On 2015-08-17, robert bristow-johnson wrote:
As I noted in the first reply to this thread, while it’s temping to
look at the sinc^2 rolloff of a linear interpolator, for example, and
think that compensation would be to boost the highs to undo the
rolloff, that won’t work in the general case. Even in Olli Niemitalo’s
most excellent paper on interpolation methods
(http://yehar.com/blog/wp-content/uploads/2009/08/deip.pdf), he seems
to suggest doing this with pre-emphasis, which seems to be a mistake,
unless I misunderstood his intentions.
Actually it's not that simple. Substandard interpolation methods do lead
to high frequency rolloff, which can be corrected to a degree with a
complementary filter. But the trouble is, at the same time they lead to
aliasing and even nonlinear artifacts, whose high frequency content will
be amplified by the compensatory filter as well. As such, that approach
is basically sound...but at the same time only within a very narrowly
parametrized envelope.
to me, it really depends on if you're doing a slowly-varying precision
delay in which the pre-emphasis might also be slowly varying.
In slowly varying delay it ought to work no matter what.
but if the application is sample-rate conversion or similar (like
pitch shifting) where the fractional delay is varying all over the
place, i think a fixed compensation for sinc^2 might be a good idea.
i don't see how it would hurt. especially for the over-sampled case.
It doesn't necessarily hurt, but here it isn't guaranteed to do any good
either. And it's close to doing something bad instead.
--
Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
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