GB2086175A

GB2086175A - Circuit for Discriminating SECAM Television Signals

Info

Publication number: GB2086175A
Application number: GB8128149A
Authority: GB
Original assignee: Victor Company of Japan Ltd; Nippon Victor KK
Current assignee: Victor Company of Japan Ltd; Nippon Victor KK
Priority date: 1980-09-21
Filing date: 1981-09-17
Publication date: 1982-05-06
Also published as: FR2490905B1; DE3137268C2; FR2490905A1; GB2086175B; DE3137268A1

Abstract

A discriminator circuit for a SECAM (Registered Trade Mark) color television receiver comprises a gate (8) for passing at least a portion of each color subcarrier of a received color television signal in response to a gate pulse to a band-pass filter (16) having a passband center frequency tuned to a trough of the characteristic crest-trough pattern of one of the color subcarrier spectrums of the SECAM system, so that the output of the band-pass filter occurs at one-half the line frequency if the received signal is in the SECAM system format. A frequency discriminator (18) provides a signal indicating that the received signal is in the SECAM format when it detects that the output of the band- pass filter occurs at one-half the line frequency to discriminate it from signals of other color television systems. <IMAGE>

Description

SPECIFICATION Circuit for Discriminating SECAM Television Signals The present invention relates to a circuit for discriminating signals of a SECAM (Registered Trade Mark) system station from signals of other color television systems.

In areas where color television signals of different systems are transmitted and receivers are so designed to operate compatibly with any one of such different signals, the receiver is provided with a circuit for discriminating between signals of different system formats to automatically switch it to an operating mode that conforms to the format of the received signal.

A prior art discriminator circuit which allows discrimination achieves the discrimination of SECAM format signals from other signals by extracting a color subcarrier in response to each horizontal synchronization pulse of the received signal. The extracted color subcarrier is applied to a band-pass filter having a passband of which the center frequency is approximately 4.5 MHz to provide a filtered burst signal to a frequency discriminator tuned to one-half the line frequency.

Since the color subcarrier of the SECAM system alternates between 4.25 MHz and 4.40625 MHz in response to each horizontal scan, the filtered burst signal is the 4.40625 MHz subcarrier which occurs at one-half the line frequency of the SECAM system, so that the discriminator provides an output if the received color television signal corresponds to the format of the SECAM system to automatically switch the receiver to operate in the SECAM mode.

However, because of the bursting nature of the output of the band-pass filter, the filtered 4.40625 MHz color burst has sideband components so that it yields a characteristic crest-and-trough pattern in the spectrum which overlaps with the spectrum of the 4.25 MHz subcarrier to such a degree than there is only a difference of about 6 dB between them at the maximum peak of each spectrum. Difficulty thus arises in discriminating between the two subcarriers of the SECAM system.

The above problem is eliminated by the invention by the use of a band-pass filter of which the passband center frequency corresponds to a trough of the spectrum of a selected one of the color subcarriers of the SECAM system. The level difference between the two spectrums at the trough of the selected spectrum is such that the output of the band-pass filter is rendered sharply distinguishable from the other spectrum.

According to a first aspect of the invention there is provided a method for discriminating signals of the SECAM color television system from signals of other color television systems, comprising the steps of: a) extracting at least a portion of each color subcarrier of a received color television signal in response to each horizontal synchronization pulse transmitted with the received color television signal, whereby the color subcarriers extracted in response to the alternately occurring horizontal synchronization pulses yield respective spectrums having sideband frequency components; b) filtering the extracted color subcarrier at a frequency corresponding to a trough point of one of said spectrums so that the filtered color subcarrier occurs at one-half the line frequency of said SECAM system if the received television signal is in the format of the SECAM system: and c) detecting when the filtered color subcarrier occurs at one-half said line frequency for indicating that the received signal is in the SECAM system format.

According to a second aspect of the invention there is provided a circuit for discriminating signals of the SECAM color television system from signals of other color television systems which comprises: means for extracting at least a portion of each color subcarrier from a received color television signal in response to each horizontal synchronization pulse transmitted with the received television signal, whereby the color subcarriers extracted in response to the alternately occurring horizontal synchronization pulses yield respective spectrums having sideband frequency components; filter means having a passband of which the center frequency corresponds to a trough point of one of said spectrum so that the output of the filter means occurs at one-half the line frequency of said SECAM system if the received signal is in the format of the SECAM system; and discriminator means for detecting when said output of the filter means occurs at one-half said line frequency to indicate that the received signal is in the SECAM system format.

