[Vorbis-dev] A few questions about libvorbis from a newbie

[Gunter Königsmann]

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On 02.12.2015 04:53, Blake Martin wrote:
> First off, I don't even know if this is the right place to ask
> these kind of questions, but I haven't been able to find answers
> anywhere else, so
> 
> 1. I have found that "pcmTotal * vorbisInfo->channels * 2" gives
> the uncompressed size of every ogg vorbis file I have used. What is
> a more robust way of getting the full uncompressed file size?
This sounds quite reliable. At least if the .ogg file is equipped with
the correct headers: If you capture the live stream from a radio
station that radio station won't know in advance how many samples you
will capture and pcmTotal (which is one of the first things you'll
get) won't be filled with the correct value.

Since ogg is vbr I assume the only way of working around bad headers
would be to decode the whole file and count the number of samples -
lest there is a way of querying every packet how many samples it
contains without actually decoding it.
> 
> 2. How do I read a certain amount of ms of data? For example, I
> have been using sizeUncompressed / seconds / 4 (using the above
> method to get sizeUncompressed) to try and get 250ms worth of data,
> but I'm not sure it is doing what I think it does.
Seems quite reasonable to me. You also can look up the seeking
mechanism of ogg vorbis that will allow you to move fast by a few
seconds. But this mechanism fails with many live streams - and it
might not get you to exactly the right point within the file you want
to be at.
> 
> 3. How do I determine the sample size of a file? For example, how
> do I know if I should reading 16 bit or 8 bit samples?
Ogg operates on floating-point numbers and is designed to do the best
job on the data format it gets - or is instructed to produce.
Unfortunately this means it doesn't care where the data came from and
how the data source looked like. Perhaps you could do some heuristics
trying to find out the remains of the typical noise generated by a
8-bit-sampling-process that are left after ogg-encoding the data. But
this approach needs loads of maths and won't be 100% reliable.

Kind regards,

     Gunter.
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