carrier frequency system
Carrier frequency systems, TF systems, are used for multiple utilization of trunk lines. This technology was used in classic, analog telephony systems and utilized frequency division multiplexing for optimum utilization of the available frequency spectrum.
In terms of technology, in carrier frequency telephony the frequency range required for telephony between 0.3 kHz and 3.4 kHz is amplitude modulated with different carrier frequencies in a ring modulator. The upper sideband is filtered in a downstream filter and combined with the other modulation frequencies. In the classical technique, the carrier frequencies have a spacing of 4 kHz and the output signals are multiplexed in a first stage to form pre-groups of three channels; in a second stage, 4x3 channels, i.e. 12 channels, are multiplexed to form a primary group; and in a third stage, 5x12 channels, i.e. 60 channels, are multiplexed to form a secondary group.
Common TF systems are the two- wire systems Z12, Z24 and the four-wire systems V60, V120, V300, V600, V2700 and V10800. They are specified in the ITU M-Series Recommendations: Maintenance of Audio Lines and Carrier Frequency Systems.