In order to maintain the center frequency of the filter means so that it exactly corresponds to the trough of one of the spectrums it is preferable that the extracting means comprises: a phase locked loop for generating clock pulses at a frequencyfhxNo, where fh represents the line frequency and No is an integer, in response to each horizontal sync pulse; a first frequency divider for dividing the frequency of the clock pulse by a factor N1 to determine a period N1/(Noxfh) which initiates in response to the leading edge transition of the horizontal synchronization pulse; a second frequency divider for dividing the frequency of the clock pulse by a factor N2 to generate a gate pulse having a duration N2/(Noxfh) which initiates in response to the termination of the period N1/(Noxfh);; and a gate responsive to the gate pulse for passing at least a portion of the color subcarrier transmitted with the received color television signal to the filter means.

The discriminator circuit is preferably provided with a jitter compensator when the circuit is used in a video tape recorder in which signals of different color television systems are recorded to eliminate undesirable consequences arising from inherent time axis fluctuation of the video tape recorder.

The invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a block diagram of a discriminator circuit of the invention; Fig. 2 is a spectrum diagram of the fundamental frequency components of the color burst gated through the gate of Fig. 1; Fig. 3 is a waveform diagram associated with the block diagram of Fig. 1; and Fig. 4 is a block diagram of a jitter compensator according to the invention which eliminates jitter components contained in the gated subcarriers.

Referring now to Fig. 1, there is shown a discriminator circuit for discriminating signals of the SECAM color television system from signals of other color television systems such as NTSC and PAL. The discriminator circuit forms part of a SECAM color television receiver which is located in an area where color television signals of different system formats are transmitted from various sources to automatically switch the receiver to the SECAM reception mode. The discriminator circuit may also form part of a video tape recorder to discriminate the signal of recorded SECAM program from signals of other systems to automatically switch the recorder to the SECAM mode. This discriminator circuit comprises generally a gate pulse generator 3 connected to receive horizontal sync pulses supplied to an input terminal 2 from the sync separator (not shown) of the receiver.The output of the gate pulse generator 3 is connected to the control terminal of a gate 8 for extracting at least a portion of each color subcarrier from the received television signal which may be a SECAM or other signal. The SECAM signal, shown at 7 in Fig. 3, is applied after amplification to a bandpass filter 12 through an input terminal 10. The band-pass filter 12 filters out the luminance component and feeds the chrominance component of the received signal to a limiter 14 where the amplitude is limited to an appropriate level.

Each color subcarrier appears on the back porch of each horizontal sync pulse 20 extending to the point where the chrominance component occurs, as shown in Fig. 2. Responsive to each gate pulse at least a portion of the color subcarrier 26 is passed through the gate 8 in the form of burst to a band-pass filter 1 6 of which the passband center frequency is tuned to fc+(1/b) where fc is the frequency of one of the color subcarriers of the SECAM system and "b" represents the duration of the gate pulse and hence the duration of the gated color burst.

Because of the bursting nature the output of the gate 8 has adjacent frequency components.

Since the color subcarrier of the SECAM system alternates between 4.25 MHz and 4.40625 MHz at every other line scan, Fourier transformation of the gated color bursts indicates that they alternately yield spectrums A and B as illustrated in Fig. 2. It is seen from Fig. 2 that the spectrum A (associated with the 4.40625 MHz subcarrier) has a maximum difference in level with respect to the spectrum B at a trough point "c" of the spectrum B which is located between its maximum peak and the next peak on the higher frequency side of spectrum B.Because of the large difference in level, each spectrum can be definitely distinguished from the other, so that the band-pass filter 1 6 unambiguously passes the 4.40625 MHz color subcarrier at one-half the line frequency (i.e., 7.8125 kHz) to the frequency discriminator 1 8. The discriminator 1 8 detects when the band-passed color burst occurs at 7.8125 kHz and provides a signal to an output terminal 19 indicating that the received signal corresponds to the SECAM system format. It is obvious that the band-pass filter 16 may also be designed so that its center frequency corresponds to fc-(l/b). It is preferable, however, that the center frequency of the band-pass filter 1 6 be 4.25 MHz+(l/b) or 4.40625 MHz-(l/b).

It is seen from the above that the next higher frequency trough "c" occurs at a point which varies in frequency as a function of the duration "b" of the gated color burst. It is thus preferable that the duration "b" be kept constant under varying operating conditions in order to keep the gated color burst exactly in tune to the passband center frequency of the band-pass filter 1 6.

