Subject: Re: [linux-audio-user] Converting sample rate: failed...
From: davidrclark_AT_earthlink.net
Date: Wed Sep 15 2004 - 23:07:06 EEST
Erik,
> Notice that step where you throw away the middle N/2 samples?
> That is a low pass filter applied in the frequency domain.
Not if the amplitudes are all already zero above N/2. In that case, the
input and output frequency spectra are absolutely identical. If nothing
was removed or even altered, then no filtering has actually occurred.
A similar situation is what I have previously stated as my starting
assumption.
> Less accurate how? Is this measurable or is this just hand waving?
I have already addressed this in previous postings where I described the
fact that I had merely listened to the recordings repeatedly until I
could distinguish the original from the sndfile-resample one and could
not distinguish the FFT-overlap one from the original --- ergo less accurate.
My intention was not to "measure" accuracy, but to listen for the alleged
distortions. You had posted a sort of celebration of the fact that I
thought the sinc-based resampler produced a better-sounding version of
the recording than my FFT-overlap resampler, but apparently had also
neglected the fact that I was also saying that it sounded better than
the original --- ergo was inaccurate (albeit very, very slightly so).
I was merely attempting to correct possible misunderstandings.
Now I could take time to measure my resampler's performance, but I think
we both know the expected results, don't we? In terms of absolute
accuracy, any FFT-overlap resampler which utilizes large windows (hundreds
of thousands or millions of samples) and which is properly implemented
should, in fixed-rate conversions, outperform a sinc-based resampler that
has been localized in the manner that Smith has developed for the very
useful purpose of allowing variable-rate conversions. However, the
difference may be measurable but not audible, hence of questionable utility.
Best regards,
Dave.
P.S. I'll be "sans computer" for the next few days, heading towards Ivan
so won't be able to correspond on this for a while....
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