Signals & Cochlear Implants

Question 1

What’s the formula for a simple sine tone?

Answer 1

x(t)= A*sin(2πft+ϕ)

Question 2

What’s the formula for a complex sine tone?

Answer 2

x(t)= ∑(n-1): N of A(n)*sin[2πf(n)t+ϕ(n)]

Question 3

What’s the formula for beats?

Answer 3

x(t)= A(1)sin[2πf(1)t+ϕ(1)] + A(2)sin[2πf(2)t+ϕ(2)]

Question 4

What’s the period of a complex tone?

Answer 4

-Period(T) is that of the lowest frequency component (F0)

Question 5

What’s the period of beats?

Answer 5

-Period (T) is 1/(F2-F1)

Question 6

What’s the formula for a SAM tone?

Answer 6

x(t)= A(t)*sin[2πf(c)t]
A(t)= A*[1 + m*sin(2πf[m]t)]

Question 7

Where do sidebands occur in SAM tones? At what amplitude?

Answer 7

• Sidebands: Fc +/- Fm

- Height: (A*m)/2

Question 8

At what rate does spectrum level decrease with increased frequency of pink noise?

Answer 8

-1/f

Question 9

What is the average envelope modulation?

Answer 9

-64% of BW

Question 10

Describe finite vs. infinite duration.

Answer 10

-Shortening duration (i.e. click trains) causes spectral spreading or splatter

Question 11

What is windowing?

Answer 11

-Similar to filtering in the time domain

Question 12

What are the effects of windowing on the frequency domain?

Answer 12

• Increased stop-band attenuation
• Limited spectral splatter
• Increased frequency resolution

Question 13

Describe the time vs. frequency domain trade-offs.

Answer 13

• Good time resolution/bad frequency resolution

- Bad time resolution/good frequency resolution

Question 14

Describe envelope vs. fine structure.

Answer 14

• Envelope: slow-moving, overall amplitude

- Fine structure: fast phase changes

Question 15

Describe the general properties of pulses with regard to CIs.

Answer 15

• Biphasic
• Causes spiral ganglion/ANFs to fire in response to incoming sound after the signal has been vocoded
• Rate ~1k pps per electrode (total rate ~10k pps)
• Follows the speech envelope (amplitude changes over time)

Question 16

Describe CI current spread.

Answer 16

• Monopolar: biggest current spread ~4.6mm (ground outside cochlea)
• Bipolar: current spread ~2.6mm (ground within cochlea)
• BW affected by where you have the current turn
• Placement of ground affects current spread (current spread matters more than number of electrodes)
• Speech intelligibility worsens as you focus the current spread

Question 17

Describe channel interactions.

Answer 17

-Only want one electrode (channel) responding at one time to avoid bad/strange interactions in the electrical field, resulting in distorted signals

Question 18

How do signals create confounds in some psychoacoustic experiments?

Answer 18

• Tone duration affects the BW of the signal because of spectral splatter
• This is particularly a problem for HI listeners who might have better off-frequency sensitivity than on-frequency sensitivity
• Not an issue with CI listeners who will only receive information from the deliberately stimulated electrode (no off-frequency listening)
• To address this problem, a signal with a wideband spectrum could be used (i.e. pulse train)

Question 19

Describe the importance of level roving.

Answer 19

• In spectral shape discrimination tasks, listeners may cheat by focusing on overall intensity instead of spectral shape by using energy for detection
• Level roving (and BW roving) is therefore done to ensure that the listener is in fact paying attention to spectral shape and not loudness, resulting in better threshold