FLAC audio: high level bitstream overview with Virinext Bitstream Analyzer

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

• FLAC audio (Free Lossless Audio Codec) is a
  lossless audio compression format
• Was developed by Josh Coalson in 2000
• Main advantage of FLAC is high compression ratios
  without losing any details of original audio
• This makes FLAC a good choice for archiving,
  professional audio production, and high-quality
  audio playback
• We will use Virinext Bitstream Analyzer for
  analyzing FLAC files.

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High level bitstream overview

• At the high level FLAC audio is the sequence of
  few metadata blocks followed by audio frames.

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FLAC bitstream metadata blocks types

• At the high level FLAC audio is the sequence of
  few metadata blocks followed by audio frames.
• There are following types of metadata
• Streaminfo metadata - provides essential stream
  parameters
• Padding metadata - for the insertion of arbitrary
  amount of padding
• Application metadata - can store additional data
  paired with 4-byte identifiers
• Seektable metadata - store seek points
• Vorbis comment metadata - contains human-readable
  information in UTF-8 format
• Picture metadata - stores image data associated
  with the audio content.

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FLAC bitstream metadata blocks types

• Screenshot provides METADATA_BLOCK_STREAMINFO
  from Virinext Bitstream Analyzer.

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FLAC bitstream audio frames

• After the final metadata block, one or more
  frames are present
• Each frame encompasses a frame header, one or
  more sub-frames, padding zero bits for achieving
  byte-alignment, and a frame footer
• The number of sub-frames in each frame
  corresponds to the number of audio channels.

Screenshot of FRAME_HEADER syntax element from
Virinext Bitstream Analyzer
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FLAC bitstream audio frames

• The frame starts with a 15-bit frame sync code of
  0b111111111111100(0x7FFC)
• Following the sync code is the blocking strategy
  bit, which remains unchanged throughout the audio
  stream
• The sample_rate field indicates the sample rate
  of the audio within the current frame
• The channel_assignment, also known as the
  channels bits code, determines both the number of
  audio channels and the channel configuration,
  defining the channel mapping for multi-channel
  audio
• The subsequent field, sample_size, specifies the
  bit depth of the audio. Lastly, the
  coded_frame_number field denotes the frame
  number
• Following the frame header, sub-frames are
  present, which contain encoded audio data.

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FLAC audio analysis with Virinext Bitstream
Analyzer

• FLAC has gained popularity over the years and is
  now recognized as a reliable and efficient
  lossless audio compression format.
• It has particularly captured the attention of
  music enthusiasts and professionals in the
  recording and music production industry.
• Virinext Bitstream Analyzer offers analysis for
  various encoding standards, including FLAC audio
• Virinext Bitstream Analyzer is capable for for
  both in-depth analysis or a high-level overview
  of coded audio and video bitstreams

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FLAC audio analysis with Virinext Bitstream
Analyzer

• Screenshot of PCM-analysis and GOP widgets of
  FLAC 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
• FLAC audio high level bitstream overview
• FLAC audio on xiph.org
• FLAC Format Specification