Design, implement, and test a digital hearing aid. Perform a web search to learn about hearing loss and hearing tests. In particular, you will solve the classic problem of reduced sensitivity that many elderly people suffer. The input sound is sampled using a microphone like Example 12.4. A FIR filter is implemented like Example 15.1. The output sound is created either with a DAC like Example 12.4 or a PWM like Example 11.3. Your system will drive headphones, so a regular op amp can be used in place of a high-current speaker amplifier. In general, the patient would first have a hearing test, and then you would use the results to generate the desired gain versus frequency for the hearing aid. However in this lab, pass frequencies below 1 kHz with gain 0.25 and pass frequencies from 1 to 5 kHz with a gain of 5. This 20 to 1 ratio will accentuate high frequencies. Implement linear phase. Test the gain versus frequency response of the input amp, the digital filter, and the output amp.
Example 12.4
Design a system that can input and output sound.
Example 15.1
Design a digital filter that passes frequencies from 2 to 4 kHz with a gain of 2. The input is audio with frequency components from 100 Hz to 5 kHz
Example 11.3
Design a system that outputs a 1-kHz sine wave using a PWM DAC.