Frequency demodulating circuit for avoiding black reversing phenomenon

In a video reproducing apparatus having a head for reproducing a recorded frequency modulated signal and a frequency demodulator for demodulating the reproduced signal output from the head, a circuit interposed between the head and frequency demodulator comprises an adder having an input receiving the reproduced frequency modulated signal output from the head, a limiter connected between the output of the adder and the frequency demodulator for providing an amplitude limited output to the latter, and a feedback loop for also applying the amplitude limited output from the limiter to a second input of the adder so as to prevent occurrence of a reversing phenomenon and of an abnormal noise bar output, and thereby improve the reproduced picture quality.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to demodulators for frequency 
modulated (FM) signals and, more particularly is directed to a frequency 
demodulating circuit for a video reproducing apparatus, such as, a video 
tape recorder (VTR). 
2. DESCRIPTION OF THE PRIOR ART 
In a video tape recorder according to the prior art, a luminance signal is 
frequency-modulated, a chrominance signal is low-frequency-converted, that 
is, frequency converted to a frequency band below that of the FM luminance 
signal, and a mixed signal composed of the FM luminance signal and the 
low-frequency-converted chrominance signal is supplied to rotary heads and 
recorded thereby in slant record tracks on a magnetic tape. In such 
previously proposed VTR, in a variable speed playback mode thereof, such 
as, a still playback mode, a slow playback mode, a high speed playback 
mode or the like, the reproduced signal output from each rotary head is 
attenuated at each portion of the recorded signal where the magnetic head 
or heads cannot accurate1y scan the respective record track or tracks, 
with the result that a noise band is produced on the displayed picture. In 
other words, when the reproduced FM luminance signal is demodulated, a 
noise component, for example, due to head impedance noise and/or amplifier 
noise, is FM-modulated. Further, such noise is of a broad band so that the 
demodulated amplitude thereof becomes relatively large. 
In order to reduce the noise on the picture, the frequency band of the FM 
luminance signal being supplied to the FM demodulator may be limited to a 
predetermined band near the carrier frequency. If such band-limiting of 
the input FM luminance signal is employed, the demodulated signal will 
become insufficient in those portions where the level of the reproduced 
signal output by the head or heads is substantially low, so that a 
reversing phenomenon will occur. 
In order to prevent such reversing phenomenon in the reproducing or 
playback mode of a VTR, it has been proposed to supply the reproduced FM 
luminance signal from the rotary head or heads through an amplitude 
limiter to a high-pass filter which, when the level of the reproduced FM 
luminance signal is lower than that limited by the amplitude limiter, is 
effective to enhance the high band component. Such enhanced high band 
component is intended to avoid a black reversing phenomenon by which the 
level of the high band component is lowered and a so-called zero cross is 
not performed, as if the frequency of the reproduced FM luminance signal 
was considerably lowered. Further, in accordance with the prior art, the 
FM luminance signal derived from the high-pass filter is further 
amplitude-limited by an additional limiter prior to being applied to an FM 
demodulator for providing the respective luminance signal. In this 
connection, it is to be noted that the extent to which the black reversing 
phenomenon can be avoided depends on the order of the mentioned high-pass 
filter. However, if the order of such high-pass filter is increased for 
ensuring effective avoidance of the black reversing phenomenon, a white 
reversing phenomenon is caused by the additional limiter when a FM 
luminance signal having a relatively low frequency is input to the 
high-pass filter. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved frequency demodulating circuit for a video reproducing apparatus 
which can avoid the above mentioned problems encountered with the prior 
art. 
More specifically, it is an object of the present invention to provide a 
frequency demodulating circuit for a video reproducing apparatus in which 
a black reversing phenomenon can be prevented without giving rise to a 
white reversing phenomenon. 
It is another object of the present invention to provide a frequency 
demodulating circuit for a video reproducing apparatus, as aforesaid, 
which can prevent the generation of an abnormal output due to a noise bar 
occurring on the reproduced picture when a magnetic head fails to 
precisely scan or trace record tracks on a magnetic tape in a variable 
speed playback mode of the video reproducing apparatus. 
