ne. need to pick up this incoming speech signal from the acoustic The ICs normally have the same facilities for last number redial and feedback. Therefore, to prevent the telephone from howling, attenu- digit storage as explained earlier. ators need to be switched in to the microphone path. These circuits DTMF signalling has the added advantage of end to end signall- are often referred to as antilarsen circuits. ing. Once the connection is set up DTMF signals can be transmitted Figure 58.13 shows a block diagram of typical handsfree system. from the telephone in-band over the link to the distant to control When a handsfree telephone connection is established speech sig- voice mail machines, answering machines and access banking ser- nals can be reflected at the two to four wire interface, either in the vices, etc. local exchange or in the digital PBX two to four wire interface, as well as round the local sidetone loop from loudspeaker to micro- phone. Hence oscillations and speech distorting can occur and in the Loudspeaking telephones worst case howling. Loudspeaking telephones offer a variety of different functions to the To have full duplex working on the loudspeaking telephone re- user and in this category of telephone we have the following fa- quires very fast switching between the transmit and receive circuits cilities: and compensation for background noise. This requirement has led to the development of microprocessor controlled handsfree circuits, 1. Call progress monitor. where the control algorithms are stored in the microprocessor. 2. Group listening. Particular attention must be paid to the acoustic design of the 3. Full handsfree operation. telephone body for handsfree operation. The loudspeaker should be housed in a speaker box and acoustically isolated from the micro- A telephone with call progress monitoring facilities is equipped phone as far as possible, to prevent sound travelling through the with a small loudspeaker and a monitor amplifier. The user presses body of the telephone to the microph