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Canon XL1 3CCD Camcorder User Manual

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HOWCOMPRESSIONWORKS

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‐Fundamentally,compressionworksbysummarizingasequenceofimagesandsoundsasefficiently

aspossible.Videothathasbeencompresseddoesn’tprovideanidenticalrepresentationofthesource

filebacktotheuser—thedatarateswouldbefartoohigh.

‐Instead,theencodedfilesdescribetheimportantdetailsofthecontentasterselyaspossiblewhile

stillprovidingareproductionthat,tothehumansensorysystem,providestheexperienceofthe

originalasaccuratelyaspossible.

‐Amatchedpairofacompressorandadecompressor,betterknownasacodec,performsthe

compression.

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Acompressorispartoftheencodingprocess,reducingtheamountofdatarequiredtostorethe

video.

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Adecompressorworksonthereceivingend,decodingthecompresseddatasothatitcanbe

presentedtotheviewer.

‐It’simportantthatthepairbematchedbecausethedecoder(decompressor)needstounderstand

theencoder’s(compressor’s)summaryofthedata.

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TYPESOFCOMPRESSION

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Videocodecscanusespatialcompression,temporalcompression,oracombinationofboth.

SpatialCompression

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Intraframe

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Affectsonlyasingleframeatatime

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Makesrandomaccesstoanypointinthevideoandeditingeasy

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Findsredundancywithinaframe(eg.similarcoloursorareasthatlooklikeotherareas)and

encodesthesesimilarareasbyreference,ratherthanbyaliteraldescriptionforeachandeverypixel

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BasedonDiscreteCosineTransformation(DCT)techniquewhicharegoodwithsmoothgradations,

butstrugglewithrandomdetailsorsharpedgesatlowerbitrates

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cancausevisibledegredationof

theimage=artifacts,‐‐suchasringing(haloorsmudge)andblocking(smoothedgesbecomeblocky)

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