Presentation and access units
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A presentation unit is the elementary block that contains each one of the digital information, still without 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 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), (Chrominance) and (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.
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 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 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.
Already, "Packers P.E.S." are found in System Layers that it constitutes the following step in the generation of program multiplex and transport 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:
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-coded): They are codified without reference to other images, that is to say: they contain all the necessary elements for its reconstruction.
- Images P type (Predicted): 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 (Bi-directional predicted): 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.)”.
- What is an MPEG-2 access unit (AU)?-Access Unit technical explanation
- Superior Technical School of Telecommunication Engineering (Polytechnic University of Madrid) Program streams and MPEG-2 transport DVD application