Most Telstra exchanges in Australia have the option to send a 12kHz signal (pulse) over the speech pair to the customers premises for each registration that is recorded by the customer's call-metering-circuitry in the telephone exchange. This allows customers to calculate their call-charges. Because each metre-registration is charged a fixed rate, simply counting the number of registrations gives the customer an approximation of the call-cost. Third-party hardware or software at the customer's premises is used to record these 12kHz pulses.
In Australia, a single call-billing registration occurs at the successful connection of a local call. In contrast to many other countries, local calls in Australia are un-timed and cover the geographic area of several telephone exchanges. As an example, a call from Avalon (one of Sydney's most northerly suburbs) to Sutherland (one of Sydney's most southerly suburbs) is an un-timed local call and will record just one meter registration.
For long distance (STD or IDD) calls, metre pulses are sent at a pre-defined interval, the time between successive intervals being determined by the location of the called party. As an example, an STD call from Sydney to Perth may receive a metre pulse every 60 seconds whilst an IDD call to Hong Kong may receive a metre pulse every 35 seconds.
Before the introduction of the 12kHz metre pulse system, a 50Hz metering pulse was used to operate customer's call-charge recorders. This 50Hz longitudinal pulse is transmitted from the telephone exchange over both legs of the telephone line. The detection equipment at the customer's premises depended on the provision of a low impedance earth at the customer's premises for correct operation. Amongst other problems, these 50Hz pulses were sometimes subject to interference from the 50Hz 240V electricity mains and were sometimes unreliable because of earthing problems at the customer's site, lightning strikes or a rise in the earth potential.
The 12kHz metering system will only work in situations where the cable between the exchange and the customer's premises is not conditioned i.e. does not have any line-amplifiers, loading coils (3.4KHz maximum transmission limit), signalling repeaters, hybrid amplifiers etc. The maximum line limits of unconditioned cable are shown in the following table. These limits equate to approximately 22db at 12kHz.
| Cable Gauge | Line Length |
|---|---|
| 0.32 | 2.84km |
| 0.40 | 4.20km |
| 0.51 | 5.60km |
| 0.64 | 7.00km |
| 0.90 | 10.50km |
* Most Telstra exchanges now support both the 50Hz and the 12kHz metering systems, with 12kHz metre pulses being the preferred option.
For correct operation of the 12kHz metering system, the addition of the 12kHz band-pass and 12kHz band-stop filters must be fitted to the CPE circuitry. This filter combination ensures that only 12kHz signals are passed to the Metre Pulse Detection circuitry whilst at the same time preventing 12kHz signals from entering the voice circuitry. The 12kHz band-stop filter must have at least 35db attenuation at frequencies between 11 & 13 kHz whilst at the same time reflect the standard complex VF impedance characteristics of a typical off-hook line condition. The 12KHz band-pass filter should provide minimal attenuation at frequencies between 11 & 13 kHz whilst at the same time reflecting a resistive termination impedance in the range of 200 to 400 ohms.
The switching matrix in a digital PABX/KSU at a customers premises typically operates at a speed of 8kHz. If the 12kHz metering pulse is allowed to interact with the PABX/KSU 8kHz switching speed (due to incorrect design or connection of the band-pass and band-stop filters), a 4kHz modulated signal is generated for each 12kHz metering pulse that is received. This causes audible noise over the conversation and unreliable 12kHz metering detection.
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