GB2075790A - Colour television transmission - Google Patents

Abstract

A colour television transmission system providing a colour television signal capable of being utilised in currently manufactured broadcast television receivers and in receivers adapted to produce a higher quality display. With the system the luminance information of a composite television signal is transmitted as an amplitude modulated carrier whilst the chrominance information is transmitted as a modulated subcarrier spaced from the carrier, the type of modulation depending on the type of transmission. The luminance modulated carrier has a lower sideband of greater bandwidth than the upper sideband and the chrominance modulated subcarrier is located above the carrier but outside the bandwidth of the upper sideband. The upper sideband contains sufficient luminance information for it to be utilised to produce a television picture of an acceptable quality on currently manufactured television receivers, whereas the adapted receivers use the lower sideband. <IMAGE>

Description

SPECIFICATION Colour television transmission The present invention relates to a colour television transmission system in which the luminance information of a composite television signal is transmitted as an amplitude modulated carrier whilst the chrominance information of said composite signal is transmitted as a modulated subcarrier spaced from said carrier. The invention also relates to television transmission equipment capable of use with the said colour television transmission system.

The current transmission of colour television signals in the United Kingdom and many other countries is performed as substantially a singe sideband transmission of luminance information into which is included a subcarrier modulated by the chrominance information. In the United Kingdom and some of the other countries the chrominance information is amplitude modulated in quadrature on the subcarrier according to the PAL system, a similarform of modulation being used in those countries which have adopted the N.T.S.C. system save that the phase of one of the colour components is not changed in alternate lines as in the PAL system The other system of colour transmission used is the S.E.C.A.M. system where one of two colour signals is alternately transmitted line-by-line as the frequency modulation of the subcarrier. The idealised spectrum for the signal as transmitted in the United Kingdom is as shown in Figure 1 of the accompanying drawings where the carrier frequency, which lies in the UHF band IV or V is indicated fc and the substantially single sideband transmission is shown to be an upper sideband where the luminance component extends to a point which is 5.5 Mllz. above the carrier frequency. At a point nominally 4.43 MHz. above the carrier frequency is positioned a subcarrierf5cwhich is modulated in the manner stated by the chrominance information. The sound carrier which is frequency modulated by the sound signal is located 6 MHz. above the carrier frequency.As will be seen from Figure 1- a small portion of the luminance signal extends below the carrier frequency to form the vestigial sideband but this portion, which forms what is left of the lower sideband contains insufficient information for it to be demodulated to produce sufficient luminance information for a satisfactory display on a television receiver.

Suggestions have been made to increase the bandwidth of the television signal transmitted and hence improve the quality of the colour display produced on a colour television receiver. One such signal which couldbe used for this purpose is shown idealised in the accompanying Figure 2 where the carrier frequency is again shown as at fc and could again be-located in the same UHF band IV or V or any other suitable frequency band.In Figure 2 it will be seen that the luminance information is transmitted as an amplitude modulated lower sideband-signal which extends to 10 MHz. below the carrier frequency fc Again a vestige of the luminance signal is transmitted above the carrier frequency but again it contains insufficient information to produce a satisfactory display on a television receiver. The chromi nance information is again transmitted modulated in the manner appropriate to the system a subcarrier fsc located nominally for the PAL system in the United Kingdom 4.43 MHz. above the carrier frequency fc but in this case the bandwidth of the modulated subcarrier is enlarged, this modulated subcarrier lying wholly outside the modulated luminance infor mation.As in Figure 1, the sound signal is again -transmitted as the frequency modulation of a sound carrier fs located 6 MHz. (for the United Kingdom) above the carrier frequency fc. Such a colour television signal would thus occupy a far greater bandwidth than the signal of-Figure 1 and there could be interference from the sound carrier in the adjacent (lower) ehannel. However, in general, in the United Kingdom transmitters operating on adjacent chan nels are geographically located distant from each other and thus such interference should not prove a major problem. It could be possible to use only.

alternate channels in the chosen frequency band for such transmission though this would reduce the number of channels-available and increase the likelihood of co-channel interference.

