JPH01264667A - Fm sound recording signal reproducing circuit for vtr - Google Patents

Abstract

PURPOSE:To prevent the deterioration of a voice at the time of the high-speed reproduction of an FM voice recording by executing a changeover to a voltage lower than a DO comparing voltage at the time of reproducing the FM voice recording at a high speed in an FM voice demodulating circuit to interrupt a sound demodulated signal only in the omission period of the signal and to correct the signal. CONSTITUTION:When an omission is generated in an FM reproducing signal A, a level with an envelope signal extracted through a limiter amplifier 1 by a filter 2c in a DO detecting circuit 2 is made lower than a DO comparing voltage V01 in the omitted part. Then, the circuit 2 generates a DO detecting pulse D, a permanent-closed switch SW of a DO correcting circuit 3 interrupts a sound demodulated signal B only in such a period, and simultaneously, a signal level just before the signal B is supplied from a capacitor C2 to an audio amplifier 5. Thus, the interruption of the switch SW is canceled before the discharge of the capacitor C2 is completed, and the signal B is fed into the amplifier 5 again. Thus, for a sound signal E outputted from the amplifier 5, the omitted part of the signal can be corrected.

Description

[Detailed description of the invention] [Industrial application field] The present invention is a rotating head type VTR [video.

tape recorder], the F
This relates to an M audio recording signal reproducing circuit.

[Conventional technology]

(al DOC[drop out control
]Circuit V In the HS type H4F1-VTR, the audio signal is FM modulated and recorded together with the video signal in the video I rank. However, if this video track has defects, it will take several I minutes (20
The signal may be lost for a period of 0 μs to 300 μs). Therefore, the FM audio recording signal reproducing circuit for reproducing the FM audio recording has conventionally been provided with a DOC circuit for correcting signal loss caused by scratches on the tape or the like.

A conventional FM audio recording signal reproducing circuit provided with this DOC circuit will be explained based on FIGS. 3 to 5.

As shown in FIG. 3, this FM audio recording signal reproducing circuit receives the reproduced FM replay signal color and the audio demodulated signal (2) obtained by FM demodulating the reproduced FM replay signal. Input FM playback signal■
is sent to limiter amplifier 1 in the DOC circuit.

This FM reproduction signal (2) is an FM modulated signal, as shown in FIG. Furthermore, this FM reproduction signal (2) has a dropout in the range of 200 μS to 300 μS in the illustrated portion. The DOC circuit includes a limiter amplifier 1, a DOo output circuit 2, and a DOO positive circuit 3.

After the waveform of the FM reproduction signal is shaped by the limiter amplifier 1, the DOO output circuit 2 detects signal loss.

The DOO output circuit 2 is composed of a comparison circuit 2a as shown in FIG. 4, and each comparison input is connected to a voltage dividing circuit 2b for the power supply voltage VCC by a variable resistor VR, a resistor R3, and a capacitor C.
0 filters 2C are respectively connected.

First, only the envelope component of the FM playback signal ◎ is extracted by a filter 2C consisting of a resistor R3 and a capacitor C1, resulting in an envelope signal ◎ as shown in FIG.

This envelope signal ◎ is then sent to the negative input of the comparator circuit 2a. Further, the positive input of the comparator circuit 2a receives the DO comparison voltage ■ divided by the variable resistor VR. is entered. This DO comparison voltage V. Adjust the variable resistor VR-t to obtain a normal envelope signal ◎
The voltage value is set to be slightly lower than the level of .

Therefore, as shown in FIG. 5, if there is no dropout in the envelope signal ◎, the output of the comparator circuit 2a remains at the "LOW" level. Also, due to signal loss, the level of the envelope signal ◎ drops rapidly, causing the DO comparison voltage ■.

When the value falls below this period, the comparator circuit 2a generates a pulse-like DO detection pulse signal for only that period. This DO detection pulse signal becomes the output of the DOo output circuit 2.

DO detection pulse generated from DOo output circuit 2, D
Oo is sent to the positive circuit 3. The DOo positive circuit 3 is composed of a normally closed switch SW and a capacitor C2. This normally closed switch SW closes the circuit when the DO detection detection pulse is not emitted from the DOO output circuit 2. When the DO detection pulse is sent from the DOO output circuit 2, the normally closed switch SW is opened only during the period of the DO detection pulse.

