JPH0224077B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for discriminating SECAM signals, and in particular uses a burst gate pulse synchronized with a horizontal synchronizing pulse and whose pulse width is maintained at a constant value to detect the frequency that is the valley of the frequency spectrum of color subcarriers. The aim is to perform extremely stable signal discrimination by setting the center frequency of discrimination to
Regarding SECAM signal discrimination.

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.
4 (Figure 3c) is output. 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 monostable multivibrator 4 is
It outputs a pulse 22 that determines the start point of extraction of the color subcarrier, and the AA monostable multivibrator 6 outputs a burst gate pulse 24 based on the pulse 22 and applies it to the gate circuit 8 .
On the other hand, a bandpass filter (BPF) 12 receives a color television signal (FIG. 3A) via the 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 extracts only the color subcarrier 26 from the color signal applied from the limiter 14 under the control of the burst gate pulse 24 (FIG. 3C) from the second monostable multivibrator 6.
In order to reliably obtain two types of discrimination pulses corresponding to the two types of color subcarriers, 4.25MHz and 4.40625MHz, with sufficient level difference,
For example, ceramics with a center frequency of approximately 4.5MHz
The signal is applied to a bandpass filter (BPF) 16.
As mentioned above, in the SECAM signal, the frequency of the color subcarrier alternates for each horizontal scanning line.
Since it changes to 4.25MHz and 4.40625MHz, BPF1
From 6 onwards, the horizontal sync pulse frequency (15.625K
Hz), two types of pulses have a frequency (7.8125KHz), namely a low-level pulse corresponding to the color subcarrier of 4.25MHz, which is far from the center frequency of BPF16, and a low-level pulse corresponding to the color subcarrier of 4.25MHz, which is closer to the center frequency of BPF16. High level pulses corresponding to 4.40625MHz color subcarrier are obtained. On the other hand, in the case of color TV signals of other formats, the frequency of the color subcarrier does not change, so BPF1
From 6 onwards, a pulse with a frequency half the frequency of the horizontal synchronizing pulse is not generated. BPF16 output
By applying 7.8125 KHz to the detection circuit 18, a signal for discriminating the SECAM signal is obtained from the output terminal 19. That is, the BPF 16 and the detection circuit 18
Frequency discrimination is performed by By the way, gating the color subcarrier means multiplying the color subcarrier signal by the signal (pulse) shown in FIG. 3C. Therefore, the spectrum of the gated color subcarrier is the spectrum of the color subcarrier multiplied by the spectrum of the signal of FIG. 3C. To be more specific, A and
If f _c is the amplitude and frequency of the color subcarrier, the spectrum of the color subcarrier is expressed as Asin 2π f _c t, and the spectrum of the pulse in Figure 3C is 2b/t _H [1/2+ _∞ 〓 ⁿ⁼¹ sin(nπb/t _H )/(nπb/t _H ) cos n2π/t _H t]. Here, b indicates the pulse width of the gate pulse. Therefore, the spectrum of the gated color subcarrier is A2b/t _H [1/2+ _∞ 〓 〓 ⁿ⁼¹ sin(nπb/t _H )/(nπb/t _H )cos n2π/t _H t]s
in2πf _c t = Ab/t _H [ _∞ 〓 〓 ⁿ⁼¹ sin(nπb/t _H )/(nπb/t _H ) sin(2πf _c t±n2
It is expressed as π/t _H t). In other words, it can be seen that the color subcarrier after being gated has sidebands of f _c ±n/t _H (n=1, 2, 3, . . . ). Also, sin (nπb/t _H )/(nπb/t _H ) becomes zero at n=mt _H /b (m=1, 2, 3,...), so (f _c ±m/b) It can be seen that a valley is formed depending on the frequency, and the shape of the sideband is determined by the gate width. FIG. 2 shows the spectrum of the fundamental wave of the color subcarrier. In Figure 2, A and B are 4.40625MHz and 4.40625MHz, respectively.
This is the spectrum of a 4.25MHz color subcarrier burst signal, and from Figure 2 it can be seen that there are many parts where the A and B spectra overlap, making frequency discrimination 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. As described above, since there are many overlapping parts of the spectra of A and B, it is difficult to separate them using the BPF 16, and the conventional SECAM signal discrimination method shown in FIG. 1 has unstable discrimination operation.