In accordance with an aspect of this invention, in a video reproducing 
apparatus having a head or heads for reproducing a recorded frequency 
modulated signal and a frequency demodulator for demodulating the 
reproduced frequency modulated signal, a circuit interposed between the 
head or heads and the frequency demodulator comprises an adder having an 
input receiving the reproduced frequency modulated signal, a limiter 
connected between the output of the adder and the frequency demodulator 
for providing an amplitude limited output to the latter, and a positive 
feedback loop connected between the output of the limiter and another 
input of the adder. 
In a preferred embodiment of the invention, the positive feedback loop 
includes first positive feedback means including first filter means for 
passing a predetermined first band of frequencies and a first attenuator 
connected in series with the first filter means and having a predetermined 
gain, second positive feedback means having second filter means for 
passing a second band of frequencies wider than the first band and a 
second attenuator connected in series with the second filter means and 
having a gain smaller than the gain of the first attenuator, switch means 
for selectively connecting the first and second positive feedback means 
between the output of the limiter and the second or additional input of 
the adder, and level detecting means for detecting the level of the 
reproduced frequency modulated signal and for controlling the switch means 
in accordance with the detected level so that the first and second 
positive feedback means are connected between the output of the limiter 
and the additional input of the adder when the detected level is less than 
and greater than, respectively, a predetermined level. 
The above, and other objects, features and advantages of the present 
invention, will be apparent in the following detailed description of 
preferred embodiments when read in conjunction with the accompanying 
drawings, in which the same reference numerals are used to identify the 
same or similar parts in the several views.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before proceeding with a detailed description of the present invention, a 
frequency demodulating circuit for video reproducing apparatus according 
to the prior art will be described with reference to FIG. 1 in order to 
provide a more complete understanding of the problems encountered 
therewith and which are overcome by the present invention. In the 
frequency demodulating circuit of FIG. 1, rotary magnetic heads 1A and 1B 
of a video reproducing apparatus are mounted at diametrically opposed 
positions on a tape guide drum (not shown), and have different azimuth 
angles. A magnetic tape 2 is helically wrapped around the tape guide drum 
with a wrap angle of approximately 180 degrees and is transported in the 
longitudinal direction of the tape at a predetermined tape speed during 
recording and normal reproducing operations. A mixed signal composed of an 
FM luminance signal and a low-frequency-converted chrominance signal is 
recorded by the heads 1A and 1B alternately as such heads scan or trace 
successive slant tracks extending obliquely across the tape. 
The mixed signal thus recorded on the magnetic tape 2 is reproduced 
therefrom by the magnetic heads 1A and 1B in alternately scanning the 
successive tracks and is supplied through a preamplifier 3 to a high-pass 
filter 4 which separates the FM luminance signal from the reproduced 
signal. The FM luminance signal is amplitude-limited by a limiter 5 and is 
fed from the latter to a high-pass filter 6. The high-pass filter 6 is 
operative, when the level of the FM luminance signal passed through the 
filter 4 to the limiter 5 is lower than the amplitude limit established by 
the limiter 5, to enhance the high band component for avoiding the 
so-called black reversing phenomenon. In accordance with such black 
reversing phenomenon, the level of the high band component is lowered so 
that a so-called zero cross does not occur as if the frequency of the FM 
luminance signal was considerably lowered. 
The FM luminance signal derived at the output of the high-pass filter 6, 
that is, the FM luminance signal whose high-band component has been 
enhanced to avoid the black reversing phenomenon, is further 
amplitude-limited by a limiter 7 prior to being fed to an FM demodulator 
8. The resulting reproduced luminance signal is supplied from the 
demodulator 8 through a deemphasizing circuit 9 to an output terminal 10. 