The improved quality signal of Figure 2 cannot be received by television receivers currently manufactured for receiving the signal of the type shown in Figure 1 and-thus a different construction of television receiverwould be required for the reception of the Figure 2-signal.

It is an object of the present invention to provide a colour television transmission system.which whilst providing an improved quality television signal would also allow that signal to be utilised on television receivers adapted to receive the television signal ofthe-type shown in Figure 1 above.

The invention provides a colour television trans mission system in which the luminance information of a composite television signal is transmitted as an amplitude modulated carrier whilst the chrominance information of said composite signal is transmitted as a modulated subcarrier spaced from said carrier, characterised in that the luminance modulated car rier has both upper and lower sidebands one of the sidebands being of greater bandwidth than the other of said sidebands whilst the chrominance modulated subcarrier is located on the same side of said carrier as the said other sideband but outside the bandwidth of the said other sideband, the said other sideband-containing-sufficient luminance information for the productiontherefrom, on reception, of a television picture of acceptable quality as herein defined.

The invention.also provides television transmis sion equipment for use with the television transmis sion system as claimed in any ofthe preceding claims comprising means for supplying luminance information,-means for amplitude modulating said luminance information onto a carrier to produce a luminance modulated carrier having both upper and lower sidebands, means for supplying a subcarrier -modulated by chrominance information, means for modulating said chrominance modulated subcarrier onto--said carrier, and means for combining said subcarrier modulated carrier with said luminance modulated carrier to produce a composite colour television signal for transmission in which one sideband of the luminance modulated carrier has a greater bandwidth than the other of said sidebands and in which the frequency relation of said subcar- rier to said carriers such that the modulated subcarrier is positioned outside the bandwidth of said other sideband, the said other sideband containing sufficient luminance information for the production therefrom, on reception of said composite signal, of a television picture of acceptable quality as herein defined.

By the expression "a television picture of acceptable quality" we mean one which approaches the quality as produced by the average domestic colour television receiver as presently available of a corresponding display screen size when viewed at a typical viewing distance appropriate to the screen size.

The invention further provides television receiving equipment for use with a television transmission system comprising means for receiving the transmitted composite television signal modulated on a carrier and for converting said signal into a corresponding composite television signal modulated on an intermediate frequency, means for applying said modulated intermediate frequency to a first path comprising a first intermediate frequency filter having a passband such as to pass those frequencies corresponding to the said one sideband whilst substantially rejecting those frequencies corresponding-to said other sideband and the chrominance modulated subcarrier, first demodulator means connected to the output of said first intermediate frequency filter for producing the luminance information from the said one sideband for display purposes, means for-applying said modulated intermediate frequency to a second path comprising a second intermediate frequency filter having a passband such as to pass those frequencies corresponding to the chrominance modulated subcarrier in said other sideband, second demodulator means connected to the output of said second intermediate frequency filter for producing the chrominance modulated subcarrier, and means for processing said chrominance modulated subcarrier for producing therefrom colour signals for display purposes.

With the present invention use is made of the fact that substantially all of the currently manufactured colour television receivers do notfully utilise all the luminance information transmitted by the television transmitters. A typical characteristic for the response of such a receiver is shown in the accompanying Figure 3 from which it will be seen that virtually only a small amount of the luminance information above about 3.5 MHz. is actually used for the reproduction of the television display.

The above and other features of the present invention will be more readily understood by a perusal of the following description having reference to Figures 4to 13 of the accompanying drawings in which by way of example: Figure 4 shows the idealised frequency characteristics of a television signal for use in a television transmission system according to the invention, Figure 5 is a block diagram of atelevision a television transmitter according to the invention, Figures 6 and 7 show idealised frequency response characteristics used to explain the operation of the transmitter of Figure 5, Figures 8 and 9 show idealised frequency response characteristics used to explain the operation of a modification to. the transmitter, of Figure 5, Figure loins a block diagram of a television receiver according to the invention, Figures 11 and 12.show idealised frequency response characteristics used to explain the operation of the receiver of Figure 10, and Figure 13 shows an idealised frequency response characteristic used to explain the operation of a modification to the receiver of Figure 10.

