JPS6326593B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a SECAM signal discrimination method, and an object of the present invention is to extract a color subcarrier from the back porch of a horizontal synchronization signal and perform frequency discrimination on its harmonics, thereby performing stable discrimination operation. .

It is one of the standard color television systems.
The following method is generally used to distinguish between SECAM color television signals and other color television signals (NTSC or PAL). In other words, in the SECAM method, color subcarriers are alternately used for each horizontal scanning line.
are different (4.25MHz and 4.40625MHz), whereas
Another method of color subcarrier is a method that takes advantage of the fact that the frequency does not change.

In other words, if the color subcarriers of the back porch of the horizontal synchronization signal are gated and extracted using a burst gate pulse, and the frequency of the extracted color subcarriers is discriminated line by line, in the case of the SECAM signal, the color subcarriers are alternately applied line by line. Since the carriers are different, a pulse with a frequency (7.8125KHz) that is half the frequency of the horizontal synchronization signal (15.625KHz) is obtained, whereas in color TV signals of other systems, the frequency of the color subcarrier does not change. It takes advantage of the fact that pulses cannot be obtained.

The present invention also utilizes this method, but first, an example of a conventional SECAM signal discrimination method using the above method will be explained with reference to FIGS. 1 to 3. A horizontal synchronization pulse is applied via input terminal 2.
When applied to the first monostable multivibrator 4, the first monostable multivibrator 4 is triggered by the leading edge of the horizontal synchronization pulse 20 and outputs a pulse 22 (FIG. 3B) having a predetermined pulse width a. do. The pulse width a of the pulse 22 is determined such that its trailing edge is located at the beginning of the gate pulse that extracts the color subcarrier. A second monostable multivibrator 6 receives pulses 22 from the first monostable multivibrator 4 and is triggered on its trailing edge to burst gate pulses 2 of pulse width b.
Outputs 4. It goes without saying that the pulse width b is determined by considering a width sufficient to extract the color subcarrier 26 (FIG. 3A). That is, the first
The monostable multivibrator 4 outputs a pulse 22 that determines the start point of extraction of the color subcarrier, and the second monostable multivibrator 6 outputs a burst gate pulse 24 based on the pulse 22 and applies it to the gate circuit 8. do. On the other hand, a band pass filter (BPF) 12 receives a color television signal (FIG. 3A) through an input terminal 10, removes a luminance signal, and outputs a color signal.
The color signal output from the BPF 12 is amplitude limited to a desired amplitude by a limiter 14 and applied to the gate circuit 8 at the next stage. The gate circuit 8 is controlled by the burst gate pulse 24 (FIG. 3C) from the second monostable multivibrator 6, extracts only the color subcarrier 26 from the color signal applied from the limiter 14, and adjusts the center frequency. is about
The signal is applied to a 4.5MHz ceramic bandpass filter (BPF) 16. As mentioned above,
In the SECAM signal, the frequency of the color subcarrier is alternately 4.25 MHz for each horizontal scan line.
Since it changes to 4.40625MHz, from BPF16,
A pulse with half the frequency (7.8125KHz) of the horizontal sync pulse frequency (15.625KHz) is obtained. On the other hand, in the case of color television signals of other formats, the frequency of the color subcarrier does not change, so
No pulse is generated from BPF16. BPF16
By applying the output of 7.8125 KHz to the frequency discrimination circuit 18, a signal for discriminating the SECAM signal is obtained from the output terminal 21. By the way, since the color subcarrier that performs frequency discrimination is a burst signal, its spectrum has widened sidebands. FIG. 2 shows the spectrum of the fundamental wave of the color subcarrier. In Figure 2, A and B are the spectra of 4.40625MHz and 4.25MHz color subcarrier burst signals, respectively.From Figure 2, it can be seen that the spectrum of A and B overlaps in many parts, so the frequency It turns out that discrimination is difficult. For example, the level difference between A and B at the respective center frequencies of two types of color subcarriers is only about 6 dB. in this way,
Since there is a lot of overlap between the spectra of A and B,
Separation using the BPF 16 is difficult, and the conventional SECAM signal discrimination method shown in FIG. 1 has unstable discrimination operation.

