Subject: Re: [linux-audio-user] Converting sample rate: failed...
From: davidrclark_AT_earthlink.net
Date: Tue Sep 14 2004 - 22:33:14 EEST
Erik,
I do not low-pass filter in my resampler, yet it works fine. The reason
is that I assume that the input is band-limited, and this is usually true
for my own work. Not only is it band-limited, but usually also tapered
in the frequency domain, i.e. already effectively low-pass filtered. I use
a raised cosine window in the time domain and no window (other than the
rectungular truncation "window" for the case of downsampling) in the
frequency domain. Effectively I am assuming oversampling in the time
domain. I also have filtering capability, but separately from resampling,
for cases in which I do have higher frequencies. (However, this is also
very crude from a user point of view...)
On the presence or absence of higher frequencies: If there is further
processing down the road and the higher frequencies are missing, then the
results may be inaccurate (for example absent cross-products in the audible
region which are not the result of aliasing). Throwing out high frequencies,
or merely altering them somehow at every stage is not necessarily advisable,
so I caution people who extol the virtues of low-pass filtering. Now in
a library resampler, such as yours, I'm sure it's a good idea to enable it...
I wouldn't volunteer to answer your email otherwise!
Best regards,
Dave.
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