Apparatus using comparator to detect drop-out of recorded video signal

A drop-out detecting apparatus comprises an amplitude detecting circuit supplied with a high frequency video signal reproduced from a record medium, a first smoothing circuit supplied with a detected output of the amplitude detecting circuit and having a short time constant so that it produces an output having a drop-out, a second smoothing circuit supplied with the detected output of the amplitude detecting circuit and having a long time constant so that it produces an output having no drop-out, a detecting circuit for level-comparing a smoothed output of the second smoothing circuit and a first reference level to produce an output of large or small level, and a comparing circuit for level-comparing a smoothed output of the first smoothing circuit and a second reference level provided by attenuating the detected output of the detecting circuit by a predetermined attenuating ratio, wherein the comparing circuit derives a drop-out detecting signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to drop-out detecting apparatus 
and, more particularly, is directed to a drop-out detecting apparatus 
which can accurately detect a drop-out from a high frequency video signal 
reproduced from a record medium even when the level of the high frequency 
video signal is fluctuated or even when the level thereof is extremely 
small. 
2. Description of the Prior Art 
In the most simplified drop-out detecting apparatus for detecting a 
drop-out from a high frequency video signal (frequency-modulated video 
signal) played back from a magnetic tape by a rotary magnetic head in a 
video tape recorder (VTR), the reproduced high frequency video signal is 
envelope-detected and the resultant envelope signal is compared with a 
constant reference level, whereby the portion of the compared signal less 
than the reference level is generated as a drop-out detecting signal. 
The above conventional drop-out detecting apparatus has a drawback such 
that, if the reference level is not changed in accordance with the change 
of the level of the reproduced high frequency video signal, then it cannot 
detect the drop-out accurately. 
In order to remedy the above-mentioned drawback, another conventional 
drop-out detecting apparatus is proposed, in which the reproduced high 
frequency video signal whose gain is automatically controlled is 
envelope-detected, the resultant envelope signal is compared with the 
constant reference level and the portion of the compared signal less than 
the reference level is generated as the drop-out detecting signal. 
The above conventional drop-out detecting apparatus becomes complicated in 
arrangement because it needs an automatic gain control (AGC) circuit and 
also has the drawback such that it cannot detect a drop-out if the level 
of the high frequency video signal is fluctuated during the time when the 
rotary magnetic head comes in contact with the magnetic tape. 
Other drop-out detecting apparatus is further proposed to remedy the above 
shortcoming. According to this previously-proposed drop-out detecting 
apparatus, as shown in FIG. 1, the reproduced high frequency video signal 
is envelope-detected and the resultant envelope signal is attenuated by a 
predetermined attenuation ratio of, for example, -16 dB or -20 dB, thereby 
produced as a reference level. Then, the envelope signal is level-compared 
with the reference level and the portion of the signal lower than the 
reference level is generated as a drop-out detecting signal. 
According to the above drop-out detecting apparatus in which the reproduced 
high frequency video signal is envelope-detected, the resultant envelope 
signal is attenuated by the predetermined attenuation ratio so as to be 
generated as the reference level, the envelope signal is compared with the 
reference level and the portion of the signal lower than the reference 
level is produced as the drop-out detecting signal, the drop-out can be 
accurately detected because the reference level changes in proportion to 
the level of the envelope signal of the reproduced high frequency video 
signal. The above drop-out detecting apparatus, however, cannot produce 
the drop-out detecting signal if a signal-to-noise (S/N) ratio of an 
FM-demodulated output of the high frequency video signal is deteriorated 
in the signal portion, the level of which is made extremely small, such as 
when the rotary magnetic head scans a guard band area of a slant track 
formed on a magnetic tape if the level of the high frequency video signal, 
i.e., envelope signal is changed comparatively gently and is considerably 
fluctuated as in the still playback mode as shown in FIG. 2. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved drop-out detecting apparatus in which the aforementioned 
shortcomings and disadvantages encountered with the prior art can be 
eliminated. 
More specifically, it is an object of the present invention to provide a 
drop-out detecting apparatus which can accurately detect a drop-out 
regardless of the fluctuation of the level of a high frequency video 
signal reproduced from a record medium or even if the level of such high 
frequency video signal is extremely small. 
As an aspect of the present invention, a drop-out detecting apparatus 
comprises an amplitude detecting circuit supplied with a high frequency 
video signal reproduced from a record medium, a first smoothing circuit 
supplied with a detected output of the amplitude detecting circuit and 
having a short time constant so that it produces an output having a 
drop-out, a second smoothing circuit supplied with the detected output of 
the amplitude detecting circuit and having a long time constant so that it 
produces an output having no drop-out, a detecting circuit for 
level-comparing a smoothed output of the second smoothing circuit and a 
first reference level to produce an output of large or small level, and a 
comparing circuit for level-comparing a smoothed output of the first 
smoothing circuit and a second reference level attenuated by a 
predetermined attenuating ratio of the detected output of the detecting 
circuit, wherein the comparing circuit derives a drop-out detecting 
signal. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following detailed description of 
an illustrative embodiment thereof to be read in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the drop-out detecting apparatus according to the present 
invention will now be described in detail with reference to FIG. 3. 
