed-delay. Each of the filter arrangements shown has the required filtering characteristic as described in the introductory part of this document. The general principle of operation is described in our patent application number 27265/76 (Serial NO 15424749).

The delay D2would be equal to one line, according to our patent application number 21118/74 or 313 lines, according to our patent application number 27265/76 (Serial NO 1524749) but is equal to 625 lines, according to the present invention.

The delay D1 shown in Figures 1 and 2 is merely instrumental; it is a small delay which compensates for the finite delay of the filter L.P.F. so as to balance the signal delays through the two appropriate paths of the comb-filter.

In Figure 1, low frequencies are delayed by D1 only; in Figure 2 they are delayed by D1 + D2. L1 and L2 indicate signals on two different television lines as determined by the delay D2.

The object of the two configurations as shown in Figures 1 and 2 is that when two such filters (one or each type) are connected effectively in tandem, (as in pre-filtering and post-filtering in a sub-Nyquist sampling process), the overall group-delay for lowfrequency components and high-frequency components of the sampled signal is equalised; the overall group-delay then is, in fact, equal to the delay (2D, + D2). Bythis means the vertical registration of chrominance and h.f. luminance with l.f. luminance is preserved.

Conceptual arrangements for adaptive line/picture-delay comb-filters are shown in Figures 3 and 4.

In these diagrams, for simplicity, the separation of l.f. and h.f. components is indicated by simple low-pass and band-pass filters. In practice, precise separation could be achieved by the use of a single low-pass filter in combination with an adder and subtractor as indicated in Figure 1, or with a single band-pass filter by another re-arrangement, after the manner of that described in our patent application number 15811/77.

In Figure 3 (showing a filter analogous to that of Figure 1) it may be seen that the comb-filter characteristic can be controlled by the variable coefficient x. With the coefficient at the extreme valuex = 1 thecomb-filterembodiesa 1-line delay; at the extreme value x = 0 the comb-filter embodies a 625-line delay. At intermediate coefficient values the comb-filter characteristic corresponds to a "mixed" line/picture-delay comb-filter. In a simple bi-modal version, in which x = 0 or 1 only, the multipliers and adder would be replaced by a switch which would select either the input or the output of the 624-line delay.

A control signal for the coefficient value (or switch position) is derived from a movement detector. In the simple example shown this compares the input and output signals of the picture-delayformed by the 1-line and 624-line delays. For stationary pictures the inputs to the movement detector would be similar, whilst for moving picture-content the inputs would differ somewhat thus providing an output from a comparator from which the coefficient value (or switch position) control signal may be derived. In practice due account would have to be taken of the presence of signals modulated on the colour subcarrier. These signals would not repeat after a picture period even for stationary picture content. The simplest method would be to compare low-pass filtered signals, although a preferred method would use a more complicated filtering or re-modulation process.

A fixed value for the coefficient xis possible to give a mixed line/picture-delay comb-filter characteristic for minimising overall picture impairment as discussed above. In this case ciearly the movement detector and control system would not be required.

In Figure 3, low frequencies are delayed by D only where D is the group delay of the filters in the two paths. The high frequency group delay varies between 9 and 3122 line periods + D, according to the value of x.

Figure 4 shows a complementary version (analagous to Figure 2) of the adaptive comb-filter. Low frequencies are delayed by D + 625 line periods. The high frequency group delay varies between 3122 and 62421 line periods + D, according to the value of x. By using a pair of filters, one as in Figure 3 and one as in Figure 4, for pre- and post-filtering in a sub-Nyquist codec, the group-delay of the l.f. and h.f. signal components is equalised, thus avoiding vertical mis-registration of chrominance and diagronal h.f.

luminance with respecto If. luminance. Other configurations for pre- and post-filtering can be devised which have different group-delay characteristics from those of Figures 2 and 3, but which still form complementary pairs having equal low- and high-frequency overall group delay, and still embody the concept of mixed line- and picture-delay filter characteristics.

