Abstract: To monitor stereo audio signals associated with a TV video signal, the two stereo audio channels are converted to digital form (20, 21), a memory (22) has bits written into it at addresses formed by successive pairs of such values in digital form to form a map therein of a Lissajou figure representing the relationship between the two channels, the memory is read out linearly in synchronism with the operation of the monitor, and the output inserted into the TV video signal (VID) being displayed. The memory may be read out diagonally, so as to produce a diamond display, by a line counter (LB, RB) and a bit counter (LL, RL) for each channel, the line counters being loaded with predetermined counts at the beginning of each field and counting lines and the bit counters being loaded from the line counters at the beginning of each line and counting bits along the lines, three of the counters (LL, RL, RB) counting in one direction and the fourth (RL) in the other.