AudioMedia - February 2012 - (Page 54) video guide A Sound Pro’s Guide To Video Television Technology: Part 5 here was a time when compression was a technical process used for specific purposes. Early rock & all there is to know about roll recordings and AM radio programmes were compressed heavily to make them both more exciting and compression in his continuing to cut through background noise when they were played on jukeboxes in diners or through the atmospherics on look at all-things TV. unsatisfactory transmissions. It was later used on FM radio for the same reasons and in general was never considered a good thing. Now compression is the bedrock for both industry and entertainment. In the latter category this includes music – with the MP file now becoming the norm despite widespread criticism of its quality – and digital television (DTV). Compression is at the heart of DTV and is the key to getting many channels on to a single frequency – a process known as multiplexing. The coming of digital has made compression techniques less obtrusive and relatively easy to perform, although there are GLOSSARY still concerns about the amount of P-frame processing used, which can affect the Short for predictive frame, or predicted frame, a quality of both audio and video. video compression method used by the MPEG Two formats have been developed standard. In a motion sequence, individual frames to compress images: JPEG – named of pictures are grouped together (called a group of af ter the Joint Photographic pictures, or GOP) and played back so the viewer is Experts Group established by the able to register the video’s spatial motion. P-frames International Standards Organisation follow I-frames and contain only the data that have (ISO) in – which is used for still changed from the preceding I-frame (such as colour pictures; and MPEG (Moving Picture or content changes). Because of this, P-frames Experts Group, established by the depend on the I-frames to fill in most of the data. ISO in ), the umbrella term now P-frames and B-frames are also referred to as delta used for a series of technologies to frames. compress video. The aim of compression in I-frame general is to reduce the bit rate of a Short for intraframe, this is also a video compression file or signal so that it takes up less method used by the MPEG standard. Also bandwidth on its carrying medium, called a keyframe, an I-frame is a single frame of which would be a computer hard disk digital content that the compressor examines or SD card in the case of still images independently of the frames that precede and and a media server, removable disk follow it and stores all of the data needed to display or card storage or transmission that frame. Typically, I-frames are interspersed with frequency for video. P-frames and B-frames in a compressed video. The Because video is fundamentally more I-frames that are contained, the better the a series of still images in succession, quality of the video. The drawback is that I-frames it can be compressed using JPEG contain the most amount of bits and so take up technology, which is based on a more space on the storage medium. technique called spatial inter-frame compression. In a sequence of B-frame moving images, each frame does Short for bi-directional frame, or bi-directional not change substantially from the predictive frame. As the name suggests, B-frames one previous to it. This means the rely on the frames preceding and following them. compression process can do away B-frames contain only the data that have changed with the segment of subsequent from the preceding frame or are different from the images that has not changed from data in the very next frame. the first image to be scanned, and so are redundant. This means only the REFERENCES changes are processed, which is done Newnes Guide to Television and Video Technology by KF Ibrahim, using a technique called temporal Newnes 2007 DCT (discrete cosine transform) The New Oxford Dictionary of English compression. DCT is the complex mathematics that forms the basis of KEVIN HILTON compresses T the process; it is a Fast Fourier Transform related to the discrete Fourier Transform, the difference being it uses only real numbers (that is, a value representing a quantity that is part of a continuum). MPEG- encoding, the basis of standard definition (SD) video used in DTV, comprises three main components: data preparation; temporal and spatial compression; and quantitisation. Video material is prepared to ensure that a raw-coded sample of the image frame selected for processing is arranged in a form that will suit data compression. The video is fed into the encoder as line-scanned coded samples that have luminance (Y) and chrominance (both Cr and Cb). The preparation stage rearranges the samples into x blocks that are used in the removal of redundant spatial material. The blocks are then in turn rearranged as x macro-blocks, ready for the removal of temporally redundant data. The structure of a macro-block depends on the form of the MPEG- coding being used. In :: sampling, a macro-block will be made up of four blocks of luminance and one block of each of Cr and Cb, which determine chrominance. Slice And Dice Once this stage has been completed the macro-blocks are formed into what are described by experts as slices. It is these slices that are the foundation units used in the compression process. Theoretically a slice can be anything from a single macro-block to an entire picture. In the majority of cases a slice will be either a full picture row or a section of a picture row. The next part of the encoding process is temporal compression, also known as inter-frame compression. This is done on successive frames and works on the principle that there will be only minimal differences between two successive frames. It is at this stage that any repeated information is discarded and the differences from one frame to another passed on to the next phase of the coding process. The two constituent parts of an image being processed are called motion vector and the difference frame; together they create a description of the differences between the frame being processed and the one that went before it. For example, in a shot of a car being driven along a road, the car will be the motion vector and the coding process will retain the basic descriptive data of both it and the background, registering only the changes in these as the scene progresses. A motorcycle entering the scene is the difference frame. This is derived through an involved process that adds the motion vector to the first frame in the > 54 AUD I O MEDIA FEBRUARY 2012 Table of Contents for the Digital Edition of AudioMedia - February 2012 Audio Media - February 2012 Contents Recording Broadcast Post SR Special Report: BVE Cut Scene: PS Vita Geofocus: Netherlands New at NAMM NAMM Show Wrap-Up Adam A77X RND Portico 5024 Untrason Signature Pro Final Cut: Drive Loudness Loudness International Presonus 16.0.2 Allen & Health GS-R24 Cockos Reaper Product Sampler: Broadcase Consoles The Mousetrap Video Guide ClassicCut: The Haunting AudioMedia - February 2012 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201307 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201306 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201305 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201304 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201303 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201302 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201301 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201212 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201211 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201210 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201209 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201208 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201207 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201206 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201205 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201204 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201203 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201202 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201201 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201112 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201111 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201110 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201109 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201108 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201107 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201106 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201105 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201104 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201103 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201102 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201101 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201012 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201010 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201011 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201009 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201008 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201007 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201006 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201005 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201004 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201003 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201002 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_201001 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200912 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200911 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200910 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200909 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200908 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200907 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200906 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200905 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200904 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200903 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200902 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200901 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200812 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200811 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200810 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200809 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200808 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200807 http://www.nxtbook.com/nxtbooks/newbay/audiomedia_200806 http://www.nxtbookMEDIA.com
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