The error signal corresponding to the deviation of the actual track position from the desired track position is applied to three pairs of threshold stages each pair consisting of one stage furnishing a threshold output signal if a positive error signal has an amplitude exceeding a predetermined value and a second stage for forming the same function for a negative error signal. The threshold output signals are stored in associated flip-flops. The flip-flops are reset whenever a flip- flop indicative of a higher amplitude error signal is set. Whenever the lowest threshold stage resets, indicating that the error signal is passing close to zero, gates are activated which advance counters connected to the flip- flops if the flip-flop has been set. In a digital embodiment, the comparison takes place on an amplitude basis only, the sign controlling the transfer to counters associated with negative and positive error signals respectively. The signal for initiating the counter advance is derived from an exclusive OR circuit which furnishes an output when the sign of the last received error signal differs from the sign of the last previously received error signal. The counters are thus advanced only when a change in sign of error signal occurs.
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