The ability of the circuit of FIG. 1 to avoid the black reversing 
phenomenon depends on the order of the high-pass filter 6, that is, the 
order of the high-pass filter 6 must be increased in order to improve the 
ability of the circuit to avoid the black reversing phenomenon. However, 
if the order of the high-pass filter 6 is thus increased, another problem 
is encountered. More specifically, when an FM luminance signal having a 
relatively low frequency is supplied to the high-pass filter 6, a ternary 
harmonic wave is increased by such filter, and a so-called white reversing 
phenomenon is caused by the following limiter 7. More specifically, when 
an FM luminance signal having a relatively low frequency, as shown 
schematically in FIG. 2A, is supplied to the high-pass filter 6 from the 
limiter 5, a ternary harmonic wave of the FM luminance signal is 
increased, as shown in FIG. 2B, by the high-pass filter 6 which has had 
its order increased for avoiding the black reversing phenomenon. If such 
FM luminance signal is supplied to the limiter 7, then some of the ternary 
harmonic waves will exceed a predetermined level, for example, the level 
indicated at Vth, so that so-called zero cross occurs. By reason of the 
foregoing, the limiter 7 generates an output (FIG. 2C) in which the white 
reversing phenomenon occurs, that is, the low frequency of the reproduced 
FM luminance signal is treated as though the original input signal had a 
high frequency. The extent to which the white reversing phenomenon occurs 
depends on the order of the high-pass filter 6 and the amplitude limiting 
level of the limiter 7. Thus, with the circuit according to the prior art 
as shown on FIG. 1, it is difficult to avoid both the black reversing 
phenomenon and the white reversing phenomenon. 
The avoidance of the above problem in accordance with the present invention 
will now be generally explained with reference to FIG. 4 in which an input 
FM signal, such as, the FM luminance signal obtained from the high-pass 
filter 4 in FIG. 1 is shown to be supplied to an input terminal 11. The FM 
signal applied to the input terminal 11 is supplied to one input of an 
adder 12 which has its output connected through a limiter 13 to an output 
terminal 14 which may be, for example, connected to the frequency 
demodulator 8 of FIG. 1. The output signal from the limiter 13 is also 
applied to a feedback circuit having a low-pass filter 15, a high-pass 
filter 16 and an attenuator 17 connected in series to another input of the 
adder 12. Thus, the feedback circuit constituted by the low-pass filter 15 
and the high-pass filter 16, which could be replaced by a single bandpass 
filter, and the limiter 17 provide a positive feedback by which the 
limiter 13 is controlled only in respect to the band of frequencies passed 
by the low-pass filter 15 and the high-pass filter 16. 
A typical frequency characteristic of the FM signal applied to the input 
terminal 11 is illustrated in FIG. 5A. As there shown, when the level of 
the input FM signal is high, the frequency characteristic is relatively 
flat. When the level of the input FM signal is decreased, the level of the 
high band component thereof is reduced considerably giving rise to the 
so-called black reversing phenomenon. When the level of the input FM 
signal is further reduced, the level of the carrier is also lowered with 
the result that a drop-out occurs. 
The purpose of the circuit shown on FIG. 4, and which generally embodies 
the present invention, is to avoid the black reversing phenomenon due to 
the lowered high band component of the frequency characteristic when the 
level of the input FM signal is decreased. More specifically, the circuit 
described with reference to FIG. 4 generally has the characteristic shown 
on FIG. 5B. As there shown, when the level of the input FM signal is 
sufficiently large, so that the limiter 13 is saturated and the amount of 
feedback through filters 15 and 16 and attenuator 17 to adder 12 is 
decreased, the characteristic of the circuit shown on FIG. 4 is 
substantially flat. When the level of the input FM signal is decreased, 
the amount of positive feedback is increased so that the high band 
component, that is, overshoot at the white peak side, is enhanced for 
causing so-called high band peaking. Therefore, the black reversing 
phenomenon is prevented by reason of the fact that the level of the high 
band component is maintained sufficiently high. In other words, the 
characteristics shown on FIGS. 5A and 5B are effectively mixed or 
combined, with the result that the output of the limiter 13 becomes 
substantially flat for avoiding the black reversing phenomenon. 
However, with the circuit shown in FIG. 4, the amount of positive feedback 
may be excessive in the case where the level of the input FM signal is 
lower than a predetermined level, for example, as when a drop-out occurs, 
and there is no signal to be demodulated. In that event, an oscillating 
phenomenon occurs, as illustrated at the lowermost portion of FIG. 5B. 
Such oscillating phenomenon particularly appears in the variable speed 
playback mode of the VTR, that is, when the tape speed is different from 
that used for recording, at which time the demodulated output becomes 
abnormal due to the noise bar resulting from the fact that the rotary 
heads do not accurately trace the slant tracks in which the video signal 
is recorded. 
