GB2112245A - Colour television systems - Google Patents

Abstract

In a colour television signal the luminance signal is modulated onto a subcarrier so as to occupy a frequency band above the chrominance band. The frequency spectrum comprises the sound signal modulated to occupy the lowest portion (1.5 MHz for 625/50 PAL System I), the chrominance signal modulated on a subcarrier of two-thirds of the existing conventional colour subcarrier (2.9 MHz), and the luminance signal modulated on what is currently the conventional colour subcarrier frequency (4.43 MHz). The whole signal occupies 10 MHz and is suitable for f.m. transmission such as for satellite broadcasting. As well as a coder (Figure 4) and a decoder (Figure 5), a transcoder (Figure 6) can be provided using frequency shift circuits to adapt a conventional receiver to the transmitted signal. <IMAGE>

Description

SPECIFICATION Colour television systems This invention is concerned with colour television systems.

In the existing broadcast television standards, when colour pictures are transmitted, the colour information is modulated onto a subcarrier within the luminance band. The chrominance and luminance signals thus share the same frequency space, so there can be cross-talk between them.

To reduce this cross-talk, most receivers restrict the bandwidths of both chrominance and luminance signals. Thus receiver design involves a compromise between four impairments: (i) crosscolour, the demodulation of high frequency luminance as spurious chrominance patterns; (ii) crossluminance, the high frequency luminance pattern resulting from imperfect suppression of the chrominance subcarriers; (iii) reduction of luminance resolution; and (iv) reduction of the chrominance resolution.

Figure 1 shows the spectrum of a conventional PAL signal by way of example. The luminance signal spectrum extends to a maximum video frequency f, which in the U.K. System I is 5.5 MHz, but the upper part of the band is shared with modulated chrominance signals, giving rise to crosscolour effects.

A demand now exists for a broadcast television system of higher quality than the conventional standards. The introduction of satellite broadcasting and cable television would permit the improvement of broadcasting standards and in particular would reduce the importance of transmitting the whole signal within the existing bandwidth.

Several coding systems for satellite broadcasting have been suggested in an attempt to improve on conventional PAL. These proposals assume frequency modulation (f.m.) of the main carrier, with bandwidths varying from 7 MHz to 10 MHz. It is widely assumed that the system will incorporate a multiple digital sound channel with a capacity of 2 Mbit/s, which would require a 1.5 MHz band in the signal. (All numerical values in this specification relate to 625/50 PAL television systems and should be adjusted for other systems).

In each case, the main features of interest are: (1) Picture equality, particularly in the respects of resolution and freedom from PAL cross-effects.

(2) Noise and distortion immunity when transmitted through the f.m. channel.

(3) Signal compatibility with current domestic and studio coding systems.

(4) Complexity required, particualrly in the domesic equipment, but also in the transmitting equipment.

The spectrum that would be used to modulate the f.m. carrier with conventional PAL, shown in Figure 1, has the advantages of signal compatibility and simplicity, with sound transcoding being the only major item in the conversion to existing receivers. However, the signal has little potential for improved receivers and would suffer from the familiar cross-effects and resolution limitations of PAL. Also, the signal is not weli suited to the modulation of an f.m.

carrier because so much of its power, the colour and sound signals, is present at the high frequency end of the band. In particular, noise introduced in the f.m. signal will result in a risingnoise spectrum when demodulated, so the colour signal components will be adversely affected.

Although experiments with injecting noise into luminance and modulated chrominance show broadly similar impairments (very dependent on picture content), when viewed on professional equipment, the same experiment with domestic receivers shows a much greater impairment in the chrominance. This is probably a result of the automatic colour control (A.C.C.) circuit which in domestic equipment senses the size of the burst and adjusts the colour gain accordingly, but which is not present in professional PAL decoders. It appears, therefore, that current domestic receivers are considerably more sensitive to noise in chrominance than in luminance. The 7 MHz bandwidth of this system would allow the signals to be transmitted at a slightly higher level than in wider bandwidth systems, with a consequent small noise improvement.

