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G6DPMM - Lecture 12

G6DPMM - Lecture 12. Digital Video. Sourcing Digital Video. Film as Digital Becoming more common as costs fall May be compressed in the camera or uncompressed Stored on magnetic tape D1, D2, D3, D5, D9 (Digital-S) Digital Betacam, BetacamSX, BetacamIMX DVCAM DVCPRO DV / MiniDV (consumer)

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G6DPMM - Lecture 12

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  1. G6DPMM - Lecture 12 Digital Video

  2. Sourcing Digital Video • Film as Digital • Becoming more common as costs fall • May be compressed in the camera or uncompressed • Stored on magnetic tape • D1, D2, D3, D5, D9 (Digital-S) • Digital Betacam, BetacamSX, BetacamIMX • DVCAM DVCPRO • DV / MiniDV (consumer) • Digital-8 (uses Hi-8 tapes) • Compressed vs Uncompressed • Film as Analogue • Digitise with capture board (frame grabber) • Wildly variable in price and quality

  3. Digital Video Playback • Playback Software / Multimedia Framework • Windows Media Player (Microsoft) • Bundled with Windows (3.1 - XP) • Free versions for Mac OSX and Solaris • Media framework – includes video infrastructure • File formats (e.g. AVI / WMV) • Quicktime • Bundled with Macintosh (OS 7 – X) • Free version for Windows • Media framework – includes video infrastructure • MOV file format • RealNetworks • RealPlayer (Windows / Macintosh / Linux / Solaris • Originally a streaming technology, now a full blown media framework

  4. Digital Video Compression • Video consists of a time-series of frames • Frames are still images • NB still image compression techniques • Sequences of frames may be compressed by: • Motion estimation • Only updating the parts of the frame that have moved • E.g. consider a talking head! • Motion compensation • Finding static parts of the frame and moving them around • E.g. consider a pan or zoom

  5. Compression Codecs • Playback software (media framework) does not usually contain compression/decompression software • Compression software implementing a specific algorithm is called a codec • Codecs are required by the framework – many different codecs can be installed into a given framework • NB compatibility and distribution (legality) issues

  6. Common Codecs • Indeo • Cinepak • P*64 • Sorenson • ClearVideo • VDOwave • RealVideo • MPEG • NB this is not a single codec • MPEG also has many spin-offs – such as Microsoft WMV or DivX

  7. Video Compression • Uncompressed video is expensive • Hard to store – very large volumes of data • Expensive equipment required to manipulate • Compression is always lossy • Multi-way trade-off • File size / transfer rate (bandwidth) • Frame rate (typically 15fps for multimedia) • Image quality • Frame size • Different codecs have differing strengths and weaknesses (e.g. Indeo vs Cinepak)

  8. Indeo • Algorithms developed for DVI – Intel i750 chipset • Indeo – implemented in software • Proprietary intel algorithms • Two levels of compression • PLV (Production Level Video) – broadcast quality • RTV (Real Time Video) • Typical compression ratio of 160:1 • Beware versions (5.1 safe on modern machines, 3.2 fairly ubiquitous)

  9. MPEG • Moving Picture Expert Group • Established by ISO in 1988 • Remit – define standards for movie with soundtracks • Digital television • Video phones • Consumer electronics • Digital cinema • Video games • Family of standards • Always lossy • Discrete Cosine Transform (DCT) algorithm

  10. MPEG-1 (1993) • The original MPEG standard • Objective: deliver video and audio from CD-ROM with comparable quality to VHS tape and CD-Audio • Encodes an audiovisual signal in about 1.5 Mbps • Images are progressively scanned • This fundamentally limits quality • Motion compensation • I-Frames (reference) • P-Frames (predictive) • B-Frames (bidirectional predictive)

  11. MPEG Frame Types • I-Frames • Reference frame • Complete image – no missing information • Subsequent P-frames are based upon an I-frame • P-Frames • Only contain the difference between themselves and the previous P-frame or I-frame • Anything that does not change is not stored • B-Frames • Contain the difference between themselves and both frames on either side • B-frames are not always implemented because of memory requirements

  12. The MPEG GOP • GOP Structure • Group of Pictures • Specifies I / P frame structure (e.g. 1 I frame per 11 P frames.IPPPPPPPPPPPIPPPPPPPPPPPIPPPPPPPPPPPI • The fewer I frames the better the compression… • but frame to frame calculations are less accurate – leading to artefacts • PEL (picture elements) • Block of pixels, used as a basis for compression • Motion of PEL is predicted based on I, P or B frames

