FR2490904A1 - SECAM-TYPE TELEVISION SIGNAL DISCRIMINATION CIRCUIT - Google Patents

Abstract

IN A SECAM COLOR TELEVISION RECEIVER, DISCRIMINATOR CIRCUIT SERVED TO REJECT SIGNALS OF OTHER FORMATS OF COLOR TELEVISION SYSTEMS, TO ALLOW THE RECEIVER TO PROCESS A SECAM SIGNAL RECEIVED IN CASES WHERE THE RECEIVER IS GRANTED ON THE FREQUENCY BANDS OTHER STATIONS. A COLORED SUB-CARRIER IS EXTRACTED, THROUGH DOOR 8, IN RESPONSE TO EACH HORIZONTAL SYNCHRONIZATION PULSE, FROM THE REAR PORCH OF IT, FROM A COLOR TELEVISION SIGNAL RECEIVED. IF THE SIGNAL RECEIVED IS A SIGNAL FROM THE SECAM SYSTEM, THE EXTRACTED COLOR SPOTS ALTERNATE IN FREQUENCY BETWEEN THE SECAM SUB-CARRIER FREQUENCIES OF 4.25MHZ AND 4.40625MHZ. A BAND-PASS FILTER 16 HAVING A BANDWIDTH WHOSE CENTRAL FREQUENCY CORRESPONDS TO ONE OF THE HARMONICS OF ONE OF THE COLOR SUB-CARRIERS OF THE SECAM SYSTEM IS PROVIDED. A FREQUENCY DISCRIMINATOR 18 IS CONNECTED TO THE OUTPUT OF THE BANDPASS FILTER 16 TO DETECT THE MOMENT WHEN THE FILTERED PULSE SIGNAL OCCURS ON HALF OF THE FREQUENCY OF SECAM SYSTEM LINES, TO INDICATE THAT THE SIGNAL RECEIVED IS IN THE FORMAT SECAM.

Description

- 1 -

  The invention relates to a discrimination circuit.

  nation of a SECAM type signal compared to signals

  other color television systems.

  In areas where color television signals from different systems are transmitted and the receivers are designed to operate compatible with any of these signals, the receiver includes a circuit for discriminating between signals from different

  formats, to automatically switch to a function mode

  operation which conforms to the format of the signal received.

  A discriminator circuit of the prior art carried out

  kills 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 pulse is applied to a bandpass filter having a narrow passband

  whose central frequency is approximately 4.5 MHz, for

  nin a filtered signal tip to a frequency discriminator

  quence tuned on half the line frequency. Since the SECAM color subcarrier alternates between 4.25 MHz and 4.40625 MHz in response to each horizontal scan, the filtered signal peak is the 4.40625 MHz subcarrier which operates at half frequency of lines of

  SECAM system, so that the discriminator provides a

  tie if the color television signal received corresponds to

  SECAM system format, to allow the receiver to pass-

use SECAM reception mode.

  However, due to the impulsive nature of the output

  band pass filter, the color pulse at 4.40625

  MHz filtered to side bands forming a characteristic spectrum

  which substantially overlaps that of the fundamental component of the 4.25 MHz subcarrier. The difference in signal level between the two spectra at 4.40625 MHz is around 6 dB; this value is subject to variation according to the working conditions, so that there will be almost no distinction between them when a situation

  unfavorable occurs. This results in difficulty in

  discriminate between the two subcarriers of the SECAM system.

  2490c04 - 2 - The above problem is eliminated by the invention

  thanks to a bandpass filter in which the frequency

  bandwidth frequency corresponds to a har-

  monique from one of the color subcarriers of the SECAM system. It has been found that a harmonic component of each subcarrier has a maximum peak which can be distinguished, under variable operating conditions, from the peak

  corresponding to the harmonic component of the other sub-

  carrier. The output of the bandpass filter thus serves as an input signal of excellent quality for the discriminator.

frequency.

  Preferably, the central frequency of the passband corresponds to a third harmonic of one of the color subcarriers of the SECAM signal, that is to say

12.75 MHz or 13.21875 MHz.

  The invention will be better understood using the

below.

  In the accompanying drawing: FIG. 1 is a block diagram of the discriminator circuit of the invention; Figure 2 illustrates the waveforms associated with the circuit of Figure 1; and Figure 3 represents the spectra of the third

  SECAM color subcarrier harmonics.

