Audio signal muting circuit for magnetic recording and reproducing apparatus

In an apparatus for recording a frequency modulated audio signal and a frequency modulated video signal on the frequency multiplex basis onto the same recording track, a muting circuit is necessary so that the reproduced audio signal is not outputted when a dropout takes place and/or when the frequency modulated audio signal is missing on the track. The muting circuit is controlled not only by the output signal of a dropout detector, but by a signal obtained by integrating the dropout detection signal so that the audio output is blocked during a frequent occurrence of dropout. The muting circuit includes a previous value holding circuit serving to hold its output at an input level immediately before the control signal has been given, the previous value holding circuit being arranged in the reproduction audio signal processing system following a demodulator.

The present invention relates to a muting circuit for use in a circuit 
which reproduces the recorded frequency modulated audio signal, and more 
particularly to a muting circuit for eliminating noise from an audio 
signal reproduced by a magnetic recording and reproducing apparatus which 
records and reproduces a frequency modulated video signal and a frequency 
modulated audio signal on the frequency division multiplex basis. 
In a typical, currently used magnetic recording and reproducing apparatus 
for a video signal, a video signal is recorded on a skewed track on a 
magnetic tape by means of a rotary head and an audio signal is recorded on 
a longitudinal track provided at the edge of the magnetic tape by means of 
a fixed head. In such magnetic recording and reproducing apparatus, it is 
desired to record the signal as long as possible on a tape of a limited 
length. This is eagerly required especially for the household apparatus. 
For a longer recording time, a higher recording density is required, which 
is achieved by a narrower video track and a slower tape speed. As a 
result, the tape speed which is slower than that of the audio compact 
cassette tape recorder is required. Although such a slow tape speed is not 
a problem in recording and reproducing a video signal since the rotational 
speed of the head dominantly determines the relative speed between the 
rotary head and the magnetic tape, it would be difficult for the fixed 
head to obtain the necessary practical frequency band in recording and 
reproducing an audio signal. 
It is contemplated that the audio signal is multiplexed with the video 
signal, so that it is also recorded on the skewed video track. In this 
manner, recording and reproduction for obtaining a high quality audio 
reproduction signal are made possible merely by assigning a part of the 
wide video signal band to the audio signal. 
In multiplexing the audio signal and the video signal, since the recording 
and reproducing system is generally adapted for recording a frequency 
modulated video signal, it is desirable for the audio signal to be 
multiplexed also as a frequency modulated signal having a center frequency 
separate from the FM video signal band. In the case of the household video 
tape recorder in which the color subcarrier is converted to a lower band, 
for example, the FM audio signal can be placed between the FM video signal 
band and the frequency converted color subcarrier band. The video signal 
track for the rotary head is very narrow and liable to cause a dropout due 
to a defect of the magnetic tape, dusts or the like. Therefore, in such 
audio signal recording and reproducing system, means for counteracting a 
dropout must be provided. In the case of the audio signal, a defect of the 
reproduced FM signal caused by a dropout will create noise having a large 
amplitude. Also a portion of the magnetic tape where no FM signal is 
recorded will cause the reproducing circuit to create large noise during 
the reproducing operation. Therefore, it is necessary to prevent the 
output of such noise. 
In order to cope with these problems, there are provided means for 
detecting a dropout in the reproduced signal and a previous value holding 
means which holds the demodulated audio signal output during the period of 
a dropout at a level immediately before it has been detected, so that the 
dropout compensation and the muting operation take place. Such system 
functions satisfactorily for a momentary dropout, however, the problem is 
not solved for a relatively long portion of the tape where only the FM 
video signal is recorded without the FM audio signal (the dropout 
detection means generates a dropout detection signal also in this case). 
