From EE122A, Mixed-Signal Circuits Laboratory, at Stanford University
Taught by Professor Greg Kovacs, Fall 2013
High Frequency hearing loss (HFHL) is an inability or reduced ability to hear high frequency tones. It can originate from exposure to loud noises or simply as a result of age-related hearing degeneration. HFHL can result in reduced appreciation of music in milder cases or reduced speech comprehension in more severe cases.
An electronic hearing aid can help compensate for HFHL. However, simply amplifying high frequencies might not be the best course of action. Research indicates that amplifying high frequency tones is useless if the hearing loss is profound enough, and that attempting amplification may actually be detrimental to speech comprehension.
In some cases for more severely impaired listeners, increasing the audibility of high-frequency speech information resulted in no further improvement in speech recognition, or even decreases in speech recognition (Hogan, Turner, '98)
As such, this project impements frequency shifting instead of amplification. This project is targeted at increasing speech recognition for users with severe HFHL (loss of sensitivity after ~3KHz). High frequency information from 3-5 kHz is shifted to 1-3 kHz and added back to the original signal.
In its current form, the project is not perfect. Imperfect carrier suppression in our modulator IC leads to a small 2 kHz tone which is distracting when used for processing speech playback. However, the system does provide noticeably clearer audio for music playback, where a low background tone is less noticeable. Regardless, this Instructable will serve as a useful primer for anyone interested in practicing with basic signal processing, analog filters, and circuit design.