GB2126822A

GB2126822A - Video signal processing

Info

Publication number: GB2126822A
Application number: GB08222134A
Authority: GB
Inventors: Ian Childs
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1982-07-30
Filing date: 1982-07-30
Publication date: 1984-03-28
Also published as: GB2126822B

Abstract

A video signal is subjected to combined horizontal/vertical, horizontal/temporal or horizontal/vertical/temporal filtering to improve the horizontal resolution at the expense of diagonal and/or temporal resolution. The signal is passed through a comb filter 8 with attenuation frequencies at multiples of line, picture or field frequency (as the case may be) and then sampled 10 with a sampling rate having a half- line, half-picture or half-field offset. High frequency components above the pass band of the transmission channel are thus carried as alias components in the gaps created by the comb filter. At the receiving end (Fig. 2) the HF components are regenerated by a similar sampler (21) followed by a comb filter (22) which removes these alias components. The process is limited to a high frequency section of the signal in order to provide a substantial degree of resolution enhancement whilst preserving compatability with existing receivers in the low frequency part of the signal. The process may be used in conjunction with systems where the high frequency luminance is transmitted out-of-band. <IMAGE>

Description

SPECIFICATION Video signal processing Much has been written recently concerning the possible improvement of resolution of television pictures in a (DBS) service without departing from a 625/50 transmission system. For example, the IBA have proposed (Lucas, K and Windram, M.D.

"Direct Television Broadcast by Satellitedesirability of a new Transmission Standard" IBA Report 116/81) a non-compatible system known as MAC (Multiplexed Analogue Components) which, while being bandwidth-limited to 4.5 MHz in the transmission channel, could achieve a horizontal resolution of 9 MHz by suitable pre-and post-filtering resolution of 9 MHz by suitable preand post-filtering. Much of this work is based on earlier studies by Wendland (Wendland, B., "Highdefinition television studies on compatible basis with present standards", EBU Technical Document No. GT-G4-044) which used similar filtering but was based on a PAL-compatible system (aithough sequentially scanned at the source and display).Although the Wendland system could, in theory at least, be received by present-day receivers the pictures would show interlace twitter in both horizontal and vertical directions because of the use of field-quincunx sampling at the source.

In contrast to this work, Oliphant (see our U.K.

patent application no 8121212 serial No. ) has suggested an extended PAL approach in which existing receivers can dispiay a luminance bandwidth restricted to 3.5 MHz with frequencies above this limit being shifted outside the original video band. Suitable receivers can recover this out-of-band signal and recombine it with the low frequency luminance.

Because only the low frequency luminance signal needs to be compatible with present-day receivers, the path is therefore open to carrying out signal processing on the out-of-band signal to extend the resolution still further. The present invention is concerned with such signal processing: such signal processing could involve sub-Nyquist filtering and sampling techniques using line, field or picture delays or various combinations of these.

According to one aspect of the present invention there is provided a method of processing a video signal having a first defined bandwidth for passing through a transmission or processing path having a second, lower defined bandwidth, comprising comb filtering at least the high-frequency portion of the signal above a defined frequency, the attenuation frequencies of the filter occurring at frequencies at which the signal has a low energy content, sampling the signal at a sampling rate such that alias components corresponding to the part of the video spectrum between the first and second defined bandwidths occur at the said attenuation frequencies over the range from the defined frequency to the upper frequency limit of the second defined band-width, and after transmission or processing, re-sampling the signal at the same sampling rate and passing it through a second, similar, comb filter.

The attenuation frequencies may for example, occur at multiples of line frequency, picture frequency, of field frequency, with the sampling rate having respectively a half line frequency, half picture frequency, or half field frequency offset.

In another aspect, the invention provides a method of processing a video signal, in which the signal is subjected to combined horizontal/vertical, horizontal/temporal or horizonta I/vertical/temporal filtering, the filtering operation being confirmed to part of the video spectrum lying above a defined frequency.

