Abstract: A time-division multiplexed (TDM) signal carrying speech information and in-band single frequency signaling tones is simultaneously applied to a digital band-pass filter and a digital band-elimination filter. The signaling tone components of the TDM signal are passed through the band-pass filter, while they are eliminated by the band-elimination filter. First and second digital rectifiers convert the outputs of band-pass and band-elimination filters into first and second single-polarity TDM signals, and integrated by first and second digital integrators to generate first and second integrator outputs having mutually distinguishable digital values. A digital comparator compares the integrator outputs with each other and generates a TDM comparator output when the first integrator output is greater than the the other by a predetermined amount.