JPH0614733B2 - Audio signal reproduction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention reproduces an audio signal superposed on a video signal by a rotary magnetic head different from a rotary magnetic head for recording and reproducing a video signal, and reproduces the audio signal in a predetermined manner. The present invention relates to an audio signal reproducing circuit suitable for being applied to a VTR or the like adapted to be converted into a television signal.

[Conventional technology]

In a two-head helical scan type VTR, in order to improve the quality of audio signals and obtain high-fidelity sound quality, audio signals such as left and right audio signals are FM-modulated and then supplied to a pair of rotary magnetic heads together with color video signals. I try to record it.

In this case, as shown in FIG. 5, the audio signal is recorded and reproduced by a pair of rotary magnetic heads H _AA and H _AB different from the pair of rotary magnetic heads H _VA and H _VB for recording and reproducing the video signal. VTRs have been proposed (for example, Japanese Patent Laid-Open No. 59-
No. 103478, etc.).

Audio heads H _AA and H _AB are video heads
H _VA, with mounted for prior to H _VB, head H _VA for video as shown in FIG. 6, the azimuth angle theta _VA of H _VB, to theta _VB, head H _AA for audio, H azimuth angle theta _AA of _AB, theta _AB is selected large, the gap width G _VA ~G _AB also head H _AA for audio, towards the H _AB is chosen to be greater.

Then, these heads H _{VA to} H _AB record and record the video signal and the audio signal on the same track.
FIG. 7 shows an example of this, and the preceding heads H _AA , H
_The audio signal is recorded _deeply by _AB , and the video signal is recorded shallowly on it by the moving heads H _VA and H _VB .

Figure 8 is shows a frequency characteristic of the case of employing such a recording method, the frequency characteristic of the carrier chrominance signal C _D that is the luminance signal Y _FM and the low frequency transformations FM modulated figure A as is well known
become that way. On the other hand, the FM audio signal S _A has the FM luminance signal Y _FM and the low-frequency carrier color signal C _D as shown in FIG.
Its occupied bandwidth is chosen to be in the frequency band between.

The FM carrier frequency ₁ of the left audio signal is 1.35 MHz in this example, and the FM carrier frequency ₂ of the right audio signal is 1.5.
The frequency deviation width Δ is set to about ± 200 KHz.

When the audio signal recording / playback method is selected in this way, the video signal is compatible with the normal method, and the audio signal and the video signal are recorded in different recording azimuths. However, even if the occupied bandwidth of the audio signal is wide and its level is high, the recording / reproduction of the video signal is hardly affected.

Therefore, an effect such that an audio signal having a wide dynamic range of, for example, 80 dB and a frequency characteristic extended to a high frequency (20 Hz _{S to} 20 KHz) can be recorded and reproduced can be obtained.

In order to reproduce an audio signal recorded in such a wide dynamic range and a wide band, a reproducing circuit (10) as shown in FIG. 9 may be used.

First, the carrier frequency ₁ is FM-modulated with the left audio signal L.
When the composite FM audio signal of the FM audio signal L _F (Δ is about ± 200 KHz) and the FM audio signal R _F in which the carrier frequency ₂ is FM-modulated by the right audio signal R is S _A and this is recorded by the head H _AA To S _AA (= L _FA + R _FA )
In the case where recorded by the other head H _AB S _AB (=
L _FB + R _FB )

The FM audio signals S _AA and S _AB reproduced by the pair of heads H _AA and H _AB are fed to the left channel and right channel playback systems (3L) and (3R) via preamplifiers (2A) and (2B), respectively. Supplied. In the left channel reproduction system (3L), (4L) and (5L) are bandpass filters each having a center frequency of ₁ ,
The left audio signals L _FA and L _FB extracted from these are supplied to the FM demodulators (6L) and (7L) respectively and FM demodulated, whereby the left audio signals L _A and L _B are obtained alternately.

The right channel reproduction system (3R) is also configured in the same way, the center frequencies of the bandpass filters (4R) and (5R) are selected as _2, and the right audio signals R _FA and R _FB are extracted from this, and this is the FM demodulator.
The right audio signals R _A and R _B are alternately obtained by FM demodulation in (6R) and (7R).

