Sound Processing (Lab)

Question 1

Describe the changes in multi channel cochlear implants over the last decades.

Answer 1

• historically features remained the same for many years

Today:

• more sophisticated; more features e.g. reverse telemetry
  
  • mechanisms to record neural responses elicited
• same number of stimulation channels as two decades ago
• electrically: behave almost unchanged in stimulus waveforms
• the way the electrical behaviour is utilised has changed significantly: largely to do with sound processing

Question 2

The healthy, young, human ear can resolve frequencies within what range?

Answer 2

~20-20,000 cycles per second (Hertz or Hz)

(as we grow older, our ability to hear the extremes of this range diminish)

Question 3

What does the ‘fourier trnasform’ allow for?

What would a ‘frequency-domain plot’ show?

Answer 3

Allows for transformation of time-domain signals e.g. ‘.WAV’ sound recording, which if plotted would express in terms of amplitude and time, to the frequency domain.

A frequency domain plot would show how much of a signal comes from a particular freuqency.

For a pure freuqency, the frequency-domain plot will show only one frequency as the contributor to the signal.

Question 4

What is the Nyquist Rate?

Answer 4

The rate at which one must sample in order to unambiguously resolve all elements of the signal:

twice the frequency of a given signal
(or the highest frequency of the signal).

NB: sampling at less than the Nyquist rate will not fully reconstruct a signal and apparent cycles will not be resolved.

Question 5

What are formants?

Answer 5

Simply extracting the peak frequencies of a given signal provides insufficient information to represent the original sound unless it is a pure sound.

If one were to construct a model of human speech, there are resonant frequencies which characterise the sound, referred to as formants.

The essence of a sound can be extracted from formants and it is the formants that form the basis of speech processing strategies that have allowed cochlear implants to become so successful.

Question 6

What are the basics of speech processing techniques? (x3)

Answer 6

1. Formants correspond to electrodes. In the actual stimulation strategies employed in the early days of cochlear implants, the stimulation was presented at a rate of F0.
2. By determining the formants of a given sound, the formants may be “mapped” to the stimulation.
3. The charge to be delivered by the stimulus is determined by the amplitude of the original signal.

Question 7

What is an Epoch and why are they needed?

Answer 7

Small time-steps of a given sound.

As sound is a dynamic flow of changing frequencies over time, we cannot represent an entire sound all at once with the formants and expect to glean useful information from it.

The formants can be found of each epoch, and then the important features of the sound within each epoch can be delivered to a speaker or cochlear implant in a step-wise fashion.