The CMX641A is a low power, system selectable Dual Subscriber Pulse 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. The detection sensitivity, frequency and bandwidth are independently selectable for each channel, under C control, as is the frequency of the security tone generator, which may be ASK modulated by an external signal. The CMX641A is also backwards compatible with the FX641 and MX641 dual SPM detectors, whilst offering a lower (3.0V) operating voltage and power. The CMX641A has two modes of operation: (1) Fixed Bandwidth Operating state, in which the two channels are set to the same system frequency and sensitivity setting. Sensitivity and system frequency can be under either C or external control. This state is fully pin and function compatible with the FX641 and MX641. (2) Enhanced Features Operating state, under C control, in which each channel has independently controllable sensitivity, bandwidth and system frequency (12kHz or 16kHz). There is also a 12kHz/16kHz transmission tone which can be keyed on and off directly by a dedicated logic pin. This device is suitable for PBX/PABX line-card installations, payphone security applications and pair-gain systems. It is available in 24-pin plastic DIL and SOIC packages and consumes 1.2mA at 3V.