The audio demodulated signal (2) is amplified by the first audio amplifier 4 and the second audio amplifier 5 in the FM audio recording signal reproducing circuit, and is output as the audio output signal (2). A normally closed switch SW in the DO correction circuit 3 is connected in series between the first audio amplifier 4 and the second audio amplifier 5. In addition, the capacitor C2 of the DOO positive circuit 3 connects this normally closed switch SW and the second
It is connected in parallel with the audio amplifier 5. Therefore, as shown in FIG. 5, when the DO detection pulse is not emitted from the DOo output circuit 2, the audio demodulated signal (2) is directly amplified by the audio amplifiers 4 and 5 and output as the audio output signal (2). However, the DOo output circuit 2 is
When the O detection detection pulse is generated, the normally closed switch SW opens.
During this time, the audio demodulated signal (■) will not be sent. For this reason,
Since the immediately preceding output of the first audio amplifier 4, which has been charged in the capacitor C2, is directly input to the second audio amplifier 5, the audio output signal (2) is subjected to the correction as shown. In this case, the loss of the FM playback signal ■ is for a short time of about 200 μS to 300 μS, so as soon as the capacitor C2 starts discharging, the normally closed switch SW closes again, and the normal audio demodulation signal ■ is sent again. .

Therefore, the DO in the conventional FM audio recording signal reproducing circuit
When a drop occurs in the FM playback signal ■, the C circuit maintains the previous audio signal level for the duration of the drop, thereby preventing noise caused by a sudden drop in the audio output signal ■, and ensuring proper output. Corrections can be made.

(b) High-speed playback of FM audio recording in 3x mode Also, FM audio recording on Hi-Fi-VTRs like Kono is different from recording on the original audio track, because it is recorded with FM modulation. Even when the playback speed is changed, it is possible to obtain a practically audible sound quality. For this reason, conventional VTRs play back previously recorded FM audio recordings such as news programs along with images at 2x or 1.5x faster speeds, making it possible to watch such programs in less time. It was from.

In this case, the VH3 system VTR is provided with a 3x mode that allows long-time recording in addition to the standard mode. However, in the standard mode, recording is performed with guard hands between each recording trunk. For this reason, when a standard mode FM audio recording is played back at high speed, when the head moves to the adjacent I rank, the signal is largely lost due to this guard hand, making it almost impossible to hear. However, in the case of the 3x mode, guard panless recording is performed with no gaps between recording tracks. For this reason, 3
When playing back an FM audio recording in double mode at high speed, even when the head moves to an adjacent track, there will be no major omission of (, 3).Therefore, when playing back an FM audio recording at high speed, uses this 3x mode.

[Problem to be solved by the invention]

However, although such high-speed playback eliminates signal dropouts due to guard-handless recording, the head
- Since the rack is crossed diagonally, a periodic level drop 4 occurs in the FM playback signal at the border.

The cause of this level down will be explained based on FIG. 6. FIG. 6(a) shows a trace trajectory when a triple speed mode recording of FM audio is played back at double speed. However, in order to make the explanation easier to understand, each recording trunk of the FM audio recording is shown connected to the front and back, and the trace locus is also drawn continuously on this.

In the H4F1 audio head, since the tape is fed at twice the speed, the tape is shifted by one trunk each time one field is traced, and the tape is traced diagonally with respect to the recording track. Therefore, the FM played from these heads
As shown in FIG. 6(b), the level of the envelope decreases every frame (approximately 33+n9) during the playback.

When such a level drop occurs in the FM playback signal, in the conventional FM audio recording signal playback circuit, the D
There has been a problem in that the OC circuit malfunctions, degrading the audio.

The malfunction of this DOC circuit will be explained based on FIGS. 3, 4, and 7.

In the DOo output circuit 2, when the envelope signal ◎ is extracted from the FM playback signal ■, a level drop appears every 33 m5. This is converted to the conventional 1]0 comparison voltage ■ by the comparison circuit 2a. Compared to
A DO detection pulse wave is generated over a period of s~10m5. If such a long DO detection pulse is sent to the DO correction circuit 3, the capacitor C2 will be completely discharged while the normally closed switch SW is open, making it impossible to perform correction.

For this reason, although the audio output signal (2) can originally be reproduced to some extent even in the level-down portion, it is cut off by the normally closed switch SW, resulting in a no-signal portion. The 5 to 10 mB no-signal portion that occurs approximately every 33 m5 makes the reproduced audio extremely difficult to hear.

