> 4.9.3 | How to Avoid Echo Problems at the Headset Interface

> Commonly, a mono-headset uses 4 wires and a stereo-headset uses 5 wires to connect to a GSM device. Two or three wires are needed for the headphone and two for a balanced or unbalanced microphone and for the push to talk button.
> Using of a 3 wire mono-headset interface or a 4 wire stereo-headset interface is possible, but not recommended. As shown in Figure 28 a 3 wire mono-headset interface may generate an electrical echo which can be heard by the far end listener.
> Figure 28: Principle circuit explaining the echo at 3 wire mono-headset interfaces
> The example in Figure 28 assumes the following scenario:

> • | Typical earpiece resistance: 32Ohms

> • | Typical cable, connector and EMI part resistances together: 0.1…2Ohms

> Recommended solution: To avoid the effect of reduced voice quality while using a 3 wire mono-headset or a 4 wire stereo-headset, Siemens recommends a 4 wire mono-headset interface or a 5 wire stereo-headset interface (either balanced headset-speaker or separated GND wires for microphone and headset-speaker)!