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Multimedia Systems

Multimedia Systems. CS-502 Operating Systems. Outline. Requirements and challenges for audio and video in computer systems Systems for multimedia Compression and bandwidth Processor scheduling File, disk, and network management Tanenbaum, Chapter 7 Silbershatz, Chapter 20.

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Multimedia Systems

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  1. Multimedia Systems CS-502 Operating Systems CS502 Spring 2006

  2. Outline • Requirements and challenges for audio and video in computer systems • Systems for multimedia • Compression and bandwidth • Processor scheduling • File, disk, and network management Tanenbaum, Chapter 7Silbershatz, Chapter 20 CS502 Spring 2006

  3. What do we mean by “multimedia” • Audio and video within a computer system • CD’s & DVD’s • Computer hard drive • Live broadcast & web casts • Webcams, Skype, … • Video on demand • Pause, fast forward, reverse, etc. • Interactive meetings • Presentations with 2-way audio • Teleconferencing • Interactive gaming • … CS502 Spring 2006

  4. Requirements • “Smooth” audio and video • Deterioration in quality >> jerky playback • Note: human is more sensitive to jitter in audio than to jitter in video! • Audio/video on PC’s doing something else • Multiple concurrent streams • Video & multimedia servers • TiVo, etc. • Wide range of network bandwidths CS502 Spring 2006

  5. System and OS Challenges • Bandwidths and Compression • Jitter • Processor Scheduling • Disk Scheduling • Network Streaming CS502 Spring 2006

  6. Some System Architectures • Simple: • Data paths for audio/video that are separate from computational data paths • Modern • Fast system bus, CPU, devices • Video server • Disk farm and multiple streams CS502 Spring 2006

  7. Memory CD-ROM drive Device Sound card System Organization (simple) memory bus CPU audio stream • Separate data path for audio stream • Main system bus and CPU were too busy/slow to handle real-time audio CS502 Spring 2006

  8. Ether-net SCSI USB Modem Soundcard Printer Mouse Key-board System Organization (typical Pentium) Main Memory video stream viaISA & bridge tographics card AGP Port Level 2 cache CPU Bridge Graphics card Moni-tor ISA bridge PCI bus IDE disk audio stream viaISA bridge to sound card ISA bus CS502 Spring 2006

  9. Video Server • Multiple CPUs • Disk farm • 1000s of disks • Multiple high-bandwidth network links • Cable TV • Video on demand • Internet CS502 Spring 2006

  10. Why Compression? – CD-quality audio • 22,050 Hz  44,100 samples/sec • 16 bits per sample • Two channels  176,000 bytes/sec  1.4 mbits/sec • Okay for a modern PC • Not okay for 56 kb/sec modem (speed) or iPod (space)! • MP-3  0.14 mbits/sec (10:1) • Same audio quality! • Compression ratio varies with type of music CS502 Spring 2006

  11. Why Compression? – Video • “Standard” TV frame = 640  480 pixels @ 25-30 frames/sec (fps)  • 9,216,000 pixels/sec = 27,648,000 bytes/sec • HDTV = 1280  720 pixels @ 30 fps • 82,944,000 bytes/sec • Typical movie  133 minutes • approx. 220 gigabytes! • DVD holds ~ 4.7 gigabytes • average of ~ 620 kilobytes/sec! • “Standard” movie of 133 minutes requires serious compression just to fit onto DVD CS502 Spring 2006

  12. Video Compression Requirements • Compression ratio > 50:1 • i.e., 220 gigabytes:4.7 gigabytes • Visually indistinguishable from original • Even when paused • Fast, cheap decoder • Slow encoder is okay • VCR controls • Pause, fast forward, reverse CS502 Spring 2006

  13. Video Compression Standards • MPEG (Motion Picture Experts Group) • Based on JPEG (Joint Photographic Experts Group) • Multi-layer • Layer 1 = system and timing information • Layer 2 = video stream • Layer 3 = audio and text streams • Three standards • MPEG-1 – 352240 frames; < 1.5 mb/sec ( < VHS quality) • Layer 3 = MP3 Audio standard • MPEG-2 – standard TV & HDTV; 1.5-15 mb/sec • DVD encoding • MPEG-4 – combined audio, video, graphics • 2D & 3D animations CS502 Spring 2006

