Matsushita Electric Industrial Co., Ltd.

Hiroaki Nakata

Kazuki Maeda

Masaru Fuse

Kuniaki Utsumi

Katsuyuki Fujito

Susumu Morikura

Hiroaki Yamamoto

Takeshige Ichida

Koichi Masuda

Naoki Ishiyama

An AM signal source frequency-multiplexes and outputs a plurality of AM signals. A soft clipper, which is formed by an amplifier or the like, nonlinearly loosely limits a large amplitude portion (impulsively changed portion) of the frequency-multiplexed AM signal by using its saturation property. Thereafter, an attenuator performs level adjustment for setting the signal at a prescribed degree of optical modulation. On the other hand, a 16QAM signal source frequency-multiplexes and outputs a plurality of 16QAM signals having different carrier frequencies. A frequency multiplexing part further frequency-multiplexes the AM multiplex signal with the 16QAM multiplex signal. An optical transmitter intensity-modulates a laser output beam with the frequency multiplex signal from the frequency multiplexing part and transmits the same. No clipping distortion occurs at this time. An optical signal from the optical transmitter is transmitted to a receiving system through an optical fiber member and is demodulated.

Optical transmission unit

Video signals for broadcasting service with a plurality of channels are frequency-multiplexed into a predetermined broadcasting band, and converted into optical signals by an optical transmitter. At the same time, video signals for VOD service requested by a plurality of subscribers are selected, taken out, frequency-multiplexed within the VOD band, and converted into optical signals by an optical modulator. These two types of optical signals are transmitted to subscribers through the optical network. In each subscriber's household, either broadcasting signals or video signals for VOD service are reproduced and desired programs are selected in the tuners.

Optical fiber cable service system provided with video on demand service

A picture image distributing apparatus provided within a passenger moving vehicle, consists of: a head end portion for multichannel FDM picture image signals; an electrical optical for converting the FDM picture image signals into optical signals; an optical amplifier for optically amplifying the optical outputs; one optical fiber transmission path connected to optical taps of unequal distribution for branching halfway on one portion of the optical power except for the branching at a final point, an optical/electrical converter for receiving optical signals branched at the final point or halfway so as to convert them into electrical signals; and a passenger seat picture image receiving terminal connected to the optical/electrical converter so as to receive the electrical signals.

Picture image distributing apparatus provided within passenger moving

The present invention discloses a frequency stabilizing circuit used for a modulator that modulates an image signal including a synchronous signal into a pulse modulation signal. The frequency stabilizing circuit comprises a detecting device for detecting the synchronous signal included in the image signal that has not been modulated to output a pulse based on the detection, a pulse generating device for outputting a pulse by extracting a modulated signal outputted from the pulse modulator corresponding to the synchronous signal, and a feedback controlling device for increasing and/or decreasing a level of the image signal to be inputted to the pulse modulator in accordance with the difference between the pulse width of the pulse signal outputted from the detecting device and that of the pulse signal outputted from the pulse generating device.

Frequency stabilizing circuit used for a pulse modulator for an image

An optical transmission system provides digital signals which have added bits for error correction by an encoder which are converted into 16-QAM signals with a predetermined carrier frequency by a modulator, and then frequency division muitiplexed with an AM signal so as to be inputted to an optical-electric converter. When the total sum of the light modulation indices of signals to be transmitted is over 1, a light modulation index of an M-QAM signal is set by calculating the light modulation index that marks the onset of diverging from the error rate given by a ratio between receiver noise power and the M-QAM signal power while considering a light modulation index corresponding to a usable transmission margin. The multiplexed signals are transmitted via optical fiber and converted into electric signals by a photo receiver, demodulated by a demodulator, and then outputted after having their errors corrected by an error corrector.

Optical transmission system

A pay-channel transmission system for a CATV has an optical transmission unit and an optical receiver unit. The optical transmission unit includes pay-channel television signals optically multiplexed by a .lambda..sub.1 wavelength semiconductor laser and non-pay-channel television signals optically multiplexed by a .lambda..sub.2 wavelength semiconductor laser. The optical receiver unit includes an optical filter for filtering the .lambda..sub.1 wavelength data. Thus, a non-subscriber having the optical filter is prevented from receiving the pay-channel television signals, and a subscriber having no optical filter is able to receive both the pay-channel television signals and non-pay-channel television signals.

Pay-channel transmission system for CATV

An optical transmission apparatus is provided which has an LD of single longitudinal mode. A bias current of the LD is set at its threshold current. A current obtained by amplitude modulation of a signal to be transmitted is superposed on the bias current. The LD is then set in the multimode, so that the apparatus can transmit a high-quality signal with reduced modal noise and distortion during multimode fiber transmission.

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Hirakata, Japan

Hiroaki Nakata