DVRs or NVRs? That is the question…

It was the rapid development of video compression algorithms (JPEG, MJPEG and MPEG-4, etc), ever-faster computer processing speeds and a rapid reduction in data storage costs that gave rise to the digital video recorder (DVR). In essence, the DVR marries the functionality of a multiplexer with a computer disk for storage in place of tape, housing everything in the same box and featuring some additional ports to take care of connectivity.

There’s little doubt that the DVR provides a convenient – if somewhat limited – replacement for the traditional multiplexer-plus-VCR combination, at the same time offering non-linear access to recorded material usually selected by camera ID, time and date. Consistency in the quality of recorded material is generally higher than with analogue systems, although much is dependent on the compression algorithm and system set-up.

Generally speaking, a greater number of programmable options are available for individual video stream recording parameters – picture resolution, the number of frames per second, trigger options, start and stop times, etc. That said, a DVR is only useful where the analogue cables are all fed back to the unit's location. Competent DVRs now feature UDP (Category 5) network ports such that the device can be provided with an IP address and thus become accessible over an Ethernet network.

Many limitations still apply, however. The biggest problem for end users is that, if a DVR should fail, you’ll most probably have lost all of your recordings. Worse still, those recordings may not have been made in the first place. This is not true of network video recorders (NVRs), which can be used in so-called ‘mirror mode’ (more of which anon).

While we are discussing the subject of reliability, if you are planning on using a DVR make sure that the type you are specifying incorporates an industrial grade hard disk drive and not one manufactured for the domestic market. Otherwise system failure may occur sooner rather than later.

The majority of DVR failures arise from overworked and overheated hard drives. That being the case, make sure you ask your equipment supplier which drives they use.

Not surprisingly, perhaps, it’s fair to say that the actual performance obtained from a DVR, its ease of use and reliability will very much depend upon the manufacturer chosen by the specifier, the individual model selected by the end user or their consultant/installer and the final price paid.

Advent of the NVR

The NVR heralds the arrival of the next natural point in the development of recording technology. At this stage, it’s important to differentiate between DVRs and NVRs since both are often termed ‘digital’.

A DVR digitally compresses analogue video feeds and stores them on a hard drive. In this case, the word ‘digital’ is used in respect of the compression and storage technology, not the transmitted video images. Therefore, the DVR must be located near to the analogue fields. In contrast, an NVR stores digital images directly from the IP network.

The most obvious difference between the DVR and the NVR is that whereas the former records from analogue streams produced by analogue cameras, the NVR records video streams that have already been encoded at the cameras. Thus you will find no video connectors anywhere on a typical NVR. Its input and output is IP data comprising compressed and encoded video. This will usually be in MPEG-4 format which has enjoyed widespread adoption in the CCTV sector as the current ‘compression technology of choice’ (largely as a result of its efficiency).

The huge advantage of architecture based on NVRs is that they can be located anywhere on a network – at the monitoring centre, adjacent to camera clusters, on the edge of a network or collected together in a ‘hardened’ environment. In use, their location is transparent to an operator. He or she simply calls up the recorded video stream to be viewed, and (provided they have the necessary level of authorisation) there it is. NVRs record and replay simultaneously. Recordings on any one machine can be remotely viewed by a number of authorised operators spread across the network (all totally independent and without affecting each other).

Just how important that independence of physical location really is should never be underestimated. IT managers are notoriously zealous in safeguarding their network capacities, and rightly so. However, by calculating the data flow requirement across the network and strategically placing NVRs according to the results, the impact of video streaming on bandwidth usage can be minimised by the security director.

Typically, an NVR might be placed on a Local Area Network (LAN) and near to – in network terms, rather than physically – a camera cluster such that the load is carried by the LAN which can absorb it pretty easily. In turn, this will save capacity on other (perhaps more restricted) parts of the network. The IT manager can specify what level of bandwidth they’ll be prepared to make available for video streaming, and this can be set as a ‘cap’. That limit will not be exceeded when under operation in worse case scenarios.

It then follows that when a recording is required at any other point on the network (typically at the ‘centre’, but not necessarily so) it can be called up seamlessly by the operator, streamed down and then analysed, viewed (which is not quite the same thing) and acted upon accordingly.

Spreadsheet-based calculators are now available to help end users with the calculation of data flow requirements and disk storage capacity needs. The numbers can be worked out on a camera-by-camera basis using such parameters as the scene type (a busy street or an internal corridor, for example), the functionality of the camera (is it a PTZ unit under continuous operator control, or a static model for ID verification?), picture resolution and update rate (in frames per second). If motion sensing is used, then the motion frequency and type may be determined.

‘Competent’ NVRs now embody features such as hot-swappable disks, Simple Network Management Protocol support, built-in diagnostics (which security managers will be pleased to know are much beloved by their counterparts in IT!) and an in-built firewall for the protection of data against unauthorised access by an unwanted third party.

The better products on the market will also offer a fire export function for end users which embodies the watermarking and digital signature based on individual video frames and an audit trail for security. There will also be synchronised audio and video recording and playback, hard disk temperature monitoring and dual, fully-redundant power supplies and network connections (the latter ensuring uninterrupted, continual operation in the event of a single power supply or network failure).

Mirroring techniques in use

It is often the case that mirroring techniques are now used to duplicate the recording of video streams on additional NVRs located at different parts of the network. A high level of protection against network failure is guaranteed. If one part goes down the other is there as a back-up. You can have as many NVRs across a network as you like – adding another is just a matter of simply plugging it in and configuring it.

There is no requirement for additional video cabling, a feature that really comes into its own during the consolidation of several independent systems into one manageable environment (or in system rationalisation and expansion as it reduces system complexity and removes all costs associated with re-cabling).

Activity Controlled Frame Rate can 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 then the recorder will fall back to a low recording rate (typically one frame per second). However, when and if any movement is detected in the scene, it will then speed back up to its pre-programmed recording rate.

This feature is most effective in those places where low levels of activity are the norm, such as in corridors or fire escape zones, or within buildings that remain unoccupied throughout the night. As a result, anything up to 50% of disk storage capacity may be saved.

Looking to the future of recording

Many tools are already available to assist the operator in identifying and replaying events of interest from a recording. Video and alarm management software, for example, analyses movement in a scene and, on the command of the system operator, displays thumbnails on the screen that represent frames from recordings containing the specified movement. Clicking on one of the thumbnails then replays that section of video.

Such systems are able to search 24 hours of recorded video, and display the necessary thumbnails in a matter of seconds. Changing the search variables allows the operator to sift through vast quantities of recorded material both quickly and efficiently.

Analytics software then searches for the events requested, saving the frustrating and time-consuming task of manually searching through hours of video and thus freeing the operator to concentrate on more specialised and immediate tasks.

End users should rightly expect huge productivity improvements to result from the use of analytics software during the searching of recorded material in post-event analysis.

For that, the NVR is the key.