one and causing instability. a button to activate the telephone, seizing the line, and dial tone is Sound emanating from the back of the speaker can be particularly applied to the line from the exchange equipment and then fed problematical and many telephones have some sound absorbing through the monitor amplifier. The user can dial the distant end, material behind the speaker. Microphones for loudspeaking tele- listen to ringing tone and when the distant end answers must then phones tend to be much more sensitive than telephone transmitters pick up the handset to be able to speak as the only transmitter in handsets as they are required to detect speech coming from a few available is in the handset. feet away in the room. Microphone Attenuator Acoustic Control Anti-side coupling Attenuator Loudspeaker Figure 58.13 Handsfree telephone Digital telephones 58/11 58.9 Digital telephones role in solving their problem. With the continuing growth in this area it is vital that telephone system designers, manufacturers and Digital telephones are characterised by having the codec in the suppliers co-operate closely with specialist telecommunication telephone to convert the speech from analogue to digital and vice headsets companies early on in the design stage to ensure that the versa. The PCM encoded speech is then sent to line at a rate of customer gets sufficient choice of quality products. 64kbit/s. In addition a 16kbit/s signalling channel is also present to With the ACD market sector growing at an ever increasing rate, send address and alerting signals to the exchange. CCITT have now organisations large and small are having to face the fact that more defined standard interfaces for digital operation in the I series of and more of their employees need to have their hands free to operate recommendations, but many proprietary interfaces exist. a computer keyboard when conducting telephone conversations. The I series specifies a 192kbit/s full duplex digital path between Companies involved in the growing business of telem