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1 Number of Channels. Most DVRs are classified by the number of channels or cameras that can be connected. Typical configurations are 1, 4, 8 or 16 channels. Consider carefully how many cameras are required, both now and in the future. It is sensible to pay a little more now and have room for expansion at a later date.
2 Method of Channel Display. This is similar to the functions carried out by Switchers, Quads and Multiplexers. A single channel recorder will probably not have any of these functions so will need the additional purchase of this type of unit.
The better DVRs have full multiplexer functions built into them. This is the desired type of product. A 4 channel DVR will split the screen into 4 and display one camera in each window. Similarly a 9 channel DVR will give a 3 by 3 matrix to display all channels. All other functions of multiplexers like alarms, time and date stamping should be available.
3 Duration of Recording Time. This will depend on the capacity of the hard drive. The more sophisticated units allow a number of user-definable options. Typical settings allow the DVR to automatically start again at the beginning and record over the oldest data or the unit can indicate that it is full and the hard disk should be changed. It is preferable that the disks are mounted in slide out drawers to aid replacement.
By using video compression techniques it is possible to greatly extend the amount of data that can be stored on a hard drive. Similarly, by reducing the number of images per second that are recorded the capacity is increased.
4 Maximum Frame Rate. This is maximum number of frames or images that can be recorded in a second. Traditional cctv VCRs used low frame rates to achieve reasonable recording times from VHS tape. This resulted in jerky images. DVRs give the user the flexibility to choose the desired resolution.
If your application is simply to observe a person or vehicle entering an area then a frame rate as low as 1 per second is all that is required. To observe fine detail of somebody taking an item and putting it in their pocket then higher frame rates are needed.
The most common DVR specification is 25 frames per second. DVRs with higher rates, 50 or 100 frames per second are only specified for very high end system specifications.
Check the quoted frame refers to the British PAL system. Some products quote 25fps for NTSC and 18fps for PAL.
Some specifications can be confusing in that they quote fields per seconds 50 fields per second = 25 frames per second.
5 Motion Detection. Depending on the application, it is not necessary to set the DVR to continually record. The better DVRs have a built-in motion detection system. This function is performed by electronically noting when the composition of the image changes and consequently setting the DVR to record mode. To ensure the required accuracy it should be possible to select the sensitivity of detection, typically at 256 levels
If this function is not included then it will be necessary to use traditional PIR sensors. Fitting PIRs means a lot of cabling and extra expense so it is often more cost effective to choose a DVR with motion detection.
6 Remote set-up and viewing. The provision of an RS232 or RS485 interface allows the DVR to be connected to a PC and setup by the use of the keyboard.
To connect to a LAN or the Internet a web server is required. This can either be an optional extra or is already built-in.
7 Transferring recorded images. Choose from a number of options.
a) Remove hard disk and send to the police or reviewer.
b) Copy from the DVR onto a conventional VCR tape.
c) Send across the LAN or Internet
d) Use a network connection to a PC with a DVD burner
e) Built-in CD Writer
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