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The properties of these signals differ between countries, but they typically have a frequency of 50 Hz, 12 kHz or 16 kHz, and a duration of several tens or 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 allow the talking parties to hear 50 Hz buzz tones. These pulses are applied at relatively high voltage to distinguish them from 50 Hz 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.