A recent product release has me thinking about assistive listening devices (ALDs) — i.e., hearing aids. The press release was pushing a new MEMS-based microphone and preamplifier in a very small surface-mount package. The main selling points (besides small size) were low noise, very low current draw (17μA), and the ability to operate from a supply as low as 1V.
This is a nice device, but if you are manufacturing an ALD, you'll need more than a mic and a preamp. Of course, you'll need a tiny speaker, too. But in between the mic and the speaker is where the real magic occurs.
With any decent present-day ALD, low current draw is essential. The device operates from a very small single cell. That means 1V operation and a 17μA draw are just what's needed. Beyond that, an essential part is the audio equalizer (EQ) that is part of the audio chain.
Just as the home hi-fi EQ compensates for listening room frequency characteristics, the ALD EQ will tailor the frequency response to the user's ears. The intent is to create an inverse frequency response. Peaking (where it occurs) is nulled with notch filters. High-frequency content is boosted to account for age- or injury-induced hearing loss.
Of course, the correction can go only so far — typically not far enough to help with speech recognition. The EQ is tweaked by an audiologist based on frequency response tests administered to the user. Any sort of sophisticated interface has a microcontroller (MCU) keeping track of everything.
Since the user would balk at the prospect of an EQ that requires a 19-inch relay rack panel, miniaturization is the next important goal after power draw. Combining these functional blocks on one piece of silicon should keep it all small enough to create an in-ear device.
- MEMS mic
- Mic preamp
- Speaker driver
- Power supply (and power supply support)
- A programming interface that the audiologist uses
Ultimately, the goal is to reduce power consumption to the point that the device can use energy harvesting techniques.
Have you worked on any projects that use this level of miniaturization and power consumption?