JPH03158087A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To reduce noise due to time axis fluctuation by storing low frequency noise due to time axis fluctuation generated in an output of an FM demodulation circuit at non audio signal and applying the stored signal to an oscillating frequency control terminal of a voltage controlled oscillator (VCO) and applying the VCO oscillating signal to a delay element as a clock. CONSTITUTION:An undesired high frequency component is eliminated from an FM demodulation output C generated or non-audio recording period at a low pass filter (LPF) 4, stored in a memory 6 for a prescribed period (e.g. 1 frame) and the frequency of a clock signal from the VCO 9 is changed continuously in response to the low noise component D read from the memory 6 and subject to D/A conversion. Then the delay time of the delay element 1 receiving an FM signal A is changed continuously by the clock signal. Thus, the time axis fluctuation is corrected in the stage of the FM audio signal. Thus, a signal E with a low noise component eliminated therefrom is obtained as the FM demodulation output E.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is directed to the low frequency band that occurs in the sound LFM demodulation output due to a change in the relative speed between the rotary head and the video tape in a high-fidelity VTR, for example. Regarding noise reduction@road to reduce noise.

(Prior art) In high-fidelity VTRs, audio signals are converted to FM signals and are recorded and played back alternately by two rotating heads, so time axis fluctuations may occur due to uneven rotation of the heads, uneven speed of the videotape, etc. Particularly when playing back a tape recorded on another VTR machine, a certain degree of time axis fluctuation always occurs in the playback signal.

The causes of time axis fluctuations when the recorder and playback device are different are the relative speed between the tape and the rotary head due to differences in tape tension between the recorder and playback device, subtle eccentricity differences in the center of the rotary head, etc. This is due to the difference in Also,
It also occurs due to differences in track linearity.

Furthermore, even when recording and reproducing with the same VTR machine, if the temperature or humidity differs between recording and reproducing, the same time axis fluctuations occur as described above.

A reproduced FM signal that has undergone such time axis fluctuations is originally an unmodulated reproduced carrier signal @(
For example, 1.7 MHz), this reproduced Gitria signal has a time axis fluctuation component as if it had an FM modulation component, and appears as noise in the FM demodulation output.

This noise is caused by the switching frequency of the rotating head (30 Hz for NTSC VTRs) and its harmonic components, and is mainly observed in the low frequency range (30 Hz to several white Hz). . Hereinafter, this noise will be referred to as low-frequency noise.

(16 Problems to be Solved by the Invention) As mentioned above, in a high-fidelity VTR, time axis fluctuations based on the switching frequency of the rotating head are
There was a problem in that it appeared in the audio signal and was demodulated as low-frequency noise. Low-frequency noise caused by this time axis variation occurs due to a difference in relative speed between the tape and the head, especially when the recording machine and the playback machine are different, and is a noise that is difficult to avoid.

This invention is intended to eliminate the above problems.

It is an object of the present invention to provide a noise reduction circuit that can reduce low-frequency noise caused by time axis fluctuations of reproduced FMa voice signals and can reproduce high-quality voice signals.

[Configuration 1 of the Invention (Means for Solving the Problem) The noise reduction circuit of the present invention supplies an input FM signal to an FM demodulation circuit through a delay cable whose delay time is controlled by a clock frequency. On the other hand, when there is no audio signal, the low-frequency noise caused by the coaxial fluctuation occurring in the output of the FM demodulation circuit is stored in a storage circuit for one frame period, for example, and this stored signal is used to generate the oscillation of a voltage 111111 oscillator (VCO). The clock is applied to the frequency control terminal, and the CO oscillation signal is supplied to the delay element as a clock.

(Function) For example, in a high-fidelity VTR, since a rotating head is used to record and reproduce audio FM signals, low-frequency noise generated due to the difference in relative speed between the video tape and the rotating head occurs at a certain period. (301-I Z in the case of NTSC system).

Therefore, the low-frequency noise that occurs in the unmodulated part of the FM signal when there is no sound angle signal is stored, for example, in a 1-frame-IJ storage circuit, and the FM signal containing the time axis fluctuation component is input using this stored signal. By changing the clock signal of the delay element, the time axis fluctuation component can be reduced.

(Example) Examples will be described with reference to the drawings.

FIG. 1 is a block diagram showing a noise reduction circuit according to an embodiment of the present invention.

In the embodiment shown in this figure, in a high-fidelity VTR,
A circuit that reproduces an FM audio signal recorded on a magnetic tape using two rotating heads and demodulates the FM audio signal will be described. The FM audio signal Δ alternately reproduced by two rotating heads (not shown) is input to the delay element 1. The delay element 1 is composed of, for example, a C0D (electronic coupling tree), and is an element capable of changing its delay time at a different frequency. The FM signal that has passed through delay element 1 is passed through a bandpass filter (
The signal is subjected to necessary band restriction by BPF) 2 and input to Flul tone 3. The output signal C or E of the FMTL circuit 3 is output to the next stage circuit, and is also supplied to an analog-to-digital conversion circuit (A/D) 5 via a low-pass (LPF) 4 that passes low-frequency components. The digitally converted low-frequency component is stored in the memory 6, and the stored signal is converted into an analog signal by a digital-to-analog conversion circuit (D/A>7).Writing and reading data to and from the memory 6 is performed by writing and reading. The analog signal converted by the D/A 7 is applied to the oscillation frequency control terminal of the VCO 9 through the integrating circuit 8. The output is supplied as a clock signal to the terminal of delay element 1. Therefore, the delay time of delay element 1 is controlled by the oscillation frequency of VCO 9.

