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In telecommunications, metering pulses are telephone signals sent by telephone exchanges to metering boxes and payphones aimed at informing the latter of the cost of ongoing telephone calls. Metering pulses are also known as billing pulses and tax pulses.
The properties of metering signals differ between the telephone administrations in various countries. Common frequencies are 50 Hz, 12 kHz, and 16 kHz. The duration of these signals is typically from several tens to hundreds of milliseconds. 50 Hz pulses are applied to the telephone circuit as common-mode signals with respect to ground, as applying them differentially would make them audible for the talking parties as buzzing tones. The pulse amplitude (RMS voltage) is sufficiently high to distinguish them from 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 an incremental cost. For calls with higher billing rates, the exchange equipment generates metering pulses at a higher rate than during calls to cheaper destinations, or during periods of lower billing rates.
Metering pulses are not used to generate billing records, but serve to inform the calling subscriber with a visual display in form of a counter or advancing dial connected to the telephone instrument.