GB2330970A

GB2330970A - Bandwidth compression of a colour television signal

Info

Publication number: GB2330970A
Application number: GB9722984A
Authority: GB
Inventors: Simon William Day
Original assignee: Silverstone Holdings Ltd
Current assignee: Silverstone Holdings Ltd
Priority date: 1997-10-31
Filing date: 1997-10-31
Publication date: 1999-05-05
Also published as: GB9722984D0

Abstract

The luminance and chroma components of a television signal are separated and the luminance filtered to reduce its bandwidth, 2a, to below the chroma component 4a. One of the components is shifted to reduce the separation, in particular the luminance 2b is shifted to be above the chroma band 4b. The two components are recombined and modulated on an rf carrier, the frequency preferably being increased in a plurality of steps. The compressed signal is restored by separating the luminance and chroma and restoring the shifted component. The compressed signal may be transmitted in a 10MHz channel and is particularly suited to surveillance systems.

Description

METHODS AND APPARATUS FOR COMPRESSING AND DECOMPRESSING THE BANDWIDTH OF A COLOUR TELEVISION SIGNAL The present invention relates to methods of compressing and decompressing the bandwidth of a colour television signal, and to apparatus for compressing and decompressing the bandwidth of a colour television signal.

A standard colour television video signal has a base bandwidth of approximately 5.5MHz. Modulating this signal produces a bandwidth of at least 11MHz per channel.

It is sometimes desirable to reduce the bandwidth of the video signal, particularly for example when two or more channels are to be transmitted within a narrow transmission band. Compressing the bandwidth of the signal inevitably results in some loss of picture quality, but for certain applications, such as security or surveillance cameras or domestic closed circuit television systems, this may be acceptable.

The incentive for the present invention came from the announcement by the UK Radio Communication Agency in 1997 that a frequency band having a bandwidth of 10MHz would be made available for closed circuit television signals at a nominal frequency of 1.394GHz. This bandwidth is sufficient for a black and white video signal but is too narrow for a conventional colour video signal.

It is already possible to restrict the bandwidth of a colour television signal to approximately 8MHz by a process known as vestigial sideband filtering" (VSB) . Such a signal could be transmitted within 10MHz band mentioned above. However, as two or more such signals cannot be carried, the band is only suitable for use with single channel television systems. It is not therefore suitable for (e.g.) closed circuit surveillance systems having two or more cameras.

The possibility of compressing the signal further is limited by the fact that a normal colour television video signal contains luminance and chrominance components that are separated in frequency be approximately 4.4MHz.

Therefore, although the bandwidth of the luminance signal can be slightly reduced, there is a fundamental limit to how much the video signal can be compressed. It may in due course be possible to compress conventional television signals using digital techniques, but such techniques are not available yet and development costs are likely to be quite high.

It is an object of the present invention to provide a method of transmitting a colour television signal, and an apparatus for transmitting a colour television signal, that mitigates at least some of the afore-mentioned disadvantages.

According to the present invention there is provided a method of compressing the bandwidth of a colour television signal, wherein the luminance and chrominance components of the signal are separated, the luminance component is filtered to reduce its bandwidth, the frequency of at least one of said luminance and chrominance components is adjusted to reduce the frequency separation of said luminance and chrominance components, and the luminance and chrominance components are recombined, the overall bandwidth of the recombined signal being less than that of the original signal.

The method allows the overall bandwidth of the colour television signal to be compressed significantly whilst still providing a picture that is of sufficient quality for many applications, such as monitors and surveillance cameras. By compressing the bandwidth to less than 5MHz, two colour television channels can be transmitted within a 10MHz bandwidth such as the 1.394GHz band made available by the UK Radio Communications Agency for closed circuit television signals. This greatly increases the possible uses of that band. It should however be understood that the possible applications of the present invention are not limited solely to that particular band.

The invention may be implemented using simple analogue electronic circuits.

Advantageously, the luminance component of the signal is low pass filtered, the frequency of the filtered luminance component is increased to a frequency above that of the chrominance component, and the filtered and frequency adjusted luminance component is VSB bandpass filtered. The chrominance component of the signal may also be bandpass filtered, to reduce the bandwidth further.

Advantageously, the frequency of the recombined signal is increased to the desired RF transmission frequency and the signal is transmitted. The frequency of the recombined signal is preferably increased in a plurality of steps.