According to a further aspect of the present invention there is provided an apparatus for processing a video signal having a first defined bandwidth for passing through a transmission or processing path having a second, lower defined bandwidth, comprising a comb filter operable to filter at least the high-frequency portion of the signal above a defined frequency, the attenuation frequencies of the filter occuring at frequencies at which the signal has a low energy content, means arranged in operation to sample the signal at a sampling rate such that alias components corresponding to the part of the video spectrum between the first and second defined bandwidths occur at the said attenuation frequencies over the range from the defined frequency to the upper frequency limit of the second defined bandwidth, and means arranged in operation, after transmission or processing, to re-sample the signal at the same sampling rate and a second, similar, comb filter.

The invention may, if desired, be applied to systems such as described by Oliphant, where high frequency video signals are shifted outside the original video band. The defined frequency referred to above may correspond to the "crossover" point of this separation, but this is not essential, and, as will be seen below, the process of the present invention may be confined to a more restricted frequency range.

Some embodiments of the invention will now be described with reference to the accompanying, drawings, in which: Figures 1 and 2 are block diagrams of a video processing system at respectively, transmitter and receiver; and Figures 3 to 5 illustrate graphicaily the results obtained with the system of Figures 1 and 2 and modifications of this.

The transmitter-end processing circuitry shown in figure 1 has inputs for receiving a luminance signal having a bandwidth extending up to 7.5 MHz. A low-frequency luminance input 1 receives components of the luminance from 0 to 3.5 MHz whilst a high frequency input 2 receives the band 3.5 to 7.5 MHz and chrominance signals are fed to a further input 3. The high frequency luminance is supplied via processing circuitry, to be described, along with chrominance and low frequency luminance (via compensating one-line delays 4, 5) to a coder 6. The total delay (transmitter and receiver) in the h.f. luminance path is only one line because of the averaging process, and it is convenient to put the compensating delays at the transmitter.The coder 6 is of the type described in our U.K. patent application no 8121212 (serial No. ), which transmits the lowfrequency components and chrominance signals as a conventional (but limited resolution) signal compatible with present day receivers, but shifts the high frequency components to a frequency range outside the normal video bandwidth.

It is assumed that the h.f. channel has a frequency capability extending up to 5.5 MHz.

The high frequency luminance applied to the input 1 is processed before reaching the coder.

Firstly, it is filtered (if necessary) in a low-pass filter 7 to remove any components above 7.5 MHz (this filter may alternativley be positioned, as shown in dotted lines, immediatley before the sampler 7). It is then passed through a comb filter 8 in which it is averaged over a one line delay 9.

The video signal, being sampled at line rate, has a frequency spectrum 4 in which energy is concentrated at multiples of the line frequency: the information contained in the gaps is due to diagonal picture detail. The comb filter attenuates the signal in the gaps (i.e. at odd multiples of half the line frequency).

The signal is then sampled, with a sampling frequency of twice the bandwidth, with a half line offset-i.e. with the 5.5 MHz bandwidth referred to, the sampling frequency would be in the region of 11 MHz, the precise frequency being an odd multiple of half the line frequency. As a result, frequency components between 5.5 and~7.5 Hz give rise to alias components in the range 3.5 to 5.5 MHz, the energy being concentrated, due to the half-line frequency sampling rate offset, in the gaps of the original spectrum. Components above 5.5 MHz are of course not transmitted over the following signal path and may suitably be removed by a low-pass filter 11 before the coder 6.

Referring now to figure 2, the transmitted signal is decoded in a decoder 20. The high frequency luminance signal is then re-sampled by a sampler 21 with the same sampling frequency as before, thus causing aliasing of the original alias components into their original positions. Unwanted alias components are removed by a comb filter 22 (averaging over a line delay 23) whilst residual components above 7.5 MHz are removed by a low-pass filter 24.

The comb filter 8 at the transmitter serves, as already mentioned, to attenuate the (diagonal detail) luminance signal components which would otherwise give rise to spurious high-frequency components after regeneration at the receiver.

The extension of the horizonal resolution from 5.5 MHz to 7.5 MHz is thus obtained at the expense of a reduction in diagonal resolution. This situation is depicted in the stationary twodimensional frequency response shown in figure 3. The horizontal resolution is extended (to a maximum of 7.5 MHz) and diagonal resolution at an angle 260 to the vertical is reduced. Diagonal resolution at an angle of 450 to the vertical is also boosted. The system is essentially a filtering operation in the two-dimensional (horizontal/vertical) frequency domain.