Here, DO comparison voltage ■. By lowering the level of the envelope signal ◎, the DO detection circuit 2
can prevent false detection. However, during normal playback where high sound quality is required,
This is not a suitable solution since relatively minor signal loss cannot be corrected.

In addition, as long as the FM audio recording is guard-panless recorded, high-speed playback can be performed in the same way with VTRs of other types, and in this case, the DOC circuit operates in the same way and the sound quality deteriorates. The problem arises that the

[Failure to solve the problem]

In order to solve the above-mentioned problems, the FM audio recording signal reproducing circuit for a VTR according to the present invention compares the envelope of the FM reproduction signal with a predetermined DOo reference voltage to detect signal loss when reproducing an FM audio recording of a VTR. By detecting this missing period, the FM audio recording signal reproducing circuit that cuts off the audio demodulated signal and corrects the signal switches the DO comparison voltage to a lower voltage when reproducing the FM audio recording at high speed. It is characterized by the provision of a DOo comparison voltage switching circuit.

[For production]

Whether the FM audio recording is played back in the same normal way as when it was recorded or at high speed is determined by the operator's operation, so it can be easily detected by referring to this operation, control mode, etc. can.

When reproducing normal FM audio recording, the DO comparison voltage switching circuit does not work, so the DO comparison voltage has the same value as before. Therefore, omissions caused in the FM reproduction signal due to tape flaws or the like are detected in the same manner as in the past, and the audio demodulated signal is reliably corrected.

When this FM audio recording is played back at high speed, the head using the rotary head system traces diagonally to the recording track, resulting in periodic level drops in the FM playback signal. However, during high-speed reproduction, the DO comparison voltage switching circuit operates and the DO comparison voltage decreases. Then, since the envelope of the FM reproduction signal is compared with this reduced DO comparison voltage, the audio demodulated signal will not be cut off due to periodic level down. If this audio demodulation signal is cut off, sufficient correction will not be made because the level down will take a relatively long time, and furthermore, this level down will be repeated periodically, which will actually deteriorate the sound quality. . However, since the audio signal is not completely lost in this level-down portion, it can be used for high-speed playback where very high sound quality is not required. Therefore, since the audio demodulated signal of the level-down portion is also amplified and output, it is possible to prevent deterioration of sound quality due to cutting off of this portion.

Furthermore, a dropout in the FM reproduction signal whose level is lower than that of the level-down portion can be detected in the same manner as in the prior art by using the reduced DO comparison voltage. but,
A relatively minor signal loss cannot be detected using this DO comparison voltage. However, 200μs to 300μs
5 ma rather than reliably correcting a slight omission of about s.
Preventing interruption due to a level drop reaching ~10m5 is far more effective in improving sound quality.

Note that the DO comparison voltage switching circuit can easily switch the DO comparison voltage by switching the input circuit of the comparator using a mechanical switch, a non-contact switch, or the like.

〔Example〕

An embodiment of the present invention will be described below based on FIGS. 1 and 2. For convenience of explanation, components having the same functions as those of the conventional example shown in FIG. 3 are given the same reference numerals.

As shown in FIG. 1, the FM audio recording signal reproducing circuit of this embodiment receives input of the reproduced FM reproduction signal, an audio demodulated signal (2) obtained by FM demodulating the same, and a high-speed reproduction signal. The DOC circuit provided in this FM audio recording signal reproduction circuit is composed of a limiter amplifier 1, a DO detection circuit 2, and a DO correction circuit 3.

The FM reproduction signal ■ is input to this limiter amplifier 1. The limiter amplifier 1 is a circuit that shapes the waveform of the FM reproduction signal (2). The output of this limiter amplifier 1 is connected to a DO detection circuit 2.

The DO detection circuit 2 extracts an envelope signal from the FM replay signal via the limiter amplifier 1, and compares the envelope signal with the DO comparison voltage V. This is a circuit that compares the

This DO detection circuit 2 includes a filter 2C for extracting the envelope signal and a DO comparison voltage ■. A voltage dividing circuit 2d for obtaining the voltage is connected thereto. Filter 2C is a low-pass filter consisting of resistor R1 and capacitor C1. The voltage dividing circuit 2d has a power supply voltage ■. Variable resistance VR to 0
and a resistor R3 are connected in series and grounded. Then, the DO comparison voltage ■ generated at the variable terminal of this variable resistor VR. is supplied to the DO detection circuit 2. Furthermore, the variable resistor VR and the resistor R3 are connected to ground via a switching transistor Q. The switching transistor Q inputs a high-speed reproduction signal, and this signal is “
This circuit makes the circuit conductive and outputs the "L" level when the signal is at the "L" level. The high-speed playback signal is emitted from the control device of the VTR, and is a signal that becomes "1 ("j/hell) only in the case of high-speed playback.Therefore, in the case of normal playback, the switching transistor Q is cut off, so In this case, the DO comparison voltage (■) is the voltage value obtained by dividing the power supply voltage VCC by the variable resistor VR and the resistor R3. However,
This voltage value is the conventional DO comparison voltage ■ shown in FIG.

