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August 25, 2009 Video Encoding and Compression Incremental Steps: Getting to more High Definition Justin Cardones [email protected] (401) 441-6801 Agenda MPEG-2 Background Deployment Techniques MPEG-4 & Transcoding A Few Ways to Get More HD Compression Familiar Examples Audio CD iPod Digital camera Video DVD Compressed? No Yes Yes Yes Compression method -- MP3, AAC JPEG MPEG-2 Audio/Video Encoding Video or Audio Input Analog-to-Digital Conversion 1 3 4 6 8 10 8 7 10 14 14 Encoding Compressed Video or Audio Audio/Video Encoding Video or Audio Input Analog-to-Digital Conversion Decoding 1 3 4 6 8 10 8 7 10 14 14 1 3 4 6 8 10 8 7 10 14 14 Encoding Digital-to-Analog Conversion Compressed Video or Audio Going from Analog to Digital Video or Audio Input Analog-to-Digital Conversion 1 3 4 6 8 10 8 7 10 14 14 SDI Video Serial digital interface (SDI) 270 MHz clock 8-bit or 10-bit Video data plus timing codes (SAV, EAV) SDI Ancillary Data Used for: Embedded audio (PCM or compressed) Captioning Aspect ratio signaling And more… Video SDI Data Audio Compression MPEG Layer II (“MUSICAM”) AC-3 (Dolby) Advanced Audio Codec (AAC) Sample rate 1 3 4 6 8 10 8 7 10 14 14 Bit rate CATV Channels 108.000 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 6 MHz CATV Channels 108.000 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 QAM 38.8 Mbps 256 QAM QAM QAM 64 27 Mbps CATV Channels QAM 38.8 Mbps QAM SDI Digital Video 270 Mbps QAM QAM CATV Channels 38.8 Mbps SDI Digital Video 270 Mbps MPEG Video Compression Digital Video System MPEG-2 Encoders Subscribers Multiplexer Program sources QAM Modulator CATV network Transporting MPEG Transport Stream (TS) Allows multiplexing 188-byte packets Transport Rate Overall rate of MPEG stream Usually constant bit rate Includes video + audio + data Video can be CBR or VBR Null packets Multiplexer MPEG-2 Encoders • • • • Combines multiple programs Adjusts control tables (PAT, PMT) Splices advertisements Rate shaping Multiplexer Program sources QAM Modulator CATV network QAM Modulator MPEG-2 Encoders Multiplexer Program sources QAM Modulator • • • • Rate shaping (limited) Encryption Forward error correction Transmit on CATV channel Equipment Interconnect Ethernet UDP/IP protocol 7 transport packets per Ethernet frame 100Base-T or Gigabit MPEG Video Compression Remove redundancy Take advantage of human visual system MPEG Video Compression Step 1 Filtering Analog to digital conversion More filtering Clean, noise-free signal Best results MPEG Video Compression Step 2 Divide image into 16x16 macroblocks MPEG Macroblocks 16 x 16 pixels 720 480 Each macroblock = • Four 8x8 luma blocks • Two 8x8 chroma blocks • 4:2:0 http://mia.ece.uic.edu/~papers/WWW/MultimediaStandards/chapter7.pdf (adapted) Frames vs. Fields Interlaced scanning 2 fields per frame MPEG supports field or frame encoding 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 MPEG Picture Types Three ways to encode a picture: I (Intra-coded) P (Predicted) B (Bidirectionally predicted) I B B P B B I-frames “Intra-coded” Similar to JPEG image Relatively large (# of bits) I B B P B B P-frames “Predicted” Changes from previous reference frame Relatively small I B B P B B B-frames “Bidirectionally predicted” Changes from previous or next reference frame Smallest I B B P B B 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 Group of Pictures Example: GOP length = 15 I/P spacing = 3 I BB P B B P B B P B B I BB P B B 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 GOP Length Shorter GOPs: Lower latency Faster recovery (not always) Longer GOPs: Better video quality (at low bit rates) Recommended range: 12 - 18 Video Quality MPEG Transport Stream Usually constant bit rate Elementary streams can be CBR or VBR Single or multiple programs Each has clock reference (PCR) MPEG Transport Packet 47 1F FF 10 4 byte header + 184 byte payload = 188 bytes (Plus 16 error-correction bytes = 204 bytes) MPEG Transport Header 47 47 Sync byte 1F 1F FF 10 FF PID (13 bits) 0 Continuity counter (4 bits) MPEG Transport Stream PAT PMT Null packet Video packet Audio packet MPEG Tables Program Specific Information (PSI) PAT = Program Association Table PMT = Program Map Table CAT = Conditional Access Table NIT = Network Information Table PSI Tables http://www.iec.org/online/tutorials/test_dv/topic01.html 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 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 Variable Bit Rate “Easy” content: News channels Cartoons “Difficult” content: Sports action Quick scene changes Open-loop VBR Encoding Rate-Shaping Modulation Rate Shaping Goal: Fit streams in fixed-bandwidth channel 38.8 Mbps (256-QAM) 38.8 Mbps Rate Shaping • • • • Partially decode MPEG stream Change quantization Reconstruct stream Favor high-priority channels Closed-loop VBR Closed-loop controller Bit rate assignments Complexity estimates Modulation Encoding All-CBR Architecture Modulation Encoding Ad Splicing Ads are usually CBR May need rate shaping Ad Splicing SCTE 35 Encoder Network Mux/Splicer QAM Ad stream Cue tones SCTE 104 SCTE 30 Ad Server CATV Network MPEG-2 vs. MPEG-4 http://www.balooga.com/mpeg4.php3 Moving to MPEG-4 Lower bit rates Same transport stream Upgrade encoders Upgrade set tops August 25, 2009 Video Encoding and Compression Presented By:Justin Cardones [email protected] (401) 441-6801 Derived from Original Content by Marty Sauser, EGT, inc.
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