Abstract: Apparatus is described for measuring distortion appearing on data signals. This apparatus is of the type which uses a counter that, responsive to a pulse train of constant repetition frequency, counts from an initial to a final value during each segment of the data signal. Those counted values which occur simultaneously to pulse edges are stored. A decoder decodes the stored counts, and the decoded result is caused to appear on a display device. The counts are intermediately stored until passed by a blocking device which opens responsive to the appearance of an interrogation pulse generated in the middle of each data element. The stored counts are coupled, upon being passed by the blocking device, to the decoder and/or a display device.