Referring to Figure 4 there is shown the idealised frequency characteristic of a television signal for use with the colour television transmission system according to the invention. As with Figure 2 it will be seen that the luminance information is transmitted as the amplitude modulation of a carrierf,the lower sideband of which extends to 10 MHz. below the carrier frequency. The chronninance information is again transmitted with the appropriate modulation of a subcarrier fsc located nominally (for the United Kingdom) 4.43 MHz. above the vision carrier fc but has the same bandwidth as that of the corresponding modulated subcarrier shown in Figure 1.In Figure 4 it will be seen that the upper sideband of the luminance information is not substantially reduced as in the case of Figure 2 but is extended to substantially the edge of the chrominance modulated subcarrier, this upper sideband being approximately 3.5 MHz. in width which is approximately one third of the bandwidth of the lower sideband.

The television signal transmitted could be received by a television receiver specially constructed to receive the full 10MHz. bandwidth lower sideband lu,minance information and the separate chrominance information whilst rejecting the luminance information contained in the upper sideband.

However, it would still be possible for television receivers adapted to receive the television signal of the type shown in Figure 1 to utilise the signal shown in Figure 4 and produce a picture of an acceptable quality from this signal. Such receivers would de modulate and operate substantially on the upper sideband of the amplitude modulated luminance information and that portion of the lower sideband of the amplitude modulated luminance information corresponding to the vestigial sidebands, the chrominance-modulated subcarrier being demodulated in the usual manner, This system thus has the advantage that a high quality television signal could be transmitted and received as a high quality signal in specially constructed television receivers to produce a high quality display but at the same time it would still be possible for conventional television receivers to produce a display of acceptable quality from the high quality signal.

Figure 5 shows a block diagram of a television transmitter for producing the modulated luminance and chrominance portions of the frequency charac teristicshown in Figure 4. In Figure 5 the reference 1 indicates an input terminal for receiving a luminance signal containing luminance signal containing infor mation normally in the form of a voltage whose amplitude varies with the light level from a scene and which may be obtained from a television camera tube system or other suitable source. The luminance signal will also contain the necessary line and field synchronising pulses.The luminance signal is ap plied from the input 1 to an amplitude, modulator 2 where the luminance information is amplitude mod ulated onto an intermediate frequency f1, oscillations at this frequency being applied to the amplitude modulator 2 from an oscillator 3. The frequency of the oscillations from oscillator 3 could be typically 38.9MHz. for television transmissions in the United Kingdom for the existing channel frequency alloca tions within the UHF bands IV and V. The-reference 4 indicates a second input terminal which receives the -chrominance signal whose two components R-Y and B-Y are already amplitude modulated in quadrature.

on-the subcarrier of nominally 4.43 MHz in accord ance with the PAL standard together with the colour reference burst. This method of modulation and the subcarrier frequency is for the United Kingdom; for other countries the method of modulation and subcarrier frequency might be different. The chromi nance signal is applied from the input 4to an amplitude modulator5 where this signal is ampli tude modulated with suppressed subcarrier onto the intermediate frequency fj, the oscillations for this frequency being applied to the modulator 5 from the .oscillator 3.