According to a further aspect of the present invention there is provided a method of decompressing a compressed colour television signal, wherein the luminance and chrominance components of the compressed signal are separated, the frequency of at least one of said luminance and chrominance components is adjusted to restore the original frequency separation of the luminance and chrominance components, and the luminance and chrominance components are recombined. Advantageously, the frequency of the filtered luminance component is decreased to a frequency below that of the chrominance component.

According to a further aspect of the present invention there is provided an apparatus for compressing the bandwidth of a colour television signal, the apparatus including separator means for separating the luminance and chrominance components of the signal, filter means for reducing the bandwidth of the luminance component, frequency adjuster means for adjusting the frequency of at least one of said luminance and chrominance components so as to reduce the frequency separation of said luminance and chrominance components, and recombining means for recombining the luminance and chrominance components, the arrangement being such that the overall bandwidth of the recombined signal is less than that of the original signal.

The filter means may include a low pass filter, and the frequency adjuster means may be arranged to increase the frequency of the filtered luminance component to a frequency above that of the chrominance component. The apparatus may include a VSB bandpass filter for filtering the filtered and frequency adjusted luminance component.

Advantageously, the apparatus includes a bandpass filter for reducing the bandwidth of the chrominance component of the signal.

Advantageously, the apparatus includes frequency converter means for increasing the frequency of the recombined signal to the desired RF transmission frequency.

Advantageously, the frequency converter means includes means for increasing the frequency of the recombined signal in a plurality of steps.

According to a further aspect of the present invention there is provided an apparatus for decompressing a compressed colour television signal, the apparatus including separator means for separating the luminance and chrominance components of the compressed signal, frequency adjuster means for adjusting the frequency of at least one of said luminance and chrominance components to restore the original frequency separation of said luminance and chrominance components, and recombining means for recombining the luminance and chrominance components.

The decompressing apparatus is able to restore the original frequency relationship of the luminance and chrominance components, so allowing the television picture to be viewed using a normal television receiver. The apparatus cannot, of course, restore the detail removed by the compression process, but the picture will nevertheless be adequate for many purposes. ~ An embodiment of the present invention will now be described, by way of example, ~ with reference to the accompanying drawings, of which: Figs. 1, 2, 3 and 4 are frequency diagrams illustrating how the components of a television signal are manipulated; Fig. 5 is circuit diagram of a transmitter circuit, and Fig. 6 is circuit diagram of a receiver circuit.

A conventional colour television video signal comprises separate luminance and chrominance components 2,4 as shown in Fig. 1. The luminance component 2 has an overall bandwidth of approximately 5.5MHz and gradually tapers off in amplitude above about 2.7MHz. The chrominance component 4 has a bandwidth of approximately 1.3MHz and is centred around a frequency that is approximately 4.4MHz above the base frequency of the luminance component 2.

According to the present invention, the overall bandwidth of the colour television signal is compressed by manipulating the signal as shown in Figs. 2 to 4. First, as shown in Fig. 2, the luminance component 2a is low pass filtered, so that it has a bandwidth of about 2MHz. This results in some loss of horizontal detail in the picture, but within acceptable limits for many applications.

Optionally, the chrominance subcarrier 4a may also be bandpass filtered, to a bandwidth of approximately 800kHz, which results in some slight loss of colour saturation.

Next, as illustrated in Fig. 3, the frequency of the luminance component 2b is increased by approximately 6MHz, so that it lies just above the chrominance component 4b.

This removes the gap- (seen in Fig. 2) between the luminance component and the chrominance component, thereby reducing the overall bandwidth of the signal to approximately 5MHz.

Optionally, the frequency adjusted luminance component 2b may also be VSB bandpass filtered to remove the lowest frequencies, thereby reducing overlap with the chrominance signal 4b.

Finally, the manipulated signal is converted up to the desired RF transmission frequency, as shown in Fig. 4.

This is achieved in two steps, first by converting the frequency to an intermediate frequency (IF) of typically 70MHz and bandpass filtering the resulting spectrum to remove the undesired mirror image produced by the conversion process, and then by converting up to the RF frequency of, for example, 1.394GHz and again bandpass filtering the signal. If two channels are to be transmitted within the 10MHz band mentioned above, the nominal frequencies of the channels may be set to 1.3915GHz and 1.3965GHz respectively, to provide adequate channel separation. These steps are conventional and will not be described in detail herein.

The transmitter circuit is illustrated schematically in Fig. 5 and includes a camera input port 10 which is connected to two parallel branches 12,14 for synchronous processing of the luminance and chrominance components of the video signal. The first branch 12 contains in series a low pass filter 16, a first frequency converter 18 and a VSB bandpass filter 20. The first frequency converter 18 is driven by a first local oscillator 22, which provides a drive signal to increase the frequency of the luminance carrier by approximately 6MHz. The second branch 14 includes a bandpass filter 24 having a pass band centred on a frequency of approximately 4.4MHz.

The first and second branches 12,14 are connected to input ports of an adder 26, the output port of which is connected to a second frequency converter 28, which is driven by a second local oscillator 30 at a frequency of approximately 65MHz. The output of the second frequency converter is connected via a bandpass filter 32 having a pass band at approximately 70MHz to a third frequency converter 34, which is driven by a third local oscillator 36 at the RF transmission frequency of, for example, 1.394GHz. The transmission frequency is set by channel selector 38. The output of the third frequency converter is connected to an RF bandpass filter 40 and via power amplifier 42 to RF output port 44.

The components after the point labelled "interface point" are conventional and may be replaced if, for example, the television signal is converted up to the transmission frequency in more or fewer than two steps, or if a different transmission frequency is selected.

In operation, a video signal from a camera is received at the camera input port 10. The luminance component passes into the first branch 12 through the low pass filter 16, which removes frequencies above about 2MHz, and is fed to the first frequency converter 18, which increases its frequency by about 6MHz. The signal then passes through the VSB bandpass filter 20, which removes the lowest frequencies from the signal.

The chrominance component of the signal passes into the second branch 14 through the bandpass filter 24, which limits its bandwidth to about 800kHz.

The modified luminance and chrominance signals are recombined in the adder 26, which produces the manipulated signal shown in Fig. 3 having a nominal bandwidth of about 5MHz. The signal is then converted up to an intermediate frequency of about 70MHz by the second frequency converter 28 and is filtered- by bandpass filter 32 to remove the mirror image that results from the conversion process. The signal is then converted again up to the RF transmission frequency of, for example, 1.394GHz by the third frequency converter 34 and similarly filtered by bandpass filter 40.

This signal is fed via power amplifier 42 to the RF output port 44.

The receiver circuit is shown schematically in Fig. 6 and comprises an RF input port 50, which is connected via amplifier 52 and RF bandpass filter 54 to frequency converter 56, which is driven by first local oscillator 58.

The output of the frequency converter 56 is connected via second amplifier 60 and 70MHz IF bandpass filter 62 to a second frequency converter 64, which is driven at a frequency of approximately 65MHz by a second local oscillator 66.

The components before the point labelled "interface point" are conventional and may be replaced if, for example, the television signal is converted down from the transmission frequency in more or fewer than two steps, or if a different transmission frequency has been selected.

The output of the second frequency converter 64 is connected to three parallel branches 68,70,72. The first branch 68 contains a 6MHz bandpass filter 74 and an FM demodulator 76, which provides output signals to an automatic gain control amplifier 78, which controls the gain of first and second amplifiers 52,60, and an automatic frequency control amplifier 80, which controls the drive frequency of the first local oscillator 58 (that frequency also being controlled by a channel selector device 82).

The second branch 70, which extracts the luminance component of the video signal, contains a vestigial sideband bandpass filter 84, a frequency converter 86, which is controlled by the 6MHz drive signal provided by the FM demodulator 6, and a low pass filter 88.

The third branch 72, which extracts the chrominance component of the video signal, contains a 4.4MHz bandpass filter 90 and an adder 92, which combines the output of the bandpass filter 90 with that of the low pass filter 88 to regenerate the original video signal. The output of the adder 92 is connected via amplifier 94 to video output port 96.

In operation, an RF input signal received at the RF input port 50 is filtered by RF bandpass filter 54 and converted down to a frequency of about 70MHz by the first frequency converter 56. The 70MHz signal is filtered by the 70MHz IF bandpass filter 62 and converted down again to a frequency of about 5MHz by the second frequency converter 64.

The luminance component of the video signal is extracted by the vestigial sideband bandpass filter 84 and its frequency is reduced by approximately 6MHz by frequency converter 86.

The chrominance component of the video signal is extracted by the 4.4MHz bandpass filter 90 and combined with the luminance component by the adder 92 to regenerate the original video signal. The regenerated signal is amplified by amplifier 94 and fed to the video output port 96.

The baseband luminance carrier is extracted by the FM demodulator 76 and used to provide for quasi-synchronous demodulation of the luminance component of the signal (via frequency converter 86), automatic frequency control of the RF local oscillator 56 (so as to maintain close accuracy of the output chrominance frequencies), and automatic gain control of the RF and IF amplifiers 52,60.