The dropout detection means typically consists of means for detecting that 
the amplitude of the FM signal picked up by the head is smaller than the 
specified level. Accordingly, the dropout detection means for the audio 
signal is arranged such that it detects the amplitude of the FM audio 
signal after the signal has been selected out of the amplified and 
reproduced FM signal. When the FM audio signal is not recorded for a long 
period, the amplitude of noise sensed by the dropout detection means often 
exceeds the detection level irregularly due to the effect of the side band 
wave of the FM video signal, residual magnetism on the magnetic tape or 
noise created by the amplifier. As a result, the system operates 
erroneously as if a normal reproduction output is obtained when the 
amplitude of a noise is actually large, resulting disadvantageously in an 
output of large noise. 
It is an object of the present invention to provide a muting circuit which 
effectively eliminates noise from the reproduced output signal caused by 
dropout and a blank portion of the tape for recording and reproduction 
system where the audio signal is frequency modulated. 
The characteristic of the present invention is that a dropout of the FM 
audio signal is detected and a compensation circuit such as a previous 
value holding circuit for conducting the demodulated audio signal is 
controlled by the dropout detection signal so that the audio output is 
held during a dropout at a level immediately before the dropout has 
occurred, and at the same time the dropout detection signal is integrated 
so that the integrated output also controls the compensation circuit when 
it exceeds the predetermined value. According to one aspect of the present 
invention, when dropout occurs frequently in a short period the 
compensation circuit is controlled to maintain the muting state even if 
the dropout detection circuit erroneously determines that this is not 
dropout and the record is reproduced normally. In this manner, the output 
of unwanted noises can be prevented and effective muting can take place in 
reproducing a portion of the tape where the FM audio signal is not 
recorded.

FIG. 1 shows the primary portion of the reproducing apparatus in which case 
the present invention is applied to a magnetic recording and reproducing 
apparatus of 2-head helical scanning system. In the figure, a frequency 
modulated video signal and a frequency modulated audio signal in the 
frequency multiplex form are recorded on a magnetic tape 1. The signals 
are recorded in such a way, for example, that a frequency modulator for 
generating a modulated wave in response to the video signal and a second 
frequency modulator which is set to a frequency range separate from the 
frequency shift by the first mentioned modulator and adapted to perform 
frequency modulation in response to the audio signal are provided, and the 
outputs of the two modulators are added and supplied to rotating heads 2 
and 3 for recording. This arrangement is identical to that of the well 
known household 2-head helical scanning video tape recorder, except that 
the audio signal is multiplexed with the video signal. 
In 2-head helical scanning system, as is well known in the art, the two 
heads 2 and 3 trace the recording track alternately and thus they output 
the signals alternately. The read out signals are amplified by 
preamplifiers 4 and 5, and then united into a continuous signal by a 
switching circuit 6 which operates in synchronization with the rotation of 
the rotary head. The output of the switching circuit 6 undergoes the 
selection for the frequency modulated video signal by means of a filter 
(not shown in the figure) connected to an output terminal 7, and then 
undergoes the reproduction process for the video signal by a signal 
processing circuit known in the art. The output of the switching circuit 6 
is further undergoes the selection for the frequency modulated audio 
signal by a band-pass filter 8, then supplied to an FM demodulator 9 and a 
dropout detector 14. The FM demodulator 9 has an amplitude limiter, and 
may include an amplifier when necessary. The audio signal demodulated by 
the demodulator 9 is filtered through a low-pass filter 10 to eliminate 
unwanted frequency components, and supplied to a previous value holding 
circuit 11 which performs the muting operation. 
