Analog compression before the A/D and digital expansion afterwards:
The ducking-under-the-bridge metaphor discussed in part 1 of this blog series is apropos for this blog entry. For those who may not remember the first blog entry in this series, either go to that blog entry or go here. I know ReSound uses this approach, and other hearing aid manufacturers may as well.
Of the many technical innovations that have been designed to improve the listening to and playing of music with hearing aids on, this technology is somewhere in the middle of the pack. The last blog dealt with a microphone that was less sensitive to the lower frequency, perhaps high sound levels, such that the low frequency volume was turned down prior to the A/D converter front-end bridge. One could argue that this is a low-tech approach because it is just swapping out one commercially available microphone for another. An almost equivalent approach would have been to use a microphone that had 10-12 dB less overall sensitivity, but still with an optimal setting of the expansion circuitry to minimize the noise floor.
This approach can come in many flavors, but is based on the idea that the output of the hearing aid microphone is still an analog signal, so why not use an “old fashioned” analog compressor to reduce the sound level to a point that it is within the optimal operating characteristic of the A/D converter and other front end components. Once the signal is reduced (ducking under a bridge), it is digitized, and then once in a digital format, it is digitally expanded to match the features of the original signal.
I am sure that different manufacturers may do it differently, but one strategy would be to attach an analog compressor to the pre-amp of the hearing aid microphone and then “undo” this volume reduction once in a digital domain. How it is exactly done is beyond this blog (and me), but I am sure that we can buy an engineer a beer (or two) at the next large convention and find out. And this may make for an interesting discussion- be sure to bring a digital recorder. But then again, perhaps an old-fashioned analog recorder would work just as well, or even a pen.
Clearly this approach would encounter some difficulty with the internal noise of the hearing aid. After all, the reason hearing aid microphones have pre-amps in the first place is to reduce the microphone noise floor, but since at least one company is using this approach, I am sure that the engineers have figured out how to do it without too many drawbacks.