MEDIA ENCODING

Why and how audio and video are encoded

Media encoding overview

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

Encoding > Decoding flow

Data

File

Stream Stream

WebcamMicrophoneOB Unit / Studio Control room

Uncompressed VideoUncompressed audio

Compressed data file

Compressed stream

Local Storage

TransportNetwork (www)

Data

File

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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

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 >>>

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))

Encoding applications

Encoding is done at the following points A\V 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

Lossless and lossy compressionLossless 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

Lossless and lossy compressionLossy 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)

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 Subjective redundancy

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

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

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