Voice transmission generally requires more runtime than we are used to in conventional telephony. Figure 10 Delay and echo contains an example illustrating how this sounds. Here the mouth to ear delay is approximately 250 ms.

If Speaker A counts from one to eight and Speaker B says stop after he hears three, A hears stop while he is saying eight. Obviously, selecting a number and saying STOP during counting is not something that happens very often in real conversation. Nevertheless, the delay effect caused by round trip delays is noticeable in normal conversation. It often leads to collisions when the speaker changes.

If an echo is generated on the B side (e.g. because of the 2-wire/4-wire hybrid of analog terminal devices, also known as hybrid coil), this is noticed by the speaker, who finds himself speaking over what he said a half a second before, for example.

With high transmission runtimes, it is therefore generally necessary to have echo-free signals on the transmission path. With IPDA, this is achieved through active echo cancellation in the HG 3500 and HG 3575 modules, which filters out the signal section received from A in the send signal from B (the echo) before transmission of the send signal from B.