DE2521288A1 - METHOD AND APPARATUS FOR GENERATING A DIGITAL PAL COLOR TELEVISION SIGNAL - Google Patents

Description

PATENT LAWYERS

dr. ing. E. LiEBAU DIPL.ING. G. LIEBAU

89 Augsburg 22, Rilkestrasse 10

Our reference H 9723 / p (please specify when answering)

Your sign

The Marconi Company Limited Bush House, Aldwych London WC2 / England

and

Standard Telephones & Cables Limited

190 beach

London WC2 / England

Method and apparatus for generating a digital PAL color television signal

The invention relates to color television and more particularly to the transmission of color television signals in pulse form, ζ, B, pulse code modulated.

It is known that generally ¹ when a signal is sampled

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Bank code 720 700 01

a certain minimum attack frequency is required if all of the information carried by the signal is to be transmitted properly. This minimum frequency limit is known as the Nyquist limit and is twice the frequency of the highest frequency component in the signal.Each frequency component f in the signal, in combination with the sampling frequency f _{s, forms} a frequency component f - f, as long as the sampling frequency f is at least sp .s.

twice the highest signal frequency f is all components f_ - f of a frequency higher than

s ρ

the highest signal frequency is f and can be filtered through removed. However, if the sampling frequency f is below drops twice the value of f, some of the

Components f_ - f in the signal frequency band, this us ρ

desired frequency components are called "alias" components designated.

For the transmission of television signals, it has been proposed to exploit the line periodicity of the signal in order to enable the use of sampling frequencies below the Nyquist limit by making the sampling frequency equal to an odd integer multiple of half the line frequency and using a filter of a special type at the receiver, this one Filter should remove all frequencies above the highest video frequency f, let through all frequencies below f - f, where f is the sampling frequency, and act as a comb filter in the range f _g - £ to f _y in that frequencies with or near Cn + - | o Remove times the line frequency, where η is an integer. In this way the syärem removes the alias frequency components of the type f - f, which arise from the frequency f in the original signal, since f strives to concentrate by integral multiples of the line frequency, £ is defined as the Cn + ■ £) times the line frequency so that f - f the effort to concentrate around odd integers

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Multiples of half the line frequency and are removed by the comb filtering.

Such a scanning system has been proposed for Black and white television signals or to separate luminance and color difference signals, however the direct application of the system to color television signal mixtures so far because of its presence in the high-frequency part of the video signal of important low-frequency color information that is modulated onto the color subcarrier is not considered possible.

Within the scope of the invention, however, it was found that certain modifications of this method directly apply to PAL television signals can be applied. It is recalled that in the PAL signal, the chrominance signal consists of two components that are modulated onto the color subcarrier with a phase shift of 90 °, namely, the U component derived from a B-Y signal and the V component derived from an R-Y signal is derived and causes a phase jump of 180 ° between the lines. The subcarrier has a frequency that is approximately equal to an odd integral multiple of a quarter of the line frequency.

In the method according to the invention for generating a digital PAL color television signal from an analog PAL signal mix is the analog signal mix with one frequency sampled which is twice the PAL signal color subcarrier frequency is, and the sampling times are substantially one-eighth of a subcarrier period before (or possibly after) held the zero crossing points of the U component of the chrominance component of the PAL signal.

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subsequently changed

A device according to the invention for generating a digital PAL color television signal from an analog PAL signal mixture has a sampling device for sampling the analog PAL signal mixture with a frequency which is twice the PAL signal color subcarrier frequency, and for maintaining the sampling times spaced substantially one-eighth of a subcarrier period of the zero crossing points of the U component of the chrominance component of the PAL signal,

The invention is also directed to a device for regenerating an analog PAL color television signal from a digital signal which is generated according to the method characterized above, which device has a digital-to-analog handler and a filter for filtering the output of the converter to the Remove frequency components that are odd integer multiples of the line frequency and lie in the frequency range f - f to f, where f _{g is} the sampling frequency of the digital signal and f is the frequency of the highest stable frequency coraponent of the analog PAL signal,

