low-pass filter question
low-pass filter question
- Subject: low-pass filter question
- From: "Dan Morgan" <email@hidden>
- Date: Tue, 10 Jan 2006 13:43:43 -0500
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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