The left audio signals L _A and L _B are the first switching means (8L)
The right audio signals R _A and R _B are transmitted to the second switching means (8
R), and by switching them with a 30 Hz pulse PG, a continuous left audio signal L is output from the first switching means (8L) and the second switching means (8R).
Outputs a continuous right audio signal R. These audio signals L and R are led to the line-out terminals (12L) and (12R) via the low-pass filters (9L) and (9R) and the noise reduction circuits (11L) and (11R), respectively.

When the demodulated audio signals L and R are supplied to the television receiver together with the video signal reproduced from the tape (1), they must be once converted into a signal form of a normal television signal. Therefore, the reproduction circuit (10) is further provided with a television signal conversion circuit (20).

Therefore, the left and right audio signals L and R are first detected by the resistors R _A and R.
_After being resistance-mixed by _B into a synthesized audio signal S _A , it is band-limited by a low-pass filter (21) provided for the reason to be described later, and further supplied to a limiter (22) to limit the level above a predetermined level. It Limiter audio signal S _AL together with the high-frequency enhanced by pre-emphasis circuit (23), is FM-modulated by the FM modulator (24), supplying the FM synthesized speech signal FM-S _A is the terminal (25) The output terminal (28) has a predetermined signal form by being supplied to the adder (26) together with the reproduced video signal S _V , frequency-multiplexed and then AM-modulated by the AM modulator (27). Television signal
S _TV is obtained.

The low pass filter (21) and the limiter (22) are provided for the following reason.

That is, the synthetic audio signal S _A reproduced from the tape (1)
Has a very wide dynamic range of about 80 dB, and its frequency characteristic extends up to 20 kHz. Therefore, if this audio signal S _A is directly pre-enhanced and FM-modulated, the audio signal S _A of high input level At this time, the frequency deviation width (about ± 30 kHz) in the FM modulator (24) may be exceeded, which causes a beat between the audio signal S _A and the video signal S _V , causing beat interference in the television image. If there is beat interference, the image quality will be significantly degraded.

To eliminate this beat interference, a low pass filter (21)
The pass band is limited to about 15kHz by
The limiter (22) limits the level.

[Problems to be solved by the invention]

However, when the audio signal S _A is band-limited and the limiter applies FM modulation as described above, the following problems occur.

First, when the limiter level is set to a high level to some extent, the distortion of the audio signal becomes almost no problem, but with such a high limiter level, the FM modulator (2
The maximum frequency deviation width of 4) may be exceeded, and in such a case, as described above, the audio signal S _A and the video signal S _{V are}
The beat caused by causes the image quality to deteriorate significantly.

If the limiter level is lowered in order to eliminate the image quality deterioration due to the beat interference, then the audio signal S _A is distorted and high sound quality cannot be expected.

Therefore, in the past, it was not possible to satisfy both at the same time so that the sound quality would be sacrificed when aiming at higher image quality.

The present invention solves these problems and proposes an audio signal reproducing circuit which can expect high sound quality without affecting the video signal.

[Means for solving problems]

In order to solve the above problems, the present invention uses a filter (21).
Instead of the limiter (22), an AGC circuit (30) having a de-emphasis characteristic is provided as shown in FIG.

[Action]

The AGC circuit (30) performs a normal AGC operation and has input / output characteristics and frequency characteristics such that the de-emphasis amount increases when the input level of the audio signal S _A is high and the frequency thereof increases. .

Therefore, since AGC is applied according to the input level of the audio signal S _A , and how the AGC is applied differs depending on the frequency, no distortion occurs even with a high-level or high-frequency audio signal S _A. Moreover, the level is limited to a level within the frequency shift width of the FM modulator (24). Therefore, beat interference with the video signal does not occur. Since the audio signal S _A is FM-modulated without distortion, the sound quality is high.

〔Example〕

FIG. 1 shows an audio signal reproducing circuit (10) according to the present invention.
An example is shown.

In the present invention, the audio signal S _A is the AGC circuit (30), although the same reference numerals are given to the portions corresponding to those in FIG.
Is supplied to the pre-emphasis circuit (23).

