> Hello
>
> I would vote for a default 14Hz high pass filter, removable with the -k
> option.
>
> Why an high pass filter:
> *theorical minimum audible freq for humans is 20Hz (also very discutable)
> *most soundcards are unable to reproduce less than 20Hz frequencies
> *most speakers are unable to reproduce less than 19Hz freq
> *most (if not all) headphones are unable to reproduce such low freq (even if
> their specs tell they are)
> *ISO doc recommends an high pass filter
At 5 Hz, IIRC.
> *even if it's a small part of the spectrum, it's still a few bits usable for
> other more critical parts.
_very_ few. Every mdct sample is about 38 Hz wide, so you wouldn't even cut
away the first (or zeroeth, whatever) sample.
The reason ISO recommends highpass filtering is to cut away the DC portions,
which mp3 can't directly represents (it is a combination of the 0'th and
575'th sample. The 575'th normally gets set to zero, especially if you're
lowpass filtering).
You get DC in a long block, if the sound contains some components < 19 Hz.
DC leads to high-frequency echoes. (Some kind of phantom image of the true
sound at lower freqs).
Dag dag,
Segher
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