Transmission of Television Sound on the Picture Carrier

Several pulse methods for the transmission of television sound on the picture carrier during the line-blanking intervals are analyzed from the points of view of signal-to-noise ratio, audio fidelity, and transmitter and receiver design. The advantages of duplex transmission are: (1) elimination of a separate sound transmitter; (2) elimination of the ambiguity and difficulty which may occur when a standard frequency-modulated sound signal is tuned in; (3) freedom of the audio output from the type of distortion which occurs in frequency-modulated receivers as a consequence of excessive drift of the frequency of the local oscillator; and (4) improvement of the phase characteristic of the picture intermediate-frequency amplifier resulting from elimination of trap circuits. The highest audio-modulation frequency in duplex systems must not exceed one half of the line-scanning frequency. This is a disadvantage under the present television standards which specify a line frequency of 15,750 cycles per second. With the exception of pulsed frequency modulation, the signal-to-noise ratios of sound in duplex systems are not so great as the ratio offered by the transmission of a standard frequency-modulated carrier. The comparison is subject to the condition that the amplitude of the frequency-modulated carrier is 0.7 of the peak amplitude of the duplex carrier. The signal-to-noise ratio of a pulsed frequency-modulated signal may equal the ratio of a standard frequency-modulated signal up to a critical distance from the transmitter, but is less at greater distance.