Fig. 2 shows an example of this AGC circuit (30), and the input terminal (31)
Is supplied audio signal S _A is supplied to an AGC amplifier (32) via a resistor R _1, the output audio signal S _LO, which is the gain control is obtained at the output terminal (33) to. The output audio signal S _LO is further supplied to the level detection amplifier (34), where the AGC level (AC component) is detected and, as will be described later, a predetermined frequency characteristic, that is, pre-emphasis characteristic with respect to this detection output. Is added, and the detection output is converted into a DC component by the detection circuit (35). Detection circuit (35)
Is composed of a pair of diodes D ₁ and D ₂ and a capacitor C _1, and this detection output is divided into a predetermined value by resistors R ₂ and R ₃ and elements G _{1 to} G ₃ provided for temperature compensation. The divided detection output is supplied to the transistor Q ₁ which is a variable impedance element via the diode D ₃ . Therefore, this detection output operates as an AGC control output, and the voltage division ratio due to this impedance and the resistance R ₁ is AG.
Controlled by C control output.

Here, the time constant circuit consisting of the capacitor C ₁ , the capacitor C ₂ connected in parallel with the resistor R ₃ and the diode D ₃ starts the AGC control operation (attack time) quickly at the time of excessive input and stops the operation (recovery time). Is provided to make it slower.

An emphasis circuit (40) is provided on the feedback path of the level detection amplifier (34) in addition to the feedback resistor R ₄ . The emphasis circuit (40) is a parallel circuit composed of a pair of resistors R ₅ and R ₆ and a pair of capacitors C ₃ and C ₄ , and the capacitor C ₄ is selected to be sufficiently larger than C ₃ . The impedance of the emphasis circuit (40) is determined only by the resistor ₆ when the feedback frequency is low, and is determined by the parallel resistance of the resistors R ₅ and R ₆ when the feedback frequency is high. Therefore, the input / output characteristic of the feedback path is a de-emphasis characteristic.

Since the emphasis circuit (40) is connected to the feedback path of the detection amplifier (34), the frequency characteristic of the level detection amplifier (34) including the emphasis circuit (40) shows the pre-emphasis circuit as shown in FIG. It becomes a characteristic.

Here, ω ₁ and ω ₂ represent the frequencies increased and decreased by 3 dB in the pre-emphasis characteristic curve. In this example, ω ₁ = 1 / R ₆ C ₃ ≈1.5 kHz, ω ₂ = 1 / R ₅ C ₃ = ≈ 4.8kHz is set.

If the level detection amplifier (34) having the pre-emphasis characteristic is inserted in the negative feedback path of the AGC circuit (30) in this way, the AGC circuit
The total frequency characteristic of (30) is the de-emphasis characteristic.

The de-emphasis characteristic provided by the AGC circuit (30) is selected so as to be slightly pre-emphasized when viewed as a total frequency characteristic considering the pre-emphasis characteristic of the pre-emphasis circuit (23).

The AGC circuit (30) is thus AGC characteristics (input / output level characteristics)
In addition, when a frequency characteristic that causes de-emphasis is added, first, when the input level of the audio signal S _A is low, the detection level of the detection circuit (35) is low, so that the impedance of the transistor Q ₁ is high. Therefore, the amount of partial pressure is reduced accordingly, and the output level of the output audio signal S _LO is increased. At this time, since the feedback level of the level detection amplifier (34) is originally small, the pre-emphasis amount of this level detection amplifier (34) is small. Therefore, the frequency characteristic including the total input / output level characteristic when the input level is low is 4th.
It becomes like a diagram curve l ₁ or l ₂ .

On the other hand, when the input level becomes higher, the impedance of the transistor Q ₁ becomes smaller accordingly, the amount of voltage division increases, the output level is lowered, and the feedback level of the level detection amplifier (34) increases. At this time, the higher the input frequency is, the more the amount of pre-emphasis of the level detection circuit (34) increases, so that the output level of the output audio signal S _LO decreases toward the higher frequency side. Therefore, the frequency characteristics including the total input / output level characteristics when the input level is high are as shown by the curves l _{3 to} l _{5 in} FIG. 4, and the output level decreases as the input frequency increases. Become.