Therefore, an object of the present invention is to achieve stable signal discrimination by utilizing the fact that the valley frequency that occurs in the frequency spectrum of color subcarriers changes depending on the pulse width of a burst gate pulse for extracting color subcarriers. able to do
An object of the present invention is to provide a SECAM signal discrimination method.

Another object of the present invention is to select the pulse width of the burst gate pulse so that the frequency at which the frequency spectrum of one color subcarrier has a valley becomes the center frequency of a bandpass filter that discriminates the frequency of the other color subcarrier. It is an object of the present invention to provide a SECAM signal discrimination method that can perform extremely stable signal discrimination by making the following.

Examples of the present invention will be described below. Second
In the figure, at a frequency where one of the two types of spectra A and B has a valley, the level ratio between the spectra A and B is theoretically infinite. Therefore, if the frequency at which the spectrum has a valley (x or y in FIG. 2) is set as the center frequency of a frequency discrimination means such as a bandpass filter, extremely stable SECAM signal discrimination is possible. In other words, the first invention of the present application extracts the color subcarrier located in the back porch of the horizontal synchronizing signal using a burst gate pulse, and extracts the color subcarrier located at the back porch of the horizontal synchronization signal, and extracts the color subcarrier at a frequency (x or y in FIG. 2) that is the valley of the frequency spectrum of the color subcarrier. This is a method of identifying SECAM signals by frequency-discriminating the extracted color subcarriers for each horizontal scanning line, using the frequency as the center frequency for discrimination. Note that the pulse width of the burst gate pulse is b, and the center frequency of either spectrum (i.e., the frequency of the color subcarrier) is
Let f _c be the frequency at which the frequency spectrum of the color subcarrier falls (x or y in Figure 2, hereinafter referred to as the discrimination center frequency) is f _c ±
It becomes 1/b.

By the way, the discrimination center frequency is a function of b, so
If there are variations in the values of the resistors and capacitors that determine the pulse width b, it is necessary to adjust b. Furthermore, since the pulse width b may change due to changes in the ambient temperature of the parts used or changes over time, there is a possibility that the discrimination center frequency may change.

Therefore, the second invention of the present application has almost no effect on the above-mentioned causes of changing the pulse width,
A burst gate pulse with a constant pulse width is generated and a stable discrimination center frequency is used.
Concerning a SECAM signal discrimination method. Hereinafter, the second invention of the present application will be explained with reference to FIGS. 3 and 4.

