Experimental study of a modulation scheme for embedding data in DSB-AM

Methods of embedding data in conventional DSB-AM signals are of interest in a number of applications including radiodata services for HF sound broadcasting. The data signals carry information which can be used by specially equipped receivers for various display and control functions such as automatic retuning when the transmission frequency is changed. In the case of DSB-AM, frequency modulation of the carrier in addition to the usual envelope modulation can provide a data channel but unless the data rate is very small the modulated signal is likely to exceed its allocated bandwidth. Zero-synchronous modulation (ZSM) is a spectrally-efficient combined AM/FM scheme proposed by Lockhart and Al-Jalili (1982) which permits data to be embedded in DSB-AM without significantly increasing the bandwidth or distorting the envelope. An experimental transmitter/receiver system constructed to study the practical performance of ZSM is described and some results presented.