FM Audio demodulator with dropout noise elimination circuit

A previous level holding circuit is connected to an output of an FM demodulator to eliminate dropout disturbance from a reproduced audio signal from a record medium on which an FM-modulated audio signal has been recorded. The previous level holding circuit is controlled by an output of a circuit for detecting dropout in the reproduced signal and comprises a gate circuit operable in response to a dropout detection signal and a capacitor. During the occurrence of dropout, the previous level holding circuit holds and produces the reproduced output level assumed immediately before the dropout to eliminate the noise due to the dropout.

BACKGROUND OF THE INVENTION 
The present invention relates to a noise elimination circuit for enabling 
pulsive noise-free reproduction of an audio signal from a record medium on 
which the audio signal has been recorded with frequency modulation. 
In the past, when the audio signal is reproduced from the record medium on 
which the audio signal has been recorded with frequency modulation (FM), 
large amplitude noise is produced by the loss of reproduced signal due to 
dropout or by the discontinuity of the reproduced audio FM signal which 
may occur at the time of switching of playback tracks in a helical scan 
type magnetic tape recording and reproducing apparatus in which the audio 
signal is recorded in superposition on the video signal. This noise is 
eliminated by fully attenuating the output signal level of an audio signal 
amplifier during the dropout period. 
FIGS. 1a to 1d show reproduced audio signal waveforms for explaining the 
operation of a prior art noise elimination circuit, in which FIG. 1a shows 
a reproduced audio FM signal waveform, FIG. 1b shows an FM-detected audio 
signal waveform, FIG. 1c shows a dropout detection signal and FIG. 1d 
shows an audio signal waveform with the noise being eliminated based on 
the dropout detection signal. 
During the dropout period shown in FIG. 1a, large amplitude noise as shown 
in FIG. 1b is produced in the reproduced audio signal. On the other hand, 
the dropout detection signal as shown in FIG. 1c is produced by 
amplitude-detecting the reproduced audio FM signal (1a). Accordingly, by 
blocking the reproduced audio signal in response to the signal (1c), the 
noise-free audio signal as shown in FIG. 1d is produced. However, the 
method of merely cutting off the noise results in the discontinuity in the 
audio signal level. Accordingly, it can reduce the noise but cannot 
eliminate the noise to a practically acceptable level. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a noise elimination 
circuit suitable for use in an audio signal magnetic recording and 
reproducing apparatus, which circuit can reduce the noise to a practically 
acceptable level. 
Another object of the present invention is to provide a noise elimination 
circuit suitable for use in a dual-head helical scan type video tape 
recorder in which a carrier frequency-modulated with a video signal and 
another carrier of another frequency frequency-modulated with an audio 
signal are frequency-division multiplexed for recording and/or 
reproduction on video tracks. 
A feature of the present invention resides in that the audio signal is not 
cut off during the noise period as is done in the prior art noise 
elimination technique but the audio signal level immediately before the 
noise appears is held during the noise period to smoothen the junction of 
the signals before and after the noise elimination, and subsequently it is 
deemphasized to attenuate high frequency components to further smoothen 
the junction.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows one embodiment of a noise elimination circuit in accordance 
with the present invention. Numeral 1 denotes a reproduced signal input 
terminal, numeral 2 denotes a band-pass filter (BPF), 3 denotes a limiter, 
4 denotes an FM demodulator, 5 denotes a previous level holding circuit, 6 
denotes a dropout detection circuit, 7 denotes a deemphasis circuit and 8 
denotes a reproduced audio signal output terminal. 
The band-pass filter 2, the limiter 3 and the FM demodulator 4 may be those 
known in FM signal processing circuits. The dropout detection circuit 6 
may be the one which has been frequently used in a video tape recorder and 
it may comprise an amplitude detector and a level comparator for detecting 
when an output level of the amplitude detector falls below a predetermined 
level. 
