1. A method of sampling marks in frames complying with CCITT standards, including B-marks, D-marks and F-marks, to provide continuous frame-to-frame adjustment of a threshold voltage which is employed to detect marks in said frames having peak amplitudes which exceed said threshold voltage, comprising the steps: (a) sampling one B-bit mark in a succeeding frame, when a B-bit mark is present in a present frame; (b) sampling one D-bit mark in said succeeding frame, when no B-bit mark is present, but a D-bit mark is present in said present frame; (c) sampling one F-bit mark in said succeeding frame, when no B-bit and no D-bit marks are present in said present frame; and (d) adjusting said threshold voltage to a predetermined proportion of the amplitude of the sample taken in step (a), (b) or (c) to detect high marks in said frame following said succeeding frame.