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Transistorised amplifiers for video transmission over coaxial cables incorporate power units with d.c.-d.c. convertors that produce an aggregate effect similar to that of band-limited random noise at low frequency. The subjective effect of such noise, centred at 3.2 and at 7.5kHz, has been investigated. Tests were also made with sinewave noise at the frequencies giving the worst effect in the regions of 1 and 3.2kHz and 1 and 3.2MHz. Results are summarised in the form of signal/noise ratios for a series of `impÂ¿ mark points that has recently been introduced.