[jnbiche]

> I wonder if they could put a pitch-shifting circuit in his hearing aid to shift sounds up/down so that they fall within his hearing range, whilst not shifting frequencies already in that range. That would help significantly, surely?

If you read the article, you'll see that's more or less what most modern hearing aids do, via a technique called multi-band compression.

Edit: Actually, here anigbrowl, an audio engineer, states that this is not how multi-band compression works: https://news.ycombinator.com/item?id=8854142

Assuming he is correct, my above statement may well be wrong.

[72deluxe]

Multiband compression works by splitting the incoming audio into different bands, much like your bass/mid/treble controls on your EQ only works on bass/mid/treble parts of the frequency range. Compression is then applied to only those frequencies and then they are summed together.

There is no pitch shifting in multiband compression - pitch shifting involves moving the frequency up or down by a number of cents, semitones, octaves etc. It's the effect used to get the "chipmonk" voice (high-pitch and squeaky) where a normal voice is fed into a pitch shifter and it is shifted up or down. It is also how harmonisers work, where they work out the frequency you're singing at and shift it up 7 notes (or an arbitrary amount) so you can sing and get a harmony of yourself.