In a preferred embodiment, the difference between the delayed and the undelayed signal is obtained at the receiving end point, which have the same amplitudes and a relative time delay of about one line period, so that the chrominance signal components are phase-shifted by 90 °, one half of the signal being the is obtained from the subtraction, for frequencies in the range fg - f _y to f is added to the delayed coder (undelayed) signal or to a signal derived therefrom in order to ensure the correct polarity and relative time delay. As a result, the signal finally obtained in the frequency range f _g - f to f _{y corresponds to}

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half the sum of the delayed and undelayed signal and is therefore subjected to a comb filter effect, while frequencies below f - f, delayed or undelayed, can easily be let through. If a delay line PAL decoder follows, the delayed signal is preferably used, while if a simple PAL decoder follows that the instantaneous signal is suitable. This preference is intended to increase the vertical misregistration be reduced to a minimum.

The preferred system described above can give good performance on most color television images, when the noise level in the original signal is low. In order to the system is fully satisfactory and there is no significant increase in noise or other levels seen in the picture Effects takes place, it is preferable that the PAL signal treated at the transmitter through a suitable filter before sampling will.

This filter is preferably of the same shape as the filter for the receiving terminal and gives a comb filtering effect at frequencies above f - f, its main function is the reduction of the interference or other components in the original analog signal, which frequencies at or close to an odd harmonic of half the line scan frequency to have. These components, if not removed, will become a more visible form in the system converted from interference so that its degradation is useful before scanning and coding. The pre-filtering has further advantages, most notably that they have a much broader tolerance in the accuracy of the temporal Control of the scanning process allows. When this pre-filtering is applied, the use of the deferred instead of the undelayed signal in the final totalization either at the receiving station or at the sending station

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approved, but not in both filters,

The invention is described in more detail below, for example in connection with the accompanying drawings although show:

Tig » 1 a somewhat schematic representation of the video frequency spectrum of a 625/50 PAL color television signalj

Fig. 2 shows a few periods of the subcarrier frequency to two adjacent rows;

3A is a block diagram of a simple form of one according to the invention Transmitting station j

Figure 3B is a block diagram of a preferred form of one transmitting station according to the invention with a pretreatment filter j

Fig. ¹ ^ a block diagram in a receiver for generating a PAL signal from the transmitted signal and

Fig. 5 are vector diagrams showing the mode of operation at the receiver.

Fig »1 shows the video frequency spectrum of a PAL television signal, in which the maximum video frequency f is about 5.5 MHz, the subcarrier frequency f about 4, U3 MHz and the sampling frequency f _g is selected according to the invention so that it is equal to twice the subcarrier frequency, i.e. , h, approximately, §, 86 MHz. Furthermore, the frequency f _s - f _{y is} C3.36 MHz), which, as can be seen, lies between the lower frequency limit of an essential chrominance information. These numeric values are appropriate for a 625/50 signal that is in

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Britain is the norm

Fig. 2 shows the phase adjustment according to the invention of the sampling for the U component of the chrominance signal, the V component also being indicated. The sampling times occur at phases 135 ° and 315 ° for the U component, where the 0 ° position is ¹ when this sine component of the auxiliary carrier waveform passes up through zero. The 45 ° and 225 ° positions are alternatives also suitable. As shown, as far as the chrominance signal is concerned, the preferred scanning positions in relation to U + V are on line 1 and UV on line 2,

As can be seen, the U component shifted in phase by about 90 °. This is due to the fact that the subcarrier frequency is about 283 3/4 times the line frequency, so that there is a change of 1 / U of a period between adjacent lines. It can also be seen that the V component executes a phase jump of 180 ° with respect to the U component between the two lines.