One proposal described in our U.K. Patent Application 81 21212 involves splitting the luminance band into high frequency and low frequency portions. The high frequency transmitted at the same frequency as the chrominance signals, but instead is transmitted above the sound signal in the 7 to 10 MHz region, as illustrated in Figure 2. This results in a compatible PAL signal in the baseband; this will lack luminance resolution, but will not suffer from cross-colour. Special receivers will add the h.f.

luminance, so resulting in a high quality YUV components signal. However, f.m. channel impairments may cause distortion when the luminance components are combined. Also, noise in the chrominance band will be a problem, as for conventional PAL.

Reference should now be made to the appended claims in which the present invention is defined.

The invention will be described in more detail, by way of example, with reference to the drawings, in which: Figure 1 (described above) schematicalry illustrates the frequency spectrum of a conventional PAL transmission; Figure 2 (described above) schematically illustrates the frequency spectrum of a PAL signal in accordance with our application 81 21212; Figure 3 schematically illustrates the frequency spectrum of a PAL television signal embodying the present invention: Figure 4 is a block circuit diagram of a PAL coder embodying the invention; Figure 5 is a block circuit diagram of a PAL decoder embodying the invention: and Figure 6 is a block circuit diagram of a converter for adapting a current domestic television receiver for use in accordance with this invention.

Figure 3 illustrates one system which embodies the present invention. The luminance and chrominance signals are transmitted separately, but with the chrominance signals on a subcarrier of lower frequency that that used for the luminance signal. The sound signal is transmitted on a subcarrier of still lower frequency.

Thus this system uses the same 10 MHz bandwidth as the system of Figure 2. Modulating the luminance as a single-sideband suppressed carrier signal onto the normal PAL colour subcarrier frequency (4.43 MHz) leaves the low frequencies available for the sound and colour signals. The colour signals can be amplitude modulated in the normal way, as double sideband, suppressed carrier signals on orthogonal subcarriers, but at 2/3 of the normal PAL colour subcarrier frequency. With some allowance for guard bands, this would provide approximately 1.2 MHz of useable chrominance bandwidth, which is in excess of that for a conventional PAL signal. The lower 1.5 MHz is allocated to digital sound signals.

The luminance bandwidth is only limited by the capacity of the f.m. channel. Therefore, the system could give greater resolution by using a wider channel. However, it might be argued that further luminance bandwidth is of little use without additional chrominance bandwidth. This would require a higher chrominance centre frequency, for example using the normal PAL subcarrier. The luminance carrier would consequently be pushed further up the band. The increased capacity is unlikely to be appropriate in the present application, but could be appropriate in other circumstances.

The concentration of signal power in the lower frequencies is beneficial for the f.m. channel.

Transmitting the digital sound signal in the lownoise part of the spectrum may allow a substantially lower sound modulation power to be used, thus reducing problems from intermodulation products. Also, placing the chrominance signal in the lower noise part of the spectrum is consistent with the relative effects of noise in chrominance and luminance discussed above in relation to Figure 1. The high frequencly luminance signal is statistically less coherent than chrominance and should, therefore, allow a greater degree of pre-emphasis to be used. It is also better suited to the use of variable pre-emphasis and pilot-tone companding systems. The separate component signals are less susceptible to f.m.

channel distortions than the mixed forms of signals used in other methods. Also, the separate component signals could be more easily noise reduced in a domestic receiver since much of the complication of composite signal noise reducers results from the presence of subcarrier confusing the movement detection.

A form of coder for this system is shown in Figure 4, and Figure 5 shows the block diagram of the decoder required in a high quality receiver. As shown, the system would give full 5.5 MHz luminance resolution and good chrominance resolution without any PAL cross-effects. With the simple demodulator shown in Figure 5, phase errors would cause Hanover bars. However, these could be suppressed in the normal way by substituting a delay line decoder.

