Media Encoding


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Overarching information on heavy media files (audio and video)

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Media Encoding

  2. 2. Why and how audio and video are encoded Media encoding overview
  3. 3. Encoding media  Encoding refers to the conversion of media files from one form to another (compression)  Encoding is performed for the following purposes  Compressing a file to a smaller size (data / frame size)  Making it usable on a particular device / software player  Practically all audio and video is encoded and compressed for distribution  Uncompressed audio and video are retained for archiving and re-use / re-encoding
  4. 4. Encoding > Decoding flow Data File Stream Stream Webcam Microphone OB Unit / Studio Control room Uncompressed Video Uncompressed audio Compresse d data file Compressed stream Local Storage Transport Network (www) Data File Encoding Engine Encoding Engine Decoding Engine
  5. 5. Transcoding  The techniques used for transcoding are the same as for encoding  The goal of transcoding is not to get a file down to a small size (compression)  Transcoding can be seen as ‘translating’ from one form to another maintaining maximum quality  Example: some editing systems may not be capable of processing a particular type of video – footage is transcoded to a form that can be used
  6. 6. Digital Media Files  Containers (Wrappers)  Encoded media is stored within container formats  Containers ‘store’ encoded audio and / or audio ‘streams’  Containers also contain metadata needed for the player to make ‘sense’ of the enclosed media formats  Container formats include Quicktime (MOV), RealMedia (RM), MPEG and OGG (open source format)  IMPORTANT: Container formats do not describe the manner in which a file has been encoded  A QT file might not play in QuickTime on a particular machine  The software requires the appropriate Codec to be installed >>>
  7. 7. Digital Media Files - CoDecs  Whether or not a file will play depends on its codec  Codec refers to the particular encoding method (algorithm) used to compress and decompress a piece of media (COmpress – DECompress)  Codecs specifically describe the type of video or audio compression used  Certain codecs play almost universally (MPEG4)  Some codecs may require plugins to be installed for playback (Vorbis (OGG), VP3 (Theora))
  8. 8. Encoding applications Encoding is done at the following points  AV production applications (from the timeline)  Final Cut Pro (native & via compressor)  Protools  Within bespoke compression applications  Adobe medi Encoder (PC / MAC)  Compressor (Apple)  MediaCoder (open source)  As import / export options on media players  iTunes (import)  QuickTime Pro (export options)  On websites such as YouTube (FFMPEG server side encoder) Some encoding applications offer more control than others
  9. 9. Lossless and lossy compression Lossless  Refers to any file type that is a true (verbatim) copy of the original  No quality has been lost in saving a file in the following formats  Lossless Audio – Flac, WavPac, Monkey’s Audio, ALAC  Lossless Video – Animation Codec, Huffyuv, Uncompressed  Lossless Graphics – Gif, PNG, Tiff  A basic example of lossless compression methods include RLE (Run Length Encoding)  Using the following as an abstraction of the data used to store a segment of audio – [AAAAABBCCCCCDEEEEEEE]= 20bytes  RLE would look at the ‘run lengths’ or repeated adjacent runs of data and summarise them as A5B2C5D1E7 = 10bytes
  10. 10. Lossless and lossy compression Lossy  File formats and codecs where a file may look or sound acceptable or as good as the original but is in fact a degraded copy  Lossy file formats include  Lossy Audio – AAC, Mp3, Vorbis  Lossy video – M2V, H.264,  Lossy Graphics – Jpeg,  Lossy compression approximates data in order to make easily represented sequences of data  A (very) basic example is to use a similar scenario as before  AAAAABAAAAA represents a signal or series of pixels (11 bytes)  The compression could represent it as A5B1A5 (6 bytes lossless)  Lossy compression decides that the discrepancy is not significant enough to record so instead approximates it back to A (A11 = 2 bytes)
  11. 11. Redundancy  File compression uses systems based around redundancy  Redundant elements are parts of the sound or image that are not required to be recorded (written) as data in the compressed file  Audio uses psychoacoustic principles to determine which sounds can be omitted without adversely affecting the overall quality (low / high frequencies, hiss, overlapping sounds)  Video uses pixel colour data to determine redundancies (see next slides)  Different codecs and encoders view and process these redundancies in different ways (algorithms) with different results  Redundancy can be broken into two categories  Objective redundancy
  12. 12. Objective redundancy in imagery • An area of pure black is detected (area spans 15,300 pixels all black) • The area is mapped between 4 points (corners of green rectangle) • 15,300 pieces of information can be reduced to 5 pieces of information • That information can then be decoded in the player and rendered exactly as it was
  13. 13. Subjective redundancy in imagery • An area is detected where the pixels are similar in colour (all black / dark grey) • The encoder decides that the difference is negligible (won’t be noticed) • The area is mapped similarly to before using 1 colour value • Information has been discarded and the quality of the compresses file is less than the original
  14. 14. Compressing  The goal of compression is to get the smallest file size while retaining maximum ‘meaningful’ information (fidelity / clarity)  Compression is always a trade-off between quality and file size  The same principle applies to audio / video as to graphics  Always work from a high quality source  Never compress already compressed media (generation loss)  Always retain (archive) a high quality original for future work