Accordingly, it is the specific purpose of a preferred embodiment of the 
invention described below with reference to FIG. 3 to avoid the black 
reversing phenomenon while preventing an abnormal demodulated output due 
to a noise bar occurring in the variable speed playback mode of the VTR. 
In FIG. 3, parts of the FM demodulating circuit there illustrated which 
correspond to those previously described with reference to FIGS. 1 and 4 
are identified by the same reference numerals and will not be further 
described in detail. 
More specifically, the FM demodulating circuit of FIG. 3 is shown to 
generally comprise a first positive feedback circuit 20 and a second 
positive feedback circuit 21 which are alternatively connected between the 
output of the limiter 13 and the respective input of the adder 12. The 
first positive feedback circuit 20 is shown to include a low-pass filter 
22, a high-pass filter 23 and an attenuator 24 connected in a series 
circuit, while the second positive feedback circuit 21 includes a 
high-pass filter 25 connected to the junction between the low-pass filter 
22 and the high-pass filter 23 and an attenuator 26. Thus, the second 
positive feedback circuit 21 effectively includes the high-pass filter 25 
and attenuator 26 connected in series with each other and in parallel with 
the high-pass filter 23 and attenuator 24, and also connected in series 
with the low-pass filter 22. 
Although the first and second positive feedback circuits 20 and 21 are 
shown and described as sharing the low-pass filter 22, the same effects 
may be achieved by providing the circuits 20 and 21 with separate or 
independent low-pass filters (not shown) in place of the filter 22. 
Further, as shown in FIG. 6, it will be appreciated that the low-pass 
filter 22 and the high-pass filter 23 may be replaced by a band pass 
filter 29 in the first positive feedback circuit 20', while the low-pass 
filter 22 and the high-pass filter 25 are replaced by a corresponding band 
pass filter 30 in the second positive feedback circuit 21'. 
The cut-off frequency of the low-pass filter 22 in FIG. 3 is selected to be 
higher than the maximum frequency of the FM luminance signal separated by 
the high-pass filter 4 from the reproduced video signal, and the cut-off 
frequency of the high-pass filter 23 is selected to be lower than such 
maximum frequency of the input FM luminance signal while the cut-off 
frequency of the high-pass filter 25 is selected to be lower than the 
cut-off frequency of the high-pass filter 23. Further, the gain K1 of the 
attenuator 24 is selected to be greater than the gain K2 of the attenuator 
26. 
Similarly, in the embodiment shown in FIG. 6, the gain K1 of the attenuator 
24 is greater than the gain K2 of the attenuator 26, and the bandwidth of 
the frequencies passed by the filter 29 in the first feedback circuit 20' 
is smaller than the bandwidth of the frequencies passed by the filter 30 
in the feedback circuit 21'. The foregoing can be realized by providing 
the bands of frequencies passed by the filters 29 and 30, respectively, 
with substantially the same maximum frequencies higher than the maximum 
frequency of the input FM luminance signal, while the minimum frequency of 
the band passed by the filter 30 is lower than the minimum frequency of 
the band of frequencies passed by the filter 29. 
Referring again to FIG. 3, it will be seen that the outputs of the 
attenuators 24 and 26 in the feedback circuits 20 and 21, or 20' and 21', 
are connected to fixed contacts a and b of a switch circuit 27 which has 
its movable contact or output c connected to a respective input of the 
adder 12. A level detector circuit 28 is operative to detect the level of 
the FM luminance signal at the output of the high-pass filter 4 and to 
control the switching operation of the switch circuit 27 in accordance 
with such detected level, as explained more fully below. 
When the level of the envelope of the output of the high-pass filter 4 
detected by the level detector circuit 28 exceeds a predetermined level, 
the resulting output of detector circuit 28 causes the movable contact c 
of the switch circuit 27 to engage the fixed contact b so that the 
positive feedback circuit 21 is connected between the output of the 
limiter 13 and the respective input of the adder 12. On the other hand, 
when the level of the detected output from the high-pass filter 4 becomes 
less than the predetermined level, for example, an output level 
characteristic of a drop-out and from which no demodulated output would be 
generated, the detector circuit 28 causes the switch circuit 27 to engage 
its fixed contact a so that the positive feedback circuit 20 is then 
connected between the limiter 13 and the respective input of the adder 12. 