In FIG. 4, 32 is a phase lock loop (PLL), which is composed of a phase comparator 34, a voltage controlled oscillator (VCM) 36, and a 1/ _N0 frequency divider ( _N0 is a positive integer) 38. There is. Phase comparator 34
receives a horizontal synchronization pulse via an input terminal 30, and the VCM 36 transmits a clock signal of frequency N ₀ f _H synchronized with the horizontal synchronization pulse to a NAND gate 40.
and 42. Note that the operation of the PLL is well known to those skilled in the art, so the details will be omitted. On the other hand, when a horizontal synchronizing pulse is applied to the set terminal 7 of the flip-flop (FF) 46 via the input terminal 44,
The output of the output terminal Q of the FF 46 goes high due to the leading edge of the horizontal synchronization pulse. While the output at the output terminal Q of the FF 46 is at a high level, the clock signal from the VCM 36 is passed through a NAND gate 40 to a 1/ _N1 frequency divider 48 (N ₁ is a positive integer smaller than N ₀ ). applied. The frequency divider 48 is FF46
From the point at which the output of output terminal Q of
N ₁ /N ₀ f Generates an output signal after _H seconds;
It is applied to the reset terminal R of FF46 and the set terminal S of FF50. Therefore, the output of the output terminal Q of the FF 46 is at a low level, and a pulse corresponding to pulse 22 in FIG. 3B is obtained. On the other hand, FF46
At the same time, the output of the output terminal Q becomes low level,
The output of the output terminal Q of FF50 becomes high level,
This time, through the NAND gate 42, 1/N ₂
A clock signal of frequency N ₀ f _H is applied from VCM 36 to frequency divider 52 . After N ₂ /N ₀ f _H seconds after the output of the output terminal Q of the FF50 becomes high level, the frequency divider 52 applies the output signal to the reset terminal of the FF50, and the output of the output terminal Q of the FF50 changes. is set to a low level and the gate of NAND gate 42 is closed. Therefore, from the output terminal 54 to C in FIG.
A pulse corresponding to pulse 24 is obtained. The above operation is repeated every time a horizontal synchronizing pulse is applied to the set terminal S of the FF 46. Therefore, output terminal 5 connected to output terminal Q of FF50
By setting the output of 4 as a burst gate pulse, the gate start time can be set from the leading edge of the horizontal sync pulse.
A burst gate pulse is obtained after N ₁ /N ₀ f _H seconds and with a pulse width of N ₂ /N ₀ f _H. Since the leading edge time and pulse width of the burst gate pulse obtained from the circuit of FIG. 4 are determined by N ₀ , N ₁ , N ₂ , and f _H ,
It is not affected by the parts used or the external environment mentioned above.
Note that the values of N ₀ , N ₁ , and N ₂ may be determined so that an optimal burst gate pulse can be obtained.

As explained above, according to the present invention, the fact that the frequency that becomes the trough that occurs in the frequency spectrum of the color subcarrier changes depending on the pulse width of the burst gate pulse that extracts the color subcarrier is used, so that stable SECAM signal discrimination can be performed. Furthermore, by selecting the pulse width of the burst gate pulse, the frequency at which the frequency spectrum of one color subcarrier has a valley becomes the center frequency of the bandpass filter that discriminates the frequency of the other color subcarrier. Therefore, extremely stable SECAM signal discrimination can be performed.

As described above, according to the present invention, very stable SECAM signal discrimination can be performed. Furthermore,
The present invention has the advantage that extremely stable SECAM signal discrimination is possible because the pulse width of the burst gate pulse is not affected by uneven characteristics of the parts used, changes in the parts used over time, or changes in ambient temperature.

[Brief explanation of drawings]

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 block diagram of FIG. 1 and the second part of this specification.
FIG. 4 is a waveform diagram for explaining the invention, and FIG. 4 is a block diagram for explaining the second invention of the present specification. 32...Phase lock loop, 38...1/N ₀
Frequency divider, 40, 42...NAND gate, 46, 50
...Flip-flop, 48...1/N ₁ frequency divider, 5
2...1/N ₂ frequency divider.

Claims (1)

[Claims] 1. A SECAM method in which the frequencies of two types of color subcarriers are alternately different for each horizontal scanning line,
The above two types of color subcarriers located in the back porch of the horizontal synchronization signal are extracted using a burst gate pulse, and the frequency of the extracted two types of color subcarriers is discriminated by a bandpass filter for each horizontal scanning line. In the SECAM signal discrimination method, if the pulse width of the burst gate pulse is b and the center frequency of one of the two color subcarriers extracted by the burst gate pulse is fc, then fc+
1/b or fc-1/b is the center frequency of the bandpass filter.
SECAM signal discrimination method. 2 The burst gate pulse is synchronized with the horizontal synchronization pulse and has a frequency (f _H ) of the horizontal synchronization pulse.
A clock signal of N ₀ times the frequency (N ₀ f _H ) (N ₀ is a positive integer) is generated using a phase lock loop, and the above clock signal is converted to 1/N ₁ (N ₁ is a positive integer smaller than N ₀ ). The _clock signal is divided into ₁ /N ₂ ( _{N 2} is a positive integer) and divide it into N ₂ /N ₀
Claim 1, wherein the pulse width of the burst gate pulse is f _H seconds.
SECAM signal discrimination method.