  14. JPEG compression (single frame) • Convert RGB into YIQ • Y = luminance (i.e., brightness) ~ black-white TV • I, Q = chrominance (similar to saturation and hue) Reason: Human eye is more sensitive to luminance than to color (rods vs. cones) • Down-sample I, Q channels • i.e., average over 22 pixels to reduce resolution • lossy compression, but barely noticeable to eye • Partition each channel into 88 blocks • 4800 Y blocks, 1200 each I & Q blocks CS502 Spring 2006

  15. JPEG (continued) CS502 Spring 2006

  16. JPEG (continued) CS502 Spring 2006

  17. JPEG (continued) • Calculate Discrete Cosine Transform (DCT) of each 88 block • What is a Discrete Cosine Transform? • Divide 88 block of DCT values by 88quantization table • Effectively throwing away higher frequencies • Linearize 88 block, run-length encode, and apply a Huffman code to reduce to a small fraction of original size (in bytes) CS502 Spring 2006

  18. JPEG (concluded) • Store or transmit 88 quantization table followed by list of compressed blocks • Achieves 20:1 compression with good visual characteristics • Higher compression ratios possible with visible degradation • JPEG algorithm executed backwards to recover image • Visually indistinguishable from original @ 20:1 • JPEG algorithm is symmetric • Same speed forwards and backwards CS502 Spring 2006

  19. MPEG • JPEG-like encoding of each frame • Takes advantage of temporal locality • I.e., each frame usually shares similarities with previous frame • encode and transmit only differences • Sometimes an object moves relative to background • find object in previous frame, calculate difference, apply motion vector CS502 Spring 2006

  20. Temporal Locality (example) Consecutive Video Frames CS502 Spring 2006

  21. B B B I B B B B B B B B B B B B P P P P MPEG organization • Three types of frames • I-frame:Intracoded or Independent. • Full JPEG-encoded frame • Occurs at intervals of a second or so • Also at start of every scene • P-frame:Predictive frame • Difference from previous frame • B-frame:Bidirectional frame • Like p-frame but difference from both previous and next frame I B B B P CS502 Spring 2006

  22. MPEG Characteristics • Non-uniform data rate! • Compression ratios of 50:1 – 80:1 are readily obtainable • Asymmetric algorithm • Fast decode (like JPEG) • Encode requires image search and analysis to get high quality differences • Decoding chips on graphics cards available CS502 Spring 2006

  23. MPEG Problem – Fast Forward/Reverse • Cannot simply skip frames • Next desired frame might be B or P derived from a skipped frame • Options: • Separate fast forward and fast reverse files • MPEG encoding of every nth frame • See Tanenbaum §7.5.1 – video-on-demand server • Display only I and P frames • If B frame is needed, derive from nearest I or P CS502 Spring 2006

  24. “Movie” File Organization • One MPEG-2 video stream • Multiple audio streams • Multiple languages • Multiple text streams • Subtitles in multiple languages • All interleaved CS502 Spring 2006

  25. Challenge • How to get the contents of a movie file from disk or DVD drive to video screen and speakers. • Fixed frame rate (25 or 30 fps) • Steady audio rate • Bounded jitter • Classical problem in real-time scheduling • Obscure niche become mainstream! • See Silbershatz, Chapter 19 CS502 Spring 2006

  26. Processor Scheduling for Real-TimeRate Monotonic Scheduling (RMS) • Assume m periodic processes • Process i requires Ci msec of processing time everyPi msec. • Equal processing every interval — like clockwork! • Assume • Let priority of process i be • Let priority of non-real-time processes be 0 CS502 Spring 2006

  27. Rate Monotonic Scheduling (continued) • Then using these priorities in scheduler guarantees the needed Quality of Service (QoS), provided that • Asymtotically approaches ln 2 as m   • I.e., must maintain some slack in scheduling • Assumes fixed amount of processing per periodic task • Not MPEG! CS502 Spring 2006

  28. Processor Scheduling for Real-TimeEarliest Deadline First (EDF) Scheduling • When each process i become ready, it announces deadline Difor its next task. • Scheduler always assigns processor to process with earliest deadline. • May pre-empt other real-time processes CS502 Spring 2006