Referring to FIG. 3, an input terminal 1 is supplied with a high frequency 
video signal (frequency-modulated video signal) which is reproduced from a 
slant track of a magnetic tape by a rotary magnetic head, though not 
shown. The high frequency video signal applied to the input terminal 1 is 
supplied to and envelop-detected by an amplitude detecting circuit 
(envelope-detecting circuit in this embodiment) 2, and an envelope signal 
from the envelope-detecting circuit 2 is supplied to first and second 
smoothing circuits 3 and 4. 
The first smoothing circuit 3 has a short time constant so that, if a 
drop-out occurs in the high frequency video signal, then it produces an 
output having a drop-out. 
The second smoothing circuit 4 has a long time constant so that, even if a 
drop-out occurs in the high frequency video signal, then it produces an 
output having no drop-out. 
A smoothed output b (see FIG. 4) of the second smoothing circuit 4 is 
supplied to a maximum value detecting circuit (analog OR circuit) 5. The 
maximum value detecting circuit 5 compares the smoothed output b of the 
second smoothing circuit 4, i.e., the input signal b with a constant 
reference level c (see FIG. 4) to produce an output of larger level. 
Alternatively, the envelope signal from the amplitude detecting circuit 2 
may be smoothed by a smoothing circuit, the time constant of which is 
sufficiently longer than those of the smoothing circuits 3, 4 and a 
smoothed output therefrom may be employed as the reference level c. 
While the input signal b and the reference level c are both positive as 
described above, if the input signal b and the reference level c are both 
negative, then the maximum value detecting circuit 5 will be replaced with 
a minimum value detecting circuit. Further, if the input signal b and the 
reference level c are positive and negative, then the maximum value 
detecting circuit and the minimum value detecting circuit may both be 
employed. 
A detected output from the maximum value detecting circuit 5 is supplied to 
a variable attenuator (variable attenuator or half-fixed attenuator) 7, in 
which it is attenuated by a predetermined attenuation ratio (e.g., -16 dB, 
-20 dB, etc.). 
The smoothed output of the smoothing circuit 3, that is, the input signal a 
and the output, which results from attenuating the detected output of the 
maximum value detecting circuit 5 by the attenuator 7, that is, a 
reference level d (see FIG. 4) are supplied to a level comparing circuit 
(e.g., operational amplifier, a differential amplifier, etc.) 8, in which 
they are level-compared. Then, the level comparing circuit 8 produces a 
drop-out detecting signal e (see FIG. 4) when the level of the input 
signal a is lower than the reference level d. 
If the input signal a and the reference level d are both negative and when 
the level of the input signal a is higher than the reference level d, then 
the level comparing circuit 8 may produce the drop-out detecting signal. 
Further, although hysteresis circuits may be provided in the maximum value 
detecting circuit 5 and the level comparing circuit 8, they are not shown 
in FIG. 3 and will not be described herein for simplicity. 
According to the above-mentioned drop-out detecting apparatus, the 
reference level d level-compared with the smoothed output a by the level 
comparing circuit 8 becomes proportional to the envelope signal if the 
level of the reproduced high frequency video signal applied to the input 
terminal 1 is high. Whereas, the reference level d becomes the reference 
level c in the portion where the level of the high frequency video signal, 
i.e., the envelope signal is extremely small such as when the rotary 
magnetic head scans the guard band area of the slant track formed on the 
magnetic tape if the above level is changed comparatively gently and is 
fluctuated considerably. 
Accordingly, even if the level of the high frequency video signal 
reproduced from the record medium is fluctuated or even if the level of 
the high frequency video signal is extremely small, then the drop-out can 
be detected accurately. When the drop-out detecting signal is obtained, 
the FM-demodulated output of the high frequency video signal, that is, the 
video signal is substituted with, for example, the one horizontal 
period-preceding signal portion, the S/N ratio of which is excellent. 
According to the present invention as described above, the drop-out 
detecting circuit comprises the amplitude detecting circuit supplied with 
the high frequency video signal reproduced from the record medium, the 
first smoothing circuit supplied with the detected output of the amplitude 
detecting circuit and whose time constant is short so that it produces the 
output having the drop-out, the second smoothing circuit supplied with the 
detected output of the amplitude detecting circuit and whose time constant 
is long so that it produces the output having no drop-out, the detecting 
circuit for level-comparing the smoothed output of the second smoothing 
circuit and the first reference level to produce the output of larger or 
smaller level and the comparing circuit for level-comparing the smoothed 
output of the first smoothing circuit and the second reference level 
provided by attenuating the detected output of the detecting circuit by 
the predetermined attenuating ratio, wherein the drop-out detecting signal 
is produced from the comparing circuit so that, even if the level of the 
high frequency video signal reproduced from the record medium is 
fluctuated or even if the above level is extremely small, the drop-out can 
be detected accurately. 
Having described the preferred embodiment of the invention with reference 
to the accompanying drawings, it is to be understood that the invention is 
not limited to that precise embodiment and that various changes and 
modifications thereof could be effected by one skilled in the art without 
departing from the spirit or scope of the novel concepts of the invention 
as defined in the appended claims.