A prototype subcarrier system to transmit digital data over commercial AM radio

Abstract

This paper describes modeling and simulation of candidate AM subcarrier systems to determine their relative performance. Based on the simulation results, a prototype system is designed that was successfully tested on the air. A data rate of 200 bits per second was achieved without introducing main channel audio distortion; the digital data was recovered in an automobile over 200 miles from the transmitter site well beyond the Continental Divide through the Colorado Rocky Mountains. For the simulation study, the primary goals were to determine the level of interference between the digital baseband spectrum and the main channel audio spectrum, and to identify effective subcarrier modulation schemes. In early systems, the phase shifts modulating the main carrier were limited to +/-15 degrees to avoid significant audio program distortion during AM detection. This paper explores the possibility of applying other digital modulation techniques including BPSK, QPSK, GMSK and QAM. Sample speech segments are used to create the AM portion of the composite signal. Effects of the secondary RBDS signal on the voice program output signal-to-noise ratio are assessed. Conversely, the effect of the AM sidebands on the RBDS bit-error rate is measured. Finally, a prototype system is described using off the shelf components to demonstrate this technology on the air. Low rate digital data is digitally modulated onto the main carrier of a commercial AM radio station. A conventional car radio is modified to extract the soft bit stream, which is subsequently processed in real time within a laptop computer hosting a DSP board. The performance of this system during field trials, transmitting from commercial AM station KTLK in Denver, Colorado, is summarised.