Ashihara's tests were with music/voice, taking into account
psychoacoustics, for an average group of music savvy listeners, and even
music professionals.
As uncorrelated jitter is practically raising the noise floor, most of
it was masked by the signal, making it more difficult to detect.
Benjamin and Gannon used sinusoidal jitter, which isn't appearing
normally in signal chains (badly designed ones excepted).
In a real case, with higher probability (added) jitter would be
correlated with the digital content transmitted over a path - S/PDIF,
and AES/EBU are prone to jitter induced by the signal path
characteristics, ISI - PSUs, and even external noise sources.
A more realistic simulation would take those into account.
OTOH there where tests on pure sine tones, with sine jitter, detectable
by trained ears at even lower levels of jitter, which might indicate the
lowest threshold of hearing, but using artificial conditions.
Who would listen to pure sine tones?
On 5/10/2012 8:25 PM, Heinzmann, Stefan (ALC NetworX GmbH) wrote:
Chris Albertson wrote:
If we are to believe the above paper,then those guys who claim to hear
pS jitter are wrong.
Note that the jitter spectrum matters for its audibility. Ashihara et.al. used
random jitter, and it is not very suprising that the sensitivity for random
jitter is lower than for jitter that has specially been shaped to improve
detectability by human ears. Thus the results by Ashihara are credible, but
they are not the lower limit on jitter audibility.
Benjamin and Gannon, the first reference in Ashihara's paper, come to lower
figures for sinusoidal jitter with carefully selected frequencies relative to
the main signal, which is also sinusoidal. Their results reach down to the
single figure nanosecond range, and that can be regarded as the real limit of
audibility.
Of course, that still leaves those who claim to hear jitter in the picoseconds
range out in fairy-tale land. And jitter of just a few nanoseconds is still
quite easy to achieve with crystal oscillators. No need for special and
expensive parts, then. Normal developer diligence is enough.
Cheers
Stefan
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