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Video Encoding and Compression

Video Encoding and Compression

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Video Encoding and Compression

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  1. Video Encoding and Compression Incremental Steps:Getting to more High Definition August 25, 2009 Justin Cardones Justin@CardonesConsulting.com (401) 441-6801

  2. Agenda • MPEG-2 Background • Deployment Techniques • MPEG-4 & Transcoding • A Few Ways to Get More HD

  3. Compression Familiar Examples

  4. Audio/Video Encoding 1 Analog-to-Digital Conversion 3 Encoding 4 6 Video or Audio Input Compressed Video or Audio 8 10 8 7 10 14 14

  5. Audio/Video Encoding 1 Analog-to-Digital Conversion 3 Encoding 4 6 Video or Audio Input Compressed Video or Audio 8 10 8 7 10 14 14 1 Digital-to-Analog Conversion 3 Decoding 4 6 8 10 8 7 10 14 14

  6. Going from Analog to Digital 1 Analog-to-Digital Conversion 3 4 6 Video or Audio Input 8 10 8 7 10 14 14

  7. SDI Video • Serial digital interface (SDI) • 270 MHz clock • 8-bit or 10-bit • Video data plus timing codes (SAV, EAV)

  8. SDI Ancillary Data Used for: • Embedded audio (PCM or compressed) • Captioning • Aspect ratio signaling • And more… Video SDI Data

  9. Audio Compression • MPEG Layer II (“MUSICAM”) • AC-3 (Dolby) • Advanced Audio Codec (AAC) Sample rate Bit rate 1 3 4 6 8 10 8 7 10 14 14

  10. CATV Channels 108.000 MHz 6 MHz 114.000 MHz 120.000 MHz 126.000 MHz 132.000 MHz 138.000 MHz 144.000 MHz 150.000 MHz 156.000 MHz 162.000 MHz 170.000 MHz

  11. CATV Channels 108.000 MHz QAM 256  38.8 Mbps 114.000 MHz QAM 120.000 MHz QAM 126.000 MHz QAM 64  27 Mbps 132.000 MHz 138.000 MHz 144.000 MHz 150.000 MHz 156.000 MHz 162.000 MHz 170.000 MHz

  12. CATV Channels QAM  38.8 Mbps QAM QAM SDI Digital Video 270 Mbps QAM

  13. CATV Channels 38.8 Mbps MPEG Video Compression SDI Digital Video 270 Mbps

  14. Digital Video System MPEG-2 Encoders Subscribers Multiplexer Program sources QAM Modulator CATV network

  15. Transporting MPEG • Transport Stream (TS) • Allows multiplexing • 188-byte packets

  16. Transport Rate • Overall rate of MPEG stream • Usually constant bit rate • Includes video + audio + data • Video can be CBR or VBR • Null packets

  17. Multiplexer MPEG-2 Encoders • Combines multiple programs • Adjusts control tables (PAT, PMT) • Splices advertisements • Rate shaping Multiplexer Program sources QAM Modulator CATV network

  18. QAM Modulator MPEG-2 Encoders • Rate shaping (limited) • Encryption • Forward error correction • Transmit on CATV channel Multiplexer Program sources QAM Modulator

  19. Equipment Interconnect Ethernet • UDP/IP protocol • 7 transport packets per Ethernet frame • 100Base-T or Gigabit

  20. MPEG Video Compression • Remove redundancy • Take advantage of human visual system

  21. MPEG Video Compression Step 1 • Filtering • Analog to digital conversion • More filtering Clean, noise-free signal  Best results

  22. MPEG Video Compression Step 2 • Divide image into 16x16 macroblocks

  23. MPEG Macroblocks 16 x 16 pixels 720 Each macroblock = • Four 8x8 luma blocks • Two 8x8 chroma blocks • 4:2:0 480 http://mia.ece.uic.edu/~papers/WWW/MultimediaStandards/chapter7.pdf (adapted)

  24. Frames vs. Fields Interlaced scanning • 2 fields per frame • MPEG supports field or frame encoding

  25. Film Mode • Movies filmed at 24 frames/sec • Converted to 30 frames/sec for TV • 1 out of 5 fields is a duplicate • Encoder can skip duplicate (save bits) 1 2 3 4 1 2 3 4 4

  26. MPEG Picture Types Three ways to encode a picture: • I (Intra-coded) • P (Predicted) • B (Bidirectionally predicted) I B B P B B

  27. I-frames • “Intra-coded” • Similar to JPEG image • Relatively large (# of bits) I B B P B B

  28. P-frames • “Predicted” • Changes from previous reference frame • Relatively small P I B B B B

  29. B-frames • “Bidirectionally predicted” • Changes from previous or next reference frame • Smallest B I B P B B

  30. Motion Estimation • For each macroblock: • Find similar 16x16 block in reference frame • Subtract them  Residual • Send motion vector and residual http://mia.ece.uic.edu/~papers/WWW/MultimediaStandards/chapter7.pdf

  31. I B B P B B P P B B B B Group of Pictures Example: • GOP length = 15 • I/P spacing = 3 I B B P B B

  32. Breaking the Pattern Fixed GOP = Fixed I/B/P pattern • Or encoder may insert I-frames • Scene changes • Splice points I B B P B B

  33. Video Quality GOP Length Shorter GOPs: • Lower latency • Faster recovery (not always) Longer GOPs: • Better video quality (at low bit rates) Recommended range: 12 - 18

  34. MPEG Transport Stream • Usually constant bit rate • Elementary streams can be CBR or VBR • Single or multiple programs • Each has clock reference (PCR)

  35. MPEG Transport Packet 47 1F FF 10 4 byte header + 184 byte payload = 188 bytes (Plus 16 error-correction bytes = 204 bytes)

  36. MPEG Transport Header 47 1F FF 10 47 1F FF 0 Sync byte PID (13 bits) Continuity counter (4 bits)

  37. MPEG Transport Stream PAT PMT Null packet Video packet Audio packet

  38. MPEG Tables Program Specific Information (PSI) • PAT = Program Association Table • PMT = Program Map Table • CAT = Conditional Access Table • NIT = Network Information Table

  39. PSI Tables http://www.iec.org/online/tutorials/test_dv/topic01.html

  40. DVB and ATSC Tables • Service Information (SI) • Program and System Information Protocol (PSIP) • Terrestrial and satellite networks • Electronic program guide • Real-time clock • Ratings MPEG PSI DVB SI ATSC PSIP

  41. Variable Bit Rate • Target bit rate range (min, max) • Target video quality • Bit rate changes to achieve target • Varies with time • Varies across program mix

  42. Variable Bit Rate “Easy” content: • News channels • Cartoons “Difficult” content: • Sports action • Quick scene changes

  43. Open-loop VBR Encoding Rate-Shaping Modulation

  44. Rate Shaping Goal: Fit streams in fixed-bandwidth channel • 38.8 Mbps (256-QAM) 38.8 Mbps

  45. Rate Shaping • Partially decode MPEG stream • Change quantization • Reconstruct stream • Favor high-priority channels

  46. Closed-loop VBR Closed-loop controller Bit rate assignments Complexity estimates Modulation Encoding

  47. All-CBR Architecture Modulation Encoding

  48. Ad Splicing • Ads are usually CBR • May need rate shaping

  49. Ad Splicing SCTE 35 Encoder Mux/Splicer QAM Ad stream Network Cue tones SCTE 104 CATV Network SCTE 30 Ad Server

  50. MPEG-2 vs. MPEG-4 http://www.balooga.com/mpeg4.php3