The specific frequencies quoted above are merely examples, of course, the other possibilities exist, e.g. for a transmission channel capacity of 6 MHz a sampling frequency of 12 MHz would give a horizontal resolution of 8.5 MHz.

It is observed that the restriciton of the resolution extension only to the high frequency part of the signal avoids loss of diagonal resolution on coarse detail: moreover, in the shifted high-frequency system described, the compatibility of the low-frequency video input with existing receivers is not degraded by the presence of alias components introduced by the sampling process.

It is not necessary that the frequency range over which the system operates be tied to the crossover frequency of the shifted h.f. system: if the loss of diagonal resolution were considered excessive, a restriction of the horizontal resolution from 7.5 to 7 MHz (e.g. by the low-pass filter 7) accompanied by restriction of comb filter operation to frequencies above 4 MHz (by addition of high pass filters 12,25) would result in the modified response shown dotted in figure 2, where the diagonal resolution is not so impaired.

The transfer functions of the comb filters 8,22 are identical of the high-pass filters 12, 25, but their construction differs in that the filter 8 at the transmitter applies a one line delay to frequencies below this frequency whereas the receiver filter 22 does not, so that the total system delay is still only one line period.

The "castellated" response shown in figure 3 is in fact an idealised two dimensional representation of a three dimensional graph: variation of amplitude with vertical frequency along line 30-30 is in fact, sinusoidal in form.

This arrangement exhibits no movement degradation.

The line delays 2, 5, 4, 9 and 23 of the arrangement shown in figures 1 and 2 may be replaced by picture delays, the operation of the system being substantially the same except that the "gaps" into which the high frequency luminance components are aliased are those resulting from the temporal sampling of the scanning process. The sampling frequency for samples 10, 21 would thus have a half picture frequency offset. The information lost in the comb filter at the transmitter represents movement, and the results obtained are shown in figure 4 where (with 11 MHz sampling) the horizontal resolution is increased to 7.5 MHz, but on moving pictures the luminance resolution in the hatched area (down to 3.5 MHz) is lost.This likely to be visible on moving pictures even in the presence of camera integration, and, as before, a better compromise might be to restrict the filtering, for example to luminance frequencies above 4.5 MHz. This would allow the resolution to be extended to 6.5 MHz with 11 MHz sampling or 7.5 MHz with 12 MHz sampling.

Field delay filtering may be achieved in the system of Figs 1 and 2 by arranging for the delays to be equal to one television field and using a field-quincunx sampling structure. The effects of field delay filtering will be midway between those of the previous two sections. Static resolution will be as shown in Fig. 5 (for 11 MHz sampling, and, on moving pictures, some of the resolution in the hatched areas will be lost. The movement required to destroy this resolution will be twice as severe as in the case of picture delays, however, and it may this time be masked by camera integration. Field delay filtering may therefore, prove to be a good compromise between the systems described above.

It is interesting to comparethe field-delay system presented here and that proposed in the IBA paper referred to above. In addition to the advantage of compatibility the extended PAL approach is not subject to any movement degradation at frequencies below 3.5 MHz: the IBA system on the other hand, averages adjacent fields at all luminance frequencies and it is thought that this produces unacceptable movement degradation. On static pictures the IBA system produces 9 MHz of horizontal luminance resolution (the equivalent vertical resolution of 625 lines is only 7.5 MHz): the worst-case diagonal resolution is 5.7 MHz at an angle of 400 to the vertical. The system proposed here is more modest, increasing the horizontal resolution to only 7.5 MHz with the worst-case diagonal resolution being a little above 5 MHz.The total area inside the boundary of Fig. 5 is greater than that available from the IBA system, however, and this should result in the resolution being judged subjectively as better. Increasing the sampling frequency to 12 MHz will allow the horizontal resolution to be increased to 8.5 MHz or the worst-case diagonal resolution to be increased to 5.8 MHz.

