JP2833932B2 - Non-linear emphasis circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-linear emphasis circuit, and more particularly to a video tape recorder (VTR).
And a non-linear emphasis circuit in video signal processing.

[0002]

2. Description of the Related Art Generally, recording / reproduction by the FM system performed in a VTR has a property that when FM demodulation is performed, the noise increases as the frequency becomes higher, so-called triangular noise, and the SN ratio of a small signal of a video signal is improved. For this purpose, the non-linear emphasis circuit emphasizes the high frequency range of the minute signal of the video signal during recording, and attenuates the high frequency range of the minute signal during reproduction to suppress noise.

FIG. 4 shows an example of a conventional non-linear emphasis circuit. Referring to FIG. 4, this conventional example includes a high-pass filter (HPF) 7, a limiter amplifier 8 for inputting an input signal Vin input from an input terminal 1 via the high-pass filter 7, and an input signal input from the input terminal 1. An addition circuit 9 adds the output signal of the limiter amplifier 8 from Vin and outputs the signal Vo to the output terminal 2 of the linear emphasis circuit.

When the input signal Vin is a very small signal, the limiter amplifier 8 does not operate the limiter, so that the output of the HPF 7 is amplified by the limiter amplifier 8, and the addition circuit 9 is added to the input signal Vin to emphasize the high frequency. .

When the input signal Vin has a large amplitude, a limiter is applied by the limiter amplifier 8, so that the amount of emphasis in a high frequency range is reduced. The frequency characteristics of the amplitude H of these input signals Vin and the output of the non-linear emphasis are as shown in FIG.

In FIG. 3, the horizontal axis is frequency and the vertical axis is response. When the amplitude H of the input signal Vin is large,
The characteristic curve becomes a characteristic curve in order as the amplitude H of the input signal Vin decreases.

[0007] Also, if the output of the limiter amplifier 8 during reproduction is reversed in phase with that during recording, and the inverted output is obtained, high frequencies can be attenuated.

[0008]

In the above-described conventional nonlinear emphasis circuit, the high-frequency small signal extracted by the high-pass filter 7 and the limiter circuit 8 is added to the addition circuit 9.
In FIG. 6, the phase around the high-pass filter 7 becomes larger as the cut-off frequency of the high-pass filter 7 becomes higher, and the shift from the input signal Vin increases as shown in FIG. At this point, the high-frequency minute signal is added, and the waveform (c) of the high-frequency minute signal of the output signal Vo is distorted, resulting in a problem that the image quality is deteriorated. That is, the waveform (a) of the input signal Vin in FIG. 6 and the waveform (b) of the output signal of the limiter amplifier 8 are added by the adder circuit 9 to obtain a distorted waveform (c) of the output signal Vo.

In the normal state of the conventional nonlinear emphasis circuit, the waveform (a) of the input signal Vin and the waveform (b) of the output signal of the limiter amplifier 3 are as shown in FIG.
There is no phase shift, and the waveform (c) of the output signal Vo can be obtained without distortion.

An object of the present invention is to provide a non-linear emphasis circuit capable of solving the above-mentioned problems and obtaining an output signal with little distortion.

[0011]

The non-linear emphasis circuit according to the present invention comprises a transconductance circuit having an input signal at an input terminal as an in-phase input, a limiter amplifier having an input signal at the input terminal as an input, and a limiter having the input signal. A capacitor having both ends connected to an output of the amplifier and an output of the transconductance circuit, and a signal obtained by adding an output signal of the limiter amplifier to an output signal of the transconductance circuit via the capacitor as an input, and an output terminal. And a buffer circuit that outputs an output signal to the transconductance circuit, wherein an output of the buffer circuit is fed back to an opposite-phase input terminal of the transconductance circuit.

[0012]

FIG. 1 is a block diagram showing a non-linear emphasis circuit according to an embodiment of the present invention. In FIG. 1, in the present embodiment, a transconductance (gm) circuit 4 having an input signal Vin of an input terminal 1 as an in-phase input, a high impedance and current output, and a limiter amplifier 3 having an input signal Vin of the input terminal 1 as an input. And a capacitor (C) 5 connected between the output of the limiter amplifier 3 and the output of the transconductance circuit 4, and a connection point between the output of the transconductance circuit 4 and the capacitor 5 as an input and the output as an output terminal. 2 and a buffer circuit 6 connected to the second circuit 2. The output signal Vo of the buffer circuit 6 is connected to the negative-phase input terminal of the transconductance circuit 4.

