[vorbis] Location of fileinfo

[Greg Wooledge]

Danny Keff (dkeff at wxs.nl) wrote:

> My question: Could you please tell me how many bytes have been reserved
> for nominal bitrate information and on what location this packet of
> bytes starts? And the same for the frequency (khz) of the ogg-files.

Is the stuff below readable to you?  If it's not, then I'm afraid
you'll have to refer to the official documentation and/or the source
code.

Hint 1: the indented >> notation is a hierarchical conditional syntax.
Thus, the lines

>>>>>48         lelong          !-1
>>>>>>48        lelong          x               ~%lu

mean "if the little endian long (32-bit) value at position 48 is not
-1, then print a tilde followed by that same number formatted as an
unsigned long".

Hint 2: (nn.b+mm) means "use the value of the byte at position nn, plus
the constant mm, as the position to check".

#------------------------------------------------------------------------------
# vorbis:  file(1) magic for Ogg/Vorbis files
#
# From Felix von Leitner <leitner at fefe.de>
# Extended by Beni Cherniavsky <cben at crosswinds.net>
# Further extended by Greg Wooledge <greg at wooledge.org>
#
# Most (everything but the number of channels and bitrate) is commented
# out with `##' as it's not interesting to the average user.  The most
# probable things advanced users would want to uncomment are probably
# the number of comments and the encoder version.
#
# --- Ogg Framing ---
0		string		OggS		Ogg data
>4		byte		!0		UNKNOWN REVISION %u
##>4		byte		0		revision 0
>4		byte		0
##>>14		lelong		x		(Serial %lX)
# --- First vorbis packet - general header ---
>>28		string		\x01vorbis	\b, Vorbis audio,
>>>35		lelong		!0		UNKNOWN VERSION %lu,
##>>>35		lelong		0		version 0,
>>>35		lelong		0
>>>>39		ubyte		1		mono,
>>>>39		ubyte		2		stereo,
>>>>39		ubyte		>2		%u channels,
>>>>40		lelong		x		%lu Hz
# Minimal, nominal and maximal bitrates specified when encoding
>>>>48		string		<\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff	\b,
# The above tests if at least one of these is specified:
>>>>>44		lelong		!-1
# Vorbis RC2 has a bug which puts -1000 in the min/max bitrate fields
# instead of -1.
>>>>>>44	lelong		!-1000
>>>>>>>44	lelong		x		>%lu
>>>>>48		lelong		!-1
>>>>>>48	lelong		x		~%lu
>>>>>52		lelong		!-1
>>>>>>52	lelong		!-1000
>>>>>>>52	lelong		x		<%lu
>>>>>48		string		<\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff	kbps
# -- Second vorbis header packet - the comments
# A kludge to read the vendor string.  It's a counted string, not a
# zero-terminated one, so file(1) can't read it in a generic way.
# libVorbis is the only one existing currently, so I detect specifically
# it.  The interesting value is the cvs date (8 digits decimal).
# Post-RC1 Ogg files have the second header packet (and thus the version)
# in a different place, so we must use an indirect offset.
>>>(84.b+85)		string		\x03vorbis
>>>>(84.b+96)		string/c	Xiphophorus\ libVorbis\ I	\b, created by: Xiphophorus libVorbis I
>>>>>(84.b+120)		string		>00000000	%.8s
# Map to beta version numbers:
>>>>>>(84.b+120)	string		<20000508	(<beta1 - prepublic)
>>>>>>(84.b+120)	string		20000508	(beta1/2)
>>>>>>(84.b+120)	string		>20000508
>>>>>>>(84.b+120)	string		<20001031	(beta2-3)
>>>>>>(84.b+120)	string		20001031	(beta3)
>>>>>>(84.b+120)	string		>20001031
>>>>>>>(84.b+120)	string		<20010225	(beta3-4)
>>>>>>(84.b+120)	string		20010225	(beta4)
>>>>>>(84.b+120)	string		>20010225
>>>>>>>(84.b+120)	string		<20010615	(beta4-RC1)
>>>>>>(84.b+120)	string		20010615	(RC1)
>>>>>>(84.b+120)	string		20010813	(RC2)
>>>>>>(84.b+120)	string		20010816	(RC2 - Garf tuned v1)
>>>>>>(84.b+120)	string		20011014	(RC2 - Garf tuned v2)
>>>>>>(84.b+120)	string		20011217	(pre-RC3 CVS)
>>>>>>(84.b+120)	string		20011231	(RC3)
# The string has not changed from beta1 to 2 - they are indistinguishable.
# Then come the comments, again length-counted (and number-counted).
# Some looping constructs and registers would allow reading them but now
# it's impossible.  However we can print the number of comments present
# (skipping by the vendor string length):
##>>>>(109.l.113)	lelong		0		\b, no comments
##>>>>(109.l+113)	lelong		>0		\b, %lu comments


-- 
Greg Wooledge                  |   "Truth belongs to everybody."
greg at wooledge.org              |    - The Red Hot Chili Peppers
http://wooledge.org/~greg/     |


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