Ideally the pre- and post-filters should use the same value ofx, i.e. in an adaptive system the movement detectors should track each other. When this condition is satisfied the complementary pair of filters will cause exactly equal group-delay to lowand high-frequency signal components. This could be achieved by the use of a form of signalling from pre-filter to post-filter. However, the effects of any mismatch ofx would be small. For example, consider the worst possible mismatch when x = 0 in one filter and x = 1 in the other. A vertical misregistration of colour and high-frequency information with respect to low-frequency information will then occur, of distance E line out of 625 lines (for a 625-line system).This is smaller thant that introduced by a delay-line PAL decoder, for example, and this is for the extreme case. A degree of mismatching between the movement detectors of the pre- and post-filters may therefore be permitted, so that neither signalling ofxfrom the pre-filter system to the post-filter system, nor precise matching of the movement detector systems, is essential.

The delays used in the filters proposed may all be expressed in the fro m asu barrier periods, where n is an integer. It therefore follows that the filters may conveniently be implemented in digital form using a sample rate of 4 times the subcarrier frequency within them, in a similar manner to the proposal of our patent application number 15811/77.

An invention has been described whereby for stationary picture-content, the inherent impairments which are associated with sub-Nyquist sampled video coding systems may be eliminated. The essential feature is that a picture-delay comb-filter is employed instead of the line-delay or field-delay comb-filters envisaged for example in our patent applications numbers 21118/74 and 27265/76 respectively.

To avoid the introduction of new impairments due to moving picture-content variants on the basic picture-delay comb-filter are proposed. In these the comb-filter consists of any of the following: (a) an adaptive bi-modal comb-filter system which employs a picture-delay for stationary picturecontent and a line-delay for moving picture-content, the adaptation being automatic; (b) an adaptive comb-filter system which progr,esses from a picture-delay characteristic to a linedelay characteristic according to the movement of the picture associated with the video signal; again the adaptation is automatically controlled; (c) a filter system which comprises a fixed combination of line- and picuture-delay comb characteristics such as to minimise overall picture impairment.

The invention is proposed with particular reference to the sub-Nyquist PAL signal sampling system but the principles may be generally applied to any other sub-Nyquist sampled television systems. In particular they could be applied to the luminance signal (Y) in the digital Y, U+VI, U-V processing system described in our patent application number 56049/74 (Serial NO 1534268). The advantage in this case is that the diagonal luminance loss incurred when the system is not used for 'reversible' PAL-to PAL processing, may be elminated.

Claims (8)

1. Colour television apparatus wherein a colour television signal is utilized in digital form with a sub-Nyquist sampling rate and the use of combfiltering to remove alias components, characterized in that the comb-filtering action is established employing a delay of one picture period accurate to a small fraction of a colour sub-carrier cycle.

2. Apparatus according to claim 1, wherein the delay is switched to one line period when motion is detected.

3. Apparatus according to claim 1, wherein a mixed delay represented by xL + (1 - x) P is employed where L and P represent line and picture delays and xis varied over a predetermined range in dependence upon picture movement, increasing with increase in picture movement, the predetermined range not exceeding the range 0 to 1.

4. Apparatus according to claim 1, wherein a mixed delay represented by xL + (1 - x) P is employed where Land P represent line and picture delays and xis a fixed value greater than 0 and less than 1.

5. Apparatus according to any preceding claim, wherein tandem filter arrangements are employed with combfiltering in only a high frequency band, the picture delay being operative upon the high frequency band to effect comb filtering in both filter arrangements and operating as a pure delay upon frequencies below the said band in one filter arrangement only.

New claims or amendments to claims filed on 21 Nov1979 Superseded claims 1 and 5 New or amended claims:- Claims 1 and 5-8

1. Colour television processing apparatus in which a colour television signal our a component thereof is utilized in digital form with a sub-Nyquist sampling rate and the use of a comb-filtering to remove alias components, wherein the combfiltering action is established employing a delay of one picture period accurate to a small fraction of a colour sub-carrier cycle.

5. Apparatus according to any preceding claim, wherein tandem filter arrangements are employed with comb-filtering in only a high frequency band, the picture delay being operative upon the high frequency band to effect comb-filtering in both filter arrangements and operating in one filter arrangement only as a pure delay upon frequencies below the said band.

6. A method of processing colour television signals or components thereof, in which the signal is utilized in digital form with a sub-Nyquist sampling rate and the use of comb-filtering to remove alias components, wherein the comb-filtering action is established employing a delay of one picture period accurate to a small fraction of a colour sub-carrier cycle.

7. Colour television processing apparatus substantially as herein described with reference to the drawings.

8. A method of processing colour television signals substantially as herein described with reference to the drawings.