GB2086174A - Circuit for discriminating secam television signals - Google Patents

Abstract

In a SECAM (Registered Trade Mark) color television receiver, a discriminator circuit is provided for rejecting signals of other color television system formats to allow the receiver to process a received SECAM signal in cases where the receiver is tuned to the frequency bands of other system stations. A color subcarrier is extracted through a gate (8) in response to each horizontal sync pulse from the back porch of each horizontal sync pulse of a received color television signal. If the received signal is a SECAM system signal, the extracted color burst alternates in frequency between the SECAM subcarrier frequencies of 4.25 MHz and 4.40625 MHz. A band-pass filter (16) is provided having a passband of which the center frequency corresponds to a harmonic frequency (e.g. third harmonic) of one of the color subcarriers of the SECAM system. A frequency discriminator (18) is connected to the output of the band-pass filter (16) to detect when the filtered burst signal occurs at one- half the line frequency of the SECAM system for indication that the received signal is in the SECAM format. <IMAGE>

Description

SPECIFICATION Circuit for discriminating SECAM television signals The present invention relates to a circuit for discriminating a SECAM (Registered Trade Mark) system signal from other color television system signals.

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

A prior art discriminator circuit achieves the discrimination of SECAM format signals from other signals by extracting a color subcarrier in response to each horizontal synchronization pulse from the received signal. The extracted color burst is applied to a band-pass filter having a narrow passband of which the center frequency is approximately 4.5 MHz to provide a filtered burst signal to a frequency discriminator tuned to onehalf 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 permit the receiver to be switched to the SECAM reception 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 forming a characteristic spectrum which overlaps substantially with the spectrum of the fundamental component of the 4.25 MHz subcarrier. The difference in signal level between the two spectrums at the 4.40625 MHz point is approximately 6 dB, and this value is subject to change under varying operating conditions, so that there will be almost no distinction between them when unfavorable situation should occur Difficulty thus arises in discriminating between the two subcarriers of the SECAM system.

The above problem is eliminated by the invention by a band-pass filter wherein the center frequency of the passband corresponds to a harmonic frequency of one of the color subcarriers of the SECAM system. It is found that a harmonic component of each subcarrier has a maximum peak which is distinguishable under varying operating conditions from the corresponding peak of the harmonic component of the other subcarrier. The output of the band-pass filter thus serves as an excellent input signal to the frequency discriminator.

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: extracting at least a portion of each color subcarrierfrom the back porch of a horizontal synchronization pulse of a received color television signal; filtering the extracted color subcarrier at a harmonic frequency of one of the color subcarriers of the SECAM system to derive a harmonic component of the extracted color subcarrier which occurs at one-half the frequency of the horizontal synchronization pulse if the received signal corresponds to the format of the SECAM television system; and detecting whether the derived harmonic component occurs at one-half the frequency of the horizontal synchronization pulse.

According to a second aspect of the invention, there is provided a circuit for discriminating signals of SECAM color television system from signals of other color television systems, comprising: means for extracting at least a portion of each color subcarrier from the back porch of each horizontal synchronization pulse of a received color television signal; filter means having a passband of which the center frequency corresponds to a harmonic of one of the color subcarriers of the SECAM system for deriving from the extracted color subcarrier a harmonic component thereof; and discriminator means for generating an output signal if the derived harmonic component changes in level at one-half the frequency of the horizontal synchronization pulse to indicate that the received signal corresponds to the SECAM system format to indicate that the received color television signal corresponds to the format of the SECAM system.

Preferably, the center frequency of the passband corresponds to a third harmonic of one of the color subcarriers of the SECAM system, i.e.

12.75 MHzor 13.21875 MHz.

The invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a schematic block diagram of the discriminator circuit of the invention; Fig. 2 is a waveform diagram associated with the circuit of Fig. 1; and Fig. 3 is an illustration of the spectrums of third harmonics of the SECAM color subcarriers.

Referring now to Fig. 1, there is schematically illustrated 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. The discriminator circuit may also form part of a video tape recorder for discriminating recorded SECAM signals from other recorded signals to automatically switch the recorder to operate to the recorded SECAM signal.

This circuit comprises monostable multivibrators 4 and 6 which are series connected to receive horizontal sync pulses supplied to an input terminal 2 from the sync separator of the receiver, not shown. The output of the monostable multivibrator 6 is applied as a gating signal to a control terminal of a gate 8 for extracting a color subcarrier from the back porch of each horizontal sync pulse of the received television signal which may be a SECAM or other signal. The latter signal.

shown at 7 in Fig. 2, is applied after amplification to a band-pass 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 26 on the back porch of each horizontal sync pulse 20 appears as shown in Fig. 2. The monostable multivibrator 4 responds to the leading edge transition of each horizontal sync pulse 20 by generating a pulse 22 of a duration "a" which terminates at the instant immediately prior to the appearance of the color subcarrier 26.

The second monostable multivibrator 6 is triggered in response to the trailing edge transition of the pulse 22 to generate the gating pulses, shown at 24, having a duration "b" which is sufficient to extract the color subcarrier 26. The gate 8 thus passes each color subcarrier 26 in response to the output of the monostable multivibrator 6 in the form of burst to a ceramic band-pass filter 1 6.

According to the invention, the band-pass filter 16 is designed so that the center frequency of its passband corresponds to a harmonic (preferably third harmonic) of one of the colorsubcarrier frequencies of the SECAM system. Since the color subcarrier 26 of the SECAM system is transmitted on frequencies which alternate between 4.40625 MHz and 4.25 MHz in response to each horizontal scan, the third harmonic frequency is 13.21875 MHz or 12.75MHz. The third harmonic of the color subcarriers which occur at every other horizontal scan is thus passed through the bandpass filter 1 6 to a frequency discriminator 1 8. The frequency discriminator 1 8 essentially comprises an envelope detector for detecting the envelope of the color burst and a band-pass filter which is tuned to 7.81 25 kHz, one-half the line frequency of the SECAM system, to provide a signal to an output terminal 1 9 indicating that the received color television signal is the signal transmitted from a SECAM system station.

Because of the inherent characteristic of burst signals, the third harmonic components of the color subcarrier frequencies of the SECAM system have undulating sideband frequency components as illustrated in Fig. 3. However, the third harmonics of the respective color subcarriers have maximum peaks A and B which are spaced substantially apart in frequency and each maximum peak is spaced substantially from the sideband components of the other third harmonic component, as seen in Fig. 3. Each third harmonic component derived from the band-pass filter 1 6 is therefore definitely distinguishable from the other third harmonic component and the output of the band-pass filter 1 6 serves as an excellent indication that the received signal corresponds to the format of the SECAM television system.

Claims (6)

1. A method for discriminating signals of the SECAM color television system from signals of other color television systems, comprising the steps of: extracting at least a portion of each color subcarrier from the back porch of a horizontal synchronization pulse of a received color television signal; filtering the extracted color subcarrier at a harmonic frequency of one of the color subcarriers of the SECAM system to derive a harmonic component of the extracted color subcarrier which occurs at one-half the frequency of said horizontal synchronization pulse if the received signal corresponds to the format of the SECAM television system; and detecting whether the derived harmonic component occurs at one-half the frequency of said horizontal synchronization pulse.

2. A method as claimed in claim 1, wherein said harmonic is a third harmonic of one of the color subcarrier frequencies of the SECAM system.

3. A circuit for discriminating signals of SECAM color television system from signals of other color television systems, comprising: means for extracting at least a portion of each color subcarrier from the back porth of each horizontal synchronization pulse of a received color television signal; filter means having a passband of which the center frequency corresponds to a harmonic of one of the color subcarriers of the SECAM system for deriving from said extracted color subcarrier a harmonic component thereof; and discriminator means for generating an output signal if said derived harmonic component changes in level at one-half the frequency of said horizontal synchronization pulse to indicate that the received signal corresponds to the SECAM system format to indicate that the received color television signal corresponds to the format of the SECAM system.

4. A circuit as claimed in claim 3, wherein said harmonic is a third harmonic of one of the color subcarrier frequencies of the SECAM system.

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

6. 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.