Figure 4. Acoustic echo
Often times, when making conference calls at workplace, we use the hands-free features of our phones, so all of the colleagues participating in the call can hear the other side. The acoustic echoes differ a lot from the hybrid echoes. First of all, the echo path delays aren't short (the echo path delay is the echo path length divided by the wave propagation speed. Electromagnetic waves propagate at about the speed of light in the wires, e.g. 3*108 meters/second, while the sound propagation speed in the air is about 3*102 meters/second. As you can see, the difference is 6 orders of magnitude!). The echo path is determined by the size of the room where the phone is used, and, obviously, the more the room size is, the longer the echo path delay is. And if we don't cancel this acoustic echo, the person who has called to the hands-free phone can hear a very annoying echo, which is delayed by the sum of the acoustic echo path delay in the room plus the round-trip delay in the network between the phones. But longer echo path delays aren't the only interesting feature of the acoustic echo. The other interesting thing, which imposes certain problems on the acoustic echo cancellation, is that there are many echo paths available in the room as the sound now can be reflected by many objects to the microphone and the paths can vary over time as the objects change their locations. Suppose you move around the room or somebody opens or closes the door in it. This makes the effective echo path change.
As the exact network and room echo paths (and their impulse responses) are generally unknown, there's no other simple means to remove the echo but an adaptive system.
Let's look at how in general the echo cancellation can be done for one direction of transmission. We will explain this on the example of the hybrid echo cancellation, most of which also applies to the acoustic echo cancellation.
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