JPH0757390A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To prevent a noise generated when the discontinuous phase difference of an FM audio signal of a HiFi VTR falls within about 180 deg.. CONSTITUTION:The discontinuous phase around 180 deg. of an FM audio signal being the fundamental cause of the switching noise of a Hill VTR is detected by means of an envelope detector 4. By inverting the detected discontinuous phase around 180 deg. of the FM by means of a limiter 5 in front of an FM demodulator 6 for demodulating the FM audio signal, the signal is demodulated after forcedly making the discontinuous phase of the FM about 0 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise reduction circuit for reducing switching noise which is a problem in a voice reproduction system of a high quality VTR.

[0002]

2. Description of the Related Art In a helical scan type VTR, a so-called "HiFi" is used, in which a stereo audio signal is deeply recorded and reproduced on the same track as a video track on a magnetic tape.
VTR "is widely used. When reproducing an audio signal in this system, an A head reproduction FM signal as shown in (1) of FIG. 4 and a reproduction signal like a B head reproduction FM signal as shown in (2) of FIG. Are alternately picked up by the A and B playheads tracing different tracks, it is necessary to connect the FM signals picked up by these A and B playheads to a continuous FM signal.

Therefore, the reproduction FM signal of the A head and the reproduction FM signal of the B head are recorded and reproduced so that a part thereof has an overlap period, and within this overlap period, the reproduction FM signal from the A head to the B head is reproduced. A continuous FM audio signal is derived by switching to a signal or from the B head to the reproduced FM signal of the A head.
The problem here is at the time of switching. Figure 4
As shown in (4), the phase of the continuously reproduced FM signal in the overlap period becomes discontinuous, and this appears as pulsed noise after demodulation. This causes a problem that switching noise at the time of head switching is mixed into the reproduced audio signal at regular intervals.

In order to solve this problem, various switching noise reduction circuits have been developed. For example, as the simplest method, there is a sample hold method in which an audio signal immediately before noise is generated is held and the held audio level is output during a noise occurrence period. As a recent general method, while holding the audio signal immediately before the noise is generated, it is differentiated to create a coefficient signal for linear approximation, and based on the held audio signal and the coefficient signal, the noise generation period There is a linear approximation method for linearly approximating the sound signal waveform inside.

These are mainly those which mask switching noise and mainly connect the demodulated output by hold interpolation or linear interpolation. FIG. 5 shows an example of a conventional system for performing a linear approximation method. FM audio signals reproduced from the A head and B head are supplied to the input terminals 51 and 52. The reproduced signal obtained by alternately selecting with the switch SW is passed through the band-pass filter 53, subjected to amplitude shaping by the limiter 54, and FM demodulated by the FM demodulator 55.

Generally, a system that performs linear approximation has a low pass filter 56 after the FM demodulator 55.
In the interpolation by linear approximation, the slope of the signal immediately before the reduction is detected, and the linear interpolation circuit 57 in the next stage corrects and interpolates based on the slope.
This is because a significant detection error occurs when the slope is detected with a signal in which the next component and other high-frequency noise remain. After sufficiently removing these high-pass components with a low-pass filter, the slope is detected and interpolation by linear approximation is performed to obtain a demodulated output.

However, when passing through the low-pass filter 56, the audio signal containing switching noise has a phenomenon in which the noise portion is tailed, and if the switching noise is large, the interpolation period must be taken very long. , It is difficult to reduce the error in the entire switching noise period with a small error. This is because the longer the interpolation period is, the larger the error between the reduced waveform due to the linear approximation and the true waveform becomes. Therefore, the interpolation period is determined in consideration of this error. The switching noise is caused by the discontinuous phase of the FM signal when the reproduced FM signals of the A head and the B head are connected.

FIG. 6 shows the relationship between the discontinuity phase difference of the FM signal and the magnitude of switching noise. FM phase difference 180
It can be seen that the switching noise becomes maximum at ° and the interpolation period must be long. In an attempt to reduce the FM phase discontinuity in a time period that includes all switching noises that occur in the vicinity of 180 °, the error between the linearly approximated interpolation waveform and the true waveform becomes very large.

Therefore, even if the pulse noise contained in the demodulated signal cannot be completely removed, it is advantageous to shorten the correction time to some extent. However, it can be said that the total noise can be further reduced by completely removing the pulse noise in the shortest possible interpolation time.

[0010]

In the conventional noise reduction circuit described above, the magnitude of switching noise and its generation time largely depend on the discontinuous phase of the FM at the time of switching, and in particular, the discontinuous phase difference is around 180 °. In this case, a particularly large noise that exceeds the limit of the correction period is generated, and there is a drawback that a large amount of switching noise remains only by the conventional correction after demodulation output.

The present invention provides a means for improving switching noise which can prevent noise particularly when the discontinuous phase difference of FM audio signals of HiFi VTR is around 180 °.

