Re: low-pass filter question
Re: low-pass filter question
- Subject: Re: low-pass filter question
- From: alex <email@hidden>
- Date: Thu, 12 Jan 2006 18:45:09 -0500
This is some really great information! I really enjoyed listening to the samples and appreciate you taking the time to collect and share this information. It would be cool to put this info on a wikipedia page or something...
Anyway great stuff,
alex
At 6:36 PM -0500 1/12/06, Herbie Robinson wrote:
>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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