The variable resistor VR is adjusted and set so that it becomes the same value as . In addition, in the case of high-speed reproduction, the switching transistor Q becomes conductive, so the DO comparison voltage in this case
. 2) This is the divided voltage value of only the variable resistor VR, which is the DO comparison voltage during normal playback. The value is lower than 1. This D
O comparison voltage ■. For example, it is supplied to the DO detection circuit 2 and compared with the envelope signal. The level of the envelope signal is this DO comparison voltage ■. If it falls below,
The DO detection circuit 2 emits a pulse-shaped I) O detection pulse ◎. The output of this DO detection circuit 2 is
It is connected to the DO correction circuit 3.

The DO correction circuit 3 includes a normally closed switch sW and a capacitor C2.
It consists of The normally closed switch sw normally closes the circuit. However, when the DO detection pulse ◎ is emitted from the DO detection circuit 2, the circuit is opened only during the pulse width period of the DO detection pulse ◎. One end of the capacitor C2 is connected to the output side of this normally closed switch SW, and the other end is grounded.

This DO correction circuit 3 includes a first audio amplifier 4 and a second audio amplifier.
It is connected between the audio amplifier 5 and the audio amplifier 5. These audio amplifiers 4 and 5 are circuits that input the audio demodulated signal (2) and amplify it at a low frequency. The output of the first audio amplifier 4 is connected to the second audio amplifier 5 via the normally closed switch SW of the DO correction circuit 3. Therefore, the DOC circuit works and the normally closed switch SW
When opened, the audio demodulated signal ■ will be cut off.

Further, the capacitor C2 is charged at the level of the audio demodulated signal ■ while the normally closed switch SW is closed. Therefore, when the normally closed switch SW is opened, the level of the audio demodulation immediately before is held, and this signal level is sent to the second audio amplifier 5 while being discharged. The audio demodulated signal (2) amplified by the second audio amplifier 5 is outputted from the FM audio recording signal reproducing circuit as an audio output signal (4).

Note that this FM audio recording signal reproducing circuit is usually incorporated into an IC. However, D.

Filter 2c and voltage dividing circuit 2d of detection circuit 2 and DO
The capacitor C2 of the correction circuit 3 has conventionally been externally connected to the terminal of the IC. Therefore, even if this voltage dividing circuit 2d is modified as in this embodiment, there is no need to modify the IC.

The operation of the FM audio recording signal reproducing circuit having the above configuration will be explained.

During normal playback, in which the FM audio recording is played back at the same speed as when it was recorded, the high-speed playback signal is at the "■" level. Therefore, the switching transistor Q of the voltage dividing circuit 2d is cut off, and the DO detection circuit 2 has the following signals: The DO comparison voltage ■.l is supplied as in the conventional case. Therefore, as shown in FIG. The level of the envelope signal ◎ extracted by the filter 2C falls below the DO comparison voltage V.1 at this missing portion.Then, the DO detection circuit 2 emits the DO detection pulse ◎, and the normally closed switch SW of the DO correction circuit 3 is activated during that period. At the same time, the signal level immediately before the audio demodulated signal (■) is supplied from the capacitor C2 to the second audio amplifier 5.The period during which this signal is lost is at most 200 μs to 300 μs.
It is about s.

Therefore, before the capacitor C2 finishes discharging, the normally closed switch SW is disconnected, and the audio demodulated signal (2) is sent to the second audio amplifier 5 again. As a result, the audio output signal outputted from the second audio amplifier 5 is corrected for missing portions of the signal as shown in the figure. Also, as is clear from Figure 2(a), l) O
Comparison voltage■. .

Since there is a sufficient margin, correction can be performed reliably in the same manner as in the past even in the case of signal loss where the level does not drop significantly.

