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HDTV Video and AC-3 Payload Formats

HDTV Video and AC-3 Payload Formats. Ladan Gharai (ladan@isi.edu) Allison Mankin (mankin@isi.edu) USC/ISI. Motivation. HDTV formats are starting to be used in studio, distributions, broadcasts, and in high bandwidth nets US commercial broadcasts of 1920x1080 as of 5/99

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HDTV Video and AC-3 Payload Formats

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  1. HDTV Video and AC-3 Payload Formats Ladan Gharai (ladan@isi.edu) Allison Mankin (mankin@isi.edu) USC/ISI

  2. Motivation • HDTV formats are starting to be used in studio, distributions, broadcasts, and in high bandwidth nets • US commercial broadcasts of 1920x1080 as of 5/99 • Use of uncompressed - tradeoffs of decoding versus bandwidth; some very high resolution applications • Bandwidths from 176 Mbps (SDTV 640x480 24P) to 1.49 Gbps (1920x1080 60I/30P) • Target bandwidths of digital broadcast - 19.4 Mbps digital cable - 39.1Mbps

  3. HDTV over RTP • US HDTV standardized by ATSC (Advanced Television Standards Committee) • www.atsc.org • Europe’s HDTV standard is DVB • www.dvb.org • Major differences • Terrestrial receiver technology (COFDM vs 8VSB) • DVB uses MPEG audio, ATSC uses AC-3 Dolby • Others are using one of these, if using digital formats

  4. RTP Payload for HDTV Video Streams draft-avt-hdtv-video-00.txt drafted, not yet submitted • Compressed HDTV video mostly covered by RFC 2250 - MPEG-2 transport and MP@HL. DVB audio? • Payload type issue for compressed • Some expectation that some HDTV decoders will break if handed an arbitrary MPEG-2 stream • Uncompressed HDTV • Cover color sub-sampling of both 4:2:0 and 4:2:2 • Related to BT.656 (ITU-R) (RFC 2431), but BT.656 lacks 4.2.0 and only covers 525/625 lines (HDTV up to 1080 lines)

  5. RTP Payload for AC-3 Audio Streams draft-avt-ac3-audio-00.txt drafted, not yet submitted. • Place an integral number of sync frames per RTP packet • Fragmentation needs to be handled - frame sizes at 48Khz range from 128-2560 bytes • Sync frames must be reassembled prior to decoding, as decoder takes all or nothing • No dependency on other frames • AAC-like audio repair seems applicable

  6. P: progressive scan I : interlaced scan

  7. left right center left surround right surround low frequency effects 384kbps AC-3 encoder Dolby Digital AC-3 • A digital compression algorithm from Dolby labs • Input: 1 to 6 PCM audio channels • Output: a serial bit stream with data rates of 32kbps to 640kbps • Low bandwidth is achieved by encoding multiple channels as a single entity • example: • 6ch * 48kHz *18bits = 5.184Mbps

  8. 256 samples SI BSI SI BSI AB0 AB1 AB2 AB3 AB4 AB5 Aux crc sync frame AC-3 Audio streams • An AC-3 audio stream is a sequence of same size “sync frames” • Each sync frame is an independent data unit, I.e. it does not need require any other data to be decoded • an AC-3 decoder must be presented with the entire sync frame AB: Audio Block SI: synchronization information header BSI: bit stream information header CRC: cyclic error check

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