JPH0227663Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a DC level fluctuation prevention circuit, and is suitable for application to, for example, a video tape recorder.

[Background technology and its problems]

For example, a two-head video tape recorder (VTR)
As a measure to improve the sound quality of audio signals, it has been proposed to extend the video track formed by each head and record a PCM audio signal on the extended track. In this case, the A channel head 1A and the B channel head 1B, which are mounted at angular positions 180° apart from each other on the rotating drum, are in sliding contact with the tape over a rotational angle range of 180° or more, as shown in Figs. 1A1 and A2. As shown in the figure, video track 2A is placed in the approximately 180° sliding contact range.
and 2B, and the video track 2A.
and 2B are extended by about 22 to 23 degrees, and PCM audio tracks 3A and 3B are formed in the extended portions. Here, since the heads 1A and 1B alternately slide into contact with the tape, the PCM audio track 3A of the A channel is formed to temporally overlap with the video track 2B of the B channel, and the PCM audio track 3B of the B channel is formed to overlap with the video track 2B of the B channel. It is formed so as to overlap in time with the video track 2A of the channel.

That is, in FIGS. 1A1 and A2, in the first time interval T1, the A head 1A scans the audio track 3A and the B head 1B scans the video track 2B, and in the second time interval T2, the A head 1A scans the video track 3A. Track 2A is scanned and the B head 1B is not in sliding contact with the tape (that is, it is in a idling state).Furthermore, in the third time interval T3, the A head 1A scans the video track 2A and the B head 1B is in contact with the audio track 3B. Furthermore, in the fourth time interval T4, the A head 1A is idling and the B head 1B is scanning the video track 2B. Below, sections T1 to T4 are repeated.

Since a video track and an audio track are formed separately in such a VTR, it is possible to monitor a signal recorded on one track and record it later on the other track (so-called dubbing). For example, when recording an audio signal using PCM while playing back a video signal recorded on a video track, it has conventionally been considered to perform dubbing of the A channel and B channel using the recording-playback circuit shown in Fig. 2. It is being

In Figure 2, A channel heads 1A and B
Channel head 1B is connected to switching terminals c of recording/reproducing switching switches 5A and 5B, respectively.
The recording terminals r of the recording/reproducing switch 5A and 5B are connected to the A channel and B channel recording amplification circuits 6A and 6B, respectively, and the reproduction terminal p is connected to the A channel and B channel reproduction amplification circuits 7A and 7B, respectively. . The output signals S1A and S1B of the regenerative amplifier circuits 7A and 7B are connected to the switching terminals a and b of the regenerative output switching circuit 8, and are sent from the output terminal u to the video signal processing circuit 10 as a video reproducing signal S2 via the coupling capacitor 9. given to.

The A channel and B channel recording/reproducing switch 5A and 5B are connected to the A head 1A and B, respectively.
Control signals S3A and S3 generated in the switching control circuit 11 during intervals T1 and T3 in which the head 1B scans the audio tracks 3A and 3B.
B (B1 and B2 in Figure 1) is switched to the recording terminal r, and in other sections, the playback terminal p is switched.
can be switched to

Furthermore, the reproduction output changeover switch circuit 8 receives a control signal S4 (FIG. 1C) from the changeover control circuit 11.
Accordingly, the A channel switching terminal a is switched to the A channel switching terminal a in the interval T2 to T3 in which the A head 1A scans the video track 2A, and the B channel switching terminal is switched to the B channel switching terminal b in the interval T4 to T1 in which the B head 1B scans the video track 2B. It will be done.

The switching control circuit 11 receives the head switching signal S5 and the video-audio switching signal S6, and generates control signals S3A, S3B, and S4 at that timing, and also generates a disable enable signal S8, which will be described later.
A and S8B are generated.