The coder 10 illustrated in Figure 4 includes inputs 12, 14, 16 and 18 for receiving the luminance signal Y, the chrominance signals U and V, and the sound signal respectively. The luminance signal is applied to a 5.5 MHz low-pass filter 20 and the output of this is modulated in a modulator 22 onto what is conventionally termed the colour subcarrier fsc, i.e. a frequency of 4.43 MHz in the 625/50 PAL system, with single sideband suppressed carrier modulation. The output of the modulator 22 is applied to one input of an adder 26.

The U signal is applied to a 1.2 MHz low-pass filter 28 and is then modulated in a modulator 30 on a carrier which is at two-thirds of the frequency fsc. The modulated signal is applied to an adder 32. Similarly the V signal is applied to a 1.2 MHz low pass filter 34 and is then modulated in a modulator 36 onto a carrier of frequency 2/3 fsc but phase displaced by 90 degrees relative to the carrier in the U signal modulator 30.

Furthermore, the carrier to modulator 36 is phase shifted or inverted by 180 degrees on alternate television lines to provide the required phase alternation of the V signal. The output of modulator 36 is applied to the other input of adder 32, the output of which is applied to the other input of adder 26.

The sound signal at input 18 is applied to a sound modulator 38. The modulated sound signal is applied to one input of an adder 40, the other input of which receives the output of adder 26.

The output of adder 40 then contains the encoded PAL signal ready for transmission by application to an f.m. modulator.

The adders 26,32 and 40 are shown separately for ease of illusration but can be constituted by a multi-input combining circuit 42.

The decoder 50 shown in Figure 5 has an input 52 for receiving the output of an f.m.

demodulator. This signal is applied to a sound demodulator 54 which demodulates the low frequency sound signal for application to an output 56. The signal is also applied to a bandpass filter 58 which selects the 1.5 to 4.3 MHz band. The output of filter 58 is applied to a demodulator 60 which synchronously demodulates with reference to a carrier frequency of 2/3 fsc, and to a demodulator 62 which synchronously demodulates with reference to the same carrier frequency but phase displaced by 90 degrees and also inverted on alternate lines. The outputs of the two demodulators 60, 62 are applied to respective 1.2 MHz low pass filters 64, 66, and the outputs 68, 70 of which constitute the U and V outputs.Finally the input signal is applied to a 4.4 MHz high pass filter 72 the output of which is demodulated with reference to the frequency fsc in a demodulator 74 and applied to a 5.5 MHz low pass filter 76. The output 78 of this filter constitutes the luminance output Y.

Although the present signal is not directly compatible with PAL by low-pass filtering, as are other systems such as that of Figure 2, the normal PAL signal can be generated by frequency shifting the luminance and chrominance, as shown in Figure 6. Since this is similar to the system used in 'PAL' domestic video tape recorders, the system is clearly sufficiently compatible for domestic use and the techniques involved present no problems to equipment manufacturers. This transcoding process could provide a suitable point for the unscrambling of frequency shift encryption, if used. Also, a luminance notch could be included to reduce PAL cross-colour. The requirement for sound transcoding, common to all systems, is likely to result in greater complexity than the video transcoding system shown in Figure 6.However, the complexity of the present 'colour under' system is nevertheless not unduly great by comparision with other proposals.

The converter 100 of Figure 6 is designed to receive a signal embodying this invention from an f.m. demodulator and from it to derive an output signal 102 which can be applied to a domestic receiver. To this end the converter 100 provides an encoded PAL video signal 104 and a sound signal 106 which are applied to a u.h.f. modulator 108.