Next, the operation of the embodiment shown in FIG. 1 will be described with respect to a small input signal in which the amplitude of the input signal Vin of the input terminal 1 is smaller than the limiter level of the limiter amplifier 3 and a large amplitude input signal larger than the limiter level. And will be described separately.

(1) At the time of a small signal In this case, if the gain of the limiter amplifier 3 is Av times, the output of the limiter amplifier 3 is Av times the input signal (Vin), so the transfer function T in FIG. (S)
The following equation (1) is obtained.

[0015]

When the frequency is sufficiently lower than 1 / 2πCr, the gain becomes [1], and the input signal Vin becomes the output signal Vo as it is. When the frequency is sufficiently higher than 1 / 2πCr, the output signal is Av times the input signal Vin. The signal becomes a signal, is emphasized in the high frequency range, and has a frequency characteristic as shown in the waveform of FIG.

(2) When Large Amplitude Signal is Input FIG. 2 shows a characteristic diagram of the limiter amplifier 3, in which the horizontal axis represents the limiter amplifier input I, the vertical axis represents the limiter amplifier output O, and the limiter level width. Above W, the limiter operates.

In this case, since the input / output characteristics of the limiter amplifier 3 are as shown in FIG. 2, the input signal Vin higher than the limiter level is output at a constant level, and the Av of the transfer function equation (1) becomes non-linear (non-linear). As for the frequency characteristic, as shown in the waveform of FIG. 3, the amount of enhancement in the high frequency range is reduced, and the characteristic of non-linear emphasis is obtained.

As described at the beginning of the prior art,
It is necessary to restore the original frequency characteristics of the signal through a circuit that emphasizes the high frequency range during recording and attenuates the high frequency region of the reverse characteristic during reproduction. However, it is easy to change the polarity of the limiter amplifier 3 in FIG. The characteristic opposite to that of the high frequency emphasis is obtained.

[0020]

As described above, according to the present invention, when the input signal is lower than the limiter level of the limiter amplifier, the characteristics of the high-pass filter are obtained, and a high-frequency component is added to the input signal as in the conventional example of FIG. Since it is not a configuration, there is an effect that the image quality is not deteriorated due to the waveform distortion of the high frequency minute signal due to the phase rotation, and the reproducibility of the high frequency is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a non-linear emphasis circuit according to one embodiment of the present invention.

FIG. 2 is a characteristic diagram showing input / output characteristics of the limiter amplifier of the embodiment shown in FIG.

FIG. 3 is a characteristic diagram showing frequency characteristics of non-linear emphasis.

FIG. 4 is a block diagram showing a conventional nonlinear emphasis circuit.

FIG. 5 is a timing chart showing operation waveforms of a conventional nonlinear emphasis circuit in a normal state.

FIG. 6 is a timing chart showing operation waveforms showing problems of the conventional nonlinear emphasis circuit.

[Explanation of symbols]

 1 input terminal 2 output terminal 3,8 limiter amplifier 4 transconductance circuit 5 capacitor 6 buffer circuit 7 high-pass filter (HPF) 9 addition circuit H input signal amplitude I limiter amplifier input O limiter amplifier output Vin input signal Vo output signal W limiter level (A), (b), (c) Waveform

Claims (2)

(57) [Claims] 1. A transconductance circuit that receives an input signal of an input terminal as an in-phase input, a limiter amplifier that receives an input signal of the input terminal as an input, and both ends of an output of the limiter amplifier and an output of the transconductance circuit. A buffer circuit that receives a signal obtained by adding an output signal of the limiter amplifier to an output signal of the transconductance circuit via the capacitor, and outputs an output signal to an output terminal; A non-linear emphasis circuit, wherein an output of the circuit is fed back to an opposite-phase input terminal of the transconductance circuit. 2. The nonlinear emphasis circuit according to claim 1, wherein the output polarity of the limiter amplifier is inverted and input to the capacitor.