FONIX® 8000 - Frye Electronics

Chapter 4 91
Advanced Coupler Tests
4.1 Enhanced DSP
The Enhanced DSP measurement screen was designed to give you more information
about digital hearing aids. The new technology in digital hearing aids
has added a great deal of capability, such as increased clarity and flexible programming.
However, the same technology can also have its pitfalls. This test
will tell you the digital processing delay (also known as group delay) and the
signal phase of the hearing aid.
The digital processing delay measurement will help you determine whether or
not a hearing aid is suitable for a monaural fitting. The phase measurement will
help you determine whether custom binaural hearing aids are working properly
together as a team.
4.1.1 Digital Processing Delay
One of the properties of digital technology not normally mentioned in the literature
is that it always takes time to process digital data. Imagine the digital hearing
aid as a miniature computer: it takes an analog sound wave, turns it into
digital information, performs some kind of algorithm to amplify the signal, and
turns it back into an analog sound wave for the ear to hear. All of this calculating
takes precious time; it’s never instantaneous. The processing delay for some
hearing aids is so slight that it is imperceptible to the human ear. The processing
delay for other aids can extend to several milliseconds.
Why is this a problem? Well, if you fit a client monaurally with an aid with a
significant digital processing delay, that person might experience some confusion
because his unaided ear will be hearing sounds slightly faster than his
aided ear. Problems can also be predicted for patients with open canal fittings.
However, if you fit the same patient with an occluded binaural set, then both
ears will be listening with the same delay, and the confusion will be alleviated.
What is a significant delay? At what magnitude does this delay start to affect
speech intelligibility? These are good questions. Unfortunately, we don’t have an
answer at this time; the field of digital hearing aids is still too young, and there
has not yet been enough research done to establish necessary guidelines. A
conservative approach would be to avoid monaural and open canal fittings with
digital aids that have delays of more than 1 to 2 milliseconds.
For now, this measurement will give you more information about what’s really
going on in that digital hearing aid circuit, and, hopefully, it will help you
troubleshoot why some aids work better than others with monaural or binaural
fittings.