Previous sub-Nyquist systems have usually used a sampling frequency of twice colour subcarrierfrequency in both studio and receiver. It has, therefore, provided relatively easy to regenerate the required sampling frequency in the receiver. The systems proposed here, however, use sampling frequencies appreciably higher than this and the question then arises as to how these freqencies may be regenerated in the receiver.

One technique would be to lock an oscillator to line syncs but it is felt that the accuracy of sampling phase produced by this method would not be sufficient A simpler technique would be to use a second burst signal to lock a separate bustlocked oscillator: this may cause problems with the system compatibility and also may give rise to problems of intermodulation in a satellite system.

A third solution would be to use simple derivations of subcarrier frequency which are nearly half-line, half-picture or half-field offset. For example, for system I, 2-1 5/32fsc is nearly 11 MHz (actually 10.94) and is sufficiently nearly half-line offset (within 0.7%). Similar derivations could be found for 12 MHz sampling and for the other sampling structures. The receiver will then produce the necessary sampling frequency by locking it to its own regenerated subcarrier. Some additional information might be required in an auxiliary data channel to fix the phase relationship between the two frequencies, for example, once per television field.

Claims (filed on 29.7.83) 1. A method of processing a video signal, in which the signal is subjected to combined horizontal/vertical, horizontal/temporal or horizontal/vertical/temporal filtering, the filtering operation being confined to part of the video spectrum lying above a defined frequency.

2. A method of processing a video signal having a first defined bandwidth for passing through a transmission or processing path having a second, lower defined bandwidth, comprising comb filtering at least the high-frequency portion of the signal above a defined frequency, the attenuation frequencies of the filter occurring at frequencies at which the signal has a low energy content, sampling the signal at a sampling rate of the video spectrum between the first and second defined bandwidths occur at the said attenuation frequencies over the range from the defined frequency to the upper frequency limit of the second defined band-width, and after transmission or processing, re-sampling the signal at the same sampling rate and passing it through a second, similar, comb filter.

3. A method according to claim 2, in which the sampling rate is substantially equal to twice the upper frequency limit of the second defined bandwidth.

4. A method according to claim 2, in which the attenuation frequencies occur at multiples of line frequency, and the sampling frequency has a half line frequency offset.

5. A method according to claim 2, in which the attenuation frequencies occur at multiples of picture frequency, and the sampling frequency has a half picture frequency offset.

6. A method according to claim 2, in which the attenuation frequencies occur at multiples of field frequency, and the sampling frequency has a half field frequency offset.

7. A method according to claim 2, in which the video signal comprises a low-frequency luminance portion and a chrominance portion contained within the second defined bandwidth and a high-frequency luminance portion extending above the upper limit of the second defined bandwidth.

