JPH04123678A - Fm reproducing device - Google Patents

Abstract

PURPOSE:To constitute the device so that the detection of drop-out can be executed surely, and also, even if an FM signal level becomes small instantaneously, it is always limited by providing a second AGC circuit having a quicker response than a response of a first AGC circuit for a limiter, in addition to this first AGC circuit for detecting drop-out. CONSTITUTION:An FM reproducing signal amplified by an amplifier 13 through a head 12 from a tape 11 is inputted to an FM-AGC circuit 14 and 15. The FM-AGC circuit 14 does not respond to drop-out since its response is show. Therefore, a drop-out detecting circuit 16 can detect surely drop-out. On the other hand, the FM-AGC circuit 15 responds more quickly. Accordingly, even in the case drop-out is generated, and even in the case a frequency of an FM signal becomes high instantaneously and an output signal drops, an FM reproducing signal of a fixed level can always be outputted to a limiter 17.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an FM playback device used for signal processing particularly during playback of a VTR.

(Prior Art) Conventionally, regarding a reproduction system of a VTR, for example, a TV';a>
A report has been submitted to Hitachi, Ltd., ``TBS51-2 IC version of preamplifier for rVTR''.

Hereinafter, a brief description of the above report and its shortcomings will be explained with reference to FIG. According to this, the FM reproduction signal obtained from the tape 1 via the head 2 is first transmitted to the amplifier (
preamplifier) 3. Further, the FM reproduction signal output from the amplifier 3 is adjusted to a constant level (amplitude) by the FM-AGC circuit 4. Thereafter, the FM reproduction signal adjusted to a constant level is input to a dropout detection circuit 5 and a limiter (or double limiter) 6. Further, the FM reproduction signal output from the limiter 6 is FM demodulated by the FMI modulator 7.

However, the FM-AGC circuit 4 is intended to equalize the output levels between the two channels and to keep the input levels of the dropout detection circuit 5 and limiter 6 constant. Therefore, the response of the FM-AGC circuit 4 necessarily needs to be slowed down. If the response is fast, even if the signal level suddenly drops due to the occurrence of dropout, the FM-AGC circuit 4 will respond to the noise signal and keep the FM reproduction signal at a constant level. In this case, the dropout detection circuit 5 is unable to detect dropouts, so that the subsequent processing circuit cannot process the noise signal, and the noise appears on the screen as it is.

On the other hand, the FM-AGC circuit 4 is required to respond quickly. In other words, if the response is fast, even if the instantaneous output level of the FM playback signal obtained from the tape 1 via the head 2 becomes small, the FM-AGC circuit 4 can respond to it, so that the zero cross at the limit and vine 6 can be This is because it can eliminate gaps. In addition, if you choose a slow response,
As shown in FIG. 6, even if the instantaneous output level of the FM reproduction signal becomes small, the signal is output as is from the FM-AGC circuit 4 (see FIG. 6(a)).

For this reason, although it is possible to detect dropout, a missing portion A of the zero cross may occur in the limiter 6 (see FIG. 6(b)).

Therefore, FM demodulation of the missing portion A cannot be performed, and an inversion phenomenon occurs, resulting in a screen that is difficult to view. Generally, the inversion phenomenon occurs when the frequency of the instantaneous FM signal increases, and the output level of the FM signal decreases (indicated by B in (a) in the same figure), which causes the limiter to malfunction, resulting in inversion. .

(Problems to be Solved by the Invention) As described above, conventionally, the response of the FM-AGC circuit had to be slowed down in order to detect dropout or the like. On the other hand, if the response is slow, zero crossings may be missing in the limiter. When a zero cross loss occurs, FM demodulation of that part becomes impossible,
Since a reversal phenomenon occurs, the resulting screen has the drawback of being difficult to view.

The present invention has been made to solve the above drawbacks,
It is an object of the present invention to provide an FM playback device that can reliably detect dropouts and that is always limited even if the instantaneous output level from a tape becomes small.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the FM playback device of the present invention has the following features:
an amplifier that amplifies a minute FM signal; a first AGC circuit to which the output signal of the amplifier is input; and the first AGC circuit.
a second AGC circuit having a faster response than that of the circuit and into which the output signal of the amplifier is input; and the first AGC circuit.
It includes a dropout detection circuit to which the output signal of the circuit is input, a limiter to which the output signal of the second AGC circuit is input, and a repeater to which the output signal of the limiter is input.

Further, the first AGC circuit has an amplifier that amplifies the minute FM signal, a first AGC circuit to which the output signal of the amplifier is input, and a response that is faster than the response of the first AGC circuit.
a second AGC circuit into which the output signal of the GC circuit is input;
A dropout detection circuit to which the output signal of the first AGC circuit is input, a limiter to which the output signal of the second AGC circuit is input, and a demodulator to which the output signal of the limiter is input. There is.

(Function) According to such a configuration, the first
Separately from the AGC circuit, a second AGC circuit having a faster response than the first AGC circuit is provided for use as a limiter. In other words, the signal input to the dropout detection circuit is the output signal of the first AGC circuit with a relatively slow response, and the signal input to the limiter is the output signal of the second AGC circuit with a relatively fast response. It is. Therefore, dropout can be detected reliably, and even if the instantaneous FM signal level becomes small, it is always limited, so that an inversion phenomenon caused by a lack of a zero cross in the limiter can be prevented.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an FM reproducing apparatus according to an embodiment of the present invention. Moreover, FIG. 2 is a waveform diagram showing the waveform of the FM reproduction signal output from each block.

Since the FM playback signal obtained from the tape 11 via the head 12 is a very low level signal, the amplifier 1
Amplified by 3. Further, the amplified FM reproduction signals are input to FM-AGC circuits 14 and 15, respectively.

The FM-AGC circuit 14 has the same functions and effects as the conventional FM-AGC circuit. In other words, FM-AGC circuit 1
The response of 4 is slow, and the variation in the signal obtained from the head 12 is suppressed (a VTR has at least two heads, and signals are taken out alternately from these two heads), and a constant level is suppressed. The purpose is to obtain a reproduced signal.

The FM reproduction signal output from the FM-AGC circuit 14 (see FIG. 2(a)) is input to the dropout detection circuit 16. The dropout detection circuit 16 detects points (where the playback signal level suddenly drops due to scratches on the tape, etc.) during playback.
This is for detecting the signal (indicated by C in FIG. 2(a)). Note that the output signal output from the dropout detection circuit 16 is used as a timing pulse in a later video processing circuit to replace it with a video signal of one horizontal period earlier by utilizing the horizontal correlation of the video signal.

On the other hand, the FM-AGC circuit 15 has faster response than the conventional FM-AGC circuit.

In other words, the frequency of the instantaneous FM signal increases, and the head 12
If the signal level played from tape 1 drops or
It is possible to quickly respond to all signal level fluctuations (indicated by D in FIG. 2(a)), such as when the signal level fluctuates due to surface irregularities. Therefore, FM-
The FM reproduction signal output from the AGC circuit 15 is always adjusted to a constant level (see FIG. 2(b)). Thereafter, the FM reproduction signal adjusted to a constant level by the FM-AGC circuit 15 is input to the limiter 17. The limiter 17 is a circuit that creates a zero-crossing point of the input FM reproduction signal (see FIG. 2(c)). The output signal of limiter 17 is input to FM demodulator 18 . The FM demodulator 18 is a circuit that forms a pulse of a constant width from the zero-crossing point of the FM reproduction signal (see FIG. 2(d)). FMI
The output signal consisting of pulses of a constant width outputted from the modulator 18 is converted into a video signal by passing through an integrating circuit (see FIG. 2(e)).

According to such a configuration, the FM-AGC circuit 14 has a slow response like the conventional FM-AGC circuit, and therefore does not respond to dropout. Therefore, the dropout detection circuit 16 can reliably detect dropouts. On the other hand, the FM-AGC circuit 15 is similar to the conventional FM-AGC circuit 15.
The response is faster than the AGC circuit. For this reason, the FM-AGC circuit 15 is activated when dropout occurs or when the frequency of the instantaneous FM signal increases and the output signal decreases (generally, the reproduction electromotive force of the tape head is , decreases as the frequency f increases.) etc., it responds quickly to such fluctuations, so it is possible to output an FM reproduction signal at a constant level at all times.

Therefore, this signal is passed through the limiter 17 to the FM demodulator 1.
8, a good screen without any inversion phenomenon can be obtained.

Note that the FM-AGC circuit 15 also responds to dropouts, so it may demodulate the dropout noise signal, but even if the noise signal is demodulated, there will be no problem because the dropout can be compensated for after demodulation. There isn't.

FIG. 4 is a block diagram showing an FM reproducing apparatus according to another embodiment of the present invention.

In this embodiment, the FM reproduction signal output from the amplifier 13 passes through the FM-AGC circuit 14, and then passes through the FM-AGC circuit 14.
The configuration is exactly the same as that of the embodiment described above except that the signal is input to the circuit 15. Further, since the response of the FM-AGC circuit 14 is slow and the response of the FM-AGC circuit 15 is fast, the operation is also exactly the same as in the above embodiment.

Even in such a configuration, the same effects as in the above embodiment can be obtained.

[Effects of the Invention] As described above, the FM playback device of the present invention provides the following effects.

Apart from the FM-AGC circuit for dropout detection, an FM with a faster response than that of this FM-AGC circuit.
- An AGC circuit is provided as a limiter. Therefore, the signal input to the dropout detection circuit is the output signal of the FM-AGC circuit with a relatively slow response, and the signal input to the limiter is the output signal of the FM-AGC circuit with a relatively fast response.
This is the output signal of the circuit. Therefore, it is possible to provide an FM reproducing apparatus that can reliably detect dropouts and prevent reversal phenomena because it is always limited even if the instantaneous FM signal level becomes small.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an FM playback device according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing the waveform of the FM playback signal output from each block in FIG. 1, and FIG. is a diagram showing the relationship between the recording signal frequency and the reproduction electromotive force, the fourth
FIG. 5 is a block diagram showing the configuration of an FM playback device according to another embodiment of the present invention, FIG. 5 is a block diagram showing the structure of a conventional FM playback device, and FIG. 6 is an output from each block in FIG. 5. FIG. 2 is a waveform diagram showing a waveform of an FM reproduction signal. DESCRIPTION OF SYMBOLS 11...Tape, 12...Head, 13...Amplifier, 14.15...FM-AGC circuit, 16...Dropout detection circuit, 17...Limiter, 18...
・FM demodulator. Applicant's representative Patent attorney Takehiko Suzue Figure r--i Figure density (＼=2gX＼・9) Recording frequency X: Decrease due to recording demagnetization, head eddy current loss, etc. Y: Thickness Decrease due to loss chart

Claims (2)

[Claims] (1) An amplifier that amplifies a minute FM signal, a first AGC circuit to which the output signal of this amplifier is input, and a response that is faster than that of the first AGC circuit, and the output signal of the amplifier is A second AGC circuit to which the output signal of the first AGC circuit is input, a dropout detection circuit to which the output signal of the first AGC circuit is input, a limiter to which the output signal of the second AGC circuit is input, and an output signal of this limiter. An FM reproducing device comprising: a demodulator into which is input. (2) an amplifier that amplifies a minute FM signal; a first AGC circuit to which the output signal of the amplifier is input; and a first AGC circuit having a faster response than the first AGC circuit;
a second AGC circuit into which the output signal of the GC circuit is input;
A dropout detection circuit to which the output signal of the first AGC circuit is input, a limiter to which the output signal of the second AGC circuit is input, and a demodulator to which the output signal of the limiter is input. An FM playback device characterized by: