2. Why and how audio and video are encoded
Media encoding overview
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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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
