-
AVT Working Group L. Barbato
Internet-Draft Xiph.Org
Expires: April 18, 2006 October 15, 2005
maximum encoder flexibility, thus allowing it to scale competitively
over an exceptionally wide range of bitrates. At the high quality/
bitrate end of the scale (CD or DAT rate stereo, 16/24 bits), it is
- in the same league as MPEG-2 and MPC. Similarly, the 1.0 encoder can
- encode high-quality CD and DAT rate stereo at below 48k bits/sec
- without resampling to a lower rate. Vorbis is also intended for
- lower and higher sample rates (from 8kHz telephony to 192kHz digital
- masters) and a range of channel representations (monaural,
- polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to 255
- discrete channels).
+ in the same league as MPEG-2 and MPC. Similarly, the version 1.1
+ reference encoder can encode high-quality CD and DAT rate stereo at
+ below 48k bits/sec without resampling to a lower rate. Vorbis is
+ also intended for lower and higher sample rates (from 8kHz telephony
+ to 192kHz digital masters) and a range of channel representations
+ (monaural, polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to
+ 255 discrete channels).
Vorbis encoded audio is generally encapsulated within an Ogg format
bitstream [1], which provides framing and synchronization. For the
This 24 bit field is used to associate the Vorbis data to a decoding
Configuration.
- Fragment type (F): 2 bit
+ Fragment type (F): 2 bits
This field is set accordingly the following list
3. Configuration Headers
Unlike other mainstream audio codecs Vorbis has no statically
- configured probability model, instead it packs all entropy decoding
- configuration, VQ and Huffman models into a self-contained codebook.
- This codebook block also requires additional identification
- information detailing the number of audio channels, bitrates and
- other information used to initialise the Vorbis stream.
-
- To decode a Vorbis stream, three configuration header blocks are
- needed. The first header, named identification, indicates the sample
- and bitrates, the number of channels and the version of the Vorbis
- encoder used. The second header, named comment, details stream
- comments and the third header, named setup, contains the decoders
- probability model, or codebook. Further details are available in the
- Vorbis I specification [12]
-
- As the RTP stream may change certain configuration data mid-session
- there are different methods for delivering this configuration data to
- a client, both in-band and out-of-band which is detailed below. SDP
- delivery is used to setup an initial state for the client
- application. The changes may be due to different codebooks as well
- as different bitrates of the stream.
+ configured probability model. Instead, it packs all entropy decoding
+ configuration, VQ and Huffman models into a data block that must be
+ transmitted to the decoder along with the compressed data. A decoder
+ also requires identification information detailing the number of
+ audio channels, bitrates and other information to configure itself
+ for a particular compressed data stream. These two blocks of
+ information are often referred to collectively as the "codebooks" for
+ a Vorbis stream, and are nominally included as special "header"
+ packets at the start of the compressed data.
+
+ Thus these two codebook header packets must be received by the
+ decoder before any audio data can be interpreted. In addition, the
+ Vorbis I specification [12] requires the presense of a comment header
+ packet which gives simple metadata about the stream. This
+ requirement poses problems in RTP, which is often used over
+ unreliable transports.
+
+ Since this information must be transmitted reliably, and as the RTP
+ stream may change certain configuration data mid-session there are
+ different methods for delivering this configuration data to a client,
+ both in-band and out-of-band which is detailed below. SDP delivery
+ is used to setup an initial state for the client application. The
+ changes may be due to different codebooks as well as different
+ bitrates of the stream.
The delivery vectors in use are specified by an SDP attribute to
indicate the method and the optional URI where the Vorbis Packed
The Packed Configuration (Section 3.1.1) Payload is sent in-band with
the packet type bits set to match the payload type. Clients MUST be
- capable of dealing with fragmentation and periodic re-transmission of
- the configuration headers.
-
-
Internet-Draft draft-ietf-avt-vorbis-rtp-01 October 2005
+ capable of dealing with fragmentation and periodic re-transmission of
+ the configuration headers.
