The final stage of the hearing aid processing involves ensuring that the sound is played back to the user in the high sound quality of the sound processing core. This is what the Automatic Output Control (AOC) does. The AOC monitors the sound signal sent to the receiver to avoid signals that are too loud for the receiver to handle. It does this in two steps.
If that is the case, the AOC will reduce the gain in the relevant channel. If the signal is loud enough to affect the level in the neighbouring channels, gain will also be adjusted in those channels.
This step is called the narrowband AOC.
The second step, the broadband AOC, takes place after the processed signals have been combined into a single output signal.
At this point the AOC can measure the exact power in the final signal. This makes it possible to avoid output signals that are so loud that they cannot be physically handled by the receiver.
By utilizing a small delay already in the signal chain, the measurement can be carried out early enough for the system to be able to handle the signal before it is played back. This way, the AOC can prevent clipping distortion of the output. We call this pre-peak detection.