The previous value holding circuit 11 ceases to conduct the input signal to 
the output when it receives the control signal, and holds the output at a 
signal level which was inputted immediately before the control signal has 
been given. The circuit shown in FIG. 2, for example, may be used as the 
previous value holding circuit 11. In FIG. 2, an input terminal P.sub.1 
receives the demodulated audio signal, and a transistor Q.sub.1 forms an 
emitter follower. Transistors Q.sub.2 and Q.sub.3 are switching devices 
and turned ON and OFF by a control transistor Q.sub.4 which in turn turned 
ON and OFF by the control signal received at another input terminal 
P.sub.2. When the input terminal P.sub.2 does not receive the signal, the 
transistor Q.sub.4 is in a cutoff state causing the transistors Q.sub.2 
and Q.sub.3 to conduct, and the audio signal coming through the emitter 
follower is conducted to an output terminal P.sub.3 through a field effect 
transistor Q.sub.5. When the input terminal P.sub.2 receives the control 
signal (here, the dropout detection signal), the transistor Q.sub.4 turns 
ON to cut off the transistors Q.sub.2 and Q.sub.3. Consequently, the 
charging voltage of a capacitor C.sub.1 which has been varying in response 
to the audio signal is fixed to a certain voltage level and outputted 
through the field effect transistor Q.sub.5 having a high input impedance. 
Accordingly, the audio signal is not conducted during the period when the 
control signal is given, and thus the muting operation takes place. The 
audio signal coming through the previous value holding circuit 11 is 
outputted through a de-emphasis circuit 12. The de-emphasis circuit is 
commonly used in processing the frequency modulated signal for correcting 
the frequency response which has been emphasized in the recording process. 
The circuit for obtaining the control signal, which is a characteristic of 
the present invention, applied to the previous value holding circuit 11 
will now be described. In order to detect a dropout of the FM audio signal 
picked up from the magnetic tape, but before it is demodulated, the output 
of the low-pass filter 8 is supplied to a dropout detector 14. The dropout 
detector 14 detects a decrease in the amplitude of the FM audio signal, 
and can be constituted by a rectifying circuit and a level discrimination 
circuit. The circuit shown in FIG. 3 which is suitable for integration may 
be used, for example, as the dropout detector 14. In FIG. 3, the reference 
number 14a denotes a rectifying circuit, in which transistors Q.sub.6 and 
Q.sub.7 perform rectification and amplification. The FM audio signal 
supplied to an input terminal P.sub.4 is rectified and amplified, then 
smoothed by a capacitor C.sub.2. Then, the signal is supplied to a level 
discrimination circuit 14b which is constituted by differential 
transistors Q.sub.8 and Q.sub.9. If the voltage level of the rectified and 
smoothed signal is higher than the predetermined value, the collector 
voltage of the transistors Q.sub.6 and Q.sub.7 are low and, consequently, 
the transistor Q.sub.8 turns ON and Q.sub.9 is cut off, resulting in no 
output at an output terminal P.sub.5. If the voltage level of the 
rectified signal becomes lower than the predetermined value, the 
transistor Q.sub.8 is cut off and, consequently, the transistor Q.sub.9 
turns ON, resulting in a high level output at the output terminal P.sub.5. 
The voltage level at which the transistor Q.sub.9 turns ON is set by a 
variable resistor VR.sub.1. Accordingly, when a dropout of the FM audio 
signal occurs, a high level voltage will appear at the output terminal 
P.sub.5 during the period of the dropout. This output is supplied through 
an OR gate 17 to the previous value holding circuit 11 as a control 
signal, as shown in FIG. 1, so that the noise does not appear in the audio 
output signal in the occurrence of the dropout. However, if the previous 
value holding circuit 11 is operated directly by the detection output of 
the dropout detector 14, numerous dropout detection outputs in a short 
period cause the circuit to fail to determine that these outputs are 
created by noises, and in this case the noise will appear in the 
reproduced output. In order to prevent such situation, an integrating 
circuit 15 and a threshold circuit 16 are added as shown in FIG. 1. In 
operation, the output of the dropout detector 14 is integrated by the 
integrating circuit 15 and the integrated detection output is 
discriminated by the threshold circuit 16 such that when the output level 
exceeds the predetermined value, the control signal for controlling the 
previous value holding circuit 11 is issued. The output of the threshold 
circuit 16 and the immediate output of the dropout detector 14 are summed 
up by the OR gate 17 and supplied to the previous value holding circuit 
11. Thus, the previous value holding circuit 11 is controlled by either of 
the output signals. 