It should be noted that since the output audio signal S _LO is de-emphasized at a high input level, noise may increase, but at this time, since the input level is originally high, the S / N does not deteriorate much even if de-emphasized.

By providing the AGC circuit (30) in this way, it is possible to output the audio signal S _A from a low input level to a high input level without distortion, and the higher the input frequency at the high input level, the higher the input frequency. Since the output level will drop, even if the output audio signal S _LO having such input / output level characteristics and frequency characteristics is pre- _emphasized by the pre-emphasis circuit (23), it will be
There is no risk of exceeding the maximum frequency deviation width of the modulator (24).
Therefore, the beat with the video signal does not occur, and the image quality is not deteriorated due to the beat interference. Further, since there is no limiter action, the waveform of the output audio signal S _LO is not distorted even at a high input level, and the sound quality is not deteriorated because the pass band of the audio signal S _LO is not limited.

As described above, according to the present invention, since the band of the signal recorded on the tape (1) is originally wide band and is suitable for reproducing an audio signal having a wide dynamic range, the present invention is as described above. Not only can it be applied to a VTR reproduction system that records and reproduces audio signals such as audio signals by a pair of or more rotary magnetic heads different from the rotary magnetic heads for recording and reproducing video signals, and rotation that records and reproduces video signals. It can also be applied to a VTR playback system that obtains high-quality sound such that an audio signal is superimposed and recorded using a magnetic head.

Further, the television standard system can be applied to the PAL system, the NTSC system and other television standard systems.

〔The invention's effect〕

As described above, according to the present invention, the audio signal S _A is pre-emphasized and FM-modulated via the AGC circuit (30) having the de-emphasis characteristic. Therefore, the audio signal from a low input level to a high input level is FM modulation can be performed without distorting S _A. Therefore, it is possible to improve the sound quality of the audio signal.

Further, since the audio signal S _A is not limited even when its level is high, there is no beat interference with the video signal due to the harmonics of the audio signal S _A caused by the limiter. Therefore, the image quality does not deteriorate due to beat interference.

Therefore, the present invention is extremely suitable when applied to a VTR reproducing circuit in which a wide dynamic range and a wide band audio signal is recorded.

[Brief description of drawings]

FIG. 1 is a system diagram of an essential part showing an example of an audio signal reproducing circuit according to the present invention, and FIG. 2 is an AGC used for this.
A connection diagram showing an example of a circuit, FIGS. 3 and 4 are characteristic diagrams for explaining the operation thereof, FIG. 5 is a configuration diagram of a rotary magnetic head device for explaining the present invention, and FIG. 6 is used for this. FIG. 7 is an explanatory diagram of a rotating magnetic head, FIG. 7 is an explanatory diagram of its magnetization pattern, FIG. 8 is its frequency characteristic diagram, and FIG. 9 is a system diagram of a conventional audio signal reproducing circuit. (10) is a reproduction circuit, H _AA and H _AB are rotary magnetic heads for audio, (3L) and (3R) are left and right channel reproduction systems, (20) is a television signal conversion circuit, and (23) is a pre-emphasis circuit. , (2
4) and 27) are modulators, (30) is an AGC circuit, (32) and (34) are amplifiers, (40) is an emphasis circuit, and Q ₁ is a variable impedance element.

Claims (1)

[Claims] 1. An FM demodulation circuit for FM demodulating an FM audio signal reproduced by a pair of heads in each channel, a signal synthesizing circuit for synthesizing the FM demodulated first and second audio signals, and the above signal. And a television signal conversion circuit for converting an audio signal output from the synthesizing circuit into an audio signal form of a television signal, wherein the television signal converting circuit amplifies the audio signal output from the signal synthesizing circuit by a predetermined amplification degree, and An AGC circuit that decreases the amplification amount by a predetermined de-emphasis amount according to an increase in frequency, and decreases the amplification factor and increases the de-emphasis amount when the level of the input audio signal increases. And a modulation circuit for modulating the output of the pre-emphasis circuit. Reproducing circuit of the audio signal, characterized in that it comprises at least and.