Multimedia Outline Compression RTP Scheduling Spring2002 CS461 - - PDF document

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Multimedia

Outline

Compression RTP Scheduling

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CompressionOverview

• EncodingandCompression

– Huffmancodes

• Lossless

– datareceived=datasent – usedforexecutables,textfiles,numericdata

• Lossy

– datareceiveddoesnot!=datasent – usedforimages,video,audio

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Lossless Algorithms

• RunLengthEncoding(RLE)

– example:AAABBCDDDDencodingas3A2B1C4D – goodforscannedtext(8-to-1compressionratio) – canincreasesizefordatawithvariation(e.g.,someimages)

• DifferentialPulseCodeModulation(DPCM)

– exampleAAABBCDDDDencodingasA0001123333 – changereferencesymbolifdeltabecomestoolarge – worksbetterthanRLEformanydigitalimages(1.5-to-1)

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Dictionary-BasedMethods

• Builddictionaryofcommonterms

– variablelengthstrings

• Transmitindexintodictionaryforeachterm
• Lempel-Ziv(LZ)isthebest-knownexample
• Commonlyachieve2-to-1rationontext
• VariationofLZusedtocompressGIFimages

– firstreduce24-bitcolorto8-bitcolor – treatcommonsequenceofpixelsastermsindictionary – notuncommontoachieve10-to-1compression(x3)

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ImageCompression

• JPEG:JointPhotographicExpertGroup(ISO/ITU)
• Lossystill-imagecompression
• Threephaseprocess

– processin8x8blockchunks(macro-block) – grayscale:eachpixelisthreevalues(YUV) – DCT:transformssignalfromspatialdomainintoand equivalentsignalinthefrequencydomain(loss-less) – applyaquantization totheresults(lossy) – RLE-likeencoding(loss-less)

Source image JPEGcompression DCT Quantization Encoding Compressed image Spring2002 CS461 6

QuantizationandEncoding

• Quantization Table

357911131517 5791113151719 79111315171921 911131517192123 1113151719212325 1315171921232527 1517192123252729 1719212325272931

• EncodingPattern

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MPEG

• MotionPictureExpertGroup
• Lossycompressionofvideo
• Firstapproximation:JPEGoneachframe
• Alsoremoveinter-frameredundancy

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MPEG(cont)

• Frametypes

– Iframes:intrapicture – Pframes:predictedpicture – Bframes:bidirectionalpredictedpicture

• ExamplesequencetransmittedasIPBBIBB

Frame1 Frame2 Frame3 Frame4 Frame5 Frame6 Frame7 Iframe Bframe Bframe Pframe Bframe Bframe Iframe MPEG compression Forward prediction Bidirectional prediction Compressed stream Input stream

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MPEG(cont)

• BandPframes

– coordinateforthemacroblockintheframe – motionvectorrelativetopreviousreferenceframe(B,P) – motionvectorrelativetosubsequentreferenceframe(B) – deltaforeachpixelinthemacroblock

• Effectiveness

– typically90-to-1 – ashighas150-to-1 – 30-to-1forIframes – PandBframesgetanother3to5x

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MP3

• CDQuality

– 44.1kHzsamplingrate – 2x 44.1x 1000x 16=1.41Mbps – 49/16x 1.41Mbps=4.32Mbps

• Strategy

– splitintosomenumberoffrequencybands – divideeachsubbandintoasequenceofblocks – encodeeachblockusingDCT+Quantization +Huffman – trick:howmanybitsassignedtoeachsubband

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RTP

• Application-LevelFraming
• DataPackets

– sequencenumber – timestamp(appdefines“tick”)

• ControlPackets(sendperiodically)

– lossrate(fractionofpacketsreceivedsincelastreport) – measuredjitter

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TransmittingMPEG

• Adapttheencoding

– resolution – framerate – quantizationtable – GOPmix

• Packetization
• Dealingwithloss
• GOP-inducedlatency

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LayeredVideo

• Layeredencodeing

– e.g.,waveletencoded

• ReceiverLayeredMulticast(RLM)

– transmiteachlayertoadifferentgroupaddress – receiverssubscribetothegroupstheycan“afford” – Probetolearnifyoucanaffordnexthighergroup/layer

• SmartPacketDropper(multicastorunicast)

– selectlayerstosend/dropbasedonobservedcongestion – observedirectlyoruseRTPfeedback

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Real-TimeScheduling

• Priority
• EarliestDeadlineFirst(EDF)
• RateMonotonic(RM)
• ProportionalShare

– withfeedback – withadjustmentsfordeadlines