+
3.1.1. Packed Configuration
A Vorbis Packed Configuration is indicated with the payload type
-
-
-
Barbato Expires April 18, 2006 [Page 9]
\f
Internet-Draft draft-ietf-avt-vorbis-rtp-01 October 2005
Barbato Expires April 18, 2006 [Page 23]
\f
-
<section anchor="Introduction" title="Introduction">
<t>
-Vorbis is a general purpose perceptual audio codec intended to allow maximum encoder flexibility, thus allowing it to scale
-competitively over an exceptionally wide range of bitrates. At the high quality/bitrate end of the scale (CD or DAT rate
-stereo, 16/24 bits), it is in the same league as MPEG-2 and MPC. Similarly, the 1.0 encoder can encode high-quality CD and
-DAT rate stereo at below 48k bits/sec without resampling to a lower rate. Vorbis is also intended for lower and higher sample
-rates (from 8kHz telephony to 192kHz digital masters) and a range of channel representations (monaural, polyphonic, stereo,
-quadraphonic, 5.1, ambisonic, or up to 255 discrete channels).
+Vorbis is a general purpose perceptual audio codec intended to allow
+maximum encoder flexibility, thus allowing it to scale competitively
+over an exceptionally wide range of bitrates. At the high
+quality/bitrate end of the scale (CD or DAT rate stereo, 16/24 bits), it
+is in the same league as MPEG-2 and MPC.
+Similarly, the version 1.1 reference encoder can encode high-quality CD
+and DAT rate stereo at below 48k bits/sec without resampling to a lower
+rate. Vorbis is also intended for lower and higher sample rates (from
+8kHz telephony to 192kHz digital masters) and a range of channel
+representations (monaural, polyphonic, stereo, quadraphonic, 5.1,
+ambisonic, or up to 255 discrete channels).
</t>
<t>
</t>
<t>
-Fragment type (F): 2 bit</t>
+Fragment type (F): 2 bits</t>
<t>
This field is set accordingly the following list
</t>
<section anchor="Configuration Headers" title="Configuration Headers">
<t>
-Unlike other mainstream audio codecs Vorbis has no statically configured probability model, instead it packs all entropy decoding configuration, VQ and Huffman models into a self-contained codebook. This codebook block also requires additional identification information detailing the number of audio channels, bitrates and other information used to initialise the Vorbis stream.
+Unlike other mainstream audio codecs Vorbis has no statically
+configured probability model. Instead, it packs all entropy decoding
+configuration, VQ and Huffman models into a data block that must be
+transmitted to the decoder along with the compressed data. A decoder
+also requires identification information detailing the number of audio
+channels, bitrates and other information to configure itself for a
+particular compressed data stream. These two blocks of information are
+often referred to collectively as the "codebooks" for a Vorbis stream,
+and are nominally included as special "header" packets at the start
+of the compressed data.
</t>
<t>
-To decode a Vorbis stream, three configuration header blocks are needed. The first header, named identification, indicates the sample and bitrates, the number of channels and the version of the Vorbis encoder used. The second header, named comment, details stream comments and the third header, named setup, contains the decoders probability model, or codebook. Further details are available in the <xref target="vorbis-spec-ref">Vorbis I specification</xref>
+Thus these two codebook header packets must be received by the decoder
+before any audio data can be interpreted. In addition,
+the <xref target="vorbis-spec-ref">Vorbis I specification</xref>
+requires the presense of a comment header packet which gives simple
+metadata about the stream. This requirement poses problems in RTP,
+which is often used over unreliable transports.
</t>
<t>
-As the RTP stream may change certain configuration data mid-session there are different methods for delivering this configuration data to a client, both in-band and out-of-band which is detailed below. SDP delivery is used to setup an initial state for the client application. The changes may be due to different codebooks as well as different bitrates of the stream.
+Since this information must be transmitted reliably, and as the RTP
+stream may change certain configuration data mid-session there are
+different methods for delivering this configuration data to a
+client, both in-band and out-of-band which is detailed below. SDP
+delivery is used to setup an initial state for the client application.
+The changes may be due to different codebooks as well as different
+bitrates of the stream.
</t>
<t>
projects / platform / upstream / libvorbis.git / commitdiff