FIG. 3 shows specific circuit examples of the previous level holding 
circuit 5 and the deemphasis circuit 7. A demodulated signal is applied to 
a gate circuit comprising field-effect transistors 11a and 11b through a 
transistor 10 of a buffer amplifier. A signal to control the gate is a 
negative polarity pulse from an inverter 17 which inverts the output of 
the dropout detection circuit 6, which is applied to a terminal 16. When 
the dropout is detected and the gate circuit is closed or non-conductive, 
a charge stored in a capacitor 12 is supplied to the deemphasis circuit 7 
through a field effect transistor 13. Meanwhile, when no dropout is 
detected with the gate circuit opened or being conductive, the demodulated 
output from the demodulator 4 supplied to the base of the transistor 10 is 
allowed to pass through the gate circuit to the field-effect transistor 
13. 
FIGS. 4a to 4e show signal waveforms in the circuit shown in FIG. 2, in 
which FIG. 4d shows an output audio signal waveform of the previous value 
holding circuit 5, and FIG. 4e shows an output audio signal waveform of 
the deemphasis circuit 7. FIGS. 4a, 4b and 4c show similar waveforms to 
those shown in FIGS. 1a, 1b and 1c. The BPF 2 extracts only the 
FM-modulated audio signal from the reproduced signal which is applied to 
the reproduced signal input terminal 1. 
As shown in FIG. 4a, the output signal waveform of the BPF 2 has a very 
small amplitude during the drop-out period. When it is amplified by the 
limiter 3 with a constant amplitude limiting function, the noise 
components are amplified in the dropout period in which the FM signal is 
absent. As a result, the demodulated audio signal from the FM demodulator 
4 includes large amplitude noise in the dropout period as shown in FIG. 
4b. 
On the other hand, the dropout period of the reproduced signal can be 
detected by the dropout detection circuit 6 which senses the amplitude of 
the reproduced signal. FIG. 4c shows the dropout detection signal thus 
produced. The previous level holding circuit 5 responds to the output 
signal from the dropout detection circuit 6 provided via OR gate 22 to 
hold the output audio signal from the FM demodulator 4, during the dropout 
period, to a signal level assumed immediately before the dropout. 
The output audio signal waveform of the previous level holding circuit 5 
thus produced is shown in FIG. 4d. It is apparent from the comparison of 
the waveform shown in FIG. 1d that the audio signal with less noise can be 
produced. However, because of certain discontinuity in the signal, small 
noise is still audible. In order to resolve this problem, the signal held 
in the previous level holding circuit 5 is deemphasized in the deemphasis 
circuit 7 so that high frequency small noise components and steep 
discontinuous portion in the signal shown in FIG. 4e are eliminated. Thus, 
the distortion due to the dropout is no longer audible. 
In addition to the noise elimination of the noise due to the dropout, the 
present invention can also be used to eliminate noise due to the 
discontinuity of an FM signal which occurs at the time of switching of 
playback tracks in a helical scan type magnetic tape video recording and 
reproducing apparatus. That is, it is usual in a video tape recorder of a 
dual-head helical scan type that two heads are alternately operated to 
individually record one field of a television signal on parallel tracks. 
During the reproduction, the two heads are alternately operated to 
sequentially reproduce the signals on the tracks. Thus, the heads are 
subjected to alternate switching for each track for electrical connection 
with a reproducing circuit. When recording and/or reproduction are carried 
out with such a dual-head helical scan type video tape recorder by 
superposing an audio FM signal on a video signal, it is possible that a 
noise occurs at the time of switching between the two heads due to a 
discontinuity of the carrier for the recorded audio signal. Such noise 
occurring at the switch-over of the heads, i.e. noise due to a phase step, 
may not influence the reproduced signal as far as the signal is concerned 
with a video signal because the head switch-over is usually effected 
within a vertical blanking time period of a television signal. As for an 
audio signal, however, since such a cut-off or pause portion is usually 
not included in an audio signal, noise occurring at the time of the head 
switch-over is perceived as undesirable noise and brings about a problem. 
In this case, the timing and the duration of the noise are previously 
known and a signal for instructing to hold the previous level can be 
readily produced at the output of OR gate 22 from a playback track 
switching signal for instructing the switching of playback tracks as seen 
in FIG. 2. The signal from the previous level holding circuit is then 
deemphasized like in the embodiment of FIGS. 2 and 3 to eliminate the 
noise. It should be understood that in the helical scan type magnetic tape 
video recording and reproducing apparatus a common circuit may be used for 
the noise elimination for the playback track switching and the noise 
elimination during the dropout period.