JPH0336148Y2 - - Google Patents

Description

[Detailed description of the invention] <Technical field> The present invention provides an automatic subcarrier switching circuit suitable for use in a multi-system color television receiver capable of receiving color television broadcasts of various systems such as the PAL system and the NTSC system. It is related to.

In special regions such as the Middle East, due to historical background, the color television broadcasting system of developed countries, i.e.
PAL and SECAM systems are mixed, and
With the spread of VTRs, there is a demand for multi-system color television receivers that can not only receive broadcast programs from both of these formats, but also enjoy NTSC programs that are played back on VTRs from soft tapes provided by the United States, Japan, etc. In response to these circumstances, this type of multi-system color television receiver has been developed.

<Prior Art> In such conventional multi-system color television receivers, since the signal formats of the NTSC system and PAL system are generally relatively similar, the color signal processing circuits of both systems, especially the subcarrier oscillator, are It also uses a phase detector for APC, a phase detector for killer, etc., and determines the reception method at that time and performs appropriate switching operations accordingly.

To explain more specifically, the PAL system generally uses 4.43MHz as its subcarrier, whereas the NTSC system usually uses 3.58MHz. Also, in some areas it is 3.58M in PAL format.
Hz is also used in some places. Furthermore, NTSC
Also used as a subcarrier for VTR soft tapes using
There is a mixture of 3.58MHz and 4.43MHz.

Conventionally, multi-system color television receivers, such as color television receivers capable of receiving both PAL and NTSC, and video tape recorders capable of playing back various types of soft tape, usually use 4.43 MHz and 3.58 MHz. Two MHz subcarrier oscillators are used, and these two oscillators are switched in conjunction with system switching to obtain the required subcarrier output as appropriate.

For this reason, conventional multi-system color television receivers of this type use two oscillators, which increases costs and requires the output of these oscillators to be switched and driven each time according to each system.

Therefore, the applicant of the present application filed utility model registration application No. 51136 dated April 7, 1980 (see Utility Model Publication No. 152070/1983) as an improvement over such conventional technology.
For example, even when receiving television signals of two or more systems with different subcarrier frequencies, such as the PAL system and the NTSC system, it is possible to obtain different subcarrier frequency outputs that match the systems from one oscillator. We have already proposed an automatic subcarrier switching circuit.

This is configured, for example, as shown in FIG. 3, where 1 is a burst gate circuit that separates and derives only the burst signal component from the chroma signal based on the burst gate pulse of the horizontal scanning period, and 2 is the burst gate circuit that separates and derives only the burst signal component from the chroma signal. The phase detector for APC, 3 is a low-pass filter, and 4 is a VCO (voltage controlled oscillator). This VCO 4 constitutes an APC circuit together with the phase detector 2 and low-pass filter 3, and a subcarrier signal is oscillated from the VCO 4. .

Here, especially for this VCO4, one for 3.58MHz and one for 4.43M
Two Hz crystal oscillators 5a and 5b are attached,
A switching circuit 6 is inserted in series with these oscillators 5a and 5b, and these oscillators 6 are appropriately switched in accordance with the frequency of the burst signal.

Furthermore, here, 7 is a phase detector for killer use, and only when the regular output of this phase detector 7 stops, the flip-flop circuit 8 is invertedly driven based on a drive pulse of about 200 msec, and the switching circuit 6 is switched. .

To explain in detail the operation of the automatic subcarrier switching circuit configured as above,
When receiving NTSC color television broadcast,
A burst signal of 3.58 MHz is separated and derived from the burst gate circuit 1 and supplied to each phase detector 2 and 7. At this time, the switching circuit 6 switches to the 3.58MHz crystal resonator 5a side according to the output of the flip-flop circuit 8, and the APC loop synchronizes to 3.58MHz in accordance with the burst signal, and the 3.58MHz subcarrier is output from the VCO 4. A signal is oscillated. Once the APC loop is synchronized in this way, a detection output is derived from the phase detector 7, and based on this, the flip-flop circuit 8
operation will be stopped from now on, and the normal operation will be stopped from VCO4.
A stable subcarrier signal of 3.58MHz is oscillated.
However, for some reason, the flip-flop circuit 8
If the control of the APC is delayed and the switching circuit 6 is switched to the 4.43MHz crystal oscillator 5b side, the APC
The synchronization within the loop is lost and no output is derived from the phase detector 7. At this time, the flip-flop circuit 8 is reversed by the next drive pulse, and the switching circuit 6 is switched to the 3.58 MHz crystal resonator 5a side. As a result, the APC loop is synchronized to 3.58MHz and a regular subcarrier signal of 3.58MHz is output from VCO4.

