The amplitudes of a signal on the microphone input of a telephone and a signal
on the line input of the telephone are compared with each other and with two thresholds.
The comparisons are sampled and the samples control an up-down counter. The count
in the counter is compared with two thresholds and the results of the comparisons
are used to control at least two accumulators. A first accumulator counts consecutive
comparisons having the same result and a second accumulator counts up or down in
accordance with the comparisons. The counts in the first and second accumulators
are logically analyzed to determine whether to receive or transmit. A plurality
of samples defines a window, at the end of which the up-down counter is reset.
The accumulators store data from a plurality of windows.