A signal processing apparatus comprises a signal path for a signal, the signal path comprising a signal processing stage. An auxiliary stage is coupled to an input of the signal processing stage for, in response to a signal in the signal path at the input of the signal processing stage, generating a control signal indicative of the time of a crossing of a first threshold by the signal in the signal path at an output of the signal processing stage by detecting a crossing by the signal of a second threshold established by the auxiliary stage. The second threshold is substantially equal to the first threshold.