Therefore, an object of the present invention is to provide a SECAM signal discrimination method with excellent stability by taking advantage of the fact that the harmonic spectra of two types of color subcarriers in the SECAM method have little overlap. It is to be.

Hereinafter, with reference to FIG. 4, SECAM according to the present invention will be explained.
The signal discrimination method will be explained. In addition, in this specification, 3
This will be explained using a harmonic (third harmonic) as an example. In FIG. 4, A' shows the spectrum of the third harmonic of the 4.40625 MHz color subcarrier burst signal, and B' shows the spectrum of the third harmonic of the 4.25 MHz color subcarrier burst signal. By the way, the shape of the frequency spectrum of harmonics is similar to the shape of the frequency spectrum of the fundamental wave, so
As shown in Figure 4, if the difference between the center frequencies of A' and B' (13.21875MHz and 12.75MHz, respectively) is three times the difference between the center frequencies of A and B in Figure 2, then
The overlap of the spectra of A' and B' becomes very small. Therefore, compared to the conventional method of discrimination using the fundamental wave of the color subcarrier, according to the method of the present invention which discriminates using the third harmonic wave, A' and
Since the level ratio of B' is large, stable discrimination of SECAM signals is possible even if the frequency to be discriminated changes somewhat.

The SECAM signal discrimination method of the present invention can be implemented using a block diagram similar to the block diagram of FIG. That is, similar to the above explanation, the third
To explain harmonic detection as an example, in FIG. 1, instead of the bandpass filter 16,
A bandpass filter (BPF) with a center frequency of approximately 13.2MHz or approximately 12.7MHz (for convenience of explanation, this
BPF is set to 16'). Incidentally, since the level of the third harmonic is approximately 12 dB lower than the level of the fundamental wave, it is more practical to insert an amplifier after the bandpass filter 16'.

Furthermore, input the harmonics of the color subcarrier.
Bandpass filter 1 that discriminates SECAM signals
Even if the Q of 6' is smaller than the Q of the conventional discrimination bandpass filter, according to the method of the present invention,
SECAM signals can be discriminated stably. In this specification, a case has been described in which a third harmonic is used, but it is clear that similar effects can be obtained with other harmonics.

Instead of extracting the fundamental wave of the subcarrier for each horizontal scanning line as in the past, frequency discrimination is performed by extracting the harmonics included in the color subcarrier, so the detuning of two adjacent frequencies is For example, if it is the third harmonic,
The third harmonic level ratio of the color subcarrier detected for each horizontal scanning line can be 12 dB or more. Therefore, even if the frequency to be discriminated changes slightly, stable SECAM signal discrimination is possible.Furthermore, since the third harmonic included in the color subcarrier is used, frequency multiplication is possible. Since no circuit is required, the invention is characterized in that the invention can be implemented with a very simple circuit configuration.

[Brief explanation of the drawing]

Figure 1 is a block diagram for explaining the conventional SECAM signal discrimination method, Figure 2 is the fundamental frequency spectrum of a color subcarrier burst signal,
FIG. 3 is a waveform diagram for explaining the block diagram of FIG. 1, and FIG. 4 is a spectrum of the third harmonic frequency of the color subcarrier burst signal. 20...Horizontal synchronization pulse, 24...Burst gate pulse, A'...Spectrum of the third harmonic of the 4.40625MHz color subcarrier burst signal,
B′...Spectrum of the third harmonic of the 4.25MHz color subcarrier burst signal.

Claims (1)

[Claims] 1 SECAM in which the frequencies of the first and second color subcarriers are alternately different for each horizontal scan line.
In the method, the center frequency of the harmonic of the first color subcarrier adopts a frequency of the harmonic of the first color subcarrier that is near the valley of the spectrum of the harmonic sideband of the second color subcarrier, The method is characterized in that the frequency is discriminated for each horizontal scanning line by extracting a color subcarrier from the back porch of the horizontal synchronization signal and passing this color subcarrier through a bandpass filter that passes the selected frequency.
SECAM signal discrimination method.