JPH0452547B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a device (such as a VTR) that alternately reproduces an audio signal from a recording medium recorded by frequency modulating the audio signal using a plurality of heads. The present invention relates to an audio signal noise removal circuit.

(b) Prior art In recent VCRs, in view of the fact that it is difficult to guarantee sufficient sound quality with the conventional audio signal track opened in the longitudinal direction of the magnetic tape, the video signal track is opened in the diagonal direction of the magnetic tape. Storage of audio signals (frequency modulated audio signals) on signal tracks in a frequency multiplexed format is being considered. And for helical scan VTRs, there are multiple (usually 2)
Adjacent tracks are scanned alternately using the same head. Therefore, periodic noise is unavoidable at the joints of the heads. In this case, the effect of the video signal can be relatively easily removed by setting the joint outside the frame of the display screen, but the effect of the audio signal can be removed due to the nature of the information. , this method cannot be adopted because the occurrence of periodic intermittent parts cannot be tolerated.

Therefore, in the conventional method, by utilizing the fact that the noise generation period can be predicted from the switching timing of a plurality of heads, instead of the transmission signal containing the noise generated due to the discontinuity of the carrier wave during the noise generation period, the transmission signal immediately before the noise generation period is used. A method of applying a signal level of Note that this tendency becomes more pronounced as the signal frequency becomes higher since the noise generation period is constant.

Therefore, recently, in order to improve this fidelity,
A method (linear approximation method) has been proposed in which the interpolation signal is composed of an alternative signal having a level that connects each signal level immediately before and after the noise generation period with a straight line (Japanese Patent Application Laid-Open No.
57-176511). However, this conventional example requires sophisticated circuit technology such as a delay circuit and a variable current source, and the number of elements inevitably increases. Especially in delay circuits,
Unless the group delay characteristics are completely matched from low to high frequencies, new errors may occur in the linear approximation.
There is a possibility that the width of the noise generation period may expand,
It becomes necessary to take a long interpolation time. Since approximation errors cannot be avoided in any approximation method, it goes without saying that the shorter the interpolation time, the more effective it is.

1 and 2 respectively show reproduced signal waveforms approximated by the above two conventional methods, in which solid lines A and B represent the reproduced signal, and portions A1 and B1 represent interpolated signals. Further, a broken line N indicates noise, a dashed-dotted line S indicates the original signal in the portions A1 and B1, and a period T indicates a noise generation period.

(c) Purpose of the Invention The present invention has been made with the above points in mind, and provides an audio signal noise removal circuit that has a noise removal effect with no subtraction compared to the linear approximation method and has a simple configuration. It is something to do.

(D) Structure of the Invention The present invention is characterized in that noise is generated immediately after the track switching signal, and that the time width of noise generated due to discontinuity of the FM carrier wave is very short, just a few microseconds, immediately after the FM demodulator. Focusing on the fact that it is constant,
After FM demodulation, a filter with a low cut-off frequency is not used, and pre-holding is performed for the minimum necessary time width, and after the pre-holding is completed, approximation errors caused by pre-holding are compensated for in terms of area, and the signal is passed through a low-pass filter. It performs interpolation.

According to the present invention, it is not impossible to set the interpolation time width to 15 μs or less, which is sufficiently shorter than one wavelength of the maximum audible frequency, so there is an extremely large approximation error from the low range to the maximum audible frequency (20 KHz). Interpolation with less is possible.

FIG. 3 is a conceptual diagram of the present invention. In the same figure, a is the area-interpolated waveform before passing through the low-pass filter, b,
Reference characters c and d are instruction pulses for specifying the noise generation period, the period immediately following the noise generation period, and the period immediately after the noise generation period, respectively, created based on the reproduction track switching signal. Further, e is the waveform of an output signal obtained by passing the signal a through a low-pass filter. If the high level periods of instruction pulses b and c are T and 1/2T, the area of right triangle D with T as the base is 1/2T, as shown in signal e.
It is equal to a rectangle E whose sides are the same and the height is the same.
In Fig. 3, if we subtract the signal level pre-held by the instruction pulse c and the signal pre-held by the instruction pulse d and extract the period immediately after the above (the high level period of the instruction pulse d), we get This corresponds to the error removed (or increased) during the noise generation period, so if this is added immediately after the signal held in advance by the instruction pulse b, the area becomes almost equal to the original waveform as shown in waveform a. . Since the interpolation time is sufficiently small for the maximum audible frequency, by passing the signal a through a low-pass filter whose cut-off frequency is higher than the maximum audible frequency, the signal waveform is blunted by the action of the low-pass filter, and this is based on pre-maintenance. Since the missing portion is interpolated area-wise, it is possible to perform interpolation that is extremely faithful to the original signal.

(E) Embodiment FIG. 4 is a circuit diagram of an embodiment of the present invention, and FIG. 5 shows signal waveforms at various parts thereof.

An input terminal 1 receives a signal obtained by demodulating a frequency modulated audio signal obtained by alternately splicing the outputs of a plurality of heads using an FM demodulator (not shown), that is, a reproduced signal shown in FIG. 5a. Portion N1 of reproduced signal a,
N2 is a noise caused by discontinuity of carrier waves that occurs when switching a plurality of head outputs, and is generated only for a certain period every certain period (for example, one field period). In the figure, the period is shown to be extremely short compared to the reproduced signal to clarify the subsequent noise generation process.

