A modem for a digital data transmission system in which binary-coded data at up to 9.6 Kbit/sec is transmitted over a telephone line pair circuit, as phase and amplitude modulation (QAM) of a carrier signal at a frequency of 1800 Hz, at a line symbol rate of 2400 baud/s. The modem utilises two data signal processors (4 and 5) and two gate arrays (6 and 7), controlled by a microprocessor (8), to carry out digitally all the processes required to convert binary coded data signals into PCM coded samples of the required carrier signal for transmission, and to recover binary coded data signals from PCM coded samples of a received carrier signal.
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