In the configuration shown in FIG. 2, in the first section T1, A
The head 1A scans the audio track 3A, and the B head 1B scans the video track 2B, and the PCM audio signal S9A is applied from the A channel recording amplifier circuit 6A to the A head 1A via the changeover switch circuit 5A. recorded in At this time, B head 1B is video track 2.
A reproduction signal S10B consisting of a video signal is reproduced from the B channel and applied to the B channel reproduction amplification circuit 7B through the changeover switch circuit 5B, but this reproduction amplification circuit 7B receives a logic "L" level disable signal S8B from the switching control circuit 11 ( D2) in FIG. 1 is applied, the circuit is controlled to be inactive, and thus no signal is sent to the video signal processing circuit 10 at the subsequent stage. Incidentally, when the A channel is in the recording mode of the PCM audio signal, the B channel is in the playback mode, so the playback output selector switch circuit 8 is also connected to the B channel selector terminal b.
However, since the recording current applied to the A head 1A has a much higher signal level than the reproduction signal of the B head 1B, this is caused by crosstalk from the circuit of the A head 1A to the rotary transformer, causing the B channel to be reproduced. The signal is applied to the amplifier circuit 7B, causing it to enter a saturated operating state. Therefore, the B channel regenerative amplifier circuit 7B is activated by the disenable signal S.
B by controlling it to a non-operating state by 8B.
To prevent the channel regenerative amplifier circuit 7 from becoming saturated.

Next, in the second section T2, while the A head 1A is scanning the video track 2A, the B head 1B is idle, and the video is reproduced from the A head 1A via the changeover switch circuit 5A to the reproduction amplifier circuit 7A. A video signal S10A is applied, and its output is applied to a subsequent signal processing circuit 10 through a changeover switch circuit 8. At this time, the B channel regenerative amplifier circuit 7B
The disable signal S8B returns to the logic "H" level.

Next, in the third section T3, the A head 1A continues to scan the video track 2A, while the B head 1B scans the audio track 3B, and the changeover switch circuit 5B is switched from the B channel recording amplifier circuit 6B. PCM to B head 1B via
A recording signal S9B consisting of an audio signal is applied and recorded on the tape. At this time, the A head 1A reproduces the reproduction signal S10A consisting of a video signal from the video track 2A and supplies it to the A channel reproduction amplifier circuit 7A through the changeover switch circuit 5A.
This regenerative amplifier circuit 7A receives a logic "L" level switch enable signal S8A from the switching control circuit 11.
(D1 in FIG. 1), it is controlled to a non-operating state, and thus no signal is sent to the subsequent signal processing circuit 10. Incidentally, in the case of this section T3, the purpose is to prevent the A channel reproducing amplification circuit 7A from becoming saturated with the recording signal of the B channel due to the crosstalk of the rotary transformer, as described above for the section T1.

Next, in the fourth section T4, the A head 1A is idling, while the B head 1B is scanning the video track 2B. A reproduced signal S10B consisting of a video signal is provided, and its output is provided through a changeover switch circuit 8 to a subsequent signal processing circuit 10. At this time, a switch enable signal S for the A channel regenerative amplifier circuit 7A
8A returns to logic "H".

In this way, A in sections T2 and T4
Channel and B channel regenerative amplifier circuit 7A
Output signals S1A and S1B (E1 and E2 in FIG. 1), which are the reproduced video signals obtained from This is displayed on the display tube in the signal processing circuit 10. Therefore, the operator can post-record the audio signal while monitoring this reproduced image.

The above has described the dubbing operation mode in which the audio signal is recorded while reproducing the video signal. In the reproduction operation mode, the A channel reproduction amplifier circuit operates based on the A channel audio signal obtained from the A head 1A in the first section T1. The output signal S1A output from the switch circuit 21 (switching control signal S31 of the switching control circuit 11)
switching input terminal P11 of
At the same time, an output signal S1B is output from the B channel reproduction amplifier circuit 7B based on the B channel video signal obtained from the B head 1B.
is sent out as a video playback signal S2 via the switching terminal b of the playback output switch circuit 8.

Further, in the second section T2, the output signal S1A output from the A channel reproducing amplification circuit 7A based on the A channel video signal obtained from the A head 1A is transferred to the video reproduction signal via the switching terminal a of the reproduction output switching switch circuit 8. Sent as S2.

Furthermore, in the third section T3, the output signal S1B output from the B channel regenerative amplifier circuit 7B based on the B channel audio signal obtained from the B head 1B is transferred to the regenerated PCM audio signal S2 via the switching input terminal P12 of the switch circuit 21.
At the same time, the output signal S1A is output from the A channel reproducing amplifier circuit 7A based on the A channel video signal obtained from the A head 1A, and the output signal S1A is sent out as the video signal S1A through the switching terminal a of the reproduction output switching switch circuit 8. It is sent out as signal S2.

