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A physiological monitoring system which can transmit up to six low-frequency data channels in the presence of a full-duplex speech channel using one telephone line is described. Bandlimiting the speech signal to 2.5 kHz leaves a bandwidth of about 900 Hz for the six data channels. To make best use of this allocation quadrature amplitude modulation (QAM) was chosen as the main transmission mode. Coherent detection must be used at the receiver to extract the data. The center frequencies for the carriers were set at 2800, 3050, and 3300 Hz. The system was tested on a local public switched telephone network line over a distance of approximately 40 km, and a sample of the signals received is shown. These were recorded at 10.00 am on a weekday when the noise level on the line can be expected to be higher than normal. The output ECG and blood pressure waveforms proved to be clinically acceptable.