To this end the gate pulse generator 3 comprises a phase locked loop 32 including a phase comparator 34, a voltage controlled multivibrator or oscillator 36 and a divide-by-No frequency divider 38 (where No is an integer) which are connected in a closed loop in the order named. The oscillator 36 generates the clock pulses at a frequencyfhxNo (where fh represents the line frequency) and feeds to NAND gates 40 and 42. The NAND gate 40 takes its another input from the Q output of a flip-flop 46 having its set input connected to the input terminal 2. The NAND gate 40 is thus enabled in response to a logical 1 output from the 0 terminal of the flipflop 46 to pass the clock pulses to a divide-by-N1 frequency divider 48 so that the latter provides an output at a frequency (fh x No)/N 1.The output of the frequency divider 48 is coupled to the set input of a flip-flop 50 and also to the reset input of the flip-flop 46. Therefore, the flip-flop 46 returns to the reset condition after the elapse of an interval N1/(Noxfh) seconds from the time of occurrence of a horizontal sync pulse, generating a pulse 22 having a duration "a" which equals N 1/(Noxfh) at the Q output thereof (see Fig. 3).

Simultaneously with the termination of the interval N1/(Noxfh, the flip-flop 50 is triggered into a logical 1 state and enables the NAND gate 42 to pass the clock pulses to a divide-by-N2 frequency divider 52 generating an output at a frequency (fhxNo)/N2 which is applied to the reset input of the flip-flop 50. Therefore, the flipflop 50 produces a pulse 24 having a duration "b"=N2(fhxNo) seconds. The 0 output of flip-flop 50 is coupled to the control terminal of the gate 8 so that the latter is open for an interval N2(fhxNo) seconds after the elapse of an interval N1 (fhxNo) seconds from the time of occurrence of each horizontal sync pulse.

Due to the constant values of the frequency dividing factors No, N1 and N2 and the constant nature of the line frequency fh, the passband center frequency of the filter 1 6 is always maintained at the trough point "c" even though the circuit parameters are varied under differring operating conditions.

If the discriminator circuit forms part of a video tape recorder in which signals of different color television systems are recorded and reproduced, it is preferable that the discriminator circuit be provided with a jitter compensator as illustrated in Fig. 4 to eliminate subcarrier frequency jitter which arises as a result of fluctuations in time axis during playback mode.

The jitter compensator comprises a phase locked loop frequency multiplier 62 of a similar circuit configuration to the phase locked loop 32 of Fig. 1. The multiplier 62 is in receipt of horizontal sync pulses from the terminal 2 and multiplies its frequency by a factor of 272, for example. Since the sync pulse is also subject to frequency jitter, the input frequency of the phase locked loop 62 is fh+(Af/272) and its output frequency corresponds to fhx272 MHz+Af, where fhx272 MHz equals the lower subcarrier frequency 4.25 MHz.A first frequency converter 64 receives the 4.25 MHz+Af input from the phase locked loop 62 and a reference frequency preferably equal to one of the subcarrier frequencies of the SECAM system, for example, 4.25 MHz from a quartz oscillator 66 to provide frequency conversion so that its output frequency is a summation of frequencies 4.25 MHz+4.25 MHz+Af. The output of the frequency converter 64 is applied to a second frequency converter 68 where it is combined with the gated color burst from the gate 8 which alternates between 4.25 MHz+Af and 4.40625 MHz+Af. The second frequency converter 68 provides a frequency difference output so that the jitter components are cancelled out. The output frequency of the converter 68 thus alternates between 4.25 MHz and 4.40625 MHz and is applied to the band-pass filter 1 6 which is tuned to 4.25 MHz (or 4.40625 MHz)+(l/b). It is to be noted that the reference frequency of the oscillator 66 does not necessarily correspond to the lower or upper subcarrier of the SECAM system in so far as the center frequency of the band-pass filter 16 is tuned to the reference frequency.

Claims

1. A method for discriminating signals of the SECAM color television system from signals of other color television systems, comprising the steps of: a) extracting at least a portion of each color subcarrier of a received color television signal in response to each horizontal synchronization pulse transmitted with the received color television signal, whereby the color subcarriers extracted in response to the alternately occurring horizontal synchronization pulses yield respective spectrums having sideband frequency components: b) filtering the extracted color subcarrier at a frequency corresponding to a trough point of one of said spectrums so that the filtered color subcarrier occurs at one-half the line frequency of said SECAM system if the received television signal is in the format of the SECAM system; and c) detecting when the filtered color subcarrier occurs at one-half said line frequency for indicating that the received signal is in the SECAM system format.

2. A method as claimed in claim 1, wherein said frequency at which said color subcarrier is extracted corresponds to fc+(l/b) of fc-(I/b), where fc is one of the frequencies of the alternately extracted color subcarriers and "b" represents the duration of said extracted color subcarrier.

