Re: Quality of CoreAudio SRC
Re: Quality of CoreAudio SRC
- Subject: Re: Quality of CoreAudio SRC
- From: William Stewart <email@hidden>
- Date: Thu, 31 Jan 2008 14:12:27 -0800
A couple of notes:
(1) We have made some significant improvements to the sample rate
converter in Leopard - so if you want re-evaluate it, that would be
very helpful. We've also added a new property to allow for the
selection of different algorithms when doing SRC, including the
provision of a new mastering quality converter ('bats') that we
believe is very good. So, aside from the caveats that Stephen
provided, we would certainly expect to see some improvements here.
(2) I have heard of other problems with their USB output device at
44.1KHz, so its probably worth some further investigation along that
pathway as well
(3) Umm.. they didn't talk to me either
Bill
On Jan 31, 2008, at 1:29 PM, Herbie Robinson wrote:
Bias published a paper on SRC about a year ago that included FFT
analysis of frequency sweeps:
http://www.bias-inc.com/products/peakPro5/resampling/peakResamplingWhitePaper.pdf
This information is 2 years old; so, it might be out of date.
Undoubtedly, the thresholds for the color assignments are chosen to
make the Bias SRC look perfectly clean, but that doesn't make the
information inaccurate. This has been widely publicized on some of
the audio engineer e-mail reflectors and nobody has ever challenged
it as being inaccurate. Unless somebody from Apple tells us
otherwise, we can probably assume Logic is using the Core Audio SRC.
Note that distortion components in that plot are getting into the
green range, or -120db. There is also aliasing around -140db. My
guess about this is that the Wavelab SRC is using single precision
floating point for the MAC loop, the Logic SRC is using the Altivec
MAC (which is a little better than single precision) and the really
clean SRCs are using double precision for everything.
This information is two years old. Also, note that if one compares
Intel vs PPC using Altivec, the result must be different, because
the Altivec floating point MAC instruction is a unique beast that
does 4 multiplies and 4 adds in double precision, but truncates the
result to single precision -- Theoretically, that would be 12dB less
roundoff error than straight single precision (but much much more
than using full double precision).
Bias has published the Matlab scripts for generating the plots at:
http://www.bias-inc.com/products/peakPro5/resampling/
--
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