What’s the formula for a simple sine tone?
x(t)= A*sin(2πft+ϕ)
What’s the formula for a complex sine tone?
x(t)= ∑(n-1): N of A(n)*sin[2πf(n)t+ϕ(n)]
What’s the formula for beats?
x(t)= A(1)sin[2πf(1)t+ϕ(1)] + A(2)sin[2πf(2)t+ϕ(2)]
What’s the period of a complex tone?
-Period(T) is that of the lowest frequency component (F0)
What’s the period of beats?
-Period (T) is 1/(F2-F1)
What’s the formula for a SAM tone?
x(t)= A(t)*sin[2πf(c)t] A(t)= A*[1 + m*sin(2πf[m]t)]
Where do sidebands occur in SAM tones? At what amplitude?
- Sidebands: Fc +/- Fm
- Height: (A*m)/2
At what rate does spectrum level decrease with increased frequency of pink noise?
-1/f
What is the average envelope modulation?
-64% of BW
Describe finite vs. infinite duration.
-Shortening duration (i.e. click trains) causes spectral spreading or splatter
What is windowing?
-Similar to filtering in the time domain
What are the effects of windowing on the frequency domain?
- Increased stop-band attenuation
- Limited spectral splatter
- Increased frequency resolution
Describe the time vs. frequency domain trade-offs.
- Good time resolution/bad frequency resolution
- Bad time resolution/good frequency resolution
Describe envelope vs. fine structure.
- Envelope: slow-moving, overall amplitude
- Fine structure: fast phase changes
Describe the general properties of pulses with regard to CIs.
- 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)
Describe CI current spread.
- 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
Describe channel interactions.
-Only want one electrode (channel) responding at one time to avoid bad/strange interactions in the electrical field, resulting in distorted signals
How do signals create confounds in some psychoacoustic experiments?
- 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)
Describe the importance of level roving.
- 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
