JPS63144457A - Frequency modulator - Google Patents

Abstract

PURPOSE:To prevent the occurrence of moires in a demodulation signal in applying the titled modulator for recording/reproducing processing of a video signal by using a filter circuit to eliminate undesired harmonic components caused by nonlinear processing thereby eliminating the mixing of the undesired components into the demodulation signal. CONSTITUTION:An LPF 6 eliminating harmonic components caused in a nonlinear emphasis circuit 16 is provided before a FM modulation circuit 20. The nonlinear emphasis circuit 16 generates harmonics. Since the LPF 36 has frequency characteristics passing through the frequency components including luminance signal information from an output of the nonlinear emphasis circuit 16, no harmonic component is applied to the FM modulation circuit 20. Thus, even when the output of the FM modulation circuit 20 is demodulated by a 2-multiple demodulation circuit, the harmonic components of the lower side band of the FM-modulated wave multipled into two are not mixed to the spec trum of the luminance signal after demodulation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency modulation device, and more specifically, to a frequency modulation device that performs nonlinear processing on an input signal and then applies frequency modulation. ] In devices such as video tape recorders that perform frequency modulation using low carrier waves to record and reproduce (or transmit) information, conventionally, in order to improve the S/N ratio, high frequency modulation is performed in advance before frequency modulation. Emphasis processing is performed to emphasize the high frequency range and reduce the high frequency range after demodulation.

Figure 3 shows the luminance signal FM modulation of the NTSC video signal.
A schematic configuration of an apparatus for recording onto a magnetic tape using a carrier color signal low-pass conversion multiplex recording method is shown. The video signal input to the input terminal 10 is processed by a luminance signal/carrier color signal separation circuit (hereinafter referred to as Y/C
The luminance signal (Y signal) and the carrier color (
C) separated into signals. The Y signal separated by the Y/C separation circuit 12 has its high frequency components removed by the LPF L 4, is subjected to nonlinear high frequency enhancement processing by the nonlinear emphasis circuit 16, and is subjected to linear high frequency component processing by the preemphasis circuit 18. Area emphasis processing is applied. The output of the pre-emphasis circuit 18 is frequency modulated by the FM modulation circuit 20, and the output is
The signal is band-limited (low frequency removed) by the PF 22 and supplied to the mixer 24 .

On the other hand, the carrier color signal separated by the Y/C separation circuit L2 is
After being band-limited by the BPF 26, the C signal processing circuit 28 performs known processing such as low frequency conversion of the carrier frequency. Since the specific contents of this C signal processing circuit 28 are well known, further explanation will be omitted. The low frequency carrier C signal output from the C signal processing circuit 28 is supplied to the mixer 24, where it is frequency multiplexed with the FM modulated luminance signal.

The output of the mixer 24 is amplified by a recording amplifier 30 and recorded on a magnetic tape 34 by a recording head 32.

(a) of FIG. 4 shows the frequency spectrum distribution of the output of the LPF 14 of the circuit shown in FIG. 3, (b) shows the spectrum distribution of the output of the nonlinear emphasis circuit 16, and (C
) indicates the spectral distribution of the output of the FM modulation circuit 20,
(d) shows the spectral distribution of the luminance signal when the signal recorded by the illustrated circuit is reproduced and demodulated.

[Problem that the invention seeks to solve]

In the circuit configuration shown in Fig. 3, the band of the luminance signal is set to 0 to f1.
The carrier frequency f of FM modulation of the FM modulation circuit 20 is
, is approximately 3f,/2. The output of the nonlinear emphasis circuit 16 has a second frequency f1 due to its nonlinearity.
, third harmonic components t, f, , f, , f, ... are generated, and in particular, the level of the third harmonic f3 becomes large. carrier frequency r. FM modulation circuit 2
As shown in FIG. 4(C), the output of 0 has the first harmonic (r+) component J(1) of the lower side wave, the second harmonic (C2) component J0, and the third harmonic ( C3) Component J(3) --- is generated, and J(H, J (ff) --- is folded back at the baseband and becomes JIH', , ) (31") in the band of the FM modulated signal. Mixed.

Fig. 4 (When the FM modulated signal of C1 is demodulated by a known double demodulator, the second wave of the lower side wave of the multiplied FM modulated signal
, the third harmonic J21, J (31・- is shown in Fig. 4 (
As shown in d), the signal falls within the luminance signal band (broken line) after demodulation. This appeared as moiré on the screen and was directly linked to deterioration of image quality.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a frequency modulation device in which unnecessary components are not mixed into a demodulated signal even when nonlinear processing is performed before FM modulation.

