Choosing the Right DVR

Number of Cameras
The first step in choosing the right DVR is to decide how many cameras you will need it to support. We offer 4 camera, 8 camera and 16 camera DVRs and whilst our three best selling DVRs are all 4 camera models we recommend that you give some thought to the likelihood of expansion of your cctv network in the future when a larger model might be more appropriate.
 

Picture Quality (video resolution, measured in pixels))
The picture quality you need is dependent upon the type of application your cctv is used for. If you require a system for general observation, watching a car park or shop floor for example, you don’t need a particularly high resolution recording so any system which records at 213 x 96 up to around 360 x 288 should do the job for you. For recognition quality (ie you would recognise people/objects known to you) resolutions of around 640 x 272 or 720 x 288 would be appropriate. For formal identification quality images, which may be used in court to provide a positive identification for example, you should be looking for a resolution of at least 720 x 576. This resolution is often referred to as D1.
 

Recording Speed (fps – frames per second)
Back in Ye Olde Days when cctv images were commonly recorded to tape, they were usually recorded at a rate of between 1 and 3 fps if you were lucky. Hard disk technology has changed all that with most machines now recording at 25fps which is often referred to as “real time”. The recording speed of most DVRs is usually quoted as the maximum recording speed of all the camera inputs combined; thus a 4 camera dvr will have a quoted speed of 100fps (4 x 25fps), an 8 camera dvr is usually 200fps, etc. Larger dvrs might not record all the cameras at real time speeds so, for example, a 16 camera dvr might be quoted as having a speed of 200fps which would give a maximum speed per camera of 12.5fps which is still more than good enough for the majority of applications although some critical applications (such as Point of Sale Till watching systems) do NEED to be real time.
 

Recording Quality and Compression
There are two compression standard in common use at the moment and what your DVR is used for will help to decide which is better for you. MJPEG currently offers the highest quality for cctv dvr images but, because of the way it saves the data (every single pixel of every frame is saved), it’s file sizes are quite large. Since hard disk storage space is so cheap, large files are not so much of a problem as they used to be. If, however, your DVR is used to provide remote viewing of images over a network or the internet, file size and compression become much more of an issue. For that reason, most DVRs compress file using MPEG4 compression systems. MPEG4 has two sub divisions, Standard Mpeg4 and Mpeg4 H264. Mpeg4 differs from MJPEG in that rather than recording every pixel of every frame, Mpeg4 only records the differences between consecutive frames and builds up the rest of the frame from the unchanged elements in the previous frame, resulting in much smaller file sizes, although image quality does suffer a little. Standard Mpeg4 is commonly found on budget and entry level DVRs whereas H264 (which gives much smaller file sizes coupled with much better image quality than standard Mpeg4) is to be found on the better DVRs which are used in medium to high risk areas and is considered to be the standard which suits most users for regular recording and remote monitoring.

Stand alone unit or PC based
Most DVRs are stand alone units with their own built in management software, own hard drive, etc. It is , however, possible to buy a DVR which is PC based using the USB port for and external unit or an expansion card fitted in to a pc for an internally based unit. PC based DVRs can be quite useful in cases where, for example, you just need to rig up a temporary system fairly quickly. Providing the software is already loaded on to your laptop pc, a cctv system incorporating a dvr can be set up in moments and will record to any hard disk connected to the laptop. It will also take only a few moments to dismantle the system again, leaving behind a clean site with no installation marks, redundant cables, etc.

Hard Disk Drive Size
How large a hard disk size you require is akin to the proverbial “how long is a piece of string” question. The required disk size is affected by all of the DVR properties discussed above (number of cameras, recording quality (fps), compression, etc) and then some others like how long do you want to keep the recordings, are you going to record every moment of the day and night or just when the motion detect system activates, etc.
For fairly low resolution images recorded at 6fps you could reckon on approx 2Gb per camera per 24hours. For better quality images (around 720 x 288 H264) at 12.5fps reckon on approx 6Gb per camera per 24hours, and for the very best image quality for court room identification purposes the sizes are much higher.
Taking the medium quality as an example, to record 4 cameras at 12.5fps 24hours per day and keep the recordings for 2 weeks you would require a hard disk size of between 300Gb and 400Gb. The most common hard disk size we are asked to supply with a 4 camera DVR is 500Gb, with 1000Gb being common for 8 camera systems.
 

Offloading Videos
There will be times when you need to copy a video from the DVR to an external media. Built in CD and DVD Recorders have been the usual way to achieve this over the last few years but with the increasing size of flash drives it is now common to find a usb port on a DVR which is used to transfer videos to a thumb drive or other form of removable media.

Additional Features and Functions