Wednesday, October 28, 2009

How to choose a DVR for CCTV applications

  1. Number of Channels. Most DVRs are classified by the number of channels or cameras that can be connected. Typical configurations are 1, 4, 9 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 essential 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. The Concept DMR4 product has a recording capacity of up to 4218 hours. The 16 channel version fitted with 2 x 250GB hard drives has over 8000 hours of recording time.
  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 18 frames per second. DVRs with higher rates, 25 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.
  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 levelsIf 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.
  • Remove hard disk and send to the police or reviewer.
  • Copy from the DVR onto a conventional VCR tape.
  • Send across the LAN or Internet
  • Use a network connection to a PC with a DVD burner

Our Recommendations

The latest addition of the DVR4/120 makes digital recorders very affordable. It offers 4 channels and multiplexer functions. It is now priced to be cheaper than some time-lapsed VCRs and as such should be considered a modern replacement for a VCR. It has a selectable choice of frame rates to allow for long recording times. The built-in multiplexer functions include picture-in-picture and adjustable dwell times between channels.
It can be triggered to record manually by an alarm from a PIR or on timer. It does not have built-in motion detection.
It has RS232 and RS485 interfaces or with the addition of the web interface (called the DMRNET) it can be connected to a LAN or the Internet allowing remote setup and reviewing of images.

The Concept DMR range, as its name suggests, is a digital multiplexer and recorder. 4, 9 and 16 channel versions are available with removable hard disks up to 240 GB. The 16 channel version can be fitted with 2 hard disks. The frame rate of 18 fps (PAL) and a choice of 4 levels of quality recording (Best, High, Normal, Basic) makes it a very versatile machine.
Fast forward viewing of up to 32 times and slow motion of 1/32 times makes it easy to review long recordings.
Its main advantage over the DVR4 is the built-in motion detection.
It has 256 levels of motion detection or can be set to record continuously, rewriting when the disk is full.
It has RS232 and RS485 interfaces or with the addition of the web interface (called the DMRNET) it can be connected to a LAN or the Internet allowing remote setup and reviewing of images.
This product is ideal for home, school, shop or business surveillance.

The Concept Pro Series, DVRM9 offers further sophistication. As standard it has LAN and Internet connectivity and comes with additional Internet software to allow the setup of user accounts. The motion detection levels can be individually set on each camera. The storage capacity is up to 320GB with the ability to archive an additional 40GB of data. It has 25 fps (PAL) performance and a rotating jog and shuttle control to allow speedy reviewing of recorded images.
This product is most suited to applications that require extensive reviewing of recorded images.

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