Vorbis dev - Sep 2002 - Test files for decoder implementation

Hi,

I am trying to implement a Vorbis decoder in Java, and I have some 
problems understanding the file-format specification included with 
libvorbis. In the header description, there are a few obvious mistakes,
which I believe to have solved, but when I try to decode the audio 
packets, the decoded floor and residue data doesn't seem to make 
sense at all, and after a few packets, my decoder is out of sync 
with the bitstream.

Are there any test files (or something like that) available, where 
the content is documented, so that it is possible to test a decoder 
implementation and more easily be able to find the bugs?

<p>Tor

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In case you try to re-invent a wheel but do not want to: there is a
ogg-decoder in the jRoar-packet and a java-library 'jorbis' which can
decode
ogg-vorbis files.

In case you want to re-invent a wheel, good luck :).

-- Merijn

On Wed, Sep 18, 2002 at 02:17:46PM +0200, Tor-Einar Jarnbjo
wrote:
> Hi,
> 
> I am trying to implement a Vorbis decoder in Java, and I have some 
> problems understanding the file-format specification included with 
> libvorbis. In the header description, there are a few obvious mistakes,
> which I believe to have solved, but when I try to decode the audio 
> packets, the decoded floor and residue data doesn't seem to make 
> sense at all, and after a few packets, my decoder is out of sync 
> with the bitstream.
> 
> Are there any test files (or something like that) available, where 
> the content is documented, so that it is possible to test a decoder 
> implementation and more easily be able to find the bugs?
> 
> 
> Tor
> 
> 
> 
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-- 
Zoals 't toetsenbordje thuis tikt, tikt 't nergens ...
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Onsdag, 18 september 2002, skrev du:
>In case you try to re-invent a wheel but do not want to: there is a
>ogg-decoder in the jRoar-packet and a java-library 'jorbis' which 
can decode
>ogg-vorbis files.
>
>In case you want to re-invent a wheel, good luck :).
I've already checked out jorbis, but it doesn't seem to be much more 
than a raw C->Java translation of libvorbis, and does not offer any 
Java-style API to access a Vorbis file or network stream. My intention 
with the decoder is to offer a JMF (Java Media Framework) plugin 
for Ogg/Vorbis support, and I also don't see how this can be easily 
done using jorbis.

A quick look at the file format specification made me believe that 
Vorbis was well documented, but as I started implementing the code,
I realized, that there are obvious errors in the documentation, 
and several points where the documentation does not match the actual 
implementation of libvorbis :-/ 

Tor

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On Wed, Sep 18, 2002 at 02:17:46PM +0200, Tor-Einar Jarnbjo
wrote:
> Hi,
> 
> I am trying to implement a Vorbis decoder in Java, and I have some 
> problems understanding the file-format specification included with 
> libvorbis. In the header description, there are a few obvious mistakes,
Please, inform us of any 'obvious mistakes'.
> which I believe to have solved, but when I try to decode the audio 
> packets, the decoded floor and residue data doesn't seem to make 
> sense at all, and after a few packets, my decoder is out of sync 
> with the bitstream.
Packet extraction and packet decode are decoupled in Ogg; completely
losing packet sync is an indication your page decode is incorrect, at
which point trying to decode the incorrectly extracted packets is a
futile exercise.

As for 'test files', no we don't have an official set.  For the most
part, out debugging strategy is to use diagnostic output from the
encoder and match that to diagnotic output from the decode.

Monty
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Torsdag, 19 september 2002, skrev du:
>On Wed, Sep 18, 2002 at 02:17:46PM +0200, Tor-Einar Jarnbjo wrote:
>> Hi,
>> 
>> I am trying to implement a Vorbis decoder in Java, and I have some 
>> problems understanding the file-format specification included with 
>> libvorbis. In the header description, there are a few obvious 
mistakes,
>
>Please, inform us of any 'obvious mistakes'.
Most turned out to be my mistakes (typos and such), but one left 
is when decoding the mappings from the setup header:

vector [vorbis_mapping_magnitude] element [j]= read ilog([audio_channels]) 
bits as unsigned integer

vector [vorbis_mapping_angle] element [j]= read ilog([audio_channels]) 
bits as unsigned integer

Both steps should read:

.. read ilog([audio_channels] - 1) bits ...

Another point that confused me, is the computation of a floor curve 
type 1. When reading the y list from the data packet, you only get 
positive values from the codebook, but the floor 1 description mentions 
negative values also. I missed, that this is taken account for in 
the algorithm computing the curve.

The libvorbis implementation also differs from the format documentation 
in variable naming and algorithm implementation. Different variable 
naming is of course not really an error, and using different algorithms 
and storage structures not necessarily (probably rather an optimization 
of the decoding process), but it would be much easier to understand 
the format specification if they would match each other. At least 
it makes it impossible to compare a partial result from my implementation 
with whatever libvorbis has computed up to this point of the decoding 
process. Specific examples for this could be the last part of the 
residue header decoding process where libvorbis uses a 1-dimensional 
array with only the used codebooks, and the format specification 
uses a 2-dimensional array, marking unused entries as such, hence 
the residue packet decoding process is also very different.
>As for 'test files', no we don't have an official set.  For the
most
>part, out debugging strategy is to use diagnostic output from the
>encoder and match that to diagnotic output from the decode.
Thanks for the tip. I'll take a look at the encoder, and see if I 
can get anywhere with that information.

Tor

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