The FX641 is a low-power, system-selectable Dual Subscriber Private Metering (SPM) Detector -two detectors on a single chip- to indicate the presence, on a telephone line, of either 12kHz or 16kHz telephone call-charge frequencies. Under Processor control via a common serial interface, each channel of the FX641 will detect call-charge pulses from a telephone line and provide a digital output for recording, billing or security purposes.