Re: [MP3 ENCODER] default high pass filtering

[Naoki Shibata]

Naoki>   Assume that the original sampling frequency is f.
Naoki>   Downsampling to sampling frequency f/n (n is integer) is easy, since
Naoki> this is done by picking up every nth sample.

Matthias> Ooh, ouch - this leads to heavy aliasing. Don't do that! Before any
Matthias> resampling to lower frequencies, you have to do filtering to that
Matthias> frequency's half.

  I posted following message 1.5 hours ago, but this message seems
disappeared.

>  Sorry, this doesn't work, since downsampling needs anti-alias filter.
>
>  What about this idea of downsampling?
>
>  At first, downsample to f/2. This requires only short filter.
>  Second, downsample to f/4. This requires only short filter too.
>  By iterating this process, downsampling can be done without using long
>filter.

  Perhaps each iteration needs 32th order FIR filter, and after 11
iterations the sampling frequency will be about 21Hz.


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Naoki Shibata   e-mail: [EMAIL PROTECTED]

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