 is at the send end (it is at the receive end in mission. The signals are amplified by transmission system ampli- short-haul). fiers. 4-wire operation is the more usual, the forward and backward 3. The polarised receive relay operates in one direction for the signalling paths being separate, the duplex signalling being make, and in the other for the break pulses. achieved by two simplex signalling paths (Figure 43.4). In 4-wire operation, the v.f. receivers are permanently associated The above requires the provision of both outgoing and incoming with the respective speech paths. As speech may contain the signall- line signalling terminals. Both terminals incorporate the trans- ing frequency, the system is subject to signal limitation by speech mission bridge. and must incorporate features to safeguard against this. There are different designs of LDDC systems. The use of po- A buffer amplifier of unit gain forward and some 60dB loss in the larised relays for signalling sensitivity and for bi-directional oper- reverse direction protects the signal receiver from near-end inter- ation to minimise decadic address pulsing distortion is a common ference from the switching equipment. The signalling is usually principle. link-by-link on multi-link connections and an arrangement of line splits adopted to isolate the v.f. signal to the particular link. Signall- 43.1.4 A.C. line signalling ing during speech is inadmissible. Protection against signal imitation (false line splits or false sig- 43.1.4.1 Low frequency a.c. signalling nalling) relies on exploiting differences between speech and signal currents, e.g.: This uses a below the speech band signalling frequency. Signalling imitation by speech does not arise. Typical frequencies are 25, 50, Adoption of a signal frequency which is less liable to exist, or 80, 135, 150 or 200Hz. D.C. signals are converted to a.c. for persist, and has less energy than other frequencies in speech. transmission, these being converted to d.c. at the distant end. The This indicates a reasonably