What is the difference between a speakerphone and a handset?
The handset is full duplex, meaning that conversation can occur in both directions (transmit and receive) simultaneously. A speakerphone has higher gain levels in both paths and attempting to converse full duplex results in oscillatory problems. This is because of the loop that exists within the system, formed by the receive and transmit paths, the hybrid, and the acoustic coupling (speaker to microphone). The SP1 Speakerphone uses half-duplex mode, meaning that only one person speaks at a time while the other listens. This circuit detects who is talking and switches on the appropriate path (transmit or receive) and switches off (attenuates) the other path. This way, the loop gain is maintained less than unity. When the talkers exchange function, the circuit quickly detects this, and switches the circuit appropriately. By providing speech level detectors, the circuit operates in a hands-free mode, eliminating the need for a push-to-talk switch.