The Equalization of Base-Band Noise in Multichannel FM Radio Systems

Pre-emphasis and de-emphasis networks are frequently incorporated in the radio equipment of multichannel systems in order to improve the distribution of noise among the channels. The pre-emphasis network has a rising frequency amplitude characteristic and this produces a redistribution of both signal power and nonlinear noise power over the working spectrum. The improvement in signal-to-noise ratio in any channel which the technique of equalization affords is dependent not only on this effect, but also on the contribution of the multiplex equipment to the total channel noise. This is discussed for typical networks having a maximum slope of attenuation of 6 db per octave and the general case when second-order distortion is predominant. Under practical conditions, the maximum possible improvement is almost entirely obtained when the mean power of the multichannel signal is unchanged by the presence of the networks; a convenient way of establishing this condition with sufficient accuracy is to maintain constant signal energy at a base-band frequency which is 55 per cent of the highest used. Practical considerations usually dictate that the networks do not have a maximum insertion loss exceeding 26 db. With this value and with the multiplex contributing 25 per cent of the total channel noise, an improvement of 3.6 db in the signal-to-noise ratio of the top channel can be expected; provided this loss is not exceeded, the relative noise power, in general, will still be greatest in the top channel.