IPfocus - IP UserGroup

IPfocusCompression explainedCorrect Compression Selection is Important for All IPSurveillance Applications, says Axis CommunicationsDominic Bruning, Managing Director of AxisCommunications, talks to IPfocus ® about theimportance of selecting the correctcompression for IP surveillance applications.Why Compression?The first question that anybody requesting anIP surveillance system may ask is “Why do Ineed to compress the video?” They may havelots of bandwidth. My network can cope. Theanswer is without compression most systemssimply can’t cope with the amount of databeing collected and stored. The CCIR-601Standard for Digital Television illustrates theproblem very well. It states that one secondof uncompressed TV pictures would create165 Mega Bites (MB) of data. That means oneminute digital TV pictures would create9.9 Giga Bites (GB) of data. One hour equals594 GB and 24 hour recording would create a14.3 Tera Bit requirement – nearly a quarter ofan 80 GB Hard Disk Drive. A standard 28.8KModem would take 16 years to down load oneday of digital TV output. It is clear from thisexample that pictures whether from digital TVoutlets or IP surveillance cameras are dataintensive and even companies with the verylargest servers at their disposal will want todraw on compression techniques to reducedata storage capacity requirements and cutdata transfer times to manageable levels.As more and more surveillance systems arenetworked the issue of managing thetransmission of digital images from thesesystems has become critical to widespreadadoption of IP surveillance. After all themovement of real-time video at up to 50frames per second across the network withpotential bandwidth consumption 165 MegaBits Per Second (Mbps) is enough to scaremany network managers away.Fortunately, there is a solution to this in theform of selection of an appropriatecompression algorithm. This article aims toassist with that selection by looking at somespecific compression algorithms and theirrelative merits. There are five main imagecompression algorithms which are all in useacross the surveillance industry today. Someare more widely used than others. These arefor moving pictures/video MPEG, H.261 &H.263 and for still pictures JPEG, JPEG 2000and Wavelet.Differential vs. Non Differential:There are two fundamental differences inimage compression techniques - Differentialand Non-Differential. With the non-differentialmethod no information is discarded and allinformation is transferred from frame to frame.Non Differential:Whereas, the Differential method wasdeveloped specifically for moving video.Typically only changes within the image aresent and static information is discarded.This means that there is a lower bandwidthusage compared to the non-differentialmethod. However the quality of video is stilldependent on the available bandwidth.Differential method:Key compression algorithmsJPEG stands for Joint Photographic ExpertsGroup –the committee responsible fordeveloping this standard and its successor theJPEG 2000 standard. JPEG is the single mostwidespread compression format in use today.It was designed, as the name implies, tohandle the compression of single still images.So it treats video output as captured stillimages. It offers the option of very highcompression ratio but low picture quality orslightly lower compression ratios with goodpicture quality. If ‘artefacts’ appear in yourimages, sometimes called ‘blockiness’, thenthis is a fairly good indication that you arerunning your compression ratio too high as inthe example given below (Figure 2):Fig 1: original imageFig 2: a JPEG overcompressedimageHigh compression ratios also mean image filesare made smaller, effectively by removing anamount and type of data stored, so the qualityof the image viewed is always affected insome way. On the other hand JPEG achievescompression through a number of complextechniques including ‘quantisation’ or removalof redundant information which exists insideevery digital image but does not add anythingnoticeable to the picture quality. Clever stuff.JPEG 2000 updates this standard toincorporate wavelets which essentiallyeliminate the ‘blockiness’ at highercompression levels and instead replace thiswith an overall fuzziness which is lessdisturbing to the eye (see Figure 3):34_issue 3