[Vorbis-dev] Total uncompressed size

[Christopher Egner]

Michael Smith wrote:

>However, to figure out the total length you certainly don't need to
>decode the entire file. You need to read the vorbis headers, and then
>seek to the end of the file (if it's a single chain; if you have
>multiple logical streams chained together, it's a little more complex
>- see the ogg docs if my terminology here is confusing). The
>granulepos value of the final ogg page of the stream will give you the
>total number of samples in the file, you can then multiply that by the
>number of channels (which you got from the header), and by the sample
>size you're decoding to (which is your choice, typically you'd pick 16
>bit data, which is 2 bytes per channel per sample).
>  
>
For a single ogg file, wouldn't you store the length, for playlist 
loading, so you don't have to do things like this to determine the 
length. I guess even the length is an approximation though (since it 
would be specified in terms of minutes and seconds generally), but you 
could take that, multiply the total number of seconds by the sample 
rate, by the number of channels, by the width of each sample. So for my 
applications, most things are 44.1 kHz, 16 bit samples, 2 channels. So a 
3 minute song is 180 seconds * 44100 samples/second * 2 channels * 2 
bytes/sample = 31752000 bytes or just over 30 megs. Again, let me 
emphasize, this is only an approximation (if ogg or vorbis even have a 
comment/meta field storing this information). Also the savings are 
probably negligable for a small app with small files. If the files are 
large however, and static, the savings may be worthwhile. If you've got 
a chained file, as before, this mess gets more complicated. Also note, 
that if you're trying to compute how much space you need to store a wave 
or something similar, you almost always need a header.

Christopher