[Means for solving problems]

A frequency modulation device according to the present invention includes a nonlinear processing concave circuit that nonlinearly processes an input signal having a predetermined frequency band, a filter circuit that removes a predetermined high-frequency component of the output of the nonlinear processing circuit, and the filter circuit. and a frequency modulation circuit that frequency modulates the output of.

[Operation] In the present invention, unnecessary harmonic components generated by nonlinear processing are removed by a filter circuit, so that unnecessary harmonics are not mixed into the signal modulated by the frequency modulation circuit, and therefore, unnecessary components are not included in the demodulated signal. is not mixed in.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows a block diagram of the configuration of an embodiment in which the present invention is applied to a video signal recording device using a luminance signal FM modulation/carrier color signal low frequency conversion frequency multiplexing recording method. It shows the spectral distribution of the circuit output. However, in FIG. 1, circuit elements that are the same as those in FIG. 3 are given the same reference numerals.

The unnecessary components mixed into the demodulated signal are fundamentally generated in the nonlinear emphasis circuit 16. Therefore, an LPF 36 is provided before the FM modulation circuit 20 to remove harmonic components generated by the nonlinear emphasis circuit 16. Since the band limitation of the LPF 14 in FIG. 3 is not necessary for the nonlinear high frequency emphasis processing in the nonlinear emphasis circuit 16, in the illustrated embodiment, the Y signal from the Y/C separation circuit 12 is directly applied to the nonlinear emphasis circuit 16. do.

Except for connecting the LPF 36 instead of the LPF 14, the circuit of FIG. 1 operates in the same manner as the conventional example of FIG. 3. Therefore, the operation of this LPF 36 will be mainly explained below.

FIG. 2 (al indicates the spectral distribution of the Y signal output from the Y/C separation circuit 12. As in the case of the conventional example, F
The carrier frequency fc of FM modulation of the M modulation circuit 20 is set to approximately 3
Let f, /2. The nonlinear emphasis circuit 16 is
Similarly to FIG. 4 (bl), harmonics as shown in FIG. 2 (bl) are generated. (For example, the FM modulation circuit 2 has the characteristics shown in FIG.
0, the harmonic component element Z+
r,, (4'r> is not applied. As a result, the frequency spectrum of the output of the FM modulation circuit 20 becomes as shown in FIG.
As shown in FIG. 2(d), the harmonic component of the lower side wave of the above-mentioned doubled FM modulated wave does not mix into the spectrum (broken line) of the demodulated luminance signal.

In comparison with the conventional example shown in FIG. 3, the LPF 14 between the Y/C separation circuit 12 and the non-linear emphasis circuit 16 is
This corresponds to moving between the pre-emphasis circuit 18 and the FM modulation circuit 20, and there is no difference in circuit cost.

In this embodiment, the carrier frequency fe is approximately 3f.

/2, when a two-week demodulator is used, higher-order lower waves than those shown in the figure do not have an adverse effect on the demodulated signal. Therefore, illustration is omitted.

〔Effect of the invention〕

As can be easily understood from the above explanation, according to the present invention, even if nonlinear processing is performed before FM modulation, unnecessary components will not be mixed into the demodulated signal, and when used for recording and reproducing processing of video signals, , it is possible to prevent moiré from occurring in the demodulated signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the configuration of an embodiment of the present invention, FIG. 2 is a diagram showing the spectral distribution of signals in each part of the embodiment, FIG. 3 is a block diagram of the configuration of a conventional example, and FIG. FIG. 3 is a diagram showing the spectral distribution of signals in each part of the circuit shown in the figure. 1〇-Input terminal 12-...Y/C separation circuit 14-
LPF 16--Nonlinear emphasis circuit 18-
・-Pre-emphasis circuit 20-・-FM modulation circuit
22-HPF 24-・-Mixer 26-・BPF
28...-C signal processing circuit 30-recording amplifier 32~recording head 34-...magnetic tape 36...LF

Claims (1)

[Claims] A nonlinear processing circuit that nonlinearly processes an input signal having a predetermined frequency band, a filtering circuit that removes a predetermined high frequency component of the output of the nonlinear processing circuit, and a frequency modulation circuit that frequency modulates the output of the filtering circuit. A frequency modulation device consisting of.