When this FM audio recording is played back at high speed, for the reason explained earlier based on FIG.
Periodic level down occurs as shown in Figure (a).

However, in this case, the high-speed reproduction signal is 'H'
level, the switching transistor Q becomes conductive, and the DOo output circuit 2 receives the previous DO comparison voltage V. , the DO comparison voltage V has a lower voltage value than . 2 is supplied. For this reason, the level down that appears in the envelope signal ◎ is equivalent to this DO comparison voltage ■ even when the level is the lowest. 2, and the DOo output circuit 2 outputs D.
O detection detection pulse signal is not emitted. Therefore, the normally closed switch SW remains closed, and the audio demodulated signal (2) is directly amplified by the audio amplifiers 4 and 5 and output as the audio output signal (2). Although the audio output signal (2) in this case has some deterioration in sound quality in the level-down portion, compared to the case where the signal in this portion is completely lost, it is possible to obtain a sufficiently usable sound quality.

Furthermore, it is clear that if a signal dropout as shown in FIG. 2(a) occurs during this high-speed reproduction, a similar correction will be made. However, in the case of a signal drop whose level does not drop significantly, it may not be possible to correct it because the DO comparison voltage VO2 is low. However, 5ma~1
Compared to a periodic level drop that reaches 0 m5, deterioration in sound quality due to a slight drop of only about 200 μs to 300 μs has almost no effect.

Note that in this embodiment, a case of double high-speed reproduction is shown. However, even in the case of 1.5x high-speed playback, the waveform is different because the slope of the playback trace changes, but the FM
They are the same in that the envelope of the reproduced signal changes periodically, and can be considered in the same way. Therefore, this 1.5
Even in the case of double high-speed reproduction, the DO comparison voltage (■) in this embodiment. The present invention can be implemented with the same circuit configuration by simply changing the level of 2.

(Effects of the Invention) As described above, the FM audio recording signal reproducing circuit for a VTR according to the present invention compares the envelope of the FM reproduction signal with a predetermined D0 comparison voltage when reproducing an FM audio recording for a VTR. In an FM audio recording signal reproducing circuit that detects a dropout and corrects the signal by cutting off the audio demodulated signal during the missing period, the DO comparison voltage is set to a lower value when the FM audio recording is played back at high speed. Switch Ruri.

The configuration includes a comparison voltage switching circuit.

As a result, during the reproduction of normal FM audio recording, the DO comparison voltage switching circuit does not work, and signal loss can be corrected in the same way as in the prior art. Furthermore, during high-speed reproduction, the DOo comparison voltage switching circuit operates to lower the DO comparison voltage, thereby eliminating the possibility of erroneously detecting a periodic drop in the level of the FM reproduction signal.

Therefore, the present invention has the effect of being able to prevent deterioration of sound quality during high-speed reproduction of FM audio recording.

[Brief explanation of the drawing]

1 and 2 show an embodiment of the present invention. FIG. 1 is a circuit block diagram of an FM audio recording signal reproducing circuit, and FIG. 2 is a circuit block diagram of an FM audio recording signal reproducing circuit. FIG. 2(a) is a time chart during normal playback, and FIG. 2(b) is a time chart 1 during high-speed playback. 3 to 7 show conventional examples, in which FIG. 3 is a circuit block diagram of an FM audio recording signal reproduction circuit, FIG. 4 is a circuit diagram of a DO detection circuit, and FIG. 5 is a diagram during normal reproduction. Figure 6(a) is a diagram showing the trace locus of the head on each recording track of FM audio recording.
) is a waveform diagram of the FM reproduction signal reproduced by the head based on this trace, and FIG. 7 is a waveform diagram of the FM audio recording signal reproduction circuit at high speed reproduction. 2 is a DOo output circuit, 2d is a voltage dividing circuit (DOo comparison voltage switching circuit), 3 is a DO correction circuit, and ■ o beam. This is a comparison voltage. Figure 2 (a) Figure 2 (b) Figure 4 Figure 35

Claims (1)

[Claims] 1. When playing back FM audio recording on a VTR, the envelope of the FM playback signal is compared with a predetermined DO comparison voltage to detect signal loss, and the audio demodulation signal is cut off during this loss period to correct the signal. In the FM audio recording signal reproducing circuit, when reproducing FM audio recording at high speed, DO
An FM audio recording signal reproducing circuit for a VTR, comprising a DO comparison voltage switching circuit for switching a comparison voltage to a voltage of a lower value.