6. Loudspeaking telephones

Loudspeaking telephones offer a variety of different functions to the user and in this category of telephone we have the following fa­cilities:

1. Call progress monitor.

2. Group listening.

3. Full handsfree operation.

A telephone with call progress monitoring facilities is equipped with a small loudspeaker and a monitor amplifier. The user presses a button to activate the telephone, seizing the line, and dial tone is applied to the line from the exchange equipment and then fed through the monitor amplifier. The user can dial the distant end, listen to ringing tone and when the distant end answers must then pick up the handset to be able to speak as the only transmitter available is in the handset.

The group listening telephone normally has call progress monitor, but in addition has a circuit to enable the user to press a button on the telephone and broadcast the incoming speech signal to others listening in the room. The circuit may also drive the receiver in the handset simultaneously with the loudspeaker in the base of the telephone. Outgoing speech is only possible through the handset transmitter.

The full handsfree telephone is equipped with both a handset and a separate microphone and loudspeaker for handsfree operation. The microphone and speaker are now all provided as part of the telephone, although some older designs consisted of separate units connected in parallel with the standard telephone.

The design of the handsfree telephone creates particular problems. The level of sound being emitted from the loudspeaker is required to be large enough to fill the volume of air in the room where it is being used, so that users can be anywhere in the room and still hear the incoming speech. The microphone in the telephone will also need to pick up this incoming speech signal from the acoustic feedback. Therefore, to prevent the telephone from howling, attenu­ators need to be switched in to the microphone path. These circuits are often referred to as antilarsen circuits.

Figure 58.13 shows a block diagram of typical handsfree system. When a handsfree telephone connection is established speech sig­nals can be reflected at the two to four wire interface, either in the local exchange or in the digital PBX two to four wire interface, as well as round the local sidetone loop from loudspeaker to micro­phone. Hence oscillations and speech distorting can occur and in the worst case howling.

To have full duplex working on the loudspeaking telephone re­quires very fast switching between the transmit and receive circuits and compensation for background noise. This requirement has led to the development of microprocessor controlled handsfree circuits, where the control algorithms are stored in the microprocessor.

Particular attention must be paid to the acoustic design of the telephone body for handsfree operation. The loudspeaker should be housed in a speaker box and acoustically isolated from the micro­phone as far as possible, to prevent sound 53raveling through the body of the telephone to the microphone and causing instability. Sound emanating from the back of the speaker can be particularly problematical and many telephones have some sound absorbing material behind the speaker. Microphones for loudspeaking tele­phones tend to be much more sensitive than telephone transmitters in handsets as they are required to detect speech coming from a few feet away in the room.