Re: low-pass filter question
Re: low-pass filter question
- Subject: Re: low-pass filter question
- From: Herbie Robinson <email@hidden>
- Date: Thu, 12 Jan 2006 18:36:26 -0500
Here are some more tests:
http://src.infinitewave.ca/
This one doesn't have anything with the core audio SRC, but it does
have the Weiss SRC, which is supposed to be the best.
http://www.bias-inc.com/products/peakPro5/resampling/peakResamplingWhitePaper.pdf
Be careful reading the Peak tests, because they have selected a color
mapping that radically accentuates low level differences. There is a
very hard transition at about -155dB from dark blue to light blue. A
more realistic color mapping would have been to range from dark blue
to light blue over the -170 to -150 range, or something like that.
The Barbabatch site claims Logic 7 is using the core audio SRC. From
the Bias paper, it would appear that Wave Editor and SoundTrack Pro
are also using it. Note that it is next to last in terms of quality.
I'm just guessing here, but I'll bet the one solid turquoise plot
(the one that is dead last) was done doing single precision SSE2
instructions and the ones that have significant light blue
noise/distortion areas were done using the Altivec multiply
accumulate instruction, which should have 12dB better
noise/distortion performance.
Just to keep this in perspective, the above SRC tools are aimed at
converting audio at a 24 bit sample depth. I don't know what you are
doing with an 8K bandwidth, but these tools might be overkill. They
certainly are if you are reducing the samples to 16 bits....
especially if you don't dither.
You probably want to take a look at this.
http://www.audioease.com/Pages/BarbaBatch4/Barba4SRCTest.html
Antoine
On 11-Jan-06, at 2:30 AM, Rustam Muginov wrote:
Bill, does it use the same AULowPass? Or other filter?
What are the parameters for the filter? What are the characteristics?
I had also found what resampler build-in into OS X does not have
acceptable for our tasks characteristics.
Thank you.
--
Sincerely,
Rustam Muginov
On Jan 11, 2006, at 4:41 AM, William Stewart wrote:
The resampler does all of this filtering for you before it
resamples. You don't need to do anything else
Bill
On 10/01/2006, at 10:43 AM, Dan Morgan wrote:
I am working on an application where audio data is being sampled
at 48kHz. Because I am only interested in frequencies below
around 6-8kHz (and am interested in data reduction), I apply the
Apple low-pass filter audio unit with cutoff frequency 7kHz (to
prevent aliasing of frequencies above 8kHz) and then downsample
to 16kHz using a format converter. This should work fine if the
low-pass filter response is sharp.
The problem is that the Apple low-pass filter has a response that
is anything but sharp. I set up a little test program that
injected sine waves of different frequencies and found that
frequencies of half the cutoff were being significantly
attenuated and frequencies of double the cutoff had a significant
fraction passed. This is not acceptable behavior for the
intended application.
A couple questions: (1) Does the Apple format conversion have any
internal anti-aliasing low-pass filtering that it does when it
downsamples (hence making other low-pass filtering unnecessary)?
(2) If not, do you have any suggestions for how to proceed, e.g.
any other low-pass filters to consider?
--
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