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DVR vs NVR DigitalVideo Recording forEnterprise SystemsAlex Swanson, IndigoVisions Head of

Engineering, highlights the differences betweenDigital Video Recorders (DVRs) and NetworkVideo Recorders (NVRs), and the implicationsfor systems designers and end users.

First came the camera and monitor, closely followed bythe Video Cassette Recorder (VCR), recording one videostream to a 3-hour tape at 25/30 frames per second andoften triggered by an input device (raid button in a bank,for example). Technology then brought us the multiplexer,which allowed several streams of video to be recorded onto the same tape and separated out into discrete, viewablestreams on replay and the time-lapse VCR which enabled the dropping of frames and in so doing permitted a 3-hour tape tobe used over much longer periods, albeit at the cost of lost information.

The DVRThe rapid development of video compression algorithms (MJPEG, MPEG-4, H.264 etc.), computer processing speeds and arapid reduction in data storage costs then gave rise to the DVR. This you could consider as being the functionality of amultiplexer together with a computer disk for storage in place of tape, all housed in the same box together with someadditional ports for connectivity.

The DVR provides a convenient, if limited, replacement for the multiplexer + VCR combination and provides non-linearaccess to recorded material usually selected by camera ID, time and date. The consistency of quality of recorded materialwill in general be higher than that obtained with analogue tape although the actual quality achieved may or may not bebetter, depending on the compression algorithm and individual configuration.

In general more programmable options for individual video stream recording parameters, (picture resolution, number offrames per second, trigger options, start/stop times etc), are available, but a DVR is only useful where the analogue camerasare all cabled back to the DVRs location. Competent DVRs now feature CAT 5 network ports so that the device can beprovided with an IP address and thereby become accessible over an Ethernet network.

Many limitations still apply, however, not the least of which being that if it fails youll have most probably lost all yourrecordings (or they may not even have been made in the first place). This is not true of NVRs which can be used in mirrormode see below. On the subject of reliability if you are going to use a DVR make sure that the one youre proposing

incorporates an industrial grade hard disk drive (HDD) and not a domestic one, or failure might be a lot sooner than youthink (most DVR failures arise from overworked and overheated hard drives) - ask the manufacturer which drives he uses.

Like most other things in this world the actual performance obtained from a DVR, its ease of use and reliability will dependupon the manufacturer, individual model selected and price paid.

NVRs now provide significant advantages over the traditional DVR solution.

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SIDEBAR

NVR in a box!

The typical NVR solution simply requiresa PC platform and hard disk storage.However, for more demanding faulttolerant applications NVRs can bepackaged in stand-alone units. Forexample IndigoVisions range of NVR-AS 3000s is a range of self-containedrack mounted, Linux based, with theoption of removable hard disk drives. Inaddition multiple disk NVRs have theoption of built in disk redundancy witheither RAID1 or RAID5 configurations.The units also have optional power

redundancy and built-in redundantnetwork connections. Recordings caneasily be mirrored to multiple NVRsaround the network.

And so to the NVRThe Network Video Recorder heralds the arrival of the next natural point in the development of recording technology.

It is important to differentiate between DVRs and NVRs, as both are often termed digital. A DVR digitally compressesanalogue video feeds and stores them on a hard-drive, the term digital referring to the compression and storagetechnology, not the transmitted video images. The DVR therefore has to be located near the analogue feeds. In contrast anNVR stores digital images directly from the IP-network.

Therefore the most obvious difference between the DVR and NVR is that whereas the DVR records from analogue streamsprovided from analogue cameras the NVR records video streams that have already been encoded at the cameras. Thus youfind no video connectors anywhere on a NVR; its input and output is IP data comprising compressed and encoded video.This will typically be in either MPEG-4 or H.264 formats which have enjoyed widespread adoption in the CCTV industry asthe compression technology of choice, due largely to their efficiency.

The huge advantage of architecture based on NVRs is that they can belocated anywhere on a network at the monitoring centre, adjacent tocamera clusters, on the edge of a network, collected together in ahardened environment, indeed anywhere at all. In use their location istransparent to an operator he or she simply calls up the recorded videostream to be viewed and, provided that they have the necessaryauthorisation there it is. NVRs record and replay simultaneously, andrecordings on any one machine can be remotely viewed by a number ofoperators spread across the network simultaneously, all totallyindependently and without affecting each other.

