JPS60167161A - Drop-out detecting circuit - Google Patents

Abstract

PURPOSE:To detect drop-out surely by simple constitution by a rectifier for rectifying restored FM signals and obtaining envelope of the restored FM signals from output signals of the rectifier. CONSTITUTION:A restored FM signal having waveform (a) is rectified by a rectifier circuit 11 to waveform (b) and applied to the first and second integrating circuits 12, 13. Output of the first integration circuit 12 is that obtained by envelope detecting the restored FM signal as waveform (e), and output of the second integration circuit 13 is integrated by time constant sufficiently larger than time constant of the first integrating circuit 12 as shown by waveform (c). Then, output of the second integrating circuit 13 is voltage divided (for instance about 70%) to a proper value (drop-out detection level) by a level adjusting circuit 14. Accordingly, even if the level of the restored FM signal changes depending on kind of tape and degree of use of the tape, time at which the restored FM signal lowered from normal level by specified rate can be detected instantaneously as drop-out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to dropout detection means for compensating for dropouts of reproduced frequency modulation signals (FM signals) of VTRs, etc., and attempts to detect dropouts reliably with a simple configuration. However, dropout compensation is generally performed in VTRs, etc., and when the amplitude of the reproduced FM signal falls below a certain value, it is assumed that a dropout has occurred, and the video signal of that period is Replaced with video signal. However, the reproduced FM signal level varies by several dB depending on the type of tape, and even with the same tape, the reproduced FM signal level varies depending on the degree of use. Therefore, increase the dropout detection sensitivity (make it so that when the FM signal level drops even a little, it is considered a dropout).

Depending on the type of tape and the degree of use, dropouts may be detected or eliminated. Therefore, when increasing the conventional dropout detection sensitivity, as shown in Figure 1, it is necessary to
AGO is applied to the M signal.

Here, the response speed of the λQG loop 1 is slow, and the MuaC circuit 1 is set to such an extent that the input to the envelope detection circuit 2 is kept constant even if the type of tape changes, and the envelope detection circuit 2 A level detection circuit 3 detects a period in which the signal obtained by envelope detection of the FM signal is below a certain level. 2
Moreover, the AcrC circuit becomes necessary, making the circuit configuration complicated and expensive.

The present invention aims to solve the above-mentioned problems with a simple configuration, and one embodiment thereof will be described below with reference to the drawings.

FIG. 2 shows one embodiment of the present invention, which will be explained together with FIG. 4 showing its operating waveforms.

The reproduced FM signal as shown in waveform a in FIG.
Added in 2.13. Here, the first integrating circuit 12
The output of the second integrating circuit 13 is obtained by envelope detection of the reproduced FM signal as shown in waveform e, and the output of the second integrating circuit 13 is integrated with a time constant that is sufficiently larger than the time constant of the first integrating circuit 12, as shown in waveform C. are doing. Next, the output of the second integration circuit 13 is adjusted to an appropriate value (dropout detection level) by the level adjustment circuit 14.
There is a partial pressure (for example, 7-sided root degree). As described above, a signal such as waveform d divided by the level adjustment circuit 14 is output to the comparison circuit 1.
5, and the output of the first integrating circuit 12 (waveform e) is added to the other side, and the voltages are compared between the two, and a dropout detection signal as shown in waveform f is obtained at the output of the comparator circuit 16. ing. Here, playback F depending on the type of tape etc.
Even if the M signal level changes, only the absolute value of the voltage at the input of the comparator circuit 15 changes as shown in waveforms e' and d' shown in FIG. 6, and the voltage ratio of the two inputs remains constant.

This is because the two input signals of the comparator circuit 15 are basically generated from the same reproduced FM signal, and only the frequency characteristics differ due to the time constant of the integrating circuit, and the voltage ratio is adjusted to a constant ratio by the level adjustment circuit 14. This is because only partial pressure is applied. Therefore, even if the level of the reproduced FM signal changes depending on the type of tape or the degree of use of the tape, a dropout can be detected when the reproduced FM signal instantly drops by a certain percentage from the normal level.

The present invention has been explained above using block diagrams, but the present invention has the advantage that dropout is usually less than a few horizontal scanning periods, and that in a dropout compensation circuit using an analog delay line, it is generally more than 4 to 5 horizontal scanning periods. It can be assumed that it is necessary and sufficient to detect only a dropout of about 1 (ms) or less using the property of 1. ing.

Next, FIG. 3, which represents the block diagram shown in FIG. 2 as a more specific circuit diagram, will be described. Again, af corresponds to af in waveforms 4 to 6. The reproduced FM signal a amplified by the amplifier 2o is passed through the diode D1. rectified by D2, then capacitor “I + resistor R,,R
2 is integrated by 1 (the second integration circuit 1 in Fig. 2).
3 and level adjustment circuit 14). A signal such as the waveform obtained in this way, capacitor C2, and resistor R
A comparator 21 compares the voltage with the signal e'' integrated by 3.The resistor R4 is sufficiently large compared to the resistor R3 because the comparator applies positive feedback in order to perform Schmitt operation, and the capacitor C1y resistor R ,, the time constant of R2 is sufficiently thicker than the time constants of capacitor C2°, resistor RS, and I't4.Here, when Schmitt operation is not performed, - old λ force aJ - output f/ of comparator 21.
Although the winter whistle 61 is shown as a reduced f button, when considering the switching noise of the dropout compensation circuit, it is more advantageous to use a dropout detection signal such as a Schmitt-operated waveform f.

As described above, according to the present invention, dropout can be detected stably even if the level of the reproduced FM signal changes depending on the type of tape or the degree of use, and the circuit can be easily constructed at low cost.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an overview of a conventional dropout detection circuit, FIG. 2 is a block diagram showing an embodiment of the present invention, FIG. 3 is a specific circuit diagram of the same, and FIGS. 4 and 6. , and FIG. 6 are waveform diagrams of the same operation. 11... Rectifier circuit, 12, 13... Integrating circuit, 14... Level adjustment circuit, 16. Old.
・Comparison circuit. Name of agent: Patent attorney Toshio Nakao Haga 1st person
ll51! 2 Figure 2 Figure 3 184 Figure 5 e' Figure 6 g"

Claims (1)

[Claims] rectifying means for rectifying the reproduced FM signal; first integrating means for obtaining the envelope of the reproduced FM signal from the output signal of the rectifying means; a second integrating means for obtaining an average value and having a time constant sufficiently larger than the time constant of the first integrating means; and a level adjusting means for dividing the output signal of the second integrating means into an appropriate ratio. , a dropout detection circuit comprising: comparison means for comparing the output of the first integration means and the output of the level adjustment means.