Various modifications of the methods and apparatus described above are possible and are intended to fall within the scope of the present invention. Examples of these possible modifications follow.

It is not essential that the luminance component of the video signal is increased in frequency and placed just above ,the chrominance component: it may instead be increased only slightly in frequency and placed just below the chrominance component. Alternatively, the chrominance component may be reduced in frequency and placed just above or below the luminance component.

All frequencies quoted in the above examples are illustrative only. Many other operational frequencies will work equally well in practice.

The reduction in bandwidth to approximately 5MHz is also illustrative only. The bandwidth can be reduced further by increasing the filtering severity. In practice, the reduction in bandwidth will be limited only by the amount of visual disturbance that is subjectively acceptable to the viewer. For most practical purposes, the minimum acceptable band width is likely to be between about 5MHz and 3MHz.

Claims

CLAIMS 1. A method of compressing the bandwidth of a colour television signal, wherein the luminance and chrominance components of the signal are separated, the luminance component is filtered to reduce its bandwidth, the frequency of at least one of said luminance and chrominance components is adjusted to reduce the frequency separation of said luminance and chrominance components, and the luminance and chrominance components are recombined, the overall bandwidth of the recombined signal being less than that of the original signal.
2. A method according to claim 1, wherein the luminance component of the signal is low pass filtered.
3. A method according to claim 2, wherein the frequency of the filtered luminance component is increased to a frequency above that of the chrominance component.
4. A method according to claim 3, wherein the filtered and frequency adjusted luminance component is VSB bandpass filtered.
5. A method according to any one of the preceding claims, wherein the chrominance component of the signal is bandpass filtered.
6. A method according to any one of the preceding claims, wherein the frequency of the recombined signal is increased to the desired RF transmission frequency and the signal is transmitted.
7. A method according to claim 6, wherein the frequency of the recombined signal is increased to the desired RF transmission frequency in a plurality of steps.
8. A method of decompressing a compressed colour television signal, wherein the luminance and chrominance components of the compressed signal are separated,- the frequency of at least one of said luminance and chrominance components is adjusted to restore the original frequency separation of said luminance and chrominance components, and the luminance and chrominance components are recombined.
9. A method according to claim 8, wherein the frequency of the filtered luminance component is decreased to a frequency below that of the chrominance component.
10. An apparatus for compressing the bandwidth of a colour television signal, the apparatus including separator means for separating the luminance and chrominance components of the signal, filter means for reducing the bandwidth of the luminance component, frequency adjuster means for adjusting the frequency of at least one of said luminance and chrominance components so as to reduce the frequency separation of said luminance and chrominance components, and recombining means for recombining the luminance and chrominance components, the arrangement being such that the overall bandwidth of the recombined signal is less than that of the original signal.
11. An apparatus according to claim 10, wherein the filter means includes a low pass filter.
12. An apparatus according to claim 11, wherein the frequency adjuster means is arranged to increase the frequency of the filtered luminance component to a frequency above that of the chrominance component.
13. An apparatus according to claim 12, including a VSB bandpass filter for filtering the filtered and frequency adjusted luminance component.
14. An apparatus according to any one of claims 10 to 13, including a bandpass filter for reducing the bandwidth of the chrominance component of the signal.
15. An apparatus according to any one, of claims 10 to 14, including frequency converter means for increasing the frequency of the recombined signal to the desired RF transmission frequency.
16. An apparatus according to claim 15, wherein said frequency converter" means includes means for increasing' the frequency of the recombined signal in a plurality of steps.
17. An apparatus for decompressing a compressed colour television signal, the apparatus including separator means for separating the luminance and chrominance components of the compressed signal, frequency adjuster means for adjusting the frequency of at least one of said luminance and chrominance components to restore the original frequency separation of said luminance and chrominance components, and recombining means for recombining the luminance and chrominance components.
18. A method of compressing the bandwidth of a colour television signal the method being substantially as described herein with reference to and as illustrated by Figs. 1 to 5 of the accompanying drawings.
19. A method of decompressing a compressed colour television signal, the method being substantially as described herein with reference to and as illustrated by Figs. 1 to 4 and 6 of the accompanying drawings.
20. An apparatus for compressing the bandwidth of a colour television signal, the apparatus being substantially as described herein with reference to and as illustrated by Figs. 1 to 5 of the accompanying drawings.
21. An apparatus for decompressing a compressed colour television signal, the apparatus being substantially as described herein with reference to and as illustrated by Figs. 1 to 4 and 6 of the accompanying drawings.