r dialled. More modern telephones send bursts of tones (DTMF) to achieve the same end. The transmitter and receiver perform the same functions as for incoming calls, and a disconnect signal is sent at the end of the call. PABXs and key systems require the telephone to perform more Figure 34.2 Simple crossbar matrix than the above fundamental purposes. In order to perform functions such as transfer and conference, a recall signal must be sent from the telephone to the telephone Silicon controlled rectifier (thyristor) technology simply replaced system. Two types of recall signal are in common use. The first the crossbar matrix with an electronic equivalent. However the cost creates a temporary disconnect in the circuit, similar to a dial pulse. effectiveness of this technology allowed non blocking matrices to The second connects an earth (ground) to the circuit. be produced, thus reducing, or even eliminating, the need for matrix For PABX and key systems operation, the technology available on engineering. standard dial pulse and DTMF telephones became a limiting factor. All the above switching matrices are referred to as analogue systems. That is, once the human voice is converted into an anal- Special telephones were developed for use with particular telephone systems. To begin with, multi-pair cables were used between these ogue electrical waveform, it remains in that form until converted telephones and the associated telephone system to transmit the back to sound in the telephone receiver. various signals required to perform additional functions. Later, The next major technology change was to digital systems. In a electronic circuitry was used to transmit these various signals over digital system, the analogue electrical waveform is sampled, and the a single pair of wires using some kind of digital code. At that time, samples converted into a digital code. At the far end of the call, this the electrical energy representing the persons voice was still trans- digital code is converted back into an analogue sample