The operation of the circuit configured as described above will be explained with reference to FIG.

FIG. 2A is a schematic representation of the reproduction of a 2 channel 1R channel FM sound signal recorded on a magnetic tape by two rotary heads for each channel. FIG. 2B shows the signals (ie, head switching pulses) for switching each channel 1P channel. Here, for example, a high-fidelity VTR
Odor C When playing back a tape recorded with another model, IN axis fluctuation occurs due to the relative speed +i between the rotary head and the j-b, as described above. Due to this time axis variation, the edge of the FM audio being played becomes as if it had received an FMS tone, and FMI! 11 Generates low-frequency noise in the output signal C of circuit 3 (see Figure 2 C). During the period when the voice signal is recorded, this low-frequency noise is Φ-folded on the FM demodulated n voice signal. By the way, no audio signal I] F in the dark
The M demodulated signal contains only low-frequency noise components proportional to the time axis fluctuation.

Also, because a rotating head is used, this low-frequency noise is generated at a constant period + l1 (30 in the case of the NTSC system).
HZ, II, etc. have 1 frame and 11 cycles).

Incidentally, noise that is audibly harsh exists at a frequency of several white Hz or less. Therefore, unnecessary high-frequency components are removed from the FM demodulation power C generated during the audio non-recording period using the LPF4, and for a certain period of time (
For example, one frame) is stored in the memory 6, read out from this memory 6, and converted into a D/A converted low-frequency noise component D (this component is compared to the low-frequency noise component included in the FMI output C as shown in the second figure) vC
Continuously changing the frequency of the clock signal from O9,
The above FM? By continuously changing the delay time of the delay element 1 whose input is the voice signal, it is possible to correct the time axis fluctuation at the stage of the FM voice signal. Therefore, as the FM demodulation output E, the second
As shown in Figure E, a signal from which low-frequency noise components have been removed is obtained.

Writing data to the memory 6 takes a long time even if it is done at the timing of turning on the manual switch when there is no sound when low-frequency noise is a concern, and it is difficult to write the data to the memory 6 by installing an audio level detection circuit at the rear stage of the FM111 circuit. The detection may be performed automatically when the detection output is at a low level (approximately low-frequency noise level).

FIG. 3 is a block diagram showing another embodiment of the invention.

As mentioned in the example of FIG.
(This is done by the /W signal, but in accordance with FIG. 3, the embodiment uses two memories 11 and 12 and the input circuit 10, and the previous data and newly read data are sent to the arithmetic circuit 10. The average value of both data is D/A.
7.

FIG. 4 is a block diagram showing another embodiment of the present invention.

The embodiment shown in this figure is approximately the same as the configuration shown in FIG.
A comparator amplifier 14 for comparing and amplifying the FMI modulation output quasi-voltage ref is provided in the previous stage of the LPF 4, and the FM
By outputting low-frequency noise when the demodulated output becomes a low level, the low-frequency noise component is supplied to the feedback path, and the averaged data is read from the memory 11 for each no-signal period. Also good. In addition, the broken line frame 1 in Figure 4
The configuration inside 3 is the same as that in FIG. 3, so it is omitted.

In the above embodiment, for example, a circuit for reducing low-frequency noise of an FM audio signal in an audio signal reproduction system of a high-fidelity VTR is described, but the present invention reduces low-frequency noise of an FM luminance signal in a luminance signal reproduction system. It can also be applied to cases.

[Effects of the Invention] As described above, according to the present invention, for example, high-fidelity V
In TR, it is possible to remove low-frequency noise caused by time-axis fluctuations in the reproduced FM audio signal, making it possible to reproduce high-quality audio signals.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a noise reduction circuit according to an embodiment of the present invention, FIG. 2 is an explanatory diagram explaining the circuit operation of FIG. 1,
FIG. 3 is a block diagram showing another embodiment of the invention, and FIG. 4 is a block diagram showing another embodiment of the invention. 1...Delay element, 3...FMI11 circuit, 5...
Analog-digital conversion U path, 6...memory, 7...
・Digital-to-analog conversion circuit, 9...Voltage control generator 1
L 1st 2nd

Claims (1)

[Claims] When demodulating an FM signal including a time axis fluctuation component, the FM
A noise reduction circuit that reduces low-frequency noise appearing in a demodulated signal, which includes a delay element that inputs the FM signal and changes the delay time of the FM signal with a clock frequency, and demodulates the FM signal that passes through this delay element. an FM demodulation circuit that converts the FM demodulated signal from the FM demodulation circuit into a digital signal during the no-signal period; storage circuit means for outputting a component equivalent to the range noise; and a voltage controlled oscillator that supplies an oscillation signal whose oscillation frequency is controlled by the output signal from the storage circuit means to the delay element as a clock. A noise reduction circuit featuring: