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A noise-excited vocoder simulates certain aspects of cochlear-implant processing, by reducing spectral information and discarding temporal fine-structure information. This has surprisingly little effect on speech intelligibility in quiet:
1. Unprocessed sentence: snd_Unprocessed_Sentence.wav
2. Processed with a 24-channel noise-excited vocoder: snd_demo2_24ch.wav
However, in more complex situations, where the speech has to be segregated from competing sounds, cochlear-implant processing has a highly detrimental effect on intelligibility.
3. Unprocessed sentence in background speech: snd_demo3_uproc_int.wav
4. Processed with a 24-channel vocoder: snd_demo4_24ch_int.wav
For more details see:
Qin, M. K., and Oxenham, A. J. (2006). "Effects of introducing unprocessed low-frequency information on the reception of envelope-vocoder processed speech," Journal of the Acoustical Society of America 119, 2417-2426.
Auditory Stream Segregation
A sequence of tones alternating between two frequencies, A and B, like ABAB..., can give rise to completely different percepts, depending on the A-B frequency separation Δf:
If Δf is small (i.e., 1 semitone, about 6%), the sequence is usually perceived as a single stream of sounds alternating in frequency
If Δf is large (e.g., 9 semitones), the sensation of pitch alternation is lost and most listeners perceive, instead, two streams of constant-frequency tones running in parallel.
The difference in percept is even more striking if the tones are grouped into triplets (ABA...ABA...), as the sequence then evokes a distinctive "gallop" rhythm at small-Δfs, but for large Δfs, the gallop is gone.
At intermediate Δfs, the sequence is usually perceived as a single stream (gallop) at first, but often splits into two streams after a few seconds of uninterrupted listening; this phenomenon is known as the build-up of stream segregation.
1. Galloping sound at small Δf (Single stream): snd_demo_gallop.wav
2. Gallop vanishing for larger Δf (Double stream): snd_demo_double_stream.wav
3. Intermediate Δf: snd_demo_intermediate.wav
Online Frequency Modulation (FM) Detection Task
ASA Sound Examples