JPH083109Y2 - AFC circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to an improvement of an AFC circuit used in a color signal reproducing system in a video tape recorder.

<Background Art> FIG. 2 shows a block diagram of a color signal reproducing system which has been generally adopted in a VHS type video tape recorder, and will be briefly described. In FIG.
1, 2 is a video head, 3 is a preamplifier, 4 is a low pass filter, 5 is an ACC circuit 6, a main converter, 7 is a band pass filter, 8 is a burst de-emphasis circuit,
9 is a mixed circuit, 10 is an APC circuit, 11 is a color killer circuit, 1
Reference numeral 2 is a sub-converter, 13 is an AFC circuit, and 14 is a band-pass filter. Here, the reproduction signal obtained from the video heads 1 and 2 is amplified by the preamplifier 3 and then the low-pass conversion chroma signal is passed through the low-pass filter 4. Only the component is supplied to the ACC circuit 5. In the ACC circuit 5, the low frequency conversion chroma signal is controlled so that the burst signal level included in the chroma signal becomes constant and is supplied to the main converter 6. In the main converter 6, this low frequency conversion chroma signal is obtained from the sub converter 12 through the band pass filter 14.
Frequency conversion is performed with the 4.2 MHz signal, and the chroma signal of the difference signal component (3.58 MHz ± 500 KHz) between the two is taken out through the band pass filter 7 and supplied to the burst de-emphasis circuit 8 and the APC circuit 10. The chroma signal obtained through the burst de-emphasis circuit 8 is mixed with the luminance signal by the mixing circuit 9 to generate a composite video signal. On the other hand, in the APC circuit 10, a 3.58 MHz signal is oscillated based on the burst signal included in the chroma signal, and the sub converter 1
Supplied to 2. In the AFC circuit 13, the phase was shifted by 90 ° every 1H based on the horizontal sync signal included in the reproduced signal.
A 9 KHz signal is generated and supplied to the sub converter 12. 3.58M obtained from APC circuit 10 in sub converter 12
The Hz signal is mixed with the 629 KHz signal obtained from the AFC circuit 13, a 4.2 MHz signal that is the sum component of the two is taken out through the bandpass filter 14, and this is supplied to the main converter 6 as described above. .

<Problem> By the way, in the AFC circuit 13 used in the conventional color signal reproduction system as described above, the AFC detection time constant is always constant, so that the special reproduction mode such as slow reproduction, fast-forward reproduction or still reproduction is performed. Sync signal with noise mixed in the AFC detection circuit (In such a special playback mode, when the video head passes the guard band between video tracks or the reverse azimuth track, large noise is mixed in the playback signal. However, the detected output voltage fluctuates greatly and a normal chroma signal cannot be obtained, and the color of the reproduced image disappears.

<Purpose> The present invention has been made in view of such a point, and aims to eliminate the above-mentioned drawbacks of the conventional circuit, so that the AFC detection time constant can be switched between the normal reproduction mode and the special reproduction mode. None Provides an AFC circuit that is not easily affected by noise during special playback mode.

<Embodiment> An embodiment will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention. Here, a capacitor C ₃ is connected in parallel with a time constant circuit composed of capacitors C ₁ and C ₂ and a resistor R ₁ attached to an AFC detection circuit 13 '. and to connect the series circuit of the transistor Q _1, transistor Q ₁ in accordance with the mode switching operation of the special playback mode and the normal playback mode
The ON / OFF control is performed to switch the detection time constant.

That is, in the normal playback mode, the "L" level control signal is
Since it is applied to the base of the transistor Q ₁ via the voltage dividing circuit by R ₂ and R ₃ , the transistor Q ₁ is normally in the off state, and at this time, the detection time constant of the AFC detection circuit 13 ′ is the capacitor C ₁ It is set to a relatively small value determined by C ₂ and resistor R ₁ . Therefore, in this case, the AFC operation is performed faithfully following the reproduction horizontal synchronizing signal.

On the other hand, in the special playback mode, the "H" level control signal is applied to the base of the transistor Q ₁ ,
Q ₁ is turned on, and the capacitor C ₃ is inserted in parallel with the capacitor C _{1 at} this time, so that the detection time constant of the AFC detection circuit 13 ′ becomes large. Therefore, in this case, since the detection time constant is large even when the sync signal contains noise, relatively stable AFC operation is performed without being significantly affected by this noise.

<Effect> As described above, according to the AFC circuit of the present invention, AF is performed in the special playback mode in which a large amount of noise is mixed in the playback horizontal sync signal.
Since the C detection time constant is made larger than in the normal playback mode, the AFC operation is stabilized even in the special playback mode, and the phenomenon that the color of the playback video is lost unlike the conventional circuit is eliminated.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of a color signal reproducing system in a general video tape recorder. 13 ′ …… AFC detection circuit

Claims (1)

[Scope of utility model registration request] 1. An AFC circuit used in a color signal reproducing system of a video tape recorder, which switches the detection time constant of the AFC detecting circuit by performing on / off control in response to switching between a normal reproduction mode and a special reproduction mode. The AFC circuit is characterized in that a switching element is provided, and the detection time constant of the AFC detection circuit in the special reproduction mode is made larger than that in the normal reproduction mode.