3. A method as claimed in claim 1 or 2, wherein said color subcarrier is extracted by passing the received signal through a gate circuit in response to a gate pulse which is generated by the steps of: generating clock pulses at a frequency which equals fhxNo where fh represents said line frequency and No represents an integer; dividing the frequency of said clock pulses by a factor N1 to determine a period N1/(Noxfh) which initiates in response to the leading edge transition of the horizontal synchronization pulse; dividing the frequency of said clock pulses by a factor N2; and generating said gate pulse having a duration N2/Noxfh) in response to the termination of said period N 1/(Noxfh).

4. A method as claimed in claim 1, 2 or 3, further comprising the following steps between the steps (a) and (b): generating a first signal at a frequency which is an integral multiple of the frequency of said horizontal synchronization pulse so that the frequency of the first signal equals the frequency of one of the subcarrier frequencies of the SECAM television system; generating a reference frequency signal; combining said first signal and said reference frequency signals to generate a second signal at a frequency which is a summation of the frequencies of the combined signals; and combining said second signal with said extracted color subcarrier to generate a third signal at a frequency equal to the difference between the frequency of said second signal and the frequency of said extracted color subcarrier to cancel out jitter components contained in said extracted color subcarrier to allow said third signal to be filtered in the step (b).

5. A method as claimed in claim 4, wherein said reference frequency corresponds to one of the frequencies of the extracted color subcarriers.

6. A circuit for discriminating signals of the SECAM color television system from signals of other color television systems, comprising: means for extracting at least a portion of each color subcarrier from a received color television signal in response to each horizontal synchronization pulse transmitted with the received television signal, whereby the color subcarriers extracted in response to the alternately occurring horizontal synchronization pulses yield respective spectrums having sideband frequency components; filter means having a passband of which the center frequency corresponds to a trough point of one of said spectrums so that the output of the filter means occurs at one-half the line frequency of said SECAM system if the received signal is in the format of the SECAM system; and discriminator means for detecting when said output of the filter means occurs at one-half said line frequency to indicate that the received signal is in the SECAM system format.

7. A circuit as claimed in claim 6, wherein the center frequency of said filter means corresponds to fc+(l/b) or fc-(l/b) where fc is one of the frequencies of the extracted subcarriers and "b" represents the duration of said extracted color subcarrier.

8. A circuit as claimed in claim 6 or 7, wherein said extracting means comprises: a phase locked frequency multiplier for generating a train of clock pulses at a frequency fhxNo, where fh represents said line frequency and No is an integer, in response to each horizontal synchronization pulse; a first frequency divider for dividing the frequency of said clock pulse by a factor N1 to determine a period N1/(Noxfh) which initiates in response to the leading edge transition of the horizontal synchronization pulse; a second frequency divider for dividing the frequency of said clock pulse by a factor N2 to generate a gate pulse having a duration N2/(Noxfh) which initiates in response to the termination of the period N1/(Noxfh); and a gate responsive to said gate pulse for passing at least a portion of the color subcarrier transmitted with the received color television signal to said filter means.

9. A circuit as claimed in claim 8, further comprising a first bistable device responsive to the leading edge transition of each horizontal synchronization pulse to switch from a first binary state to a second bindary state and responsive to an output signal from said first frequency divider to switch from said second binary state to said first binary state, and a second bistable device responsive to said first bistable device switching from the second binary state to the first binary state to switch from a first binary state to a second binary state and responsive to an output signal from said second frequency divider to switch from the second binary state to the first binary state, the output of said second bistable device being connected to said gate for passing said color subcarrier to said discriminator means,

10.A circuit as claimed in claim 6, 7, or 9, further comprising: means for generating a first signal at a frequency which is an integral multiple of the frequency of said horizontal synchronization pulse so that the frequency of the first signal equals the frequency of one of the subcarrier frequencies of the SECAM television system; a reference frequency source for generating a reference frequency signal; a first frequency converter for combining said first signal and said reference frequency signals to generate a second signal at a frequency which is a summation of the frequencies of the combined signals; and a second frequency converter for combining said second signal with said extracted color subcarrier to generate a third signal at a frequency equal to the difference between the frequency of said second signal and the frequency of said extracted color subcarrier to cancel out jitter components contained in said extracted color subcarrier, said third signal being applied to said filter means.

11. A circuit as claimed in claim 10, wherein said reference frequency corresponds to one of the frequencies of the extracted color subcarriers.

12. A method of discriminating signals of the SECAM color television system substantially as hereinbefore described with referencz to the accompanying drawings.

13. A circuit for discriminating signals of the SECAM color television system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.