Abstract: Apparatus for deriving a parameter indicative of the quality of parts of, or the whole of, a television system from an insertion test signal repeated at regular intervals during field blanking periods of a transmitted signal includes a sampling means for providing a digital signal representing the amplitude at a sampling position of the test signal, means for actuating the sampling means, and a computer. The computer is programmed to caculate the quality parameter from a number of the amplitude representative digital signals taken at different sampling positions by the sampling means, to provide the calculated parameter as an output, and to respond to timing origins included in the television signal to thereby control the actuating means. The actuating times are so related to the timing origins that the digital amplitude signals are provided at an average rate which is low compared with the rate at which the signals occur during a single test signal.