The integrating circuit 15 can be formed of a capacitor and a resistor, as 
is well known in the art, and it may include an amplifier for further 
amplifying the output of the dropout detector when necessary. For the 
threshold circuit 16, any circuit which outputs a signal for controlling 
the previous value holding circuit 11 when an input higher than the 
predetermined level is given may be used. For example, the circuit shown 
in FIG. 4 which is similar to the level discrimination circuit 14 in FIG. 
3 for constituting the dropout detector 14 may be used. This circuit is 
adapted to receive the integrated output at its terminal P.sub.6. If no 
dropout occurs or the occurrence of dropout is rare, a low base voltage of 
the transistor Q.sub.10 brings it into conduction, causing the transistor 
Q.sub.11 to be cut off, and no output appears at the output terminal 
P.sub.7. On the other hand, if dropout occurs frequently in a short 
period, the integrated dropout detection signal goes high, and when it 
exceeds the level set by a variable resistor VR.sub.2, the transistor 
Q.sub.10 is cut off, causing the transistor Q.sub.11 to turn ON, and a 
high level output will appear at the output terminal P.sub.7. 
In this manner, when dropout occurs frequently in a short period, the 
holding state is maintained by the integrated dropout detection signal, 
thus the muting operation takes place continuously. Accordingly, during a 
long absence of a recorded signal on the tape, such malfunctioning that 
the dropping detector fails to operate due to noise or the like and the 
holding state is released for a short period to output the noise can be 
prevented. 
Operation of the foregoing arrangement will now be described with reference 
to the signal waveforms shown in FIG. 5. An assumption is made that in the 
FM audio signal selected by the low-pass filter 8, a dropout occurs at 
time t.sub.P, and no modulation signal is reproduced after time t.sub.Q as 
shown in FIG. 5a. It is also assumed that noises N.sub.1 -N.sub.4 having 
relatively large amplitude occur after the time t.sub.Q due to the effect 
of the side band wave of the FM video signal and due to noises originated 
in the recording medium, magnetic head, amplifier, or the like. When such 
frequency modulation signal is supplied to the dropout detector 14, the 
noises N.sub.1 -N.sub.4 are erroneously determined as if dropout has 
ceased, resulting in a detection output as shown in FIG. 5b. It should be 
noted that demodulation of the signal of FIG. 5a without any compensation 
will create the noises in the dropout period at t.sub.P and after t.sub.Q 
as shown in FIG. 5f. When the same signal is compensated by holding solely 
with the dropout detection signal, the noise caused by the dropout at 
t.sub.P is suppressed from being outputted owing to the holding operation, 
but noises will be produced at the points of time corresponding to the 
original noises N.sub.1 -N.sub.4. The dropout detection signal is 
integrated as shown in FIG. 5c, where a frequent occurrence of dropout in 
a short period results in an increase in the integration output. This 
integration output is supplied to the threshold circuit 16, which produces 
a high level signal when the integrated signal exceeds the predetermined 
threshold level E. This signal and the dropout detection signal are summed 
up by the OR gate 17 to produce a control signal as shown in FIG. 5e. By 
controlling the previous value holding circuit 11 with the control signal, 
the demodulation signal of FIG. 5f is varied as shown in FIG. 5g, 
indicating that a dropout compensation and muting operation are performed 
correctly. 
In the foregoing description, the arrangement was made such that the 
control signal obtained by integrating the dropout detection signal and 
the dropout detection signal itself are summed up to produce a signal 
which controls the previous value holding circuit. Another arrangement is 
also possible, in which the previous value holding circuit is controlled 
solely by the dropout detection signal, while a control signal obtained by 
integrating the dropout detection signal controls the demodulated audio 
signal by means of a gate provided in arbitrary location following the 
demodulator so as to perform the muting operation.