The FX631 is a low-power, system-selectable Subscriber Pulse Metering (SPM) detector to indicate the presence, on a telephone line, of both 12kHz and 16kHz telephone call-charge frequencies. Deriving its input directly from the telephone line, input amplitude/sensitivities are component adjustable to the user's national `Must/Must-Not Decode' specifications via an on-chip input amplifier, whilst the 12kHz and 16kHz frequency limits are accurately defined by the use of an external 3.579545MHz telephone-system Xtal or clock-pulse input. The FX631, which demonstrates high 12kHz and 16kHz performance in the presence of both voice and noise, can operate from either a single or differential analogue signal input from which it will produce two individual logic outputs.