Metering pulse

In telecommunications signalling, metering pulses are signals sent by telephone exchanges to metering boxes and payphones aimed at informing the latter of the cost of ongoing telephone calls.

The properties of these signals differ between countries, but they typically have a frequency of 50Hz, 12 kHz or 16 kHz, and a duration of several tens or hundreds of milliseconds. 50Hz pulses are applied in common-mode to the telephone circuit, with respect to ground, as applying them differentially would allow the talking parties to hear 50Hz buzz tones. These pulses are applied at relatively high voltage to distinguish them from 50Hz power-mains-induced signals. 12- and 16 kHz metering pulses are applied differentially across the telephone circuit, as these frequencies cannot be heard by listeners with conventional telephone instruments.

Each pulse represents a certain incremental cost. Therefore, during more expensive calls the exchange will generate more metering pulses per minute than during cheaper calls.