The modulated output from the amplitude mod 'ulator 2 is applied to a first sideband filter 6 which for the transmission according to Figure 4 has an idealised characteristic as shown in Figure 6 from which itwill be seen that the filter 6 has a passband which is a mirror image to that shown for the luminance information Figure 4, namely that it extends from 10 MHz. above the intermediatefre -quency fito 3.4 MHz. below the intermediate fre quency.The modulated output from the amplitude modulator 5 is applied to a second sideband filter 7 which for the transmission according to Figure 4 has an idealised characteristic as shown in Figure 7 from which it will be. seen that the filter7 has a passband -which is-a mirror image to that shown for the chrominance information in Figure 4, namely that it extends between 3.4 and 5.5MHz. below the inter mediate frequency fi. The outputs from the.sideband filters 6 and 7 are applied-to respective inputs of a signal combining circuit 8.The composite television -signal formed from the luminance and chrominance -signals modulated on the. intermediate frequency at the output of the combining circuit 8 is applied to an input of a frequency changer circuit 9 to a further input of which are applied oscillations from an oscillator 10 at a frequency which is equal to the sum of the desired carrier frequency fc p!us that of the intermediate frequency fi. The selected outputfrorn the frequency changer 9 is at the carrier frequency fc and which is modulated by the composite television signal-of the type-shown-in Figure 4, this output being applied to a power amplifier stage 1.1 for amplification prior to application to a transmitting aerial 12.The sound signal shown in Figure 4will be generated separately and coupled to the feed to the aerial 12 for transmission with the composite televi sion signal.

It would be envisaged that transmission having the characteristic shown in Figure 4 could be transomitted until such time as television receivers of current design for receiving transmission of the characteristic typically shown in Figure 1 have been phased out. The transmission produced by the transmitter shown in Figure 5 could then be changed such that it then has the characteristic as shown in Figure 2. This could be achieved by replacing the sideband filters 6 and 7 by filters having the ideaiised characteristics shown in Figures 8 and 9 respective ly, or by modifying the passbands of the existing sideband filters. The characteristics shown in Fi gures 8 and 9 correspond to the relevant portions of the characteristic of Figure 2.

Figure 10 shows a television receiver for receiving the higher quality television signal of Figure 4. The composition television signal is received in the usual manner by a receiving aerial 13 and applied to a tuner 14 where the signal is changed in frequency to an intermediate frequency or say 48 MHz, this higher than normal intermediate frequency being chosen to improve image rejection with channels in the same group. The resulting modulated intermediate fre quency.is applied to separate paths for the lumi nance and chrominance information, the first path comprising a first i.f. filter 15 which passes only the wanted luminance modulated signals whilst reject ing the chraminance modulated, signals.The pass band chracteristic for this filters shown idealised in Figure 1.1 and because of the frequency change operation inthe tuner 14 the characreristic is a mirror image of the required portion of the luminánce signal present in Figure 4. From Figure 1'1 it will be seen that the response is such as to substantially reject the components present in the upper sideband of the cha,racteristicpf.Figure 4 i.e. a virtual rejection of the luminance, chrominance and sound compo nents above the carrier frequency. The filtered luminance modulated signal is applied from the i.f.

filter 15 through a first i.f. amplifier 16 two an amplitude demodulator 20 whose output supplies the demodulated luminance information for utilisa- tion to produce a colour television display.

The second path comprises a second i.f. filter 18 which passes only the chrominance modulated signals whilst rejecting the luminance modulated signals. The passba nd cha racte ristic for filter 18 is shown idealised in Figure 12 and again because of the frequency change operation in the tuner 14this characteristic is a mirror image ofthe relevant- portion of the characteristic shown in Figure 4. From Figure 12 it will be seen that the passband for the chrominance signals is of the same width asthat for the corresponding signals in Figure 4 whilst also passing a component atthe i.f. frequency. The output of the filter 18 is applied through a second i.f.

amplifier 19 to a demodulator 20 capable of providing the amplitude modulated in quadrature chromi nance signals at subcarrier frequency (or as other wise modulated), the chromin3ncé modulated sub carrier being subsequently processed in one of several known ways to produce the colour signals for utilisation in producing the colour television display.

The characteristic in Figure 12 is such that the filter 18 would reject the upper sideband luminance information when the signal according to Figure 4 was transmitted. When transmissions having the characteristic of Figure 2 were commenced the filter 18 could have a bandpass characteristic similar to that shown idealised in Figure 13 having a larger bandwidth than that shown in Figure 12. The increased bandwidth for the chrominance information would produce a further increase in picture quality. The change in bandpass characteristic could be achieved by having for the filter 18 a filter whose passband can'be changed by means of a switch.

