GB2144942A

GB2144942A - Video disk modulator device

Info

Publication number: GB2144942A
Application number: GB08419209A
Authority: GB
Inventors: Ryoji Higashi; Satoru Nomura
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1983-07-30
Filing date: 1984-07-27
Publication date: 1985-03-13
Also published as: JPS6026604U; KR890007400Y1; JPH0341323Y2; GB2144942B; FR2549994A1; GB8419209D0; KR850008137U

Abstract

A video disk modulator is disclosed in which deterioration of the reproduced sound quality of an audio signal reproduced from a video disk is prevented by filtering a modulated video signal. An adder 24 sums first and second signals, the first signal being produced by filtering 23 the output of a video modulator 21 which FM modulates a video signal. The second signal is produced by FM modulating 22 an audio signal. If desired, the audio signal may also be PCM encoded 26. The output of the adder 24 is multiplexed employing pulse width modulation in a limiter 25. <IMAGE>

Description

SPECIFICATION Video disk modulator device The present invention relates to a video disk modulator device for preventing deterioration of sound reproduced from a video disk.

A conventional modulation system is shown in Fig. 1, in which reference numeral 1 designates a video modulator which FM modulates an input video signal and provides its output to an adder 3. Reference numeral 2 designates an audio modulator which FM modulates an input audio signal and provides its output to an adder 3. The output of the adder 3 is applied to a limiter 4, which is provided for multiplexing the input signals according to a pulse width modulation technique.

In one prior art system a PCM encoder 5 is included when the application demands. In such a case, the input audio signal is subjected to PCM modulation by the PCM encoder 5 and the resultant signal is fed to the adder 3 as a transfer signal.

In such a circuit the video and audio highfrequency signals which have been subjected to FM modulation by the video modulator 1 and the audio modulator 2 are summed by the adder 3 and then applied to the limiter 4.

In the case where the PCM encoder 5 is employed, the output of the PCM demodulator 5 is added and multiplexed.

However, the output of the video modulator 1 contains high-order sideband components and high-frequency audio signal components.

Particularly because the secondary component of the colour subcarrier has been subjected to preemphasis processing, its level is considerably high. The signal to be modulated which contains high-frequency components, such asa multiburst signal, similarly has high-level secondary and tertiary components.

In the conventional video disk modulator device, the high-order sidebands of the output of the video modulator 1 interfere with the audio signal and the PCM signal so that the S/N (signal-to-noise) ratio and the sound quality are reduced.

To overcome this problem and in accordance with the present invention a video disk modulator device comprises a video modulator for FM modulating a video signal and providing a modulated high-frequency signal, a filter for filtering the modulated high-frequency signal, an audio modulator for FM modulating an audio signal, an adder for adding an output of the filter and an output of the audio modulator, and a limiter for multiplexing the output of the adder according to pulse width modulation.

If desired, a modulator for PCM modulating the audio signal can also be provided, in which case the output of this modulator is also added by the adder to the outputs of the first audio modulator and the filter.

An example of a device constructed in accordance with the present invention will now be described with reference to the accompanying drawings in which:~ Figure 1 is a block diagram showing a prior art video disk modulator device; Figure 2 is a block diagram showing an example of a video disc modulator device according to the present invention; Figure 3A is a diagram illustrating the frequency spectra of the video modulator in the embodiment shown in Fig. 2; and Figure 3B is a diagram illustrating the frequency spectra of the output of an adder shown in Fig. 2.

Fig. 2 is a block diagram showing the arrangement of the present invention in which the reference numeral 21 designates a video modulator which modulates an input video signal. The thus-modulated signal is fed to an adder 24 through a filter 23 which processes the high-frequency video signal. The audio signal is applied to an audio modulator 22, and the resultant modulated signal is also applied to the adder 24. If desired, an output of a PCM encoder 26, which encodes a PCM audio signal is also applied to the adder 24.

The output of the adder 24 is fed to a limiter 25.

The bandwidth of the high-frequency video signal modulated by the vide modulator 21 need only have a bandwidth such that the primary sideband modulated by the maximum frequency of the video signal to be modulated passes (for example, 3.9 through 13.5 MHz in the NTSC system). Therefore, a BPF (bandpass filter) 23 is provided in order to suppress the high-order sideband components.

Due to the provision of the BPF 23, interference of the high-frequency audio signal and the PCM signal caused by the high-frequency video signal is suppressed.

In Fig. 3A, the frequency spectra of the output of the video modulator 21 are shown, in which reference character a designates the primary spectrum; b, the secondary spectrum; and c, the tertiary spectrum. In Fig. 3B, the frequency spectra of the signal after processing by the BPF 23 is shown. In this case, suppression of the upper sideband above 13.5 MHz has the effect of suppressing interference relative to the PCM transfer band in the case where the PCM encoder is used.

However, little effect is attained for the interference relative to the high-frequency audio signal band. This is due to the fact that the upper sideband is reproduced prior to the lower sideband in the limiter processing, and the upper sideband, which interferes with the high-frequency audio signal band, overlaps with the high-frequency video signal band, and therefore the high-frequency audio signal cannot be eliminated.

While a filter has been described which suppresses high-order upper and lower side bands for the processing of the high-frequency video signal, it is also possible to eliminate only the lower sideband, which in terferes with the high-frequency audio signal band and PCM signal band, with the use of a HPF (high-pass filter) or a BPF. In this case, however, since the upper sideband of the high-frequency video signal is not eliminated, the suppressed lower sideband appears in the output being multiplexed by the limiter processing. That is, rather than being completely eliminated, the level of the lower sideband appearing in the output being multiplexed by the limiter processing is reduced by 6 dB.

As described an added signal of the filterprocessed signal of the output of the video modulator and the output of the audio modulator is multiplexed employing a pulse width modulation technique so that the components contained in the high-frequency video signal, which interfere with the high-frequency audio signal and the PCM signal, are suppressed.

Consequently, audio interference caused by the high-frequency video signal is suppressed, and therefore the bit error rate of the PCM signal is decreased, improving the sound quality and the S/N ratio.

Claims

1. A video disk modulator device comprising a video modulator for FM modulating a video signal and providing a modulated highfrequency signal, a filter for filtering the modulated high-frequency signal, an audio modulator for FM modulating an audio signal, an adder for adding an output of the filter and an output of the audio modulator, and a limiter for multiplexing the output of the adder according to pulse width modulation.

2. A device as claimed in claim 1, further comprising a modulator for PCM modulating the audio signal, and wherein the adder adds the output of the filter, the output of the first audio modulator and the output of the modulator for PCM modulating the audio signal.

3. A device according to claim 1, substantially as described with reference to Figs. 2 to 4 of the accompanying drawings.