Stereo sound at its simplest means that two microphones are used for a recording, each picking up a slightly different signal. Stereo sound from discs and from tape was well established by the 1950s, but stereo sound broadcasting took a little longer. Ian Sinclair, in Electronics Simplified (Third Edition), 2011 Stereo Radio In the case of a Zenith-GE encoded stereo signal it will be at least 30 Hz–53 kHz, even if adequate filtering has been used to remove spurious noise components based on the 38 kHz sub-carrier harmonics.

In the case of a mono signal this bandwidth is 30 Hz–15 kHz. The reason for the greater background ‘hiss’ associated with the stereo than with the mono signal is that wide-band noise increases as the square-root of the audio bandwidth. It is then filtered to remove any audio frequency components before addition to the (L+R) channel.įinally, the 38 kHz sub-carrier signal is divided, filtered and phase corrected to give a small amplitude, 19 kHz sine-wave pilot tone which can be added to the composite signal before it is broadcast. In the case of the (L-R) signal, however, it is first converted into a modulated signal based on a 38 kHz sub-carrier, derived from a stable crystal-controlled oscillator, using a balanced modulator to ensure suppression of the residual 38 kHz sub-carrier frequency. Errors in this respect will degrade the 35–40 dB (maximum) channel separation expected with this system. In both cases it is essential that the relative phase of the regenerated 38 kHz sub-carrier is accurately related to the composite incoming signal. Because the system shown in Fig. 2.8 is more easily incorporated within an integrated circuit, it is very much the preferred method in contemporary receivers. Provided that adequate input filtration is employed in both cases there is no operational advantage to either method. This is only true if the input bandwidth of the sampling system is sufficiently large to allow noise signals centred on the harmonics of the switching frequency also to be commutated down into the audio spectrum. In the circuit of Fig. 2.8, an equivalent process is done by sequentially sampling the composite signal, using the regenerated 38 kHz sub-carrier to operate a switching mechanism.Īdvocates of the matrix addition method of Fig. 2.7 have claimed that this allows a better decoder signal-to-noise (S/N) ratio than that of the sampling system. In the circuit of Fig. 2.7, this process is carried out by recovering the separate signals, and then using a matrix circuit to add or subtract them.

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