Since the hybrid echo is inherent in the designs involving 2-to-4-wire conversion, we always need to cancel this echo in the switching stations and any other devices having this kind of conversion.
Regular phones, which are connected by 2-wire lines with the switching stations, may also have this kind of conversion, but there's an excuse for not doing full-blown echo cancellation in the regular phones. The delay in the electrical path between the microphone and earpiece (or speaker) in the phone is essentially zero, so a cheap transformer-based attenuator can be used, and hearing your own undelayed voice of low amplitude does not cause much, if any, of the discomfort. Actually, hearing talker's own voice is desirable as people expect to hear themselves but being not able to do so makes them think that the phone is not working.
However, using echo cancellation is required in the hands-free phones and all phones, which amplify the signal right before the earpiece or loudspeaker. Not doing echo cancellation in such devices leads not only to very well audible echoes for those who make calls to such phones (acoustic echo cancellation will be treated in the following section), but also to self-excitation of the amplifier. The self-excitation results in from non-ideal signal separation in the phone's hybrid, e.g. part of the signal from the microphone reflects at the hybrid to the other signal path and gets amplified by the amplifier, so what the microphone is picking can be heard from the speaker. The acoustic feedback between the amplifier's output and input effectively turns the amplifier to a generator. Therefore, the hands-free and all other amplifying phones (for example, phones for people with hearing impairments, who tend to speak louder) must include line echo cancellers.
Unlike the phones, the dialup modems and faxes always employ built-in echo cancellers to combat the local echo, because these digital devices are much more sensitive to the distortions of the received signals than humans. The same echo cancellation may be desirable in the answering machines, which record the voice from the phone line.
There are a few peculiar properties of the hybrid echoes. One is that the echo path delays are very short and each hybrid has a single echo path. Another is that the echo paths don't change or change very slowly over time because of very slow changes of the electrical circuitry parameters and wire lines parameters in the network.
The second kind of the echo is the acoustic echo. It is easier to understand why and where this echo is created, although as we will see later, this doesn't make it easier to efficiently cancel it.
The acoustic echo is created by the loudspeaker in a phone. The sound comes out of it, bounces the walls, ceiling and other objects in the room, reflects and comes back to the phone's microphone. The same thing is possible to have not only in the buildings, but also in cars, basically, everywhere, where the sound from the loudspeaker can be reflected to the microphone, and this also includes the phone's case as the sound can and, usually does, go from the speaker to the microphone inside the hands-free phone! Similarly, if there's bad acoustic decoupling between the microphone and earpiece in the handset, the acoustic echo will exist in the handset, no matter whether it's a regular or cellular phone.
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