Sound signal-to-noise ratio in intercarrier sound television receivers

Abstract

The effect of the recent FCC order permitting the reduction of sound/picture carrier power ratio from 50% to 10% for UHF on the sound-channel performance of intercarrier sound television receivers is evaluated. The sound channel performance of intercarrier sound television receivers is investigated analytically and experimentally. It is shown that in a perfect intercarrier sound receiver the ultimate signal-to-noise ratio at maximum white level (12.5% of sync level) for 10% sound power is 36 db for 30% modulation when the picture signal-to-noise ratio (TASO definition) is 20 db. At lower input signal levels, the performance falls off more rapidly than the reduction in input level because of intermodulation of noise, video, and synchronizing components in the second detector. It appears doubtful that the sound power can be reduced to 10% without degrading the performance of a significant percentage of receivers now being used to receive fringe signals. In any case it is desirable that a single standard of sound-to-picture carrier ratio be established for VHF and UHF. Under the present standards a receiver must be capable of providing both freedom from beat interference and good sound performance for a transmitted power ratio ranging from 70% to 10%, exclusive of the additional variations introduced by propagation and environmental factors.