Product description

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 µC 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 operating voltage and power requirement.

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 µC or external control. This state is fully pin and function compatible with the FX641 and MX641.
  2. Enhanced Features Operating state, under µC 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.

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Block diagram


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  • Two (12.kHz/16kHz) SPM Detectors
  • Detects 12kHz and 16kHz SPM Frequencies
  • Xtal Controlled Accuracy
  • “Controlled” (µC) and “Fixed” Signal Sensitivity Modes
  • Selectable Tone-Follower or Packet-Mode Outputs
  • High Speechband Rejection Properties
  • ‘Output Enable’ Multiplexing Facility


  • Call Charge Applications on PABX Line Cards
  • Complex and/or Simple Telephone Systems, Hotel and Office Call-Charge/Logging Sytems
  • Billing Systems
  • Public Call Monitoring
  • Payphone Systems
  • PCMCIA Fax and Data Modems

Power supply requirement

  • 2.7 to 5.5 V

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Design resources

CMX631A/FX631: SPM (Subscriber Pulse Metering)

All Fax machines, PC, and laptop Fax and data cards have a facility to report the “duration” of a Fax transmission in seconds and minutes. The system relies on simple timing but gives no information as to the actual cost of each data transmission. By including an SPM detector (X631) in the relevant equipment, a report can now be generated which actually reports the real cost of the ‘call’ in local currency. This allows the machine to cost the differences between local and international calls and the use of peak- and cheap-rate dialling periods.

FX/CMX641: SPM 12kHz and 16kHz Blocking Filters

This application note presents SPM (Subscriber Pulse Metering or Tax Metering) detection and methods to attenuate SPM burst signals. Common hybrids or SLICs (subscriber line interface circuit) input circuits have limited dynamic range and are adversely affected by large amplitude SPM signal bursts.

Package: D2

24-pin SOIC

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