Signal multiplexing system

An encoder for multiplexing a composite television video signal with a plurality of audio signals, comprising:

• a video input terminal for receiving the composite video signal;
• a sync signal separation circuit connected with the video input terminal for extracting the horizontal sync signal and a signal which corresponds to the leading edge of the initial pulse of the vertical sync signal of a selected one of an odd- and even-numbered field from the composite video signal;
• a timing pulse generator for producing various pulses in response to the horizontal sync signal and the signal which corresponds to the leading edge of the initial vertical sync pulse;
• a plurality of audio input terminals for receiving audio signals of a plurality of channels;
• a plurality of sample and hold circuits connected with respective ones of the audio input terminals and receiving sampling pulses from the timing pulse generator to sample respective ones of the audio signals, the sampling pulses having a frequency f.sub.s which is related to the frequency f.sub.h of the horizontal sync signal by the following expression:
• f.sub.s =f.sub.h .times.(n/m)
• where m represents a selected integer which represents a divisor of the number of scanning lins per frame of the composite video signal and n represents a positive integer;
• a plurality of analog-to-digital converters for receiving samples from respective ones of the sample and hold circuits and for converting them into a word formed by a plurality of bits;
• a multiplexer responsive to the signal supplied from the timing pulse generator and which corresponds to the leading edge of the initial vertical sync pulse for receiving words from the individual analog-to-digital converters sequentially and in parallel fashion for rearranging them into a serial form to output a pulse code modulated time division multiplexed audio signal composed of a succession of frames comprising words from respective channels by determining the relative position of the initial frame of the PCM multiplexed audio signal relative to the composite video signal;
• and a matrix circuit for adding the frequency modulated composite video signal with the PCM multiplexed audio signal.

A decoder for demodulating a multiplexed video-audio signal which is produced by multiplexing of a frequency modulated composite video signal and pulse code modulated time division multiplexed audio signals, comprising:

• an input terminal for receiving the multiplexed video-audio signal;
• a bandpass amplifier for separating the frequency modulated composite video signal from the multiplexed video-audio signal;
• an FM demodulator connected to receive and demodulate the frequency modulated composite video signal;
• a sync signal separation circuit for separating a horizontal sync signal and a signal which corresponds to the leading edge of the intiial pulse of a vertical sync signal of a selected one of an odd- and even-numbered field from the demodulated composite video signal;
• a low pass amplifier for separating the PCM multiplexed signal from the multiplexed video-audio signal;
• a bit synchronization circuit for producing a signal having a frequency which is the same as or double the clock frequency in response to the PCM multiplexed signal;
• a timing pulse generator for producing various pulses in response to the outputs from the sync signal separation circuit and the bit synchronization circuit;
• a converter controlled by gate pulses having a given timing and which are supplied from the timing pulse generator for separating the PCM multiplexed signal into digital outputs corresponding to individual channels;
• and a plurality of digital-to-analog converters for converting the digital outputs of the converter into corresponding analog audio signals.