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The design details for a 768-Mbit/s single bit digital by analogue correlator for a wideband interferometer system are given. Design tolerances for a high-speed 1-bit digitizer realized using several lower speed converters are presented, and it is shown that the arrangement may be extended to 1200 Mbits/s if a 5% loss in sensitivity can be tolerated. A variable path compensating delay using a high-speed cyclic memory is described. The operation of a 1-bit by analogue multiplier is considered in detail and design tolerances for this device are also presented. The measured sensitivity of the correlator, relative to that of a perfect analog system, is 0.77±0.02, which compares well with the predicted value of 0.78. The correlator has been used with the 76-m Mk 1A and 26-m MK II radio telescopes at Jodrell Bank to provide a high-sensitivity, single-baseline inerferometer.