 high signal frequency, typically, system may be applied to metallic circuits above the range of U.K. 2280Hz, N. America 2600Hz. short-haul d.c., and to circuits isolated for d.c. when d.c. signalling Speech currents containing the signal frequency usually have is not possible. The more usual application is on 2-wire circuits, the other frequencies. The signal receiver has two elements: Exchange Exchange 4 wire receiver circuit Receive Transmit line split line split Buffer amplifier Buffer amplifier Receive Transmit line split line split receiver Figure43.4 General arrangement of v.f. line signalling system 43/6 Signalling systems (a) tuned to the signal frequency, and (b) responsive to other Unlike inband v.f. signalling, outband signalling: frequencies to oppose receiver operation to (a). Signals made longer than the normal persistence of the signal 1. Is not subject to signal imitation by speech or other inter- frequency occurring in speech. At least 40ms longer for import- ference. ant signals such as release. 2. Eliminates the line splits. 4. Signals of two frequencies compounded are less liable to occur, 3. Permits signalling during speech (e.g. meter pulse trans- and persist, in speech than one frequency. 2 v.f. signalling mission). systems are known, but 1 v.f. systems are more usual. On the other hand v.f. signalling can be applied to any type of The v.f. signals may be continuous or pulse. Pulse signals of transmission media, whereas outband signalling can only be applied different lengths permit a greater signal repertoire. A continuous to FDM channel circuits. signal requires a continuous response to cease it. As outband signalling is independent of speech, there is virtually complete freedom in the choice of signalling mode with simple arrangements. The signalling may be: 43.1.4.3 Outband signalling 1. 2-state continuous tone-on idle (off during speech). This is a.c. signalling within a 4kHz channel of FDM transmission. 2. Tone-off idle (on during speech), which however tends to The channel bandwidth