From 74f4b1bc3b627ba4c7e03498234d88cacdfbe97b Mon Sep 17 00:00:00 2001 From: Aki Date: Wed, 29 Sep 2021 22:52:49 +0200 Subject: Squashed 'vorbis/' content from commit d22c3ab5f git-subtree-dir: vorbis git-subtree-split: d22c3ab5f633460abc2532feee60ca0892134cbf --- doc/v-comment.html | 285 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 285 insertions(+) create mode 100644 doc/v-comment.html (limited to 'doc/v-comment.html') diff --git a/doc/v-comment.html b/doc/v-comment.html new file mode 100644 index 0000000..aad5e88 --- /dev/null +++ b/doc/v-comment.html @@ -0,0 +1,285 @@ + + + + + +Ogg Vorbis Documentation + + + + + + + +

+ +

Ogg Vorbis I format specification: comment field and header specification

+ +

Overview

+ +

The Vorbis text comment header is the second (of three) header +packets that begin a Vorbis bitstream. It is meant for short, text +comments, not arbitrary metadata; arbitrary metadata belongs in a +separate logical bitstream (usually an XML stream type) that provides +greater structure and machine parseability.

+ +

The comment field is meant to be used much like someone jotting a +quick note on the bottom of a CDR. It should be a little information to +remember the disc by and explain it to others; a short, to-the-point +text note that need not only be a couple words, but isn't going to be +more than a short paragraph. The essentials, in other words, whatever +they turn out to be, eg:

+ +

+"Honest Bob and the Factory-to-Dealer-Incentives, _I'm Still Around_, +opening for Moxy Früvous, 1997" +

+ +

Comment encoding

+ +

Structure

+ +

The comment header logically is a list of eight-bit-clean vectors; the +number of vectors is bounded to 2^32-1 and the length of each vector +is limited to 2^32-1 bytes. The vector length is encoded; the vector +contents themselves are not null terminated. In addition to the vector +list, there is a single vector for vendor name (also 8 bit clean, +length encoded in 32 bits). For example, the 1.0 release of libvorbis +set the vendor string to "Xiph.Org libVorbis I 20020717".

+ +

The comment header is decoded as follows:

+ +

+  1) [vendor_length] = read an unsigned integer of 32 bits
+  2) [vendor_string] = read a UTF-8 vector as [vendor_length] octets
+  3) [user_comment_list_length] = read an unsigned integer of 32 bits
+  4) iterate [user_comment_list_length] times {
+
+       5) [length] = read an unsigned integer of 32 bits
+       6) this iteration's user comment = read a UTF-8 vector as [length] octets
+
+     }
+
+  7) [framing_bit] = read a single bit as boolean
+  8) if ( [framing_bit] unset or end of packet ) then ERROR
+  9) done.
+

+ +

Content vector format

+ +

The comment vectors are structured similarly to a UNIX environment variable. +That is, comment fields consist of a field name and a corresponding value and +look like:

+ +

+comment[0]="ARTIST=me"; 
+comment[1]="TITLE=the sound of Vorbis"; 
+

+ +

A case-insensitive field name that may consist of ASCII 0x20 through +0x7D, 0x3D ('=') excluded. ASCII 0x41 through 0x5A inclusive (A-Z) is +to be considered equivalent to ASCII 0x61 through 0x7A inclusive +(a-z).
The field name is immediately followed by ASCII 0x3D ('='); +this equals sign is used to terminate the field name.
0x3D is followed by the 8 bit clean UTF-8 encoded value of the +field contents to the end of the field.

+ +

Field names

+ +

Below is a proposed, minimal list of standard field names with a +description of intended use. No single or group of field names is +mandatory; a comment header may contain one, all or none of the names +in this list.

+ +

TITLE: Track/Work name
VERSION: The version field may be used to differentiate multiple +versions of the same track title in a single collection. +(e.g. remix info)
ALBUM: The collection name to which this track belongs
TRACKNUMBER: The track number of this piece if part of a specific larger collection or album
ARTIST: The artist generally considered responsible for the work. In popular music +this is usually the performing band or singer. For classical music it would be +the composer. For an audio book it would be the author of the original text.
PERFORMER: The artist(s) who performed the work. In classical music this would be the +conductor, orchestra, soloists. In an audio book it would be the actor who did +the reading. In popular music this is typically the same as the ARTIST and +is omitted.
COPYRIGHT: Copyright attribution, e.g., '2001 Nobody's Band' or '1999 Jack Moffitt'
LICENSE: License information, eg, 'All Rights Reserved', 'Any +Use Permitted', a URL to a license such as a Creative Commons license +("www.creativecommons.org/blahblah/license.html") or the EFF Open +Audio License ('distributed under the terms of the Open Audio +License. see http://www.eff.org/IP/Open_licenses/eff_oal.html for +details'), etc.
ORGANIZATION: Name of the organization producing the track (i.e. +the 'record label')
DESCRIPTION: A short text description of the contents
GENRE: A short text indication of music genre
DATE: Date the track was recorded
LOCATION: Location where track was recorded
CONTACT: Contact information for the creators or distributors of the track. +This could be a URL, an email address, the physical address of +the producing label.
ISRC: ISRC number for the track; see the +ISRC intro page for more information on ISRC numbers.

+ +

Implications

+ +

Field names should not be 'internationalized'; this is a +concession to simplicity not an attempt to exclude the majority of +the world that doesn't speak English. Field contents, +however, use the UTF-8 character encoding to allow easy representation +of any language.
We have the length of the entirety of the field and restrictions on +the field name so that the field name is bounded in a known way. Thus +we also have the length of the field contents.
Individual 'vendors' may use non-standard field names within +reason. The proper use of comment fields should be clear through +context at this point. Abuse will be discouraged.
There is no vendor-specific prefix to 'nonstandard' field names. +Vendors should make some effort to avoid arbitrarily polluting the +common namespace. We will generally collect the more useful tags +here to help with standardization.
Field names are not required to be unique (occur once) within a +comment header. As an example, assume a track was recorded by three +well know artists; the following is permissible, and encouraged: +
```
+              ARTIST=Dizzy Gillespie 
+              ARTIST=Sonny Rollins 
+              ARTIST=Sonny Stitt 
+
```

+ +

Encoding

+ +

The comment header comprises the entirety of the second bitstream +header packet. Unlike the first bitstream header packet, it is not +generally the only packet on the second page and may not be restricted +to within the second bitstream page. The length of the comment header +packet is (practically) unbounded. The comment header packet is not +optional; it must be present in the bitstream even if it is +effectively empty.

+ +

The comment header is encoded as follows (as per Ogg's standard +bitstream mapping which renders least-significant-bit of the word to be +coded into the least significant available bit of the current +bitstream octet first):

+ +

Vendor string length (32 bit unsigned quantity specifying number of octets)
Vendor string ([vendor string length] octets coded from beginning of string +to end of string, not null terminated)
Number of comment fields (32 bit unsigned quantity specifying number of fields)
Comment field 0 length (if [Number of comment fields]>0; 32 bit unsigned +quantity specifying number of octets)
Comment field 0 ([Comment field 0 length] octets coded from beginning of +string to end of string, not null terminated)
Comment field 1 length (if [Number of comment fields]>1...)...

+ +

This is actually somewhat easier to describe in code; implementation of the above +can be found in vorbis/lib/info.c:_vorbis_pack_comment(),_vorbis_unpack_comment()

+ +

+ + + -- cgit v1.1