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== Presentation unit ==

=== Definition ===
A presentation unit is the elementary block that contains each one of the [[digital information]], still without [[Data compression|compressing]], of video, audio or data in "[[system layer]]".
In this layer the operations are realised in order to lead to obtain signal flows
[[MPEG-2]], which they consist of the organization in packages, of the compressed
data and the later one muxed [[multiplexing|multiplex]] of all signals associated to the
program.

=== Video presentation units ===
For video signal digitized without compressing, format 4:2:2 is used with 8 bits
gathered [[quantification]] [[CCIR 601]]-1, whose “Presentation Unit” is an “Image or
Picture” (830 Kbytes for 625 systems lines). In agreement with this standard,
the signals and ([[Luminance]]), <math> C_R </math>([[Chrominance]]) and <math> C_B </math> (Chrominance) are
digitized (got from difference colour signals).

=== Audio presentation unit ===
As far as the audio one, the “presentation unit” is a “Scheme of Audio [[AES3]]/ EBU”. In this case, they are required from 16 to 24 bits by sample to provide the
dynamic range and wished signal/noise link.

[[Image:Tramas audioINGLES.jpg|thumb|center|500x500px|Plot Structure of Audio AES / EBU]]

== Access unit ==

=== Definition ===
An access unit is a logical substructure of "elementary stream" formed by binary
audio video flows and compressed to facilitate the bit stream manipulation
access. It is the smallest data organization than it is possible to be attributed
to the systems of synchronization of layer of [[Data compression|compression]]. In this layer,
MPEG decoding operations are made, taking care about general procedures
of data compression, and being useful in addition, for the images, his
space [[Data redundancy|redundancy]] (same areas) and temporally (successive images),
correlation between near points and the smaller sensitivity of the eye to fix
details of the fixed images.

[[Image:Tramas mpegingles.jpg|thumb|center|550x550px|MPEG-2 compression of digital video signal]]

Already, "Packers P.E.S." are found in System Layers that it constitutes the
following step in the generation of program multiplex and transport [[Multiplexing|multiplex]]
MPEG-2. One is to turn each “Elementary Stream (E.S.)” composed exclusively
by “Units Access”, in a “Packetised Elementary Stream (P.E.S.)”. A P.E.S is
completely compound of “PES-Packets”, as it is in the figure:

[[Image:Conversion de es a pesingles.jpg|thumb|center|550x550px|Converting a E.S. a P.E.S.]]

=== Video access unit ===
In the case of the video signal, the compressed “access units”, since we have
verified, are 3 types, corresponding to other so many types of images MPEG:

*Images type I (Intra): They are codified without reference to other images, that is to say: they contain all the necessary elements for its reconstruction.
*Images P type (Anticipated): They are codified with respect to the image of type I or another previous P, thanks to the techniques of prediction with compensation of movement. Its compression rate is clearly greater
than the one of images I.
*Images B type (Bidirectional): They are codified by interpolation between two images of I type or preceding and following P frames that them. They offer a higher rate compression.

The video sequence MPEP codification result is a succession of “Video AccessUnits”. This succession of access units constitutes the denominated “Video Elementary Stream (Video E.S.)”.

=== Audio Access Units ===
In the case of the audio signal, the “Access Units” typically contains a few audio
milliseconds audio tablet. In the same way, the result of codification MPEG
for the audio one is a succession of “Audio Access Units” that compose the
denominated “Audio Elementary Stream (Audio E.S.)”.

== See also ==
[[Program stream]]

[[Transport stream]]

[[MPEG-2]]

== External links ==
* [http://www2.tek.com/cmswpt/faqdetails.lotr?ct=FAQ&cs=faq&ci=6366&lc=EN What is an MPEG-2 access unit (AU)?]-Access Unit technical explanation

== References ==
*Superior Technical School of Telecommunication Engineering (Polytechnic University of Madrid) [http://www.etc.upm.es/tsmpeg2d.pdf Program streams and MPEG-2 transport DVD application]

{{DEFAULTSORT:Presentation Unit And Access Unit}}
[[Category:MPEG-2]]

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