A simple form of a transmitting station according to the invention is shown in FIG. 3A, which has an input 9 which is connected to a low-pass filter 13, the cut-off frequency of which is equal to the highest video frequency f. The output of the filter 13 is connected to a sampler and analog-to-digital converter 11 which provides a digital output on the line. The sampling frequency f. With which the circuit 11 operates is kept exactly at twice the color subcarrier frequency f _QQ and the sampling times are kept at 45 ° from the zero crossings of the U-1 component of the chrominance signal.

From Fig. 2 it follows that in the case of the chrominance signal for

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an evenly colored area, the sampling values of both U and V in line 1 would also be obtained from samples exactly one line sampling period later from line 2. In FIG. 2, these scans take place vertically below the 135 ° and 315 ° scan points of line 1, but correspond to the scans at 45 ° and 225 ° in line 2. The chrominance signal is therefore unaffected if the arrangement is made in this way that it appropriately delays the mean value of the signal E ₁ from line 1 C) and the signal E "from line 2 with a time difference of one line period before sampling is carried out at points in time which correspond to the phases 135 ° and 315 ° on line 1 . In the case of the mean value signal 1/2 (E ₁ + Ε «), however, all odd harmonics of half the line frequency are removed. Such harmonics (including interference components) in the frequency range f - f generate additional alias components at the receiver within the same frequency range, which are integral harmonics of the line frequency. These components are relatively more noticeable as image disturbances and are not removed by the filter in the receiver. Another property of the signal 1/2 (E ₁ + E ₂ ) is, as can also be seen from FIG. 2, that the resulting subcarrier component of U or V or some combination has a phase which is a maximum or a minimum at the sampling times entpsricht »As a result, small timing errors have a much smaller effect on the sampling length than if it is sampled directly.

Shown in Fig, 3B improved arrangement shown to produce a digital signal which is almost identical to that which is obtained by directly sampling and encoding the PAL-Sign "ls at phases 135 ° and 31S ^Q but containing a Filters which on because the analog signal acts before sampling.

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In FIG. 3b, additional components are provided between the input 9 and the filter 13. These consist of a one-line delay 15 connected to input 9 and a subtracter 17, the non-inverting input of which is also connected to input 9 and the inverting input of which is connected to the output of delay 15. The output of the subtracter 17 is via a divider in the ratio 1; 2 "which" is labeled 19, is fed to a high-pass filter 21, which loads ^ frequencies from f _s - f _y up to at least f "Dt" output 4 ·· Filters 21 becomes an «· adder! 23, the other input of which is connected to the output <& f * delay 15 via an equalizing delay. The output of the adder 23 is the tilt »? Il fed,,

The purpose of the filter arrangement of FIG. 3B, represented by units to the left of adder 23 including the latter, is to produce 1/2 (E ₁ ♦ Ej) nü * over the upper frequency range, ie above f _g - f _yt for which it is beneficial. At lower frequencies, the * signal E ^ occurs at the output. The required Wijyfcüag is obtained by the fact that the video signal arriving at input 9 is fed to the delay circuit IS by one line diode and the difference between the input E ₂ and the delay output (representing the previous line) E ^ is used. The output of the subtracter 17 is halved in the attenuator 19 and fed to the high-pass filter 21 with the cutoff frequency f _g - f _y . After passing through the delay equalizer 25, the delayed signal E ^ is added as * £ «* compensation for the delay in the filter 21 to the di * **« output. The adder 2 3 then delivers E * f ^ l? ? r # (| U * iMMBi under f ₅ - f _y and 1/2 (E ₁ + E ₂ ) for higher frequencies. This output is passed through the low-pass filter 13, which rre ^ uensee

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half of the nominal maximum f away, the scanner and analog-to-digital converter 11 supplied. The sampling times are determined by a reference with twice the color subcarrier frequency controlled.

Although in this example an analog filtering before the Scanning is used, it will be apparent from the description given in connection with Fig. 2 that the same effect can be obtained if samples of the incoming signal with four times the subcarrier frequency (at the Phases 45 °, 135 °, 225 ° and 315 °) are available and the signals are processed digitally.

