[vorbis] Lowpass filter option required

[Michael Smith]

At 08:29 PM 11/15/00 +1300, you wrote:
>Unfortunately the LAME filters are implemented on the output of the MP3
>filterbank, which of course is not used by Vorbis.

Ah. I wasn't sure if that was the case. Oh well.

>
>Monty said at the time that it would be a simple matter to add filter
options.
>The first release of the OGG encoder had a hardcoded lowpass filter of around
>17.5khz I think it was, so here must be code there already?

I think that 'lowpass filter' was in the psychacoustic code (though at the
time I didn't understand the code as well as I do now, so I could be
misremembering. Monty could presumably give a more definitive answer). It
had more or less the effect that you want, but doing this as a general
means of lowpass filtering is not the best idea, from a quality perspective. 

There is also some general filter code present in vorbis (used for other
purposes) which could probably be adapted easily for lowpass filtering.
However, it's still not completely trivial to do it decently, and it's of
little use to most people. Additionally, since you can just go through some
external program (sox?), there's little incentive to give this high
priority. I'm not sure what you think is wrong with doing this...

I guess it's something that should go in there, but there are lots of other
things with much higher priority for Monty, and other people don't really
have the time (like myself. Damn exams). 

Michael

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