The importance of the independence of physical location, well away fromthe cameras if necessary, should not be underestimated IT Managersare notoriously zealous in safeguarding their network capacity and rightlyso, but by calculating the data flow requirement across the network andstrategically placing NVRs accordingly, the impact of video streaming onbandwidth usage can be minimised. Typically an NVR might be placed ona Local Area Network (LAN) and near (in network terms, not necessarilyphysically) a camera cluster so that the load is carried by the local LANcapable of absorbing it easily, thus saving capacity on other, perhapsmore restricted, parts of the network. The IT Manager can specify whatlevel of bandwidth he is prepared to make available for video streamingand this can be set as a cap, so that it is not exceeded under worst-case

conditions when in operation.

Then, when a recording is required at any other point on the network(typically at centre, but not necessarily so) it can be called upseamlessly by the operator, streamed and then analysed, viewed (not thesame thing) and acted upon accordingly.

To assist in the calculation of data flow requirement and disk storage capacity requirement spreadsheet-based calculatorsare available enabling these numbers to be estimated on a camera-by-camera basis using such parameters as the scenetype (busy street / internal corridor etc.), functionality of the camera (PTZ under continuous operator control / static for IDpurposes etc.), picture resolution and update rate in frames-per-second requirement, and if motion-sensing is used themotion frequency and type.

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Competent NVRs now embody features such as:

Hot-swappable disks Simple Network Management Protocol (SNMP) support Built-in diagnostics (much beloved of IT Managers) Protection of files against deletion (whether accidental or otherwise) In-built firewall for the protection of data against unauthorised access File export function which embodies the digital signature based on individual video frames and audit trail for security Synchronized audio and video recording and playback Hard-disk temperature monitoring Dual, fully redundant power supplies and network connections, providing uninterrupted and continued operation in

the event of a single power supply or network failure.

Mirroring techniques are now often used to duplicate the recording of video streams on additional NVRs located at differentparts of the network, which provides a high level of protection against network failure; if one part goes down the other isthere as a backup. You can have as many NVRs across a system as you like adding another is just a matter of plugging itin and configuring it. There is no requirement for additional video cabling. This feature really comes into its own during theconsolidation of several independent systems into one managed environment, or in system rationalisation or expansion as itreduces system complexity and removes all of the cost associated with re-cabling.

Activity Controlled Framerate (ACF) is also used to reduce the size (and hence the cost) of disks.This facility relies on processing at the camera encoder. In use, should no movement be detected in the camera scene thenthe recorder falls back to a low recording rate (typically one frame per second). However, when movement is detected in the

scene it changes back to its pre-programmed recording rate, achieving this in typically only 100ms (or 1/10th

of a second).This feature is most effective in places where low activity occurs, such as in corridors or on fire escapes, or internally inbuildings which are unoccupied at night, and can save as much as50% of the disk storage capacity that would otherwise be required.

So what can we expect for the future?Many tools are already available to assist the operator identify andreplay events of interest from a recording. IndigoVisions SMS4TMManagement software, for example, analyses movement in a sceneand on command from an operator displays thumbnails on the screenthat represent frames from recordings containing the specifiedmovement. Clicking on one of the thumbnails then replays that sectionof video. The system can search twenty four hours of recorded videoand display these thumbnails in just a few seconds. Changing the

search variables allows the operator to sift through vast quantities ofrecorded material quickly and efficiently. Analytics software searchesfor the events requested, saving the frustrating and time-consumingtask of manually searching through hours of video and freeing the operator to concentrate on more specialised andimmediate tasks. These are not just features that benefit the user but they also help to reduce the overall demand on thenetwork. More advanced analytics such as Congestion Detection, Motion Detection, Abandoned Object Detection, CounterFlow (person moving against a defined route), Virtual Tripwire, Object Tracking and Theft Detection are also available.

This is just the tip of the iceberg new developments encompass: Shape-Based Detection, automatic number platerecognition; gun-shot detection; facial recognition etc. It can be expected that huge productivity improvements will result fromusing analytics software during the searching of recorded material in post-event analysis, and for which the Network VideoRecorder is the key.

About the AuthorAlex Swanson has worked in IndigoVision for nine yearsand was appointed Head of Engineering in 2010.Previously he was Chief Technical Officer at Vebnet, amarket-leading global service provider of flexible benefitsschemes. Before that he led engineering teams in 3Com,Axon Networks and Spider Systems working in networktechnology. He holds a BSc (Hons) in Physics and anMSc in A.I. both from Edinburgh University.

Alex Swanson, Head of EngineeringIndigoVisionE-mail: enquiries@indigovision.com orVisit: www.indigovision.com

The latest Digital IP-Video transmitter units incorporatean Activity Controlled Framerate (ACF) facility which

reduces the amount of information that NVRs need torecord.