  29. Earliest Deadline First Scheduling (continued) • No assumption of periodicity • No assumption of uniform processing times • Theorem: If any scheduling policy can satisfy QoS requirement for a sequence of real time tasks, then EDF can also satisfy it. • Proof: If i scheduled before i+1, but Di+1 < Di, then i and i+1 can be interchanged without affecting QoS guarantee to either one. CS502 Spring 2006

  30. Earliest Deadline First Scheduling (continued) • EDF is more complex scheduling algorithm • Priorities are dynamically calculated • Processes must know deadlines for tasks • EDF can make higher use of processor than RMS • Up to 100% • However, it is usually a good idea to build in some slack CS502 Spring 2006

  31. Multimedia File & Disk Management • Single movie or multimedia file on PC disk • Interleave audio, video, etc. • So temporally equivalent blocks are near each other • Attempt contiguous allocation • Avoid seeks within a frame TextFrame AudioFrame CS502 Spring 2006

  32. File organization – Frame vs. Block • Frame organization • Small disk blocks (4-16 Kbytes) • Frame index entries point to starting block for each frame • Frames vary in size (MPEG) • Advantage: very little storage fragmentation • Disadvantage: large frame table in RAM • Block organization • Large disk block (256 Kbytes) • Block index entries point to first I-frame of a sequence • Multiple frames per block • Advantage: much smaller block table in RAM • Disadvantage: large storage fragmentation on disk CS502 Spring 2006

  33. Frame vs. Block organization larger smaller CS502 Spring 2006

  34. File Placement on Server • Random • Striped • “Organ pipe” allocation • Most popular video in center of disk • Next most popular on either side of it • Etc. • Least popular at edges of disk • Minimizes seek distance CS502 Spring 2006

  35. Disk Scheduling (server) • Scheduling disk activity is just as important as scheduling processor activity • Advantage:– Predictability • Unlike disk activity of ordinary computing • In server, there will be multiple disk requests in each frame interval • One request per frame for each concurrent video stream CS502 Spring 2006

  36. Disk Scheduling (continued) • SCAN (Elevator) algorithm for each frame interval • Sort by cylinder # • Complete in time for start of next frame interval • Variation – SCAN–EDF: • Sort requests by deadline • Group similar deadlines together, apply SCAN to group • Particularly useful for non-uniform block sizes and frame intervals CS502 Spring 2006

  37. Network Streaming • Traditional HTTP • Stateless • Server responds to each request independently • Real-Time Streaming Protocol (RTSP) • Client initiates a “push” request for stream • Server provides media stream at frame rate CS502 Spring 2006

  38. Push vs. Pull server CS502 Spring 2006

  39. Bandwidth Negotiation • Client (or application) provides feedback to server to adjust bandwidth • E.g., • Windows Media Player • RealPlayer • Quicktime CS502 Spring 2006

  40. Conclusion • Multimedia computing is challenging • Possible with modern computers • Compression is essential, especially for video • Real-time computing techniques move into mainstream • Processor and disk scheduling • There is much more to this subject than fits into one class CS502 Spring 2006

  41. Break CS502 Spring 2006

  42. Digression on Transforms • Fourier’s theorem:– • Every continuous periodic function can be reduced to the sum of a series of sine waves • The Fouriertransform is a representation of that function in terms of the frequencies of those sine waves • Original function can be recovered from its Fourier transform • Fourier transforms occur frequently in nature! CS502 Spring 2006

  43. Nyquist’s Theorem (1924) • If a continuous function is sampled at a frequency 2f, then the function can be recovered from those samples provided that its maximum Fourier frequency is  f. CS502 Spring 2006

  44. Discrete Cosine Transform • A form of the Fourier Transform • When applied to an 88 block of samples (i.e. pixel values) yields an 88 block of spatial frequencies • Original 88 block of samples can be recovered from its DCT. • Assuming infinite arithmetic precision CS502 Spring 2006

  45. More on Nyquist • If arithmetic precision is not infinite, we get quantization error during sampling • Recovered signal has quantization noise • i.e., a lossy transform CS502 Spring 2006

  46. Return to JPEG CS502 Spring 2006

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