Also, in the fourth section T4, the output signal S1B output from the B channel reproducing amplification circuit 7B based on the B channel video signal obtained from the B head 1B is passed through the switching input terminal b of the switch circuit 8 as the video reproduction signal S2. Sent out.

Next, in a recording operation mode in which an audio signal is recorded simultaneously with a video signal, the B channel video signal externally applied as the recording video signal S22 in the first section T1 is transmitted to the switch circuit 22.
B via the input terminal P23 (switching is controlled by the switching control signal S32 of the switching control circuit 11).
It will be input to the channel recording amplification circuit 6B, and will be input to the recording/reproducing switch 5 as a recording signal S9B.
B channel head 1B through recording terminal r of B
The video signal is supplied to the B channel video track 2B and thereby recorded on the B channel video track 2B. At the same time, record PCM
The A-channel PCM audio signal given from the outside as the audio signal S23 is input to the A-channel recording amplifier circuit 6A via the input terminal P22 of the switch circuit 22, so that the A-channel PCM audio signal is input as the recording signal S9A to the recording terminal r of the recording/reproducing switch 5A. is supplied to A channel head 1A through
As a result, it is recorded on the A channel PCM audio track 3A.

Also, in the second section T2, the recorded video signal S
The A channel video signal given from the outside as 22 is input to the A channel recording amplifier circuit 6A through the input terminal P21 of the switch circuit 22, and the A channel video signal is input as a recording signal S9A to the A channel through the recording terminal r of the recording/reproducing switch 5A. The signal is supplied to the head 1A, and thereby recorded on the A channel video track 2A.

Also, in the third section T3, the recorded video signal S
The A channel video signal given from the outside as 22 is input to the A channel recording amplifier circuit 6A through the input terminal P21 of the switch circuit 22, and the A channel video signal is input as a recording signal S9A to the A channel through the recording terminal r of the recording/reproducing switch 5A. The signal is supplied to the head 1A, and thereby recorded on the A channel video track 2A. Recorded at the same time
By inputting the B channel PCM audio signal externally applied as the PCM audio signal S23 to the B channel recording amplifier circuit 6B via the input terminal P24 of the switch circuit 22,
The recording signal S9B is supplied to the B channel head 1B through the recording terminal r of the recording/reproducing switch 5B, and thereby recorded on the B channel PCM audio track 3B.

Also, in the fourth section T4, the recording video signal S
The B channel video signal given from the outside as 22 is input to the B channel recording amplifier circuit 6B through the input terminal P23 of the switch circuit 22, and the B channel video signal is input as a recording signal S9B to the B channel through the recording terminal r of the recording/reproducing switch 5B. The signal is supplied to the head 1B and recorded on the B channel video track 3B.

However, the video reproduction signal S2 of the switch circuit 8 has the same signal levels KA and KB of the output signals S1A and S1B when the reproduction amplifier circuits 7A and 7B are in the non-operating state in the intervals T1 and T3. The signal level suddenly changes to a completely different level from the average level of the reproduced video signal.

In this way, the signal level on the output end of the changeover switch circuit 8 (therefore, the input end of the coupling capacitor 9) is in the intervals T1 to T2, T2 to T3, and T3.
to T4, and from T4 to T1, the output signal S1 of the output terminal of the coupling capacitor 9 (therefore, the input terminal of the signal processing circuit 10)
As shown in FIG. 1G, the signal level of 1 changes transiently along the differential waveform every time the level of the signal S2 at the input terminal of the coupling capacitor 9 changes, and as a result, the signal level of the signal S2 at the input terminal of the coupling capacitor 9 changes transiently along the differential waveform. The video signal cannot be played properly when starting. In reality, this unstable period corresponds to a time several times the time constant determined by the input impedance of the coupling capacitor 9 and the next-stage signal processing circuit 10.

[Purpose of invention]

The present invention attempts to prevent transient fluctuations in the signal level at the output side of the coupling capacitor when the regenerative amplifier circuit is temporarily controlled to be inactive during the regenerative mode.

[Summary of the idea]

In order to achieve this purpose, in this invention,
When the regenerative amplifier circuit is controlled to be inactive, a correction voltage having a DC level approximately the same as the reproducing video signal level is applied to the input side of the coupling capacitor.