8. A method of processing a video signal substantially as herein described with reference to the drawings.

9. Apparatus for processing a video signal having a first defined bandwidth for passing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. compromise might be to restrict the filtering, for example to luminance frequencies above 4.5 MHz. This would allow the resolution to be extended to 6.5 MHz with 11 MHz sampling or 7.5 MHz with 12 MHz sampling. Field delay filtering may be achieved in the system of Figs 1 and 2 by arranging for the delays to be equal to one television field and using a field-quincunx sampling structure. The effects of field delay filtering will be midway between those of the previous two sections. Static resolution will be as shown in Fig. 5 (for 11 MHz sampling, and, on moving pictures, some of the resolution in the hatched areas will be lost. The movement required to destroy this resolution will be twice as severe as in the case of picture delays, however, and it may this time be masked by camera integration. Field delay filtering may therefore, prove to be a good compromise between the systems described above. It is interesting to comparethe field-delay system presented here and that proposed in the IBA paper referred to above. In addition to the advantage of compatibility the extended PAL approach is not subject to any movement degradation at frequencies below 3.5 MHz: the IBA system on the other hand, averages adjacent fields at all luminance frequencies and it is thought that this produces unacceptable movement degradation. On static pictures the IBA system produces 9 MHz of horizontal luminance resolution (the equivalent vertical resolution of 625 lines is only 7.5 MHz): the worst-case diagonal resolution is 5.7 MHz at an angle of 400 to the vertical. The system proposed here is more modest, increasing the horizontal resolution to only 7.5 MHz with the worst-case diagonal resolution being a little above 5 MHz.The total area inside the boundary of Fig. 5 is greater than that available from the IBA system, however, and this should result in the resolution being judged subjectively as better. Increasing the sampling frequency to 12 MHz will allow the horizontal resolution to be increased to 8.5 MHz or the worst-case diagonal resolution to be increased to 5.8 MHz. Previous sub-Nyquist systems have usually used a sampling frequency of twice colour subcarrierfrequency in both studio and receiver. It has, therefore, provided relatively easy to regenerate the required sampling frequency in the receiver. The systems proposed here, however, use sampling frequencies appreciably higher than this and the question then arises as to how these freqencies may be regenerated in the receiver. One technique would be to lock an oscillator to line syncs but it is felt that the accuracy of sampling phase produced by this method would not be sufficient A simpler technique would be to use a second burst signal to lock a separate bustlocked oscillator: this may cause problems with the system compatibility and also may give rise to problems of intermodulation in a satellite system. A third solution would be to use simple derivations of subcarrier frequency which are nearly half-line, half-picture or half-field offset. For example, for system I, 2-1 5/32fsc is nearly 11 MHz (actually 10.94) and is sufficiently nearly half-line offset (within 0.7%). Similar derivations could be found for 12 MHz sampling and for the other sampling structures. The receiver will then produce the necessary sampling frequency by locking it to its own regenerated subcarrier. Some additional information might be required in an auxiliary data channel to fix the phase relationship between the two frequencies, for example, once per television field. Claims (filed on 29.7.83)

1. A method of processing a video signal, in which the signal is subjected to combined horizontal/vertical, horizontal/temporal or horizontal/vertical/temporal filtering, the filtering operation being confined to part of the video spectrum lying above a defined frequency.

through a transmission or processing path having a second, lower defined bandwidth, comprising a comb filter operable to filter at least the highfrequency portion of the signal above a defined frequency, the attentuation frequencies of the filter occurring at frequencies at which the signal has a low energy content, means arranged in operation to sample the signal at a sampling rate such that alias components corresponding to the part of the video spectrum between the first and second defined bandwidths occur at the said attenuation frequencies over the range from the defined frequency to the upper frequency limit of the second defined bandwidth, and means arranged in operation, after transmission or processing, to resample the signal at the same filtering rate and a second, similar, comb filter.

10. Apparatus according to claim 9, in which the sampling means samples at a sampling rate which is substantially equal to twice the upper frequency limit of the second defined bandwidth.

11. Apparatus according to claim 9, in which the comb filter comprises a delay of one line, and the sampling means samples at a sampling frequency which has a half line frequency offset.

12. Apparatus according to claim 9, in which the comb filter comprises a delay of one picture, and the sampling means samples at a sampling frequency which has a half picture frequency offset.

13. Apparatus according to claim 9, in which the comb filter comprises a delay of one field, and the sampling means samples at a sampling frequency which has a half field frequency offset.

14. Apparatus according to claim 9, in which the output of the sampling means is combined with the low frequency part of the video signal below the said defined frequency for transmission.

1 5. Apparatus for transmitting a video signal having a first defined bandwidth for passing through a transmission or processing path having a second, lower defined bandwidth, comprising a comb filter operable to filter at least the highfrequency portion of the signal above a defined frequency, the attenuation frequencies of the filter occurring at frequencies at which the signal has a low energy content, and means arranged in operation to sample the signal at a sampling rate such that alias components corresponding to the part of the video spectrum between the first and second defined bandwidths occur at the said attenuation frequencies over the range from the defined frequency to the upper frequency limit of the second defined bandwidth.

16. Apparatus for transmitting a video signal substantially as herein described with reference to Figure 1 of the drawings.

17. Apparatus for receiving a video signal, comprising an input, separating means connected to the input for separating out from the video signal two luminance component portions namely a low freuqency portion below a defined frequency and a high frequency portion, a comb filter connected to receive the high frequency luminance portion only from the separating means and constructed such that the attenuation frequencies of the filter occur at multiples of the video line, picture or field frequency.

18. Apparatus for receiving a video signal substantially as herein described with reference to Figure 2 of the drawings.