JPH0462633B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a color television system discrimination circuit, and particularly relates to a color television system discriminating circuit, and particularly to whether an input color video signal is a PAL system or not.
This relates to a circuit that determines whether it is a SECAM method.

2. Description of the Related Art As is well known, color television systems are not standardized worldwide, and each country or region adopts a predetermined color television system. However, depending on the region, it is possible to receive television broadcast signals of different color television systems from neighboring countries, so it is possible to receive television broadcast signals of two different color television systems (particularly PAL system and SECAM system). There are areas where this is possible.

For this reason, when recording television broadcast signals using, for example, a VCR used in such regions or countries,
Is the TV broadcast signal you are trying to record in PAL format?
A color television system discrimination circuit is required in order to automatically determine whether it is the SECAM system and switch the tuner, video signal processing circuit, etc. to operate optimally according to the color television system.

Conventionally, in this type of discrimination circuit, the color subcarrier frequency and color burst signal frequency of the PAL carrier color signal are 4433619MHz (hereinafter referred to as 4.43M
Hz), whereas in the SECAM method, the chroma non-modulated color subcarrier frequencies are 4.40625MHz (hereinafter referred to as 4.41MHz) and 4.25MHz, which are 1H
Taking advantage of the fact that it is repeatedly transmitted alternately,
With the configuration shown in Figure 6, PAL system and
There was something that distinguished it from the SECAM method.

In FIG. 6, the color video signal input to the input terminal 1 is supplied to the burst gate circuit 2,
Here, 1H generated by a known means is supplied from the input terminal 3 (H is the horizontal scanning period:
The same applies hereafter) due to the burst gate pulse period,
The color burst signal or chroma unmodulated color subcarrier is extracted. This extracted signal is
As shown in Figure 7, 4.16MHz is the center frequency of the passband, and 4.25MHz is passed, but 4.41MHz and
The 4.43MHz is supplied to the bandpass filter 4, which has a frequency characteristic that does not allow it to pass. Therefore, when the input color video signal is of the PAL format, the output of the bandpass filter 4 is at an extremely low level;
In the case of the SECAM method, the output signal of the bandpass filter 4 is as shown in Figure 8A, with the 4.25MHz unmodulated color subcarrier at a high level and the 4.41MHz unmodulated color subcarrier at a low level, alternating every 1H. The waveform is arranged as follows.

The output signal of the bandpass filter 4 is subjected to envelope detection by the envelope detection circuit 5, and then
2) It is supplied to the f _H detection circuit 6, where a frequency component (approximately 7.8 kHz) that is 1/2 times the horizontal scanning frequency f _H is detected. The output detection signal waveform of the envelope detection circuit 5 becomes a waveform as shown in FIG. Since the frequency f _H /2) is obtained, this is detected by the (1/2)·f _H detection circuit 6. In contrast, the input color video signal
In the case of a PAL color video signal, the output detection signal of the envelope detection circuit 5 is at an extremely low level and there is no repeating frequency component of (1/2) f _{H .} It is not detected by the detection circuit 6.

Therefore, from the (1/2) f _H detection circuit 6, a detection output (for example, a substantially sine wave with a frequency of f _H /2) is obtained only when the input color video signal is of the SECAM format, and this detection output The signal is integrated by the comparator circuit 7 and compared in level with a predetermined reference voltage, and then output to the output terminal 8. In this way, when the input color video signal is in the SECAM format, a high level signal is outputted to the output terminal 8 as a SECAM format discrimination signal, and when the input color video signal is in the PAL format, for example, a low level signal is output as a PAL format discrimination signal. It is extracted as a system discrimination signal.

Problems to be Solved by the Invention However, the above conventional discrimination circuit
Since the discrimination between 4.41MHz and 4.41MHz is performed by the bandpass filter 4 (or a bandpass filter that has a passing center frequency at 4.5MHz and attenuates 4.25MHz), the discrimination ratio is insufficient, especially the S/N ratio. However, this method has a problem in that it may misjudge a poor color video signal.

Therefore, the present invention shapes the signal obtained by performing frequency demodulation (FM demodulation) on the color burst signal and the chroma non-modulated color subcarrier.
It is an object of the present invention to provide a discrimination circuit for a color television system that solves the above problems.

