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Re: Quality of CoreAudio SRC
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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This information is 2 years old; so, it might be out of date.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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References: 
 >Quality of CoreAudio SRC (From: Brian Willoughby <email@hidden>)
 >Re: Quality of CoreAudio SRC (From: Herbie Robinson <email@hidden>)

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