JPH03139081A - Drop-out detecting circuit - Google Patents

Abstract

PURPOSE:To realize a stable detection by providing an envelope circuit whose detection sensitivity is high for an envelope detection of a reproducing RF signal which becomes a drop-out detection object. CONSTITUTION:A second envelope detecting circuit 1B covers a wide dynamic range of a reproducing RF signal and contributes to formation of a reference voltage for a drop-out decision. A first envelope detecting circuit 1A having high detection sensitivity provided separately from this circuit 1B executes an envelope detection of the reproducing RF signal which becomes a drop-out detection object. In such a way, when an input level of the reproducing RF signal is high, even if an output level of a first envelop detecting circuit is saturated, there is no hindrance, therefore, when the input level of the reproducing RF signal is minute, for instance, when whether it is a level to be decided to be drop-out or not is in a critical level, a drop-out detecting circuit whose envelope detection sensitivity is enhanced is obtained.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a dropout detection circuit in a reproduced RF signal such as an FM-modulated video signal, and in particular, to a dropout detection circuit for detecting a drop in the envelope of the reproduced RF signal. The present invention relates to a dropout detection circuit that improves detection stability when detecting dropout.

[Summary of the invention]

The present invention has an envelope detection circuit and a time constant circuit separately from a dropout detection system for a reproduced RF signal and a detection reference voltage setting system, and detects dropout by comparing the outputs of the two systems. In this dropout detection circuit, the stability of dropout detection is improved by making the detection sensitivity of the envelope detection circuit of the detected system larger than the detection sensitivity of the detection reference voltage setting system.

[Conventional technology]

A circuit that detects dropouts in reproduced RF signals such as FM-modulated video signals transmitted or recorded and reproduced includes:
For example, an envelope detection type circuit that detects a drop in the envelope of a reproduced RF signal is known. As an example, FIG. 2 shows a block diagram of a conventional dropout detection circuit, which has been rewritten to facilitate understanding, with reference to the technique disclosed in Japanese Utility Model Application No. 57117878.

The explanation will be given below according to FIG.

After the reproduced RF signal a passes through an envelope detection circuit (eg, rectifier circuit) 1, it is branched into a first line and a second line. First, on the first line, the signal is sent to one input terminal of the comparator circuit 5 through a first time constant circuit 2 that integrates with a relatively small time constant that can follow changes in the envelope of the reproduced RF signal. Next, the second line is smoothed by a second time constant circuit 3 that integrates with a sufficiently large time constant, the threshold level setting circuit 4 performs level adjustment for setting a dropout detection level, and the comparator circuit 5 sent to the other input terminal. The output of the comparison circuit 5 is then used as a dropout detection signal (Do pulse).

Incidentally, the input/output characteristics of the envelope detection circuit 1 of the conventional dropout detection circuit are as shown in FIG. 3, for example. In FIG. 3, the reference amplitude of the input signal of the reproduced RF signal is assumed to be OdB, and input/output characteristics applied when it is desired to determine that dropout has occurred when the amplitude falls below -20 dB are illustrated.

As is clear from Figure 3, if you try to obtain a wide dynamic range from when the input of the reproduced RF signal has a minute amplitude to, for example, +6 dB from the reference amplitude, there is a problem that the sensitivity at minute amplitudes will deteriorate. . In particular, when a low power supply voltage is used, it is difficult to achieve both a wide dynamic range and high envelope detection sensitivity, resulting in decreased dropout detection sensitivity and detection instability.

[Problem to be solved by the invention]

An object of the present invention is to solve the problems described in the prior art and to obtain a dropout detection circuit that eliminates instability in dropout detection.

[Means to solve the problem]

In the present invention, a first envelope detection circuit performs envelope detection on a reproduced RF signal with a first detection sensitivity and outputs the detection output to a first time constant circuit; a second envelope detection circuit that performs envelope detection at the second time constant circuit and outputs the detected output to a second time constant circuit having a sufficiently larger time constant than the first time constant circuit; A reference voltage is formed based on the output of the constant circuit, and a dropout is detected by comparing this reference voltage with the output of the first time constant circuit, and the first detection sensitivity is set to the first detection sensitivity. The dropout detection circuit is characterized in that the detection sensitivity is greater than the second detection sensitivity.