  Referring now to FIG. 1, we have represented

  felt a discriminating circuit used to distinguish between signals from the SECAM color television system and signals from other color television systems, such as NTSC and PAL. The discriminator circuit is part of a SECAM color television receiver which is located in a region where color television signals of different formats are transmitted from different sources. The discriminator circuit can also be part of a video tape recorder to discriminate recorded SECAM signals

  other signals recorded and automatically switch the

  recorder to operate on the SECAM signal

  registered. This circuit includes monostable multivibrators

2490O04

  4 and 6 which are connected in series to receive pulses

  horizontal synchronization signals supplied to a terminal

  input 2, from the synchronization separator of the

  trap, not shown. The output of the monostable multivibrator 6 is applied, as a control signal, to a control terminal of a door 8, to extract a subcarrier from

  color of the back porch of each synchronization pulse

  horizontal position of the received television signal, which may be a SECAM signal or the like. This last signal, represented at 7 in FIG. 2, is applied, after amplification, to a bandpass filter 12 through an input terminal 10. The filter 12 eliminates the luminance component and sends the chrominance component from the signal received at a limiter 14 o

  the amplitude is limited to an appropriate level.

  Each color subcarrier 26, on the rear porch of each horizontal synchronization pulse, appears as shown in FIG. 2. The monostable multivibrator 4 responds to the front edge transition of each horizontal synchronization pulse 20 by generating a pulse 22 of a duration "a" which ends at the instant immediately preceding the appearance of the subcarrier of

  color 26. The second monostable multivibrator 6 is triggered

  ché in response to the rear edge transition of the pulse 22, to generate the control pulses shown at 24, having a duration "b" which is sufficient to extract the colored subcarrier 26. The gate 8 thus transmits each

  color subcarrier 26 in response to the output of the multi-

  monostable vibrator 6, in the form of a signal tip,

  to a ceramic bandpass filter 16.

  According to the invention, the bandpass filter 16 is

  designed so that the center frequency of its band passes

  health corresponds to a harmonic (preferably the third) of one of the color subcarrier frequencies of the SECAM system. Because the color subcarrier 26 of the system

  SECAM is transmitted on frequencies which are alternative-

  between 4.40625 MHz and 4.25 MHz in response to

  each horizontal scan, the third harmonic of fr

  quence is at 13.21875 MHz or 12.75 MHz. The third harmo-

  color subcarriers which occurs in every other horizontal scan is thus transmitted through the bandpass filter 16 to a frequency discriminator

  18. The latter essentially comprises an envelope detector.

  lop used to detect the envelope of the color signal tip and a bandpass filter which is tuned to 7.8125 kHz, i.e. half the line frequency of the

  SECAM system, to provide a signal on an output terminal

  tie 19 indicating that the color television signal received

  is the signal transmitted by a transmitter of the SECAM system.

  Due to the characteristic of the signals in

  pulses, the third harmonics of the frequencies sub-

  color carriers of the SECAM system have lateral bands

  the undulating ones as shown in figure 3. However the

  third harmonics of the respective color subcarriers

  tives have maximum peaks A and B which are substantially

  spaced in frequency and each maximum peak is sensitive-

  spaced from the side bands of the other third harmonic, as shown in Figure 3. Each third harmonic transmitted by the bandpass filter 16 can thus be clearly distinguished from the other third harmonic and the output of the pass filter strip 16 is a

  excellent indication that the signal received corresponds to the

  SECAM television system mast.

Claims (2)

  1- A method of discriminating signals from the SECAM color television system with respect to signals from other color television systems, comprising: removing at least part of a subcarrier from   color of the back porch of each synchronization pulse   horizontal position of a color television signal received, the extraction, from the color subcarrier thus sampled, of a pulse signal which has the   half the frequency of said synchronization pulse   horizontal tion if the signal received corresponds to the format of the SECAM system and the detection of the moment when the signal thus extracted intervenes with half the frequency of said horizontal synchronization pulse, characterized in that   that said pulse signal is extracted by filtering the-   called color subcarrier at a harmonic frequency of one of the system's color subcarrier frequencies SECAM.   2- Method according to claim 1, characterized in that said harmonic is a third harmonic of one of the subcarrier frequencies of color of the system SECAM. -