  13. The MPEG-1 standard • Five parts (separate ISO standards) • 1. Systems • 2. Video • 3. Audio (NB MPEG 1, level 3 – MP3) • 4. Conformance testing • 5. C implementation (encoding and decoding)

  14. MPEG Audio • MP3 is MPEG 1, audio layer 3 • Layer 1: 384, 4:1 compression • Layer 2: 192 – 256kbit/s, 6:1..8:1 compression • Layer 3: 112 – 128 kbit/s, 10:1..12:1 compression • Compression • Modified discrete cosine transfom (MDCT) • 32 band polyphase quadrature filter (PQF) • Alias reduction postprocessing • Severe design limitations of MP3 • Bitrate, time resolution, lack of scaleability – especially for high frequencies

  15. Applications of MPEG-1 • MPEG-1 compression is cheap and rapid • Consumer Devices • Multimedia / CD-ROM • Internet Video • Video CD (production is extremely cheap) • Internet audio (MP2) • Audio only devices (MP3 players)

  16. MPEG-2 (1994) • A completely different standard from MPEG 1. • Objective: • Deliver video and audio at comparable quality to broadcast video at roughly 4 times the typical MPEG-1 bit rate • Requires between 3 and 15 Mbps • Improvements over MPEG-1 • Higher resolution • Higher image quality • Supports interlaced video formats • Multiresolution scalability • Multiple sound track features • Scaleable

  17. The MPEG-2 Standard • ISO 13818 – 10 parts • 640 patents, held by 24 corporations and one university • Video Encoding (grossly simplified) • I, P and B frames – similar to MPEG-1 • Video is separated into: • 1 luminance and 2 chrominance channels, • 4 luminance macroblocks • variable number of chrominance macroblocks • Macroblocks of P and B frames are compared, and a motion vector calculated • Each block is treated with DCT, the result is run-length encoded and a huffman compression is applied

  18. Scaleable MPEG • MPEG-2 is scaleable (layered coding) • Single coded bitstream is scaleable to various bitrates • Layers can be incrementally decoded • Basic layer – low quality video • Subsequent layers – higher quality video • If the bandwidth is low, only the bitstream from the base layer is sent • If the bandwidth changes, more or fewer layers may be used • The same principle can be used on noisy connections

  19. Applications of MPEG-2 • DVD Video • HDTV • Ditgital satelite / cable TV • Digital video recorders • Video conferencing • Some games • Some multimedia (usually where very high quality is essential) • NB encoding is processor intensive – thus more expensive than MPEG-1

  20. Editing MPEG 1 & 2 • High performance compression formats • Not designed for editing • Only I-frames are stored in their entirety • This makes editing, rewinding or even bookmarking extremely difficult • MPEG editors tend to be crude • Serious editing involves decmopressing and recompressing • Quality loss • Moral – don’t edit MPEG if at all possible • Therefore don’t store digital video as MPEG!

  21. MPEG-4 (1998) • A different approach to video • Designed primarily for streaming video (e.g. on WWW), 3G phones, and other multimedia. • MPEG-4 absorbs most of MPEG-1 and MPEG-2 • Adds: • VRML (3D) support • Object-based coding • User interactivity • Hypermedia capability • Streams from different sources can be combined • Supports uploading and downloading • Rights management protocol

  22. Implementation of MPEG-4 • Designed to be modular • Standard in 20 parts (levels) • Each level contains profiles • Developers implement components of MPEG-4 • Probably no full implementation • Various partial implementations, such as: • DivX • Microsoft ASF • Quicktime 6

  23. Object-based coding • Media objects • Audio and visual objects • Natural objects (captured by camera) • Synthetic objects (created by computer) • Visual objects can be any shape (not just rectangles) • Video manipulation (insertion, deletion, rotation etc.) • Video indexing and retrieval • Texture maps • Sprites • 3-d models and animation • Probably the future of multimedia!

  24. MPEG-7 • Multimedia content description interface • Aimed at addressing issues of content retrieval • Intended primarily for digital libraries • Object based (as MPEG-4) • Supported data: • Still images • 3D models • Sampled audio • Speech • Video • Composition information (combinations of the above) • Data representation • Binary objects, XML or both

  25. MPEG-21 • Designed to manage digital media resources • Digital item declaration • Digital item identification and description • Content management and usage • Intellectual property management and protection • Terminals and networks • Content representation • Event reporting

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