The operation of the frequency demodulating circuit of FIG. 3 will now be 
explained with reference to FIG. 5C which illustrates characteristics 
associated with the first and second positive feedback circuits 20 and 21. 
More specifically, when the level of the input FM signal from the 
high-pass filter 4 is less than the predetermined level at which the level 
detector circuit 28 causes the switch circuit 27 to engage its fixed 
contact b, that is, as long as the switch circuit 27 continues to engage 
its contact a, and thereby render operative the first positive feedback 
circuit 20, the limiter 13 is saturated and the amount of the feedback is 
decreased so that the characteristic of the frequency demodulating circuit 
is substantially flat. When the level of the input FM signal is increased, 
but is still not greater than the predetermined level at which the level 
detector circuit 28 causes the switch circuit 27 to change-over from 
engagement with its contact a to engagement with the contact b, the first 
positive feedback circuit 20 remains operative. Therefore, in response to 
such increase in the level of the input FM signal, the amount of positive 
feedback through the first positive feedback circuit 20 is increased, and 
the high-band region, that is, overshoot at the white peak side, is 
enhanced for achieving high-band peaking. By reason of the foregoing, the 
level of the high-band component is maintained sufficiently high so that 
the black reversing phenomenon is avoided. In other words, at the output 
side of the limiter 13, the characteristics shown on FIG. 5A and FIG. 5C 
are, in effect, combined to provide a substantially flat combined 
characteristic by which the black reversing phenomenon is avoided. 
When the level of the input FM signal is further increased so as to exceed 
the predetermined level, the level detector circuit 28 responds thereto by 
causing change-over of the switch circuit 27 so that its movable contact c 
engages the feedback circuit 21. The cut-off frequencies of the low-pass 
filter 22 and the high-pass filter 25 are selected so that such filters, 
when the feedback circuit 21 is operative, effect high-band peaking near 
the center of the frequency characteristic while the gain constant K2 of 
the attenuator 26 is selected so as to avoid oscillation. In other words, 
the second positive feedback circuit 21 does not cause the oscillating 
phenomenon shown in the lowermost portion of FIG. 5B, as will be apparent 
from the waveform shown at the lower portion of FIG. 5C. At the time of 
the change-over of the switch circuit 27 for making operative the feedback 
circuit 21 in place of the feedback circuit 20, the noise bar may look 
like so-called white noise. In that event, if the output level lies within 
the range between the black and white levels, the gain K2 of the 
attenuator 26 may be increased for permitting some oscillation under the 
conditions prescribed by the waveform appearing at the lowermost portion 
of FIG. 5C. In that case, an examination of the output of the frequency 
demodulator 8 will show the noise bar appearing as a gray single signal. 
As earlier described, in accordance with the prior art the enhancement by 
the high-pass filter 6 (FIG. 1) is made constant regardless of the level 
of the input FM signal and, as a result thereof, the white reversing 
phenomenon arises. However, according to the present invention, the 
enhancement by the high-pass filter 16, 23 or 25 in the positive feedback 
circuit is varied by changing the amount of feedback in response to the 
level of the input FM signal. Therefore, the present invention 
advantageously avoids the white reversing phenomenon. 
Furthermore, in the preferred embodiment of the invention shown in FIG. 3, 
the first and second feedback circuits 20 and 21 are selectively connected 
between the limiter 13 and the adder 12 so that, when the level of the 
input FM signal is less than a predetermined level, the first-positive 
feedback circuit 20 is selected to avoid the appearance of the reversing 
phenomenon and, when the level of the input FM signal is higher than the 
predetermined level, the second positive feedback circuit 21 is selected 
to cause the average level of the noise bar of the input FM signal to fall 
within a predetermined level range. Thus, the effectiveness in preventing 
the black reversing phenomenon can be increased without encountering the 
white reversing phenomenon. Furthermore, an abnormal noise bar output can 
be avoided in the variable speed playback mode so that the image quality 
of the reproduced picture is substantially improved. 
Although preferred embodiments of the invention have been described in 
detail herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to those precise embodiments, 
and that various changes and modifications may be effected therein by one 
skilled in the art without departing from the scope or spirit of the 
invention as defined in the appended claims.