Intelligibility of time-reversed
Time-reversed speech has the same long-term amplitude spectrum as natural
speech, but is largely unintelligible.
However, Saberi & Perrott ("Cognitive restoration of reversed speech,"
398: 760, 1999) recently showed that
local time reversal does not destroy intelligibility, if the
time reversal window is brief. They divided everyday English
sentences into segments of fixed duration, and each segment was time
reversed. Near-perfect intelligibility was
found for windows less than 50 ms and performance declined progessively
as the time reversal window length was
increased to 300 ms.
The effects of local time reversal are illustrated with audio examples
below. The example is of a male voice saying
"The watchdog gave a warning growl." In each case the waveform of the
sentence was divided into successive,
non-overlapping time-reversal windows of progressively longer duration:
0 ms (unaltered original),
10 ms, 20 ms, 50 ms, 100 ms, 250 ms, 500 ms, and in the final example,
1 second. The figure shows
the waveform of a time-reversed sentence using a 0-ms time reversal
window (unaltered original, top);
a 1-s time reversal window (middle); and a 50-ms window (bottom).
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