The signal from the input 110 is applied to a sound demodulator 112, to a bandpass filter 114, and to a high pass filter 116, which are respectively similar to the components 54, 58 and 72 in Figure 5. The sound demodulator 112 provides the sound signal 106. The output of the band pass filter 11 4 is applied to two modulators 118 and 120 which each receive a signal the frequency of which is one-third of the frequency fsc, these two modulating signals being phase displaced by 90 degrees. The outputs of circuits 118 and 120 are added in an adder 122 and band pass filtered in a filter 124 having a pass band of 3.3 to 5.5 MHz, i.e. the conventional chrominance band. This is applied to one input of an adder 126.

The output of filter 116 is applied to a demodulator 128 and lowpass filter 130 which correspond to the components 74 and 76 of Figure 5, and hence to the other input of adder 126. The output of adder 126 constitutes the PAL video signal 104.

While the transmission system described is almost directly compatible with the digital YUV signals to be used in studios, a PAL decoder would be needed to provide YUV signals from the current PAL system. This problem is common also to other systems such as that of Figure 2.

Thus it is seen that modulating the luminance signal separately into a band above the colour and sound signals can produce a high quality signal, will suited to the requirements of the f.m. channel.

The system is compatible with conventional PAL u.h.f. receivers through a frequency shifting process similar to that used in domestic video tape recorders. Special receivers could be relatively simple and could provide high bandwidth signals free from PAL impairments.

Claims (14)

Claims

1. A colour television system in which the luminance signal is modulated onto a subcarrier so as to occupy a frequency band above the chrominance band.

2. A colour television system according to claim 1, in which the sound signal occupies a frequency band below the chrominance band.

3. A colour television system according to claim 1 or 2, in which the chrominance signal is modulated onto a second subcarrier of lower frequency than the first-mentioned subcarrier.

4. A colour television system according to claim 3, in which the second subcarrier has a frequency of from 60% to 75% of the frequency of the first subcarrier above the baseband frequency.

5. A colour television system according to claim 3, in which the second subcarrier has a frequency of substantially two-thirds of the frequency of the first subcarrier above the baseband frequency.

6. A colour television system according to any preceding claim, in which the luminance signal is modulated with single sideband suppressed carrier modulation.

7. A colour television signal comprising, from the baseband frequency, a first frequency band comprising the sound signal, a second frequency band above the first and comprising the chominance signal, and a third frequency band above the second and comprising the luminance signal.

8. A colour television encoder comprising means for receiving luminance and chrominance input signals, first modulating means for modulating the luminance signal onto a first subcarrier, second modulating means for modulating the chrominance signals in phase quadrature onto a second subcarrier of lower frequency than the first subcarrier such that the luminance signal occupies a frequency band above the chrominance, and means for combining the outputs of the modulating means.

9. A colour television encoder according to claim 8, including means for receiving a sound signal, and third modulating means for modulating the sound signal onto a third subcarrier below the second subcarrier.

1 0. A colour television decoder comprising input means for receiving an input television signal demodulated from baseband frequency, first demodulating means for demodulating by reference to a first subcarrier frequency to provide a luminance signal, and second demodulating means for synchronously demodulating with reference to quadrature-displaced subcarrier signals of a second frequency less than the first subcarrier to provide chrominance output signals.

11. A colour television decoder according to claim 10, including third demodulating means for demodulating with reference to a third subcarrier frequency below the second frequency to provide a sound signal.

12. A colour television transcoder comprising input means for receiving an input television signal demodulated from baseband, demodulation means for demodulating by reference to a subcarrier frequency to provide a luminance signal frequency-shift means for shifting components modulated on a second subcarrier of frequency less than the first subcarrier upwardly by the amount of the difference between the first and second subcarriers, and means for recombing the outputs of the demodulation means and the frequencyshift means.

1 3. A colour television signal substantially as described with reference to Figure 3 of the drawings.

14. A colour television encoder substantially as described with reference to Figure 4 of the drawings.

1 5. A colour television decoder substantially as described with reference to Figure 5 of the drawings.

1 6. A colour television transcoder substantially as described with reference to Figure 6 of the drawings.