Such a switch could be manually operated and reset by an operator following the change in the transmission characteristic.

Claims (11)

1. A colour television transmission system in which the luminance information of a composite television signal is transmitted as an amplitude modulated carrier whilst the chrominance information of said composite signal is transmitted as a modulated subcarrier spaced from said carrier, characterized in that the luminance modulated carrier has both upper and lower sidebands one of the sidebands being of greater bandwidth than the other of said sidebands whilst the chrominance modulated subcarrier is located on the same side of said carrier as the said other sideband but outside the bandwidth of the said other sideband, the said other sideband containing sufficient luminance information for the production therefrom, on reception, of a television picture of acceptable quality as herein defined.

2. A colourtelevision transmission system as claimed in Claim 1, in which the sound signal is transmitted as the frequency modulation of a carrier, characterized in that the sound modulated carrier is located on the same side of the transmission as the said other sideband but outside the bandwidths of the said other sideband and said chrominance modulated subcarrier.

3. A colour television transmission system as claimed in Claim 1 or 2, characterized in that the said one sideband is the lower sideband whilst the said other sideband is the upper sideband.

4. A colour television transmission system as claimed in Claim 1,2 or 3 characterized in that the bandwidth of the said other sideband is substantially one third the bandwidth of the said'one sideband.

5. A colour television transmission system substantially as herein described with reference to Figure 4 of the accompanying drawings.

6. Television transmission equipment for use with the television transmission system as claimed in any of the preceding claims comprising means for supplying luminance information, means for amplitude modulating said luminance information onto a carrier to produce a luminance modulated carrier having both upper and lower sidebands, means for supplying a subcarrier modulated by chrominance information, means for modulating'said chrominance modulated subcarrieronto said carrier, and means for combining said subcarrier modulated carrier with said luminance modulated carrier to produce a composite colour television signal for transmission in which one sideband of the luminance modulated carrier has a greater bandwidth than the other of saidsidebandS and in which the frequency relation of said subcarrier to said carrier is such that the modulated subcarrier is positioned outside the bandwidth of said other sideband, the said other sideband containing sufficient luminance information for the production therefrom, on reception of said composite signal, of a television picture of acceptable quality as herein defined.

7. Television transmission equipment as claimed in Claim 6, in which said combining means comprises first filter means for receiving the luminance modulated carrier and for shaping the sidebands thereof to produce sidebands of the required unequal bandwidth, second filter means for receiving the subcarrier modulated carrier and for shaping the subcarrier modulated carrier to the required bandwidth, and a combining circuit for adding the outputs from said first and second filter means to produce said composite colour television signal.

8. Television transmission equipment substantally as herein described with reference to Figures 4, 5,6 and 7 of the accompanying drawings.

9. Television receiving equipment for use with a television transmission system as claimed in any of the preceding Claims 1 to 5, comprising means for receiving the transmitted composite television signal modulated on a carrier and for converting said signal into a corresponding composite television signal modulated on an intermediate frequency, means for applying said modulated intermediate frequency to a first path comprising a first intermediate frequency filter having a passband such as to pass those frequencies corresponding to the said one sideband whilst substantially rejecting those frequencies corresponding to said other sideband and the chrominance modulated subcarrier, first demodulator means connected to the output of said first intermediate frequency filter for producing the luminance information from the said one sideband for display purposes, means for applying said modulated intermediate frequency to a second path comprising a second intermediate frequency filter having a passband such as to pass those frequencies corresponding to the chrominance modulated subcarrier in said other sideband, second demodulator means connected to the output of said second intermediate frequency filter for producing the chrominance modulated subcarrier, and meansforpro- cessing said chrominance modulated subcarrier for producing therefrom colour signals for display purposes.

10. Television receiving equipment as claimed in Claim 9, in which switching means are provided for selectively increasing the width of the passband of said second intermediate frequency filter.

11. Television receiving equipment substantially as herein described with reference to Figures 10, 11, 12 or 13 of the accompanying drawings.