CCTV - Choosing A DVR ( Digital Video Recorder)

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Choosing a DVR system on whether on Ebay or from an internet site can be a daunting prospect with the wide range of models on offer. This choice can be further exasperated by the use of jargon and misleading information. 

This guide will hopefully allow the reader to make an informed decision by understanding some of the jargon used within listings and spot the most common “misleading” details used in some specifications. 

What is a DVR? 

At it’s most basic level, a Digital Video Recorder, as used in the CCTV Industry is a device which can receive video signals from numerous cameras simultaneously and store the recordings on internal hard drives. Depending on the unit chosen, a DVR can offer numerous other benefits to an end-user, a breakdown of the most common jargon and additional functions are listed below. 

Compression Algorithms: 

This describes the method used to compress the video for storage on the DVR. Most embedded DVRs use MPEG4 or J2000 as a compression method. J2000 is the most modern of the 2 compression methods and is found mainly on professional units, although MPEG4 is still a very popular compression method for 4 channel units. 

Frame Rates: 

This is the number of images that the DVR is capable of recording per second. It is also where the most common listing “errors” exist. Whether deliberate or due to lack of knowledge, numerous listings and specifications quote “Live” recording at 100FPS. Upon closer inspection of specifications however, a lot of these recorders are only capable of 25FPS across all cameras and base the “Live” description on a 4 way screen split (CIF) being recorded at 25FPS not the individual cameras which can only be recorded at 6.25FPS

The net result of this is that to obtain “live” recording of all cameras, each camera is recorded at 25% of its image size which obviously affects the quality of recorded detail.A true “live” recording unit is capable of recording each individual camera full screen (D1) at 25FPS. So a 4 Channel DVR would offer 100FPS across all cameras, an 8 Channel DVR would offer 200FPS and a 16 Channel Unit would offer 400FPS. 

Recording Periods:

When assessing stated recording times, it is wise to note that practically any DVR can be configured to store recordings over quite large time periods. However, this normally involves reducing image quality, resolution and the number of frames per second at which the video footage is recorded thus directly affecting the usefulness of any recorded video for evidence purposes.

The recording time of any DVR is therefore a variable and will depend on numerous factors such a motion detection, camera placement, frames per second per camera, image resolution and the quality of recordings.

Network/ Remote Access

This refers to the ability of the unit to provide remote viewing of both live images and recorded footage across the internet or Local Area Networks. Most network DVRs are supplied with their own client software which can be loaded onto a PC for this purpose. Higher Specification DVRs will normally allow access via a standard web browser such as Microsoft Internet Explorer which negates the need to load client software. Several machines also provide a DDNS service whereby a static IP address is not required and password protected access to the DVR can be achieved via the domain server site.

Simplex/ Duplex/ Triplex/ Pentaplex

Put simply this is the number of simultaneous functions which can be used on the DVR unit. Unfortunately, it is also commonly misused in equipment specifications. Traditionally these terms were used in relation to analogue systems (VCR & multiplexer).

Simplex – These units require recording to be stopped in order to view saved video footage

Duplex – These units allow simultaneous recording and playback/ search of recorded images.

Triplex – These units allow simultaneous recording, playback and network/ remote access.

Pentaplex – Recently numerous specifications purport to be pentaplex – simultaneous recording, playback, network and backup. In reality this criteria can be applied to most triplex units and tends to be a play on words: ie the majority of triplex recorders are capable of more than 3 simultaneous functions – sensor input/ output, PTZ control etc.

It should also be noted that the term ”pentaplex” does not even appear in the Oxford English Dictionary allowing it’s use to be open to interpretation.

Motion Detection

This allows better management and longer storage times of recorded images. The DVR only records when movement is detected on camera, eg. If used in a retail environment which is open 12 hours per day, the recorder will record all activity during opening hours but when the premises are closed will only record if motion is detected, effectively doubling the recording time of the recorder.

In a domestic environment, motion detection is an equally, if not more important feature when choosing a DVR. Most domestic camera installations involve external cameras. With motion detection, a householder can quickly and easily determine if there has been movement in target areas. Motion detection recorders record motion events in a log. This allows the user to quickly determine if there has been motion in monitored areas such as a back garden, then immediately view the recorded footage of what triggered the recorder, without having to trove through hours of recorded images.

More advanced DVRs use a programmable motion detection grid. This allows the “masking” of high movement areas such as trees or shrubbery, further elongating the recording times of the DVR.

One last but very important detail when choosing a DVR, is to be aware of “optional” hardware or functions. Many listings state DVD backup, VGA output, etc. but on closer inspection this hardware is only supplied at additional cost and is not included in the listed price. Other considerations are standard with the purchase of any item, what warranty is offered, where is the supplier based, what technical support is available, etc. 

We hope this guide offers some help in choosing a DVR suitable for your needs but if you require any further help or advice, we will be happy to help. We can be contacted via our Ebay Shop which also gives our Freephone contact number. 

Please check our other guides on Choosing CCTV Cameras, DIY CCTV Installation and The Law & CCTV

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