Guide to NVR's & DVR's
An NVR (Network Video Recorder) records video images in digital format to a hard drive (HDD). Images are relayed from the IP camera to the NVR by means of an Ethernet network via Cat5 or Cat6 cables. IP based systems using IP cameras and NVR's offer much higher resolution and superior video clarity when compared to a DVR with analogue cameras.
A DVR (Digital Video Recorder) records video images in digital format to a hard drive (HDD). Unlike an NVR, analogue video is relayed from analogue cameras to the DVR by means of Coaxial cable. This type of system is more cost effective and easier to setup, however the resolution is usually limited to D1 (720×480) or at best 960H (960x572).
Our experience has shown this to be a significant issue for many customers, who have bought high resolution cameras only to find that the recorded images are somewhat less clear than they expected. Primarily this is because the sales person has not fully understood the basics of putting together an effective CCTV system. Simply put, there is no point in having cameras which will produce a high resolution image if the DVR will only record low resolution!
Typically, NVR's and DVR's are available in five basic levels of resolution:
1080p Full High Definition with a frame size of 1920 x 1080
720p High Definition with a frame size of 1280 x 720
960H with a frame size of 960 x 480 pixels
D1 with a frame size of 720 x 480 pixels
CIF is the lowest
quality with a frame size of 360 x 240 pixels.
Most DVR’s have an internal hard-drive to store video information. The most popular being a SATA hard-drive. Although standard computer hard drives can be used, it is very inadvisable. There are many reasons for this, one being that the average computer is not operating and storing information 24hrs a day, 7 days a week. Only hard-drives specifically designed for CCTV applications should be used.
Normally, CCTV footage would quickly fill a hard drive. However, this is overcome by compression the information. Typically there are two compression formats used in CCTV, these being Motion JPEG (or MJPEG) and H.264. The CCTV industry has in large adopted the H.264 format due to its increased compression over MJPEG, thus it uses far less space on the hard-drive.
Frames Per Second (fps)
This area is a trap to many people. CCTV sales people will often make an unnecessarily big issue of this. In surveillance, what is important is the quality of images and the ability to recognise someone or something. Remember, the point of a CCTV system is to clearly monitor, record and display video information for the purpose of surveillance, not to make a Hollywood blockbuster.
For most applications a frame rate of between 3 and 6 fps is quite suitable for CCTV. And 6 frames per second will result in space saving of approximately 75% compared to 24fps.
Real time NVR & DVR recorders
Firstly, a real time NVR or DVR is one which records 24fps, per channel. Next consider what you want your CCTV system to achieve. Sales people will often try to sell you a "real time DVR recorders". Many cheaper DVR’s can only do this at the expense of image quality. To maintain image quality, the required storage capacity is huge. For example, a recorder with hard-drive which can store 2 weeks of video at 6fps will only achieve 3.5 days in real time. What’s more is the fact that in most cases you don't capture any more information recording at 24fps than you would at 6fps.
Video Motion Detection
Simply put, the NVR or DVR monitors camera images. If there is movement then it stores the images. You can select which parts of an image will trigger the recording, rather than the whole image. This reduces false alarm recording.
Remote Monitoring and Access
This is a feature of most NVR's and DVR’s. By using your existing LAN or internet service, you can remotely monitor and control your CCTV surveillance system from another computer, whether it is in another office or your home. You can even do all this from your tablet or smart phone.
However, may not be quite as simple as many make it sound. There are lots of considerations such as bandwidth, servers, IP addresses etc. Many of our customers come to us in frustration at having bought a DIY system from a DIY or electronics store in the belief that this is an easy matter, only to find they have been missled.
Backing up your captured images
It is important to be able to archive important video footage for such purposes as evidence in prosecution. This can be done by a number of methods. But to keep things simple, it is recommended that the DVR should have a USB port so you can connect either a USB memory stick or external hard-drive.
Comparison of video resolutions