Means for Solving the Problems The color television system discrimination circuit of the present invention includes gate means for gate-outputting a color burst signal or a two-frequency chroma non-modulated color subcarrier;
An FM demodulator, a clamp circuit that performs peak clamping, and a pulse train that removes the DC component and has either a first pulse train with a constant polarity of one horizontal scanning period, or a second pulse train whose polarity is alternately reversed every horizontal scanning period. It consists of a waveform shaping means for generating a pulse train, and a means for obtaining a discrimination output signal based on the level of the first or second pulse train.

Operation The output signal of the gate means is converted to FM by the FM demodulator.
The demodulated signal obtained by demodulation is peak-clamped by a clamp circuit, and then waveform-shaped by a waveform shaping means to become a first or second pulse train. This first pulse train is for one horizontal scanning period (1H)
The pulse train has a constant period and polarity, while the second pulse train is a pulse train whose polarity is alternately reversed every 1H. When the input color video signal is a color video signal of the first color television system, a color burst signal of a constant frequency is taken out from the gate means at a period of 1H, so that the above-mentioned first pulse train is obtained.

On the other hand, if the input color video signal is a color video signal of the second color television system,
Since the two-frequency chroma non-modulated color subcarriers are alternately extracted every 1H from the gate means, the second pulse train described above is obtained. The first pulse train or the second pulse train is compared in level and converted into a discrimination signal.

Embodiment FIG. 1 shows a block system diagram of an embodiment of a discriminating circuit for a color television system according to the present invention.
In the figure, the same components as in FIG. 6 are designated by the same reference numerals. In FIG. 1, a color video signal input to an input terminal 1 is gate-outputted by a burst gate circuit 2 as a signal having a period of 1H and a predetermined period.

Here, if the input color video signal is of the SECAM format, the burst gate circuit 2 outputs a chroma non-modulated color subcarrier of 4.25MHz and a chroma non-modulated color subcarrier of 4.41MHz, as shown in FIG. 4A. and are taken out alternately for a certain period of time every 1 hour.
The output unmodulated color subcarrier of the burst gate circuit 2 is supplied to the FM demodulator 10. This FM demodulator 10 is selected to have an input frequency versus output voltage characteristic as shown by curve I in FIG. 2, and the demodulation center frequency is set to 4.33 MHz, which is between 4.25 MHz and 4.41 MHz. Therefore, the demodulated signal waveform obtained by FM demodulating the above chroma non-modulated color subcarrier by the FM demodulator 10 is 4.25M as shown in FIG. 4B.
The chroma non-modulated color subcarrier of Hz is converted into a positive polarity pulse, and the chroma non-modulated color subcarrier of 4.41 MHz is converted into a negative polarity pulse.

The FM demodulated signal then passes through the peak clamp circuit 11
supplied to This peak clamp circuit 11
For example, as shown in FIG. 3, a capacitor _C1 connected between the input terminal 18 and the output terminal 19,
The configuration includes diodes _D1 and _D2 connected in parallel with opposite polarities between the connection point between the capacitor _C1 and the output terminal 19, and the input terminal 18' and the output terminal 19'. As a result, the peak clamp circuit 11 outputs a signal as shown in FIG. 4C whose amplitude is limited by clamping the upper and lower peaks of the input FM demodulated signal.

The DC component of the output signal of the clamp circuit 11 (shown in FIG. 4C) is blocked by the DC blocking capacitor 12, so that the average value is located approximately at the center of the waveform, as shown in FIG. 4D. It is considered to be a waveform.
Then, the output signal of the clamp circuit 11 from which the DC component has been removed by the DC blocking capacitor 12 is supplied to the gate circuit 13, where the signal from the terminal 14 is
Due to the 1H cycle gate pulse, only the signal portion in the section corresponding to the input unmodulated color subcarrier signal is gated out. As a result, a pulse train whose polarity is alternately inverted every 1H is taken out from the gate circuit 13, as shown in FIG. 4E.

This pulse train is supplied to a comparator circuit 15, where the level is compared with a preset reference voltage shown in FIG. 4E by the dashed line. When the signal is at a high level, it is converted to a low level signal, resulting in a 2H cycle pulse train as shown in FIG. 4F. In order to prevent malfunction due to noise, this pulse train is further detected by a (1/2) f _H detection circuit 6 to determine whether its period is 2H (repetition frequency is f _H /2), and then a comparator circuit 7 Integration and level comparison with another reference voltage are performed, and the resultant signal is output to the output terminal 8 as a high-level SECAM method discrimination signal.

