Abstract:
In a light communication system in which data to be transmitted is converted to an amplitude-modulated light signal which is radiated to a space, and the radiated light signal is received and demodulated to reproduce the original data, the amplitude-modulated light signal is transmitted only for a predetermined period shorter than a unit time of the data to be transmitted.

Description:
This application is a continuation of application Ser. No. 939,309 filed Dec. 5, 1986, which is a continuation of application Ser. No. 570,811 filed on Jan. 16, 1984 now both abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light communication system, having a high transmission rate. 
     2. Description of the Prior Art 
     In the past, when digital signals &#34;1&#34; and &#34;0&#34; are light-transmitted, a light pulse at a predetermined frequency is sent for the signal &#34;0&#34;, for example, and the light pulse is not sent for the signal &#34;1&#34;. That is, a 100% modulation AM-modulation system is employed. 
     In such a system, a filter circuit which passes only a carrier frequency is used in a receiver to eliminate unnecessary light signals. However, since the filter circuit usually produces a gradually decaying output signal even after an input signal has disappeared, due to self-resonance, an apparent input signal period is longer than an actual input signal period. Accordingly, signals &#34;1&#34; and &#34;0&#34; which do not allow neglect of this delay time cannot be processed in this system. This restricts the rate of data transmission. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a light communication system which avoids the restriction on the highest transmission rate by, for example, a delay in a filter circuit to improve the transmission rate by shortening a transmission time to reflect an expansion of time in a receiver. 
     It is an aspect of the present invention to provide a light communication system comprising means for shortening a signal duration of data consisting of a signal train, and transmission means for light-modulating the data having the shortened signal duration. 
     It is another aspect of the present invention to provide a light communication system comprising means for shortening a signal duration of a data consisting of a signal train and means for transmitting the data having the shortened signal duration. 
     It is another aspect of the present invention to provide a communication system comprising series-connected storage means for transferring data consisting of a signal train, means for shortening a signal duration of the data to be shorter than the duration of outputs of the storage means and modulation means for modulating a carrier based on the shortened signal train. 
     It is another aspect of the present invention to provide a communication system comprising cascade-connected D-type flip-flops for receiving data consisting of a signal train, means for NORing outputs of the D-type flip-flops, means for modulating a carrier based on the output of the NOR means, and means for converting the signal produced by the modulation means to a light-signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a block diagram of a transmitter of a light communication system of the present invention, 
     FIG. 2 shows signal waveforms for explaining the operation of FIG. 1, 
     FIG. 3 is a block diagram of a receiver of the light communication system of the present invention, and 
     FIG. 4 shows signal waveforms for explaining the operation of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a block diagram of a light signal transmitter in accordance with the present invention. T×D denotes a digital signal which is a signal train of data to be transmitted, Tp denotes a clock pulse having a period of 200μ seconds, FF1 and FF2 denote D-type flip-flops, G1 denotes a NOR gate, G2 denotes an AND gate, TR1 denotes a transistor, L denotes a light emitting diode for converting a signal to light, and R1 and R2 denote resistors. 
     The digital signal T×D to be transmitted is applied to an input terminal D of the D-type flip-flop FF1 and the clock pulse Tp is applied to an input terminal C. An output terminal Q of the D-type flip-flop FF1 is connected to an input terminal D of the D-type flip-flop FF2. The clock pulse Tp is applied to an input terminal C of the D-type flip-flop FF2. The Q-outputs of the D-type flip-flops FF1 and FF2 are supplied to input terminals of the NOR gate G1. An output of the NOR gate G1 is &#34;1&#34; when the outputs of both flip-flops are &#34;0&#34; as shown in FIG. 2. As a result, a &#34;0&#34;-level time of the digital signal T×D is shortened by 200μ seconds as shown in FIG. 2. The output of the NOR gate G1 is supplied to one input terminal of the AND gate G2 and a 38.4 KHz clock pulse is applied to the other input terminal of the AND gate G2. Thus, the 38.4 KHz clock pulse appears at the output terminal of the AND gate G2 for the period during which the output of the NOR gate G1 is &#34;1&#34;, that is, for a period which is 200μ seconds shorter than the period during which the digital signal T×D is &#34;0&#34;, as shown in FIG. 2. When the output of the AND gate G2 assumes &#34;1&#34;, the transistor TR1 is rendered conductive through the resistor R1 and the light emitting diode L is energized through the resistor R2. 
     In this manner, the light emitting diode L is turned on and off repeatedly at the frequency of 38.4 KHz for the period which is 200μ seconds shorter than the &#34;0&#34; period of the digital signal T×D and hence the amplitude modulated light signal is produced and radiated into space. 
     FIG. 3 shows a circuit configuration of a receiver in the present invention. PD denotes a photo-diode which detects the light signal radiated into space, and A denotes a differentiation circuit for deriving a signal SA representing a change in a light intensity from an output electrical signal of the photo-diode PD. The output signal SA is supplied to a band-pass filter F which passes only the carrier at 38.4 KHz to provide a filter output, or the 38.4 KHz component SF. The output SF is supplied to a detection circuit G which detects a signal SG, which in turn is supplied to a waveform shaping circuit H to produce a digital signal R×D. The digital signal R×D includes the original data and it is processed as a conventional digital signal. 
     The signal SF from the filter circuit F has a tail portion of approximately 200μ seconds as compared with the differentiated signal SA as shown in FIG. 4, and it is longer than an actual flash period of the light signal. However, in accordance with the present invention, since the transmitting light signal is previously corrected to compensate for the extended period of 200μ seconds, the digital signal R×D has a proper duration. 
     As described hereinabove, in accordance with the present invention, since the generation period of the transmitting light signal is previously shortened in the transmitter to compensate for the delay in the filter, the transmission rate is improved by the period corresponding to the delay in the filter, and the restriction to the transmission rate which has been a problem in the prior art system can be significantly avoided by the simple circuit configuration. 
     Alternatively, pulse generating means may be provided to shorten the pulse width of the pulse train.