JPS5936334B2 - Frequency multiplex signal demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for demodulating frequency multiplexed signals recorded on an optical recording medium.

2. Description of the Related Art Recently, it has been proposed to frequency-multiplex and record narrowband signals such as audio signals on an optical recording medium by utilizing the wide band of a wideband recording/reproducing device such as an optical video disc device. Conventionally, to demodulate such a frequency multiplexed signal, there are two methods: preparing as many bandpass filters as the number of divisions and selecting one of them to extract the desired signal, or using one bandpass filter and a variable local oscillator. Therefore, there was a method of extracting the desired signal by the heterodyne method, but the former method required many bandpass filters and was extremely expensive and impractical.

Although the present invention belongs to the latter heterodyne method, it provides a demodulation device for frequency multiplexed signals whose structure is simplified by performing the function of the frequency conversion stage in the heterodyne method on the recording surface of an optical recording medium. This is what we provide.

FIG. 1 is a block diagram of essential parts of a wideband high-density playback device (hereinafter referred to as a "video disc device") showing one embodiment of the present invention.

In FIG. 1, light emitted from a reading light source 1 such as a laser light source passes through a beam splitter 13 and is focused onto a disc-shaped optical recording medium (hereinafter referred to as a disc) 3 by a condensing lens 2. On the disk surface, there are various original signals such as audio signals with different carrier waves ω1, ω2, . . .
, ωi, ..., ωm, and the second
It is recorded in the form of pits 21 as shown in the figure. For example, the first original signal has a carrier wave of ω1 in Japanese,
The i-th original signal is recorded in English as having a carrier wave of ωi. By the way, disk 3 is motor 4.
Since it is rotating at a constant speed, the light reflected on the disk surface is reflected by the ω1, ω2, ..., ω of the pit 21.
It enters the photodetector 5 with information of m and is converted into an electrical signal. Here, assuming that the output light of the light source 1 is modulated at the frequency ωs, the light incident on the photodetector 5 is a bead with pit information on the disk surface ω1-ωs, ω2-ωs, ゜゜', ωi-ωs, '゜゜, ωm-ωs is included. These electrical signals are input to a bandpass filter 10. When the passing frequency of the bandpass filter 10 is ωf, ωi−ωs=ω
If ωs is selected so that f becomes ωi−ω, the demodulator 11
Only the s component is selected and input, and the i-th original signal appears at the demodulation output terminal 12. 6 is a modulator that modulates the light source 1, T is a local oscillator that supplies a modulation frequency signal to this modulator, and a selection switch 9 selects any crystal in the local oscillation crystal group 8 to adjust the frequency ωs of the output light. ωs is selected so that ωi−ωs always becomes ωf.

Note that 8 may be an oscillation source other than crystal. As described above, according to the present invention, it is possible to easily demodulate a frequency multiplexed signal recorded on an optical recording medium, and in particular, by using an element that is easy to modulate such as a semiconductor laser as a readout light source, it is possible to easily demodulate a frequency multiplexed signal recorded on an optical recording medium. This has the advantage that the configuration can be made extremely simple.

[Brief explanation of the drawing]

FIG. 1 is a main block diagram of a video disc device showing an embodiment of the present invention, and FIG. 2 is an enlarged view showing an example of pits recorded on the disc surface. In the figure, 1 is a readout light source, 2 is a condensing lens, 3 is a disk, 4 is a motor, 5 is a photodetector, 6 is a modulator, T is a local oscillator, 8 is a local oscillation crystal group, 9 is a selection switch, 1
0 is a bandpass filter, 11 is a demodulator, 13 is a beam splitter, and 21 is a pit.

Claims (1)

[Claims] 1 In a device for demodulating narrowband signals such as a plurality of audio signals recorded by frequency division multiplexing with different carrier waves on an optical recording medium having wideband recording characteristics, means for modulating a readout light source at a constant frequency. and a switching means for switching the modulation frequency of the light source according to the band of the readout signal, and focusing the readout light from the light source modulated by the modulation means onto the recording surface of the optical recording medium. means for converting light from the recording surface into an electrical signal; a bandpass filter having a constant pass band and receiving the electrical signal; and demodulating the electrical signal that has passed through the bandpass filter. and the switching means,
A demodulating device for a frequency multiplexed signal, characterized in that the modulation frequency is switched so that a frequency difference between each of the carrier frequencies and the modulation frequency becomes a pass frequency of the bandpass filter.