GB1509212A - Compatible am stereophonic transmission system - Google Patents

Abstract

1509212 Stereophonic broadcasting L R KAHN 1 July 1975 [10 July 1974] 27726/75 Heading H4R In a compatible amplitude modulated stereophonic broadcasting system a carrier wave is amplitude modulated with the sum of the stereophonically related signals and is phase modulated with a composite stereo difference signal formed of the fundamental difference signal together with a difference signal developed from the frequency doubled stereo related signals, the frequency doubled component being maintained in substantially square law relation relative the amplitude of the fundamental difference signal and about 13% of the fundamental signal amplitude at full stereo modulation. As described with respect to Fig. 1 L and R stereophonically related signals are applied to an adding network 12 to form a sum signal which, after shifting in phase by an amount #(f) -45 degrees provides a signal to amplitude modulate a transmitter carrier. In addition the L and R signals are fed through respective 5 kHz low pass filters to a subtractor 28 to provide a fundamental difference signal 30 which is passed through a network 32 to provide a phase shift of #(f) +45 degrees, i.e. +90 degrees relative the sum signal at 18. The filtered L and R signals are also fed via networks 38 and 40 having a phase shift of #(f)+0 to frequency doublers 46 and 48 before application to subtractor 54 to derive a frequency doubled difference signal 56. The signal 56 is applied through a variable gain amplifier controlled in accordance with the level of the fundamental difference signal so that the level of the signal 68 at the output of amplifier 58 is proportional to the square of the fundamental difference signal amplitude. The fundamental difference signal and the frequency doubled difference signal are combined at 36, the delay equalized at 72, and the combined signal phase modulates a carrier signal from oscillator 78 in modulator 76 before frequency multiplying to the appropriate broadcast frequency and amplitude modulating with the sum signal for broadcasting. A 15 Hz stereo indicator signal can be added either to the amplitude modulating signal or as a phase modulating signal. The resulting signal has the stereophonically related signals appearing respectively in the upper and lower first order sidebands.