Similarly, when receiving PAL color television broadcasting, burst gate circuit 1 outputs 4.43MHz.
When the burst signal is derived, the switching circuit 6 is activated based on the control output of the flip-flop circuit 8.
is switched to the 4.43MHz crystal resonator 5b side,
The APC loop is synchronized to 4.43MHz and the VCO
4 outputs a 4.43MHz subcarrier signal.
Once synchronization is applied to 4.43MHz in the APC loop, a detection output is derived from the phase detector 7, after which the operation of the flip-flop circuit 8 is stopped, the switching circuit 6 is fixed to the 4.43MHz oscillator 5b side, and the output from the VCO 4 is A stable subcarrier signal of 4.43MHz is derived.

<Problem> By the way, in a multi-system color television receiver equipped with the above-mentioned automatic subcarrier switching circuit,
Since the low-pass filter forming the APC loop has a relatively long time constant, when switching to a channel with a different reception method, the reception method is identified and the switching circuit 6 performs a switching operation, and the VCO 4 It took a considerable amount of time to obtain the proper subcarrier output, and the disadvantage was that the image was distorted during this time.

<Purpose> The present invention was developed in view of these points, and provides an automatic subcarrier switching circuit that can quickly obtain stable images even when switching channels, especially when switching to channels with different reception methods. It provides:

<Examples> Examples of the present invention will be described below. FIG. 1 shows one embodiment of the present invention, in which the same parts as in the conventional example of FIG. 3 are given the same reference numerals.

Here, in particular, a time constant switching means is provided in the low pass filter 3 forming an APC loop with the phase detector 2 and the VCO 4 to temporarily switch its time constant when switching channels.
That is, here the low-pass filter 3 is connected to the capacitor 9,
A series circuit consisting of a capacitor 12 and a switching element 13 is further connected in parallel with the capacitor 10 to an ordinary filter circuit consisting of a filter circuit 10 and a resistor 11. The switching element 13 is controlled on/off based on the channel switching pulse, thereby temporarily shortening the time constant of the low-pass filter 3 each time the channel is switched.

Note that in this embodiment, the above-mentioned low-pass filter 3
In addition, the time constant of the killer phase detector 7 is also temporarily switched when switching channels. That is, here, a series circuit of a capacitor 17 and a switching element 18 is further inserted in parallel with the capacitor 16 constituting the time constant circuit of the phase detector 7, and when switching channels, the pulse generator 1
The switching element 18 is controlled on/off based on the channel switching pulse obtained from the channel switching pulse 5, thereby temporarily shortening the time constant of the phase detection circuit 7 every time the channel is switched.

Describing the operation of the automatic subcarrier switching circuit configured as above, when an NTSC signal is being received, the flip-flop circuit 8 outputs "H" by the same operation as that of the conventional example described above. ” is derived, and the switching circuit 6
At this time, the switching circuit 6
It is switched to the 3.58MHz crystal resonator 5a side. As a result, the APC loop is synchronized to 3.58MHz, and the VCO 4 outputs a stable subcarrier signal of 3.58MHz.

Note that during such a steady reception operation, the switching elements 13 and 18 of the low-pass filter 3 and the killer phase detector 7 are both in the on state.
Therefore, in this case, the time constants of the low-pass filter 3 and the phase detector 7 are relatively long.