[0012]

According to the present invention, the discontinuous phase of the FM audio signal, which is the fundamental cause of switching noise, in the vicinity of 180 ° is detected, and 180 ° inversion processing is performed before the demodulation of the FM audio signal. Thus, demodulation is performed after forcibly setting the discontinuous phase of the FM to around 0 °.

[0013]

By the means described above, the oversized noise in the demodulation output due to the switching of the FM is prevented, and by using it together with the conventional interpolation method in the demodulation output such as linear approximation, the switching noise remains in a short interpolation period. You can make corrections with almost no minutes.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit block diagram for explaining an embodiment of the present invention. For example, the FM signals reproduced by the A head and B head in the video tape recorder are supplied to the input terminals 1 and 2. The FM signal supplied to the input terminals 1 and 2 is switched and selected by the switch SW, and is taken out as a continuous FM signal. After that, the band-pass filter 3 separates only the FM signal component of its own channel, and then separates into the detection system and the main signal system for detecting the FM discontinuous phase around 180 °. FM input to the detection system
The signal is input to the buffer amplifier, amplified, and then input to the envelope detector 4. As a detecting means in the vicinity of 180 °, as shown in FIG.
The FM signal in the vicinity of ° utilizes the fact that the envelope is narrowed to almost 0 after the band-pass filter 3.

The limiter 5 for performing the inversion process is provided with a two-system differential input circuit in which the input polarities are switched in its input stage, and these are switched alternately every time a control signal is output from the envelope detector 4. Leave.

When the vicinity of 180 ° is detected as the discontinuous phase of the FM, the output of the envelope detector 4 changes from Lo to Hi or from Hi to Lo, and the control flip-flop is inverted to provide two differential inputs to the limiter 5. The 180 ° inversion process can be easily performed by performing control such that the circuit is switched.

The FM signal input to the main signal system is shaped in amplitude by the limiter 5 and is inverted when it is detected around 180 ° and then input to the FM demodulator 6 for FM demodulation. The demodulated output is then input to the low-pass filter 7 to sufficiently remove unnecessary high-frequency components such as the secondary component of the FM carrier. This is to prevent an error when detecting the inclination. The signal in which the high frequency components have been sufficiently dropped is subjected to linear approximation interpolation by the linear interpolation circuit 8 based on the slope of the signal immediately before the switching noise, and the output terminal 9
Extract the demodulation output from.

FIG. 3 shows a comparison by simulation between the demodulated output when the FM phase difference is 180 ° and the case where the demodulated output is not inverted and the case where the inverted process is performed.
The comparison result by this simulation shows that the demodulation noise is drastically reduced when the inversion process is performed.

In this embodiment, an oversized noise at the time of 180 ° phase discontinuity is prevented in the FM region, so that a fundamental improvement can be achieved. In addition, a noise removal system with less residual noise can be achieved by using it together with the conventional interpolation method for demodulation output.

[0020]

As described above, since the noise reduction circuit of the present invention envelope-detects the discontinuous phase by making good use of the characteristics of the bandpass filter, it is possible to reduce the noise with a small circuit scale. You can

[Brief description of drawings]

FIG. 1 is a system diagram showing an embodiment of the present invention.

FIG. 2 is a waveform diagram after a band pass filter when the phase difference of FM during head switching is 90 ° and 180 °.

[FIG. 3] The phase difference of FM at the time of head switching is 180 °
FIG. 6 is a characteristic diagram showing simulation results of demodulation noise with and without inversion processing.

FIG. 4 is an explanatory diagram for explaining the principle of generation of switching noise in a HiHiVTR.

FIG. 5 is a system diagram for reducing switching noise by conventional linear approximation.

FIG. 6 is a characteristic diagram showing the relationship between FM discontinuous phase and generated demodulation noise.

[Explanation of symbols]

1, 2 ... Input terminal, SW ... Switch, 3 ... Bandpass filter, 4 ... Envelope detector, 5 ... Limiter, 6 ... FM
Demodulator, 7 ... Low-pass filter, 8 ... Linear interpolation circuit, 9
… Output terminal.

Claims (3)

[Claims] 1. A first and second FM audio signal reproduced by different heads mounted on a rotary cylinder and a switch for alternately switching and extracting the first and second FM audio signals, respectively. It is characterized by comprising a detection means for detecting a phase discontinuity at the time of switching the switch and generating a control signal, and a means for inverting the phase of the FM audio signal based on the control signal of the detection means. And noise reduction circuit. 2. A band pass filter for selecting a specific signal, wherein the output of the filter is envelope-detected as the discontinuous phase detecting means.
The described noise reduction circuit. 3. The noise reduction circuit according to claim 1, wherein the reproduced FM audio signal is demodulated through a means for inverting the phase.