[Effect]

In the present invention, in addition to the second envelope detection circuit that covers a wide dynamic range of the reproduced RF signal and contributes to the formation of a reference voltage for dropout determination as in the past, the envelope detection circuit of the reproduced RF signal to be detected as a dropout detection circuit is provided. For detection, high detection sensitivity (first detection sensitivity) is used.
A first envelope detection circuit is provided.

As a result, when the input level of the reproduced RF signal is high, there is no problem even if the output level of the first envelope detection circuit is saturated, so when the input level of the reproduced RF signal is small, it is determined that, for example, a dropout (DO) has occurred. Since the sensitivity (the amount of displacement of the output voltage with respect to the displacement of the input voltage) can be increased when the voltage is at a critical level, the instability of dropout detection is eliminated and stable detection can be performed.

〔Example〕

An embodiment of the present invention will be described with reference to FIG. FIG. 1(a) is a block diagram, and is significantly different from the conventional example shown in FIG. 2 in that a first envelope detection circuit IA is provided.

Here, the reproduced RF signal a is immediately connected to the first line A and the second line A.
It is branched into line B.

First, on the line to the first line, a first envelope detection circuit IA performs highly sensitive detection as shown in FIG. 1(b). ) in FIG. 1, the detection output may be saturated if the input level is higher than the level (Not Do) that does not require determination as Do (dropout).

This detected output g is smoothed through a first time constant circuit 2 having a small time constant that can follow changes in the envelope of the reproduced RF signal, and is sent to one input terminal of a comparator circuit 5.

Next, on the line 2B, the first time constant is The average level is extracted by a second time constant circuit which has a sufficiently large time constant compared to the circuit, a reference voltage for dropout determination is formed by the threshold level setting circuit 4, and the voltage is applied to the other input terminal of the comparator circuit 5. Sent. The comparator circuit 5 may include a well-known comparator, a Schmitt trigger circuit, and the like, as well as a flip-flop circuit depending on the output format of the dropout (DO) pulse.

Note that the envelope detection circuit in this example is a re/
4: If the input level of the RF signal is high and the dropout determination level is also high, a rectifier circuit or the like can be used, but a balanced modulator is suitable for increasing the accuracy and stability of dropout detection. In particular, it is stated in the specification of the utility model recently filed by the applicant that it is preferable to use a circuit configuration that can simultaneously perform envelope detection and double the input signal frequency, such as a double-balanced differential amplifier. That's where it was. The reason is τ. When the reproduced RF signal is a reproduced signal recorded on a magnetic tape, it passes through an electromagnetic conversion system such as a magnetic head and a rotary transformer, so the zero cross is +
Although dropouts occur in waveforms that are not necessarily symmetrical from one side to the other, such dropouts can be reliably detected by doubling.

〔Effect of the invention〕

By implementing the present invention, it is possible to obtain a dropout detection circuit that has high envelope detection sensitivity when the input level of a reproduced RF signal is at a critical level as to whether or not it is at a level that should be determined as a dropout.

This improves the stability of dropout detection and provides a highly reliable dropout detection circuit.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention, (a) is a block diagram, and (a)
(C) shows the input/output characteristics of the envelope detection circuit (1), and (C) shows the input/output characteristics of the envelope detection circuit (2). FIG. 2 shows a conventional dropout detection circuit, and FIG. 3 shows the input/output characteristics of a conventional envelope detection circuit. IA - first envelope detection circuit IB...---
−−1−−−Second envelope detection circuit 2−・−・−
--------...-First time constant circuit 3 Second time constant circuit 4 ・-Threshold level setting circuit i5--・--・-
・−・・−Comparison circuit

Claims (1)

[Scope of Claims] A first envelope detection circuit that envelope-detects a reproduced RF signal with a first detection sensitivity and outputs the detection output to a first time constant circuit; a second envelope detection circuit that performs envelope detection with sensitivity and outputs the detected output to a second time constant circuit having a sufficiently larger time constant than the first time constant circuit; A reference voltage is formed based on the output of the time constant circuit, and a dropout is detected by comparing the reference voltage with the output of the first time constant circuit, and the first detection sensitivity is determined. A dropout detection circuit characterized in that the detection sensitivity is greater than the second detection sensitivity.