Home
DVR Basics
Residential DVRs or PVRs
Commercial DVRs
DVR Retailers
 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Overview of Commercial DVRs | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Commercial DVRs operate under the same principles as their residential counterparts, with the main difference being (1) the DVRs storage capacity, (2) its features (which will depend upon the software), and (3) the DVRs ability to record multiple video inputs. While a residential DVR will only have a single video input (the cable or satellite signal), commercial DVRs are used to record video from multiple security cameras, and so they will have anywhere from 4 to 32 video inputs. To process so many video signals requires a specialized piece of hardware, which is called the 'video capture card'. The DVR also requires specialized software to manage the multiple video signals, and to provide the interface for viewing, searching, copying, and otherwise interacting with the video. These 2 components, the 'video capture card' and the specialized DVR software, are the unique components that make the DVR something more than a common computer.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
VCRs have traditionally been used in video surveillance, and they have become an integral part of almost every security department. There have always been 2 main deficiencies of VCRs however. The first is the fact that a VCR can only input a single video signal. To get around this, devices called 'multipexers' and 'quads' were invented. A multiplexer can take in up to 16 video signals and a quad can take in 4 (there are also other slight differences between quads and multiplexers), but since the 2 devices are nearly identical, lets just look at multiplexers. The multiplexer combines the 16 camera signals into a single video signal, and that signal is sent to the VCR. The resulting video signal, or video output, looks like a 4 by 4 grid. So if you are looking at a monitor with a muliplexed signal coming into it, you will see all 16 video signals (or however many cameras you have, between 1 and 16), but each video signal will only take up a small portion of the screen. The limitations here should be obvious, it becomes difficult to see the video in such a small area, and any copying off or searching of video becomes complicated because all of the video is recorded onto the same VHS tape. Commercial DVRs 'fix' these problems, by recording each camera signal independently from the other camera signals. This means that even though you may have 16 cameras all being recorded to the same DVR, each camera can be searched, copied off, and otherwise interacted with independently. Also, each video signal can be set up differently, with its own lighting, color, motion, alarm, and resolution settings.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
The second main limitation of the VCR involves the recording media, or VCR tape. Each tape can record only hours worth of video, and then it must be replaced or recorded over. The only way around this is to use a 'time-lapse' enabled VCRs, which only record images once every few seconds, and can be combined with alarms so that the recording time can be measured in days rather than hours. These VCRs have there own limitations however, including the fact that they are severely limited on image quality, and are only useful in very specialized situations. If using any type of VCR, some amount of employee time must be dedicated to interacting with the system, to either change tapes, or to search through tapes for past incidents. With a DVR, the video footage is recorded to a hard drive, or to multiple hard drives. With today's large hard drive capacity, it is easy to record weeks, months, and sometimes even years of video footage onto a single DVR (see Table 1 for DVR recording times). In addition to longer recording times, recorded video from any camera, and from any date and time, can be viewed within seconds. With many of today's higher end DVRs, the video can also be accessed remotely via the Internet.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
So are there any downsides or limitations to DVRs? The short answer is 'yes'. The main limitation involves the quality at which you record video, which is determined by 2 things, image resolution and FPS (frames per second). With a VCR, the image quality is pre-determined by the VCR, usually corresponding to between 240 and 400 lines of television resolution, and from between 25 to 30 FPS (except of course with a time-lapse VCR, which may only record 1 FPS or less). With a DVR the image quality is adjustable, with typical resolution settings of either medium (204x352 pixels) or high (408x704 pixels), and anywhere from between 1 to 30 FPS. While a DVR can actually record video that is of much better quality than a VCR, there is a trade-off between the video quality and the amount of time that the DVR will be able to record. The resolution settings of VCRs and DVRs are also a little hard to compare, with all of the differences between analog and digital factored in, but a good rule of thumb is that a VCRs resolution will lie somewhere between the medium and high resolution of a DVR. Another limitation of the DVR is the maximum number of frames per second that it can process overall (which is determined by the 'video capture card'). This number is then divided by the total number of camera signals. For example, a DVR with a maximum FPS of 120 can record 4 cameras at 30 fps, 8 cameras at 15fps, and 16 cameras at 7fps. For most commercial applications a setting of 3 FPS to 5 FPS per camera is usually acceptable (think of taking 5 pictures for every second), whereas for more critical video, say a casino blackjack table, a setting of 15 FPS to 20 FPS may be required. The FPS settings must also be balanced with the amount of time that needs be recorded, if a DVR can record 1 month of video with 10 cameras set at 8 FPS, then that same DVR can record for 2 months if all 10 cameras are set to 4 FPS. Of course the FPS of each camera can be set independently from the others, so that more critical cameras can be set to a higher FPS if desired. The following table may give you a better idea of how FPS, resolution, and hard drive capacity are all related.
Table 1. DVR recording capacities *The following are rough approximations of DVR storage times
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
There are several different standards for video compression, which factor into the amount of time that a DVR can record. The steadfast rule is, the greater the compression, the greater the loss of image quality. The table above is approximated using the MPEG-2 compression standard, which is the most widely used.
Pg. 1 - Overview of commercial DVRs. Pg. 2 - Special features of commercial DVRs. Pg. 3 - A more detailed look at FPS. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
- DVR101 Sponsors -
Looking for the best
wireless deals?
Need a great webhost?
DVR101 uses Lunarpages.
