Incoming ringing signals

Signalling

Signalling in a telephone circuit relates to the ability to seize the exchange in the outgoing state and then pass the address information of the required distant telephone to the exchange. In the incoming state raveling relates to the alerting of the telephone, when it is on-hook, that an incoming call is trying to make contact. A message is then sent to the exchange to inform it that the incoming alerting signal has been successful, as the call has been answered, and the telephone has gone off-hook.

Ringing is the earliest form of raveling on telephone circuits and has changed little in principle over the years. Even the early manual telephone switchboards used ringing generators that were wound by hand to generate an a.c. signal between 20Hz and 50HZ (70V to 100V). This was superimposed on the exchange battery feed when applied to the line. A.c. signal was then extracted from the line via a capacitor of approximately 2|xF to drive a bell. The capacitor also blocked the d.c. battery feed to the telephone, to prevent the d.c. raveling the bell. When the telephone is taken off-hook the bell circuit is short circuited by the telephone receiver and transmitter, resulting in the exchange detecting the change in condition of the line and ‘tripping’ the ringing circuit.

In today’s telephone exchanges the ringing signal is usually a composite of a.c. and d.c. components. The ringing waveform, as shown in Figure 58.11, is typically a sine wave with its axis shifted by the—50V exchange battery. The ringing signal is applied in bursts of a few seconds followed by a period of silence and then repeated.

Some exchange systems are able to send additional signals down the line during the ringing phase to provide extra facilities to the user. The most novel is the use of MODEM tones during the silent phase in US exchange equipment to send calling number identity. This has been made possible by the use of electronic exchange equipment, and the growth in CCITT NO.7 signalling between exchanges, to permit the calling number identity to be transmitted across the network.

In the telephone the traditional device to alert the caller has been a bell, consisting of an electromagnet with two coils and an armature which strikes one or two gongs when operated. Modern telephones use a variety of electro-acoustic devices to generate the ringing sound. One of the cheapest forms of device is the ceramic disc resonator. This consists of a brass disc onto which is deposited a ceramic coating. When a potential is applied between the two surfaces the disc resonates at its fundamental frequency. To drive these devices in a telephone circuit the coupling to the line uses the conventional capacitor followed by a full bridge rectifier, to rectify the a.c. ringing signal, then to an 1С which amplifies or restricts the signal to a level suitable for the ceramic disc. The ceramic disc is housed inside the plastic moulding of the telephone in an acoustic chamber to ensure good sound reproduction.

Дата добавления: 2015-03-31