The reproduced signal a is applied to the first, second, and third switch circuits 2, 3, and 4, respectively, and when each switch circuit is on, the first, second, and third capacitors 5 attached to the output side of each switch circuit are applied. , 6, and 7, the signal level is maintained every moment. The first switch circuit 2 output (Fig. 5b) is the mixing circuit 8
There, it is added to an error signal (h in the same figure), which will be described later, to output an interpolated signal as shown in FIG. 5c. This interpolated signal is applied to a low-pass filter 9 whose cut-off frequency is higher than the maximum audible frequency, where the error signal is averaged and the signal shown in FIG. 5d is delivered to the output terminal 10.

The signals shown in FIG. 5e and f, which are the outputs of the second and third switch circuits 3 and 4, are applied to the subtraction circuit 11, which outputs the signal shown in FIG. 5g as the output of the subtraction circuit. This signal g is applied to the fourth switch circuit 12, and a portion selected there is applied to the mixing circuit 8 as an error signal h.

A first switching signal (FIG. 5i) is input to the input terminal 13, which inverts the state according to the reproduction track switching timing. This switching signal corresponds to a so-called RF pulse for selectively deriving a plurality of head outputs. Accordingly, 2
In a head helical scan type VTR, the state is changed for each field, and therefore, as mentioned above, each change causes discontinuity in the carrier wave, which periodically adds noise to the audio signal output from the FM demodulator. This noise generation period is called a noise generation period T.

The first switching signal i is transmitted from the first mono multivibrator 14 (hereinafter referred to as mono multivibrator).
MM) and the second MM15. No.
The time constant of the 1MM14 is selected to be 1.5 times the noise generation period T, while the 2MM15 is selected to match the noise generation period, so the 5th
The second and third switching signals shown in FIGS. j and k are output.

The second switching signal j is connected to the third switching signal j so as to switch off during the period when it is at high level.
It is applied to the switch circuit 4, and the logic circuit 1
6 is given. Further, the third switching signal k is applied to the first switch circuit 2 and the logic circuit 16 so as to turn off the switch during the period when the third switching signal k is at a high level. This logic circuit 16 performs an exclusive OR operation on the second and third switching signals j and k, and outputs the fourth switching signal shown in FIG. 4. This fourth switching signal l is applied to each of the second and fourth switch circuits 3 and 12, and during its high level period, the second switch circuit 3 is switched off and the fourth switch circuit 12 is switched on. I have to.

With the above configuration, the reproduced signal a applied to the input terminal 1 is applied to the first capacitor 5 through the first switch circuit 2 whose on period is controlled by the third switching signal l, and the signal level immediately before the noise generation period is is applied to the mixing circuit 8.
Further, the second and third capacitors 6 and 7 are connected to the second and third capacitors.
Since the previous signal level is maintained during the off period of each switch circuit 3, 4, the signals shown in e and f in FIG. be granted to This fourth switching circuit 12 outputs a fourth switching signal l.
Since it is turned on only during the high level period, the error signal h is applied to the mixing circuit 8. Therefore, mixing circuit 8
mixes the signals b and h and acts to interpolate in terms of area the portion that is deleted (or increased) during the noise generation period. The output of the mixing circuit 8 is applied to a low-pass filter 9, and a reproduced signal faithful to the original signal from which noise has been removed is derived at an output terminal 10.

(F) Effects of the Invention The present invention pre-maintains the noise generation period that occurs periodically at the time of switching the reproduction track at the signal level immediately before the period, and also maintains the signal level of the period immediately following the noise generation period. Since the error signal between the immediately previous signal level and the signal level immediately before the noise generation period is added to interpolate the portion deleted (or increased) in the noise generation period,
Noise removal with extremely small approximation error noise is possible with a simple configuration.

[Brief explanation of the drawing]

FIGS. 1 and 2 are output waveform diagrams according to the first and second conventional methods. FIG. 3 is a conceptual diagram of the present invention. FIG. 4 is a configuration block diagram of an embodiment of the present invention.
FIG. 5 is a waveform diagram of each part of FIG. 4. Explanation of main drawing numbers, 2, 3, 4, 12...first, second, third, fourth switch circuit, 5,6,7...first, second, third capacitor, 8...mixing circuit, 11
...Subtraction circuit, 14, 15...1st, 2nd MM, 16
...Logic circuit, 9...Low pass filter.

Claims (1)

[Scope of Claims] 1. In an audio signal reproducing circuit that alternately reproduces a frequency modulated audio signal obtained by frequency modulating a carrier wave with an audio signal using a plurality of heads, the discontinuity of the carrier wave occurs at the time of switching the reproduction track. pre-holding means for pre-holding a noise generation period in a demodulated signal; and error signal addition for adding an error signal corresponding to a signal level difference before and after the noise generation period to a period immediately after the noise generation period. An audio signal noise removal circuit comprising: means for area-wise interpolating a deleted (or increased) portion of the signal to which the error signal is added based on the pre-holding.