The arrangement shown in FIG. 4 serves to receive a signal which has been sampled at twice the subcarrier frequency at times which are phase-shifted by 45 ° with respect to the U component, as obtained in the arrangement according to FIG. 3B, and regenerate a standard PAL signal from this. The transmitted signal is received at 10 and fed to a digital-to-analog converter 12. The analog output of the converter 12 is fed to a one-line delay 16 via a low-pass filter IU with a cut-off frequency f. The delayed and undelayed signals E. and E2, which represent two temporally adjacent lines at each instant, are subtracted in a subtracter 18 to produce (E * _{-E 3} ), the amplitude of which is divided by two in an attenuator 20 and a high-pass filter or bandpass filter 22 is supplied, which passes frequencies from f _s - f to at least f. Finally, the output of the filter 22 is added to the signal E «at the output of the delay equalizer 2 & in an adder 24 in order to obtain the restored PAL video signal on a Lefcung 28. The delay equalizer 26 has a delay similar to that. Delay in filter 22 and therefore compensates for the effect

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of the latter.

The output of the converter 12 is a non-standard color video signal in which the phase of the submount on the 135 ° -315 ° axis is limited. A comb filter action ^ is over the area f - f through f required to select "alias" components to remove the luminance or brightness signal, and it is also necessary to adjust the chrominance signal to the to convert conventional PAL form. Both requirements are met if the average is over two times consecutive lines is used. As can be seen, the low frequency luminance components, i.e., below f - f passed from the filter IU- directly to the output, since in no output is obtained from the filter 22 in this frequency range.

In the frequency range f - f _y to £, the main luminance components are in the range of even integer multiples of half the line frequency, but since the sampling frequency f _s = 2f _sc = 567j f _L , where f _{L is} the line frequency, the alias components f _g - f mainly in the vicinity of odd integer multiples of half the line frequency. These are removed by the comb filter effect. In the frequency range fg - £ _y to f _v , the brightness and chrominance signals therefore consist of CE ₁ - E ₂ ) / 2 + E ₂ = (E ₁ / E ₂ ) / 2j that is the average over two lines, this gives the necessary Comb filter effect and the resulting signal is a sufficient approximation of the original signal,

The operation of the system according to FIG. 4 with regard to the The chrominance signal is shown in Fig. 5, which at (a) and Cb) separate the original U and V components or shows combined. The effect of scanning at 315 ° is

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_ ι,

shown at (c), where the factor k has the value 2 ~ 2. The delayed and undelayed sampled signals are added, but it is recalled (see Figure 2) that the act of delaying one line causes a relative phase shift of 90%. Therefore, the output for each line is derived from the delayed scanned signal for the previous line (d) plus the undelayed scanned signal for the current line Cc) as shown at (e). As can be seen, the vector combination (for uniform color areas) generates the resulting PAL carrier identical to that of the original signal in (b). If the input components U and V are the same when the two rows are combined, the output components are k I (U + V) + (UV) I = U and k ² ['(U + V) - (UV) 1 = V, if 2k ² = 1. The circuit shown in FIG. 4 does not remove the alias components of the chrominance signal, so that the effect of the filter as a divider stage in the ratio 1: 2 furthermore the mean value of the true and the alias components of the chrominance signal, thereby eliminating the effective doubling of the amplitude caused by such information aliasing.

For non-uniform areas, the treatment results in an average value of the color information in two consecutive ones Lines, however, the effect of this degradation on the color is extremely small.

The circuit arrangements according to FIGS. 3B and 4, as described, form (E ₁ + E ₂ ) / 2 by first _{forming (E 2} - E ^ / 2 or (E ^ - E ₂ ) / 2 and then E «, or, E ₂ , although other methods could also be used to form (E ₁ + E n) / 2. Furthermore, the necessary processing can be carried out digitally instead of in analog form

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If a delay is specified by one line, the actual value of the delay for a
Fine-tune optimal performance. The optimal value should be between exactly one line period and the next one
Value for this which results in a phase shift of the subcarrier with respect to that of the previous line of exactly 90 °.

Using a lower sampling frequency than that
normally used for mixed color image signals
a proportional reduction in the bandwidth required. Although the system and circuitry described above are designed for use with PAL color signals, they can also be used in connection with black and white television signals.

Claims;

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Claims (1)

Patent claims: / lJ Method for generating a digital PAL color television signal from an analogue PAL signal mix, thereby characterized in that the analog PAL composite signal with a frequency is sampled which is twice the PAL signal color subcarrier frequency, and the Sampling times are kept at a distance from one another, which is essentially one eighth of a subcarrier period from the zero crossing points of the U component of the chrominance component of the PAL signal. 2. Method according to claim 1, characterized in that the analog PAL signal is filtered before the sampling in order to reduce at least the frequency components which are odd integer multiples of the line frequency and are in the frequency range f - f to f, where f is the sampling frequency and f is the frequency of the highest essential frequency component of the analog PAL signal. 3 _# A method according to claim 2, characterized in 'that the filtering being achieved in daae the signal is substantially one line period is delayed and a signal is given via the above-mentioned frequency range is equal to the mean of the delayed and dee unverxögerten signal. 609848/0810 subsequently changed »Device for generating a digital PAL color television signal from an analog PAL signal mixture, characterized by a scanning device for scanning the analog PAL signal mixture at a frequency. Which is twice the PAL signal color subcarrier frequency, and for Maintaining the sampling times at a distance from one another which is essentially one eighth of a subcarrier period of the zero crossing points of the U component of the chrominance component of the PAL signal is » Device according to claim 4, characterized in that the A filter device is connected upstream of the scanning device, which serves to filter the analog PAL signal, to at least decrease the frequency components that are odd integer vxel times the zexlen frequency and in the frequency range f - f to f, where f is the sampling frequency and f is the frequency of the highest essential frequency component of the analog PAL signal is » Apparatus according to Claim 5, characterized in that the filter device has a delay stage in order to delaying the input signal by substantially one line period, and means for delivering a signal over the mentioned frequency range, which is equal to the mean value of the delayed and undelayed signal. 7, device according to claim 6, characterized in that the the last-mentioned device has a subtracter, 509848/0810 nars - 16 - subsequently changed which serves to form the difference between the delayed and the undelayed signal, also a filter to suppress the frequencies below f _s »f and a divider in the ratio 1: 2, which is connected to the output of the subtractor, and an adder for Add the filtered and halved signal to the delayed or undelayed signal. Device for generating an analog PAL color television signal from a digital one generated by the method of claim 1 Signal, characterized by an analog-digital converter and a filter device for filtering the output of the converter to remove frequency components that are odd integer multiples of the digit frequency are, and are in the frequency range f - f to f, where f the sampling frequency of the digital signal and f the Is the frequency of the highest essential frequency component of the analog PAL signal, 9, device according to claim 8, characterized in that the Filter means a delay stage for delaying the output of the converter by essentially one line period has, and a device for outputting a signal over the frequency range mentioned, the same is the mean of the delayed and undelayed signals. 10. Apparatus according to claim 9, characterized in that the last-mentioned device comprises a subtracter for 509848/0810 Formation of the difference between the delayed and the undelayed signal, a filter to suppress Frequencies below f - f and a divider in a ratio of 1: 2 connected to the output of the subtracter is »as well as an adder for adding the filtered and halved signals to the delayed or instant signal, 11, Method of regenerating an analog PAL color television signal from a digital signal which has been generated by the method according to claim 1, characterized in that that the signal is converted into an analog form and the analog signal is filtered to remove frequency components that are odd integers Are multiples of the line frequency and are in the frequency range f - f to f, where f is the sampling frequency of the digital signal and f is the frequency of the highest essential frequency component of the analog PAL signal is. 12, the method according to claim 11, characterized in that the filtering takes place in that the signal by substantially a line period is delayed and a signal is output over the mentioned frequency range, which equals the mean value of the delayed and undelayed signal. 509848/0810