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Digital Video

Digital Video. Digital Video. Until the arrival of the Pentium processor, in 1993, even the most powerful PCs were limited to capturing images no more than 160 x 120 pixels. Digital Video.

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Digital Video

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  1. Digital Video

  2. Digital Video • Until the arrival of the Pentium processor, in 1993, even the most powerful PCs were limited to capturing images no more than 160 x 120 pixels

  3. Digital Video • When processor speeds finally exceeded 200MHz, PCs could handle images up to 320 x 240 without the need for expensive compression hardware

  4. Digital Video Formats • There are numerous video formats: • M-JPEG • MPEG • AVI • QuickTime • DV

  5. M-JPEG • Motion-JPEG not surprisingly based on the JPEG still image format • Stores every frame as a compressed bitmap image • Typical compression ratios of between 2:1 and 12:1 • Can be applied in hardware or as a software codec

  6. MPEG • Moving Picture Experts Group standards for compressing motion video and audio signals using DCT (Discrete Cosine Transform) • The MPEG formats are asymmetrical - it takes longer to compress a frame of video than it does to decompress it

  7. MPEG Compression • MPEG uses both spatial DCT compression (as JPEG) and temporal compression • Temporal compression involves removing data that does not change between consecutive frames • MPEG video streams consist of a sequence of sets of frames known as a GOP (group of pictures)

  8. MPEG Compression • Each GOP (typically 8-24 frames), has only one complete frame represented in full, which is compressed using only spatial compression • The spatially compressed frame is just like a JPEG still and is known as an I frame (intra frame) • Around the I frame are temporally-compressed frames, representing only change data

  9. MPEG Compression • During encoding, prediction techniques compare neighbouring frames and pinpoint areas of movement and define vectors for how each will move from one frame to the next

  10. MPEG Compression • By recording only these vectors, the data which needs to be recorded can be substantially reduced

  11. MPEG Compression • There are two types of vector frames: • P frames (predictive), refer only to the previous frame • B frames (bi-directional) rely on previous and subsequent frames

  12. MPEG Compression

  13. MPEG Compression I B B P B B P B B I

  14. MPEG Compression • This combination of compression techniques makes MPEG highly scalable • Using longer GOPs with more B and P frames, reduces data rates

  15. MPEG-1 • MPEG-1 (White Book Standard) was introduced in 1993 • Supports video coding at bit-rates up to about 1.5 Mbit/s and virtually transparent stereo audio quality at 192 Kbit/s • Provide video resolution of 352x288 at 25 fps • This produces video quality slightly below the quality of conventional VCR videos

  16. MPEG-2 • Resolutions of 720x576 and 1280x720 at 50 fps, with full CD-quality audio • This is sufficient for all the major TV standards, including PAL, and even HDTV • MPEG-2 is used by DVD-ROMs, digital satellite and Cable • MPEG-2 can compress a 2 hour video into a few gigabytes • Decompressing an MPEG-2 data stream requires only modest computing power • Encoding video in MPEG-2 format requires significantly more processing power

  17. MPEG-4 • MPEG-4 is a standardised way to define, encode and playback time based media • It can be used in numerous applications: • Video • Delivering 2D still images • Controlling animated 3D models • Handling two-way video conferences • Streaming video, etc. • Standardized in October 1998

  18. MPEG-4 • Provides standardised ways of representing units of aural, visual or audio-visual content, as discrete “media objects” • These can be of natural or synthetic origin, for example, they could be recorded with a camera or microphone, or generated with a computer

  19. Video Codecs • These are the algorithms that handle the compression and decompression of the digital video • Every architecture has certain codecs built-in and some codecs are common to many architectures • E.g. QuickTime originally used Sorenson video codec, but now supports MPEG-2, Cinepak etc.

  20. Cinepak • Cinepak – developed by SuperMac Technology • Vector quantization based codec developed specifically to deliver 24-bit video in quarter screen (320 X 240 pixel) windows from files restricted to single-spin CD-ROM data rates • Cinepak is a highly asymmetric codec, with a compression process that takes 300 times longer than decompression • Better than Indeo for action sequences • Poorer than Indeo for "talking head" and other low-motion sequences

  21. Indeo • Indeo – developed by Intel • Indeo is a proprietary blend of colour sampling, vector quantization, and run-length encoding • One of Indeo’s valuable characteristics is its scalability. Indeo will deliver a movie clip at higher frame rates when more processing power is available • It is slightly faster than Cinepak at compression but is still slow

  22. Video File Formats • Like audio and image files, video files also have a series of diverse formats: • .mov • .qt • .avi • .mpg

  23. QuickTime • A video and animation system developed by Apple Computer • In QuickTime, a structure of time-based data is called a movie, hence the file extensions .mov or .qt • The QuickTime application can create, display, edit, copy, and compress video data in most of the same ways that text and still-image graphics are currently manipulated

  24. QuickTime • Its open architecture supports many file formats and codecs, including Cinepak, Indeo, Motion JPEG and MPEG, and is extensible to support future codecs, such as DVCAM • In February 1998, the ISO standards body gave QuickTime a boost by deciding to use it as the basis for the new MPEG-4 standard

  25. QuickTime • Besides processing video data, QuickTime can handle still images, animated images (sprites), vector graphics, multiple sound channels, MIDI music, 3D objects, virtual reality panoramas and objects, and even text

  26. AVI (Video for Windows) • Audio Video Interleave • A special case of the RIFF (Resource Interchange File Format), defined by Microsoft • Video for Windows supports several data compression techniques, including RLE, Indeo, and Cinepak

  27. DV • In the late 1990s a new generation of entirely digital cameras and camcorders emerged, and with them a new video format, Digital Video (DV) • It uses a spatial compression technique (each frame being compressed on an individual basis rather than being compared to adjacent frames) based on the DCT

  28. DV Compression • DV can compress different parts of each frame to different ratios • So, the blue sky in an image backdrop can be brought down to, say, 25:1, while the complex forest in the foreground, which needs more detail, is reduced to only 7:1 • In this way DV can optimise its video stream frame by frame

  29. Fin Fin

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