However, when the channel selection device 14 is operated to switch to a PAL channel during such an NTSC reception operation, the channel switching pulse generator 15 is driven at this moment and a relatively short channel switching pulse is generated. As a result, the switching elements 13 and 18 of the low-pass filter 3 and the phase detector 7 are temporarily turned off, and during this time the time constants of the low-pass filter 3 and the phase detector 7 are shortened. Therefore, in this case, the phase detector 2,
Formed by low pass filter 3 and VCO 4
The response characteristics of the APC loop and the killer phase detector 7 become faster, and as a result, the switching circuit 6 is activated by the "L" output from the flip-flop circuit 8.
Switched to the 4.43MHz crystal oscillator 5b side, APC
Within the loop, synchronization is quickly applied to 4.43MHz.
A stable subcarrier signal of 4.43MHz is output from VCO4.

In this way, in the subcarrier automatic switching circuit of this embodiment, the time constants of the low-pass filter 3 and phase detector 7 are temporarily shortened each time the channel is switched, including the reception method switching, so that the APC loop pull-in operation is fast. Therefore, even when switching to a channel of a different system, the subcarrier signal can be quickly switched, and a stable subcarrier output can be obtained thereafter.

In the above embodiment, a series circuit consisting of a capacitor 12 and a switching element 13 is inserted in parallel with the capacitor 10 in the low-pass filter 3, and the switching element 13 is temporarily turned off when switching channels. The time constant of the low-pass filter 3 is shortened, but instead, a series circuit consisting of a resistor 19 and a switching element 13 is inserted in parallel with the capacitor 10, as shown in the embodiment of FIG. The time constant of the low-pass filter 3 may be shortened during this period by turning off the switching element 13 and temporarily turning on the switching element 13 only when switching channels. Note that in the killer phase detector 7 as well, a resistor may be inserted instead of the capacitor 17.

<Effects> As described above, according to the subcarrier automatic switching circuit of the present invention, in a multi-system color television receiver capable of receiving color television signals of different systems such as PAL system and NTSC system, When switching to a channel, the subcarrier switching operation is quickly performed, and during this time, it is possible to obtain a stable image with almost no image disturbance.

In addition, in the subcarrier automatic switching circuit of the present invention,
A first time constant switching means is provided for switching the time constant of the low-pass filter of the APC loop, and a second time constant switching means is provided for switching the time constant of the killer phase detector.
Not only the first time constant switching means is switched based on the channel switching pulse generated from the channel switching pulse generator to temporarily shorten the time constant of the low-pass filter, but also the second time constant switching means is switched at the same time. Since the time constant of the killer phase detector is temporarily shortened, it is easier to determine the broadcasting method when switching reception channels, compared to a case where only the time constant of the low-pass filter is temporarily shortened. This has the excellent effect of making it more reliable.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of one embodiment of the automatic subcarrier switching circuit of the present invention, Fig. 2 is a circuit diagram of another embodiment of the low pass filter according to the invention, and Fig. 3 is a conventional subcarrier automatic switching circuit. FIG. 3 is a block diagram of the circuit. 1...Burst gate circuit, 2...APC phase detector, 3...Low pass filter, 4...
VCO, 5a, 5b...Crystal resonator, 6...Switching circuit, 7...Killer phase detector, 8...
. . . flip-flop circuit, 13 . . . switching element, 15 . . . channel switching pulse generator.

Claims (1)

[Claims for Utility Model Registration] A voltage-controlled oscillator VCO that oscillates by appropriately switching different oscillation frequencies depending on the reception method such as PAL or NTSC, and that detects the phase of the oscillation phase of the VCO and the phase of the burst signal. A multi-system color equipped with an APC loop formed from an APC phase detector, a low-pass filter that filters the low-frequency component of the detection output of the APC phase detector and supplies it to the VCO, and a killer phase detector. In the television receiver, a first time constant switching means is provided for switching a time constant of a low-pass filter of the APC loop, and a second time constant switching means is provided for switching a time constant of the killer phase detector, and a receiving channel is provided. At the time of switching, the first time constant switching means and the second time constant switching means are switched based on the channel switching pulse generated by the channel switching pulse generator, and the time constant of the low-pass filter and the killer filter are switched. A subcarrier automatic switching circuit characterized in that the subcarrier automatic switching circuit is configured to temporarily shorten the time constant of a phase detector.