[Tremor] Resampling while decoding

[Martin Vilcans]

Hi list,

I'm developing a game that will use Tremor for decoding Vorbis and got 
this idea about supporting different sample rates. We'll have to support 
several different sample rates but the Vorbis streams are at a fixed 
frequency. We can resample the decoded samples in realtime, but I'm 
wondering if there is a more efficient way to play back a Vorbis stream 
at a specific sampling rate. I suspect that there might be some 
efficient way to resample while decoding, but this might be just because 
I don't know much of how a Vorbis stream is constructed.

Also, perhaps it's possible to increase quality without sacrificing 
speed. For example, if the Vorbis stream uses a rate of 44 kHz, but the 
playback rate is 22 kHz, we'd normally decode the stream, apply a 
high-pass filter to remove frequencies above the Nyqvist frequency and 
resample by averaging every two samples. Instead, perhaps the decoder 
can simply not decode the upper half of the frequency spectrum (the 
11-22 kHz band). That would give the same effect as applying a low-pass 
filter to the decoded samples. Then we can do a quick resample by simply 
removing every other sample.

As I said, I don't know much of how Vorbis works, so perhaps all of this 
is just stupid, but I'd be happy to be enlightened if that is the case. :-)


-- 
Martin Vilcans
Lead Programmer
mvilcans at digital-legends.com

Digital Legends Entertainment
C/ Aribau 112, 5-1
08036 Barcelona (Spain)
Tel : +34 93 394 17 77
Fax: +34 93 451 19 80
www.digital-legends.com <http://www.digital-legends.com>