〔Example〕

An embodiment of the present invention will be described in detail below with reference to the drawings. In this case, as shown in FIG. 3 with the same reference numerals assigned to parts corresponding to those in FIG. In addition to the first and second switching terminals a and b, it has a third switching terminal g connected to the correction voltage source 21. A control signal S41 is given from the switching control circuit 11 to the reproduction output switching circuit 8.
As shown in Figure 4A, corresponding to Figure 1C,
In the recording sections T1 and T3 of the A channel and the B channel, the reproduction output changeover switch circuit 8 is connected to the third switching terminal g, and the correction voltage V _g of the correction voltage source 21 is applied to the input terminal of the coupling capacitor 9. At the same time, the control signal S41 switches the reproduction output changeover switch circuit 8 to the first and second switching terminals a in the video signal reproduction sections T2 and T4 of the A channel and the B channel, as in the case of FIG. 1C.
and b.

In the above configuration, the reproduction output changeover switch circuit 8 applies the correction voltage V _g from the third terminal g to the video output signal S2 as shown in FIG. 4B in the interval T1.
In the following section T2, an output signal S1A consisting of a video reproduction signal of the A channel regenerative amplifier circuit 7A having an average level almost the same as the level of the correction voltage V _g is transmitted from the first terminal a as a video output signal S2. Then, in the following section T3, the correction voltage V _g is applied to the video output signal S from the third terminal g.
2, and in the subsequent section T4, the output signal S1B consisting of the video reproduction signal of the B channel reproduction amplifier circuit 7B having an average level almost the same as the level of the correction voltage V _g is output from the second terminal b as the video output signal S2. Send.

Therefore, at the input end of the coupling capacitor 9, a signal S2 whose DC level hardly changes in the interval T1 to T4 can be obtained.
As shown in FIG. 2, it is possible to obtain an output signal S11 without transient signal level fluctuations.

In the above description, the present invention is applied to a case where there are two heads, but it can be similarly applied to a case where the number of heads is two or more.

[Effect of idea]

As described above, according to the present invention, when the reproduced video signals are obtained from the video tracks of the A channel and the B channel and the audio signals are dubbed to the audio tracks of the A channel and the B channel while viewing images based on the reproduced signals, It is possible to obtain a DC level fluctuation prevention circuit with a relatively simple configuration that can prevent signal level fluctuations caused by the coupling capacitor 9 provided between the regenerative amplifier circuits 7A, 7B and the subsequent circuit.

[Brief explanation of the drawing]

Fig. 1 is a signal waveform diagram to explain the operation of a VTR capable of dubbing, Fig. 2 is a block diagram showing the conventional configuration, and Fig. 3 is a VTR to which an embodiment of the DC level fluctuation prevention circuit according to the present invention is applied. FIG. 4 is a signal waveform diagram for explaining its operation. 1A, 1B...A, B head, 2A, 2B...
Video track, 3A, 3B...Audio track, 5A, 5B...Recording/playback switch circuit, 6
A, 6B...recording amplifier circuit, 7A, 7B...playback amplifier circuit, 8...playback output selector switch circuit, 9
... Coupling capacitor, 11 ... Switching control circuit, 21 ... Correction voltage source.

Claims (1)

[Claims for Utility Model Registration] A video track on a tape is extended, an audio track is formed on the extended portion, the video track and the audio track are recorded or reproduced by at least two heads, and each of the heads In a video tape recorder, the reproduced signal is amplified by a reproduced amplifier circuit and at least the video signal is supplied to a subsequent signal processing circuit via a reproduced output selector switch circuit and a DC cut coupling capacitor. While reproducing the video signal of the video track, the reproducing amplifier circuit is controlled to be inactive during a time period during which the audio signal is dubbed by the switching control circuit, and a correction voltage source is provided to control the reproducing output selector switch circuit. By switching to the correction voltage source and supplying a correction signal of a predetermined DC level to the input terminal of the coupling capacitor, the above-mentioned A DC level fluctuation prevention circuit characterized in that the DC level at the input end of a coupling capacitor is not fluctuated, thereby preventing transient signal level fluctuations from occurring at the output end of the coupling capacitor.