Digital Processing
Virtually all newer hearing aids are digital, meaning the sound passes through the microphone and into a digital processor. The processor converts the analogue signal into a digital signal which can be manipulated by the processor. It is here that the hearing aid companies have attempted to remove sounds from the original signals that would be interpreted as noise. The signal is then conerted back to analogue and amplified through the hearing aid speaker (ideally without all the noise).
The problem? All this must occur in real time (i.e. less than 4/100ths of a second), and it is not easy to get a computer to recognize.
Directional Microphones
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