Performance of a digital stereo FM modulator with reduced output resolution

A digital stereo FM modulator has been built using a direct digital synthesiser (DDS). The system incorporates a digital stereo generator to provide the modulating signal. The stereo generator was sampled at 228 kHz and interpolation by 2 was carried out to move the unwanted FM sidebands further away from the FM carrier, in order to facilitate filtering. The paper presents the effects of quantisation noise on the system output by reduction in output resolution. Demodulation has been carried out so that the signal to noise and distortion ratios (SINAD) could be measured for all spectral components ⩽53 kHz with different monotone modulation frequencies and varying output resolution. An FM simulation of this process has been developed and the FM spectra produced by it are shown to match the experimental spectra almost exactly. These spectra complied with broadcast standards even with as little as a 6 bit output. The experimental system, with a 10 bit output, gives a SINAD performance which is ⩾77 dB