On 22/01/2011 18:29, Iain Laird wrote:
Hello Sursounders,
I'm a long time lurker on the list hoping to pick your collective brains on
something.
I have a B-format recording made outdoors on an ST350 which I am attempting
to speed up using a phase vocoder. The aim being to make the recording
occur over a much shorter period of time without affecting the pitch or the
spatial image.
I have made numerous attempts on many different algorithms including Logic
pro 8, Audacity, a number of Matlab scripts and Max patches and SoundHack. I
am getting a range of results, some produce audible distortion due to the
resynthesis but most appear to break down the spatial image, for instance a
source which clearly travels right to left in the recording has no clear
direction after processing. During playback it feels like phase errors are
being introduced which I guess is to be expected with this type of
processing.
Yes, pvoc does tend to mess up phases during time-scaling (much as it
does in pitch shifting - blurs transients, etc). It depends on the
algorithm; some of the newer ones incorporate various forms of phase
locking to tighten up processed transients. Whether they are good enough
to preserve B-format phase relationships is something I can't answer,
but it is certainly a familiar problem with the "standard" pvoc.
If Logic can't do it, it must be quite a problem, as I was under the
impression it used the "Dirac" time-scaling library from Stephan Bernsee
(www.dspdimension.com), and which seems to be one of the best systems
around.
Richard Dobson
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