Opus audio: high level bitstream overview with Virinext Bitstream Analyzer

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Opus audio high level bitstream overview with
Virinext Bitstream Analyzer
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Opus audio overview

• Opus audio is a lossy audio compression format
• Open and rayality-free
• Was developed and released by Xiph.Org Foundation
  in 2012
• Main advantages of Opus is low delay on encoding
  and decoding
• Low delay makes Opus a good choice for real-time
  communication such as online gaming or video
  conferencing
• We will use Virinext Bitstream Analyzer for
  analyzing Opus files.

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High level Opus bitstream overview

• At the high level Opus audio is the sequence of
  packets which contains compressed audio frames

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Opus identification header(IDHeader)

• Opus codec parameters are stored in the
  identification header
• The IDHeader starts with the 8 bytes "OpusHead"
• IDHeader packets are commonly stored in the
  decoder configuration parameters at the container
  level.

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Opus TOC header

• Each frame starts with a one byte
  TOC(table-of-contents) header
• TOC header describes the modes and configurations
  used packet
• The "config" describes which one of 32 possible
  configurations of operating mode, audio
  bandwidth, and frame size is used in the packet
• The stereo flag "s" signals mono or stereo is
  used, where 0 indicates mono and 1 indicates
  stereo

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Opus TOC header

• Count code "c" describes the number of frames per
  packet (codes 0 to 3) as follows
• 0 1 frame in the packet
• 1 2 frames in the packet, each with equal
  compressed size
• 2 2 frames in the packet, with different
  compressed sizes
• 3 an arbitrary number of frames in the packet

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Opus TOC header
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Opus compressed frame

• After the TOC byte data of compressed frames is
  stored.

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Opus bitstream sampling rate

• "Original input sample rate" field of ID Header
  describes the sample rate of the input before
  encoding
• Each packet in the Opus stream can have a
  different audio bandwidth.
• This field is also used to pass the sample rate
  of the input stream as metadata.
• During playback encoded audio, Opus player select
  the playback sample rate according to the
  following procedure
• If the hardware supports 48 kHz playback,
  decode at 48 kHz,
• else if the hardware's highest available
  sample rate is a supported rate, decode at this
  sample rate,
• else if the hardware's highest available
  sample rate is less than 48 kHz, decode at the
  next higher supported rate and resample,
• else decode at 48 kHz and resample.

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Opus bitstream channels count

• Number of output channels is described by the
  "channel count" field of the ID Header
• This can be different than the number of encoded
  channels, which can change on packet-by-packet
  basis and described by the "c" field of the TOC
  header.

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Opus bitstream channels layout

• The "channel map" field in ID Header indicates
  the order and meaning of the encoded channels.
• There are three mapping families
• Family 0 (RTP mapping) Allowed channel numbers
  are 1 or 2. For mono, there is one channel
  (monophonic), and for stereo, there are two
  channels (left, right)
• Family 1 (Vorbis channel order) Allowed channel
  numbers range from 1 to 8. The meanings of the
  channels depend on the number of channels and
  follow the Vorbis mapping specifications
• Family 255 (no defined channel meaning) Allowed
  channel numbers range from 1 to 255. In this
  family, the channels are unidentified.

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Opus audio analysis with Virinext Bitstream
Analyzer

• Screenshot of PCM-analysis and GOP widgets of
  Opus audio file opened in Virinext Bitstream
  Analyzer

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References

• Virinext Bitsteam Analyzer - GUI tool for
  displaying and analyzing coded video and audio
  streams
• Opus audio high level format overview
• Opus homepage
• Official Opus file format specification