I read this on a UK blog recently and have quoted it verbatim, below:

“A few years ago the audiology dept in Liverpool tried to issue me with the new generation of analogue/digital devices, and I couldn’t use them as they cut out the sounds I wish to hear, as they have new devices within them called active gain control and a compressor.

Natural hearing works on an analogue level. There is no such thing as a digital sound, the only thing digital in hearing aids is the processor, a/d converter, and d/a converter. The input and output stages are still analogue.”

I have never met this person, but feel as though I have, as I meet this issue often. I don’t agree with his second paragraph though. The issues pointed out in the first paragraph of the UK blog have nothing to do with digital technology – active gain control and compression were first implemented in analogue devices.

Let’s have a quick recap of what’s actually going on here: In an analogue hearing aid, the microphone picks up the sound and converts it into an electrical input signal. This signal is amplified and some spectral shaping is done. The amplified signal is converted by the loudspeaker (“receiver” in hearing-aid-speak) back to an acoustic signal. The acoustic and the electrical signals can be thought of as being continuous in time. This is like thinking of the continuous sound profile stored along a groove on a vinyl record.

An analogue hearing aid is full of capacitors and transistors; a digital hearing aid has a very small, but quite powerful, computer in it to process the signals. This provides a lot of opportunity to “do things better”.

In the digital hearing aid, the microphone still converts the acoustic signal to an electrical signal, but then an analogue-to-digital converter converts the electrical signal into a series of numbers that represent the amplitude of the signal. The number of times that this happens in a time period affects the fidelity of the digitised signal.   This is referred to as sampling. The small computer can then carry out sophisticated signal processing, and hearing aid engineers work to make sure that sophisticated processes can be performed without too much power drain on the battery. Professor Blamey and his team were among the first to realise that this meant you could implement very innovative algorithms to get better hearing aid processing than had ever been possible before. He had been working to provide innovative solutions in the bionic ear for some time before. We understood early that you didn’t have to use the same old amplifiers in hearing aids anymore.

One of the many challenges for a hearing aid is that people with hearing loss have a reduced range of loudness perception. This has traditionally been addressed by analogue compression circuits. The advent of digital meant that new algorithms could be introduced that can sometimes overcome problems inherent in analogue devices. One of the most important has been the introduction of excellent feedback cancellation technology.

The transition from analogue to digital, in hearing instruments, was gradual, and started with digital control over the hearing aid controls, so that the hearing aid could be described as “Programmable”. In the next stage of development, much of the hearing aid circuitry was replaced by a tiny computer chip.

According to the Hearing Industries Association (HIA) statistical report, digital signal processing (DSP) hearing aids became the most common hearing aid sold in the U.S.A during the first quarter of 2002, passing both digitally programmable analogue and non-programmable hearing aids.

But more was to come: A significant milestone occurred with the work of Rob Brennan, Todd Schneider and their colleagues in Canada, whose work became the foundation of the DSP factory (now owned by OnSemi) which sold ultra-low power DSP chips into the hearing aid industry, leaving the way open for third party developers.

Dspfactory Ltd was a start up in Canada in the late 1990’s. They developed ultra-miniaturized, ultra-low power, software-programmable, digital signal processing (DSP) technology. An early product, the Toccata DSP chip was a technology that changed the hearing aid industry forever. Suddenly, there was a very small DSP chip, that enabled researchers from around the world to implement the latest research in hearing aids on a powerful and tiny computer. The hearing aid world was no longer the preserve of a handful of giant multi-national hearing aid companies, and Peter Blamey’s team were ready to take advantage of the new era.

By enabling third party developers, like the team lead by Peter Blamey in Melbourne, the pace of signal processing in hearing aids accelerated very rapidly. Peter had for many years been developing and evaluating DSP solutions for cochlear implants. The arrival of the miniature DSP chip meant that he could develop additional advanced solutions for hearing aids as well, a career step that eventually lead to the founding of Blamey Saunders hears.

For the person with hearing loss, this meant a lot of change. The price of top quality hearing aids could come down. The sound quality could go up – by abandoning traditional compression we could make our hearing aids sound more natural.

So where does that leave the person from Liverpool? People fitted with analogue hearing aids more than 20 years ago, probably ended up with a lot of output – in other words they were loud. Everyone involved was doing the best they could, but the result makes it hard to adjust to more subtle solutions today, even though they are probably a lot more suitable. The best digital hearing aids today have good sound processing. We go further and avoid compression. Why wouldn’t you? The reason for using compression is long gone in my view. So my advice to the analogue users is to take the plunge and persist with really good digital hearing aids (ours for example!), suited to severe loss – or check out whether a cochlear implant is suitable.  The Blamey Saunders Hears power aids need earmoulds and are only available from our clinic or from other audiologists by arrangement.