Signal transmission system and a transmitter and a receiver for use in the system

A system for transmitting at least two signal channels comprising:

• means for dividing each channel into first and second complementary sub-channels which occupy frequency bands symmetrical about a common subcarrier frequency;
• means for adding signals in the sub-channels to form a first modulating signal;
• means for subtracting signals in the sub-channels to form a second modulating signal;
• means for phase shifting the second modulating signal from said means for substracting;
• a carrier generator for generating first and second carrier signals of a different phase but identical frequency;
• means for modulating said first carrier signal with the signal from said means for adding, and for modulating said second carrier signal with the signal from said means for phase sifting;
• and
• means for combining the modulated first and second carriers from said means for modulating, whereby a single modulated carrier signal is produced containing said complementary sub-channels which are frequency symmetrical with respect to the frequency of said carrier signals.

A receiver for receiving a carrier frequency signal modulated with signals in first and second sub-channels which are frequency symmetrical with respect to said carrier frequency signal, said first sub-channel having a frequency position below said carrier signal frequency, said second sub-channel having a frequency position above said carrier frequency signal;

• said receiver comprising:
• a local oscillator producing first and second signals having the same frequency but different phase;
• mixer means connected to receive said modulated carrier frequency signal having said first and second complementary sub-channels, and for receiving from said local oscillator said first and second signals having the same frequency, said mixer means producing first and second phase displaced output signals;
• a signal adding circuit connected to said mixer means for adding the first and second phase displaced signals together to form a summation signal;
• a signal substraction circuit for forming a difference signal from the first and second phase displaced signals produced by said mixer means;
• demodulator means connected to said signal adding and subtracting circuits to receive said summation and difference signals and for forming therefrom first and second baseband signals containing information in said first and second sub-channels;
• and
• a demultiplexer connected to receive said first and second baseband signals and form a single channel from said complementary sub-channels.