JPS6267704A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To reduce switching noise by using a sound signal obtained from the demodulation of an FM signal without phase discontinuity in a switch circuit so as to replace a switching noise generation period. CONSTITUTION:The 2nd FM demodulator 31 demodulates an output signal of the 2nd preamplifier 4 and gives the result to the 2nd switch circuit 32. A magnetic tape 12 wound on the peripheral of a cylinder is wound by >=180 deg. and the FM sound signal reproduced from the 1st, 2nd sound heads 1, 2 is reproduced respectively at the changeover period of a switch circuit 5. In demodulating an output signal of the 2nd preamplifier 4 by the 2nd demodulator 31, the demodulated output is a demodulated output not including spike noise. Further, the timing of the changeover of the 2nd switch circuit 32 is selected so that the phase of the output signal of the 1st and 2nd demodulators 6, 31 is coincident. Thus, the sound signal without noise is outputted at an output terminal 9.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention is particularly suitable for improving the quality of reproduced sound in a magnetic recording and reproducing apparatus (hereinafter abbreviated as VTR) that records and reproduces video signals and audio signals using a magnetic tape. Concerning methods.

[Background of the invention]

In recent years, with improvements in magnetic heads and magnetic tapes, the magnetic recording and reproducing characteristics of video signals have improved.

For this reason, less tape is required to obtain a desired S/N ratio, and tape speeds have been slowed down to improve recording density. However, as for the audio signal, since the audio head is fixed, the band of the reproduced audio output becomes narrower as the tape speed becomes slower, and the S/N ratio deteriorates. Furthermore, due to the instability of the tape running system, there were also problems with wow and flutter performance.

In order to reduce and eliminate the drawbacks of the above technology, the following video track dual recording technology is known. The first method, as shown in Japanese Patent Application Laid-open No. 51-155519, is to record an FM audio modulation signal that is FM modulated according to the audio signal on a recording track for a video signal, and then The above-mentioned FM audio modulation signal records a luminance modulation signal that is FM modulated according to the signal and a chroma low frequency signal that is low frequency converted.

This method overwrites the recorded data on the same track where it was recorded. By first recording the FM audio modulation signal and then overwriting the video modulation signal, the FM audio modulation signal is recorded deep into the magnetic layer of the magnetic tape, and then the FM audio is recorded on the surface of the magnetic layer. The brightness modulation signal having a higher frequency than the modulation signal will be recorded without being erased.

However, since the FM modulated FM audio modulation signal is recorded together with the video signal, there is a drawback that noise occurs in the audio signal during playback. The conventional audio reproduction circuit shown in FIG. 1 will be explained below using the waveform diagram in FIG. 2.

In FIG. 1, 12 is a magnetic tape, 1 and 2 are first and second audio heads, and 3 and 4 are first and second audio heads 1.
, first and second preamplifiers that amplify the output signals of 2;
5 is a switch circuit that alternately switches and selects the output signal of the preamplifier 3.4, and 6 is an FM selected by the switch circuit 5.
A demodulation circuit that demodulates the audio signal, 7 is a switch for holding the previous value, 8 is a capacitor for holding the previous value, 10 is a mono multi,
13 is a pulse source of 30#z for driving the switch circuit 5 and the multi-channel 10, and 9 is an output terminal.

The first and second sound heads 1, 2 are mounted on a rotating cylinder 18
They are arranged at 0 degree intervals and mutually scan the magnetic tape tzi. The pulse source 11 generates a pulse signal synchronized with the sweeping of the audio heads 1 and 2, and controls the switch circuit 5. Audio - F played on RAD 1 and 2 respectively
The M audio signal is amplified by the preamplifier 3.4 and supplied to the switch circuit 5. Waveforms 14 and 15 in FIG. 2 are output signal waveforms of the preamplifiers 3 and 4, respectively.

Further, a waveform 11 in FIG. 2 is an output signal waveform of the pulse source 11. The single output signal of the preamplifier 3.4 is switched by the switch circuit 5 and becomes a continuous signal as shown in waveform 16 in FIG. However, the phase of the FM audio signal is disturbed at the time when the switch circuit 5 switches, that is, at the rising edge and the falling edge of the output pulse of the pulse source 11, and the phase of the FM audio signal is disturbed, resulting in the waveform 16 shown in FIG.

The phase becomes discontinuous at the switching portions indicated by 23 and 24.

When such an FM audio signal is demodulated by the demodulator 6, the demodulated output contains spike noises 25, 26, 27 in the phase discontinuous portion of the FM audio signal, as shown by waveform 17 in FIG.
occurs.

Therefore, the switch 7 is turned off at the time when the switch circuit 5 is switched by the output of the monomulti 10 triggered by the output pulse of the pulse source II, and the voltage is maintained by the voltage forming capacitor 8 immediately before the switching of the switch circuit 5, thereby eliminating the spike noise. I'm trying to prevent it from being output.

The output waveform is shown as waveform 18 in FIG. Code four.

The parts indicated by 29 and 30 are the parts where the previous values are held.

However, this previous value holding part is different from the original signal,
This becomes a new switch noise.

[Purpose of the invention]

An object of the present invention is to eliminate the drawbacks of the above techniques and provide a noise reduction circuit that reduces switching noise.

