Re: low-pass filter question
Re: low-pass filter question
- Subject: Re: low-pass filter question
- From: William Stewart <email@hidden>
- Date: Tue, 10 Jan 2006 17:41:00 -0800
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?
Thanks.
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