The above description is for the case where the input color video signal is of the SECAM format.Next, the determination operation when the input color video signal is of the PAL format will be explained. In this case, as shown in FIG. 5A, a color burst signal with a constant color subcarrier frequency of 4.43 MHz is extracted from the burst gate circuit 2 for a constant period of 1H period, so the output demodulated signal waveform of the FM demodulator 10 is ,
As can be seen from the input frequency vs. output voltage characteristic in FIG. 2, the pulse has a negative polarity as shown in FIG. 5B. In Fig. 5B, the positive polarity pulse b is a pulse in the opposite direction caused by noise mixed in the color burst signal, and in this way, noise sometimes causes pulses in the opposite direction. Sometimes.

As a result, the output signal waveform of the peak clamp circuit 11 becomes as shown in FIG. The waveform appears to move toward the potential side. In addition, in FIG. 5D, the broken line indicates the average value.

As described above, the gate circuit 13 is configured to gate-output the input signal during a period corresponding to the gate output period of the burst gate circuit 2, so its output signal waveform becomes a pulse train as shown in FIG. 5E. This pulse train is originally a pulse train of the same polarity (negative polarity here) and a 1H period, but
As mentioned above, noise can sometimes cause the pulses to be reversed. This phenomenon is explained above.
A similar problem occurs when a SECAM color video signal is received.

This pulse train is level-compared with the reference voltage shown by the dashed line in FIG. 5E (same as the reference voltage shown in FIG. It becomes a signal. Here, in the output pulse train of the gate circuit 13,
The positive pulse generated in the opposite direction has a low level that does not exceed the reference voltage as a result of DC blocking, and therefore does not appear in the output of the comparator circuit 15.

The low level signal from the comparison circuit 15 is (1/
2).f The signal is output through the _H detection circuit 6 and comparison circuit 7 to the output terminal 8 as a low-level PAL system discrimination signal.

Effects of the Invention As mentioned above, according to the present invention, 4.25MHz and
The color burst signal or chroma non-modulated color subcarrier is FM demodulated using an FM demodulator whose demodulation center frequency is a frequency intermediate between 4.41 MHz and the waveform of the demodulated output signal is shaped to obtain a discrimination signal. The present invention has features such as improved discrimination ability and ability to accurately discriminate even input color video signals with poor S/N ratios.

[Brief explanation of drawings]

FIG. 1 is a block system diagram showing one embodiment of the circuit of the present invention, and FIG. 2 is a block system diagram of the block system shown in FIG.
FIG. 3 is a diagram showing an example of the input frequency vs. output characteristic of an FM demodulator. FIG. 3 is a circuit diagram showing an example of the peak clamp circuit in the block system shown in FIG. 1.
5 and 5 are signal waveform diagrams for explaining the operation of the block system shown in FIG. 1 when a SECAM system color video signal and a PAL system color video signal are input, respectively. FIG. 6 is a block system diagram showing an example of a conventional circuit. 7 is a diagram showing an example of the frequency characteristics of the bandpass filter in the block system shown in FIG. 6, and FIG. 8 is a signal waveform diagram for explaining the operation of the block system shown in FIG. 6. 1...Color video signal input terminal, 2...Burst gate circuit, 6...(1/2)/f _H detection circuit, 7, 15...
Comparison circuit, 8...Discrimination signal output terminal, 10...FM demodulator, 11...Peak clamp circuit, 12...DC blocking capacitor, 13...Gate circuit.

Claims (1)

[Scope of Claims] 1. When the input color video signal is of the first color television system, a color burst signal with a period of one horizontal scanning period is gate-outputted, and when the input color video signal is of the second color television system, gate means for alternately gate-outputting two-frequency chroma non-modulated color subcarriers that are alternately transmitted every horizontal scanning period; an FM demodulator for FM demodulating the output signal of the gate means; a clamp circuit that performs peak clamping on the output demodulated signal of the device; and a clamp circuit that removes a DC component from the output signal of the clamp circuit, and generates the input color video signal based on the output signal of the clamp circuit from which the DC component has been removed. is the first color television system, generates a first pulse train of constant polarity with a period of one horizontal scanning period;
waveform shaping means for generating a second pulse train whose polarity is alternately inverted every horizontal scanning period when the input color video signal is of the second color television system; or means for obtaining a discrimination output signal based on the level of the second pulse train.