[Summary of the invention]

In order to achieve the above object, the present invention is characterized in that the switching noise generation period is replaced by an audio signal obtained by demodulating an FM signal in which no phase discontinuity occurs in a switch circuit.

The noise reduction circuit of the present invention will be explained below with reference to an embodiment shown in FIG. In FIG. 3, 31 is a second FM demodulator, whose input terminal is connected to the output terminal of the 21st preamplifier 4, and whose output terminal is connected to the second switch circuit 3.
is connected to the other input terminal of 2. The second switch circuit 32 is controlled by the output of the monomulti 1O.
The demodulated output of the M demodulator 6 and the demodulated output of the second demodulator 31 are selected. The selected demodulated output is then supplied to the output terminal 9.

In such a circuit, the second 1M demodulator 31 demodulates the output signal of the second preamplifier 4 and supplies it to the second switch circuit 32. By the way, the magnetic tape 12 wound around the outer periphery of the cylinder is wound over 180 degrees around the outer periphery of the cylinder. Therefore, waveforms 14 and 15 in Fig. 2
As shown in FIG. 2, the py audio signals reproduced from the first and second audio heads 1 and 2 are respectively reproduced during the switching period of the switch circuit 5. Therefore, when the output signal of the second preamplifier 4 (waveform 15 in Figure 2) is demodulated by the second demodulator 31, the demodulated output becomes a demodulated output that does not include spike noise, as shown in waveform 19 in Figure 2. . The second switch circuit 32 selects the output signal of the second demodulator 31 from the output of the first demodulator 6 at the time when the first switch circuit 5 switches, and after a predetermined period of time has passed, the second demodulator 31 selects the output signal of the second demodulator 31 from the output of the first demodulator 6. 6 output signal is selected. By adjusting the switching timing of the second switch circuit 32 so that the phases of the output signals of the first and second demodulators 6 and 31 match), the second switch circuit 32 is connected to the output terminal 9.
As shown in waveform 2o in the figure, a noise-free audio signal is output. The mono multi circuit 10 controls switching of the second switch circuit, and the switch circuit 32 controls switching of the first 1M demodulator 6 when the output of the mono multi circuit 10 is at a high level.
The output of the second FΔf demodulator 31 is selected when it is at low level. A waveform 21 in FIG. 2 is an output signal waveform of the mono multi-circuit IO.

FIG. 4 is a circuit diagram of switch circuit &832.

The terminal 33 and the components are each 1M demodulator 6゜31.
Connect to i? The DC potential is Vl, V, the node and 36 are switches, and 37 is a capacitor whose capacitance value is C. North is the resistance and its value is R. The operation of this circuit will be explained using the waveform diagram in FIG. switch 35
, 36 are controlled by pulses zl and 4o to turn on when the p pulse is high and turn off when the p pulse is low, respectively.
First, the F and Sf audio signals of the second preamplifier 4 are input to the 1M demodulator 3, and after a normal reconnaissance audio signal 19 is output, the switch 36 is turned on by the pulse 40. At this time, the potential difference between terminals 33 and 34 is V, -V.

The output impedance of FM demodulator 6 and the weak switch impedance are '1Git37. Assuming that the resistor C and R(r) are much smaller than the series impedance, V, -V, has a capacitance of 37. All resistance is applied to the terminal 9, and no noise is generated at the terminal 9 due to the switch 36 being turned on. Here, if the time t from when the switch 36 is turned on until the switch north is turned off is sufficiently larger than the summation time constant CR of the capacitor 37 and the resistor 38, the capacitor i37 is charged and V, -V, has a potential difference of In this state, even if the switch 35 is turned off, the input terminal R' of the terminal 9 remains at VIO, so the buffer output is transferred from the first demodulator 6 to the second demodulator.
It is smoothly replaced with the FM demodulator audio signal of the demodulator 31, and when the switch is turned on, the output is returned to the demodulator output of the first demodulator 6, and the switch 36 is turned off.

Therefore, according to the present invention, spikes and noise can be smoothly compensated for with normal signals. Also, since the compensation audio signal is output as an audio signal only during the period when spike noise occurs, the S/N ratio is somewhat low.

It has the advantage of being able to tolerate τ even if the distortion is bad.

〔Effect of the invention〕

As explained above, according to the present invention, spikes and noise caused by switching the preamplifier output can be replaced by another FM system.
This has the effect of being able to be suppressed by compensating with the demodulated signal.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional FM demodulation circuit, and Fig. 2 is a block diagram of a conventional FM demodulation circuit.
1 and 5, FIG. 3 is a block diagram of the noise reduction circuit of the present invention, FIG. 4 is a circuit diagram of the switch circuit of the present invention, and FIG. 5 is the operation of FIG. 4. It is an explanatory waveform diagram.

Claims (1)

[Claims] 1) In a helical scan magnetic recording and reproducing device that has a frequency modulation audio carrier and records audio signals on a video recording track using first and second audio heads, the first and second head outputs during reproduction. A first and second preamplifier that amplifies the signal, a first switch that alternately switches and outputs the amplifier output, a first demodulator that performs FM demodulation of the signal, and a second preamplifier that converts only the output of the second preamplifier to FM. It has a second demodulator that demodulates, and a second switch that switches between the first and second demodulator outputs and outputs them to the output terminal. A noise reduction circuit characterized in that the switch outputs the second demodulated output and outputs the first demodulated output during other periods.