Digital video recorders (DVRs) – although much more affordable and accessible for end-users than they ever have been – still are high-value products. Client expectations are usually high, too, because end-users believe they will be able to record everything, keep it forever, find it easily, and zoom in on it for breathtaking detail. Some clients do not realize the inevitable trade-offs between frame rate, image quality, and storage capacity. Sometimes they do not realize the size of the images they are recording and how that limits quality and storage capacity. (For more information about this topic, see related article in this issue, “DVR Performance Criteria.”)

Some security dealers and systems integrators also have unreasonable expectations about DVRs – about what they can or can’t do for their clients, as well as what to expect during a job set-up and installation. Setting the record straight on the facts and fiction of DVR technology can improve your technicians’ skills and your clients’ satisfaction.

Fact: Terminology Makes a Difference

The objective of video surveillance recording is to capture as much quality information as possible using the least amount of recording media. In security applications, the standard for measuring real time for all moving images is defined at 30 frames or 60 fields per second.

When selecting and evaluating digital systems there are variations in terminology because different manufacturers use different terminology. You will most often find references to images per second (ips) and frames per second or fields per second (fps). You must ask if fps stands for frames per second or fields per second and if ips equates to fields or frames. You see, 30 frames per second is equal to 60 fields or images per second, which is true, real-time NTSC video. Some DVRs record at 30 ips, while others record at 60 ips.

Fiction: There is no difference in video quality among compression modes.

Yes, there is a difference – but not as much in viewing live video as when viewing archives.

Competing compression modes should be examined side-by-side at distributors’ or representatives’ locations or on-site at an installation to determine whether significant losses in quality are being experienced when images are archived. Remember, live CCTV images are not yet compressed.

You must go back to archived video and zoom in on picture details before the differences in compression modes can be seen. Some may make larger pixels for compression than others and larger than were seen in the live video. This pixellation can wipe out details that were apparent on the live video. Because so much video is viewed after the event, the quality of the archived image is the one that must be considered.

When viewing compression modes without comparison, the eye sees more quality than may be there. Some security dealers report not being able to see much difference among different compression modes when viewed without side-by-side comparisons.

If you plan to mix and match DVRs, be sure they use compatible compression modes or that your hard drive or computer software can handle multiple codecs.

Fiction: DVRs Let You Search for Events and Find Them within Seconds

A typical mistake dealers and integrators make is overestimating a system’s search-retrieval capabilities. Some DVRs need a specific time and date entered in the system before they can display any video. Pinpointing an exact event may require 15 to 20 tries at approximate times before it can be located.

An advantage that DVRs have over time-lapse tape recorders, however, is that some systems can provide views while the search is being conducted. Still others let you “draw” a box around an item and search until it is moved or an event occurs with it. If a DVR is used frequently for searching, make sure it or the software it runs has an efficient and friendly retrieval system. The best search software is not always found on the most expensive DVR.

Fact: High-quality images require large amounts of storage capacity or a

reduction in frame rate.

It is best to determine for an installation whether frame rate or image quality is most important. If license plate numbers or individual faces are important, then frame rates of 5 to 7 per second should be adequate. If 24 to 30 frames per second are necessary, then hopefully high image quality will not be needed.

Of course, if your client needs both – such as at a gambling casino – and if a client’s resources are limited, then they can get both high quality and high frame rates, but they will need large data servers to which they can download all the images during slow periods.

Fact: Compression Affects both Bandwidth and Resolution

A few compression modes you will see most often are H.263, wavelet, MPEG2, MPEG4, JPEG and MJPEG.

Compression affects both bandwidth and resolution. Some compression methods or hybrid versions are better at passing high-quality images at lower bit rates. Some compression technologies are better suited for streaming live video over LANs or WANs.

Most of the accepted enterprise-level DVRs utilize MPEG2 or MPEG4 compression and portray resolution as CIF (common intermediate format) or SIF (source input format). A variation of CIF or SIF seems to be the most regularly used. CIF/SIF images are 352 pixels wide and 240 pixels tall. QCIF is one-quarter CIF and some integrators consider it to be too low a resolution for security applications. In most cases, CIF is adequate for applications such as parking areas. 4CIF uses 704 x 480 pixels resolution or 640 x 480 pixels, roughly four times the area of CIF. This loosely equates to DVD quality video. This will produce the highest quality images available in today’s marketplace. D1 is a newer term being used, which produces 720 x 480 for NTSC and for product evaluation purposes, is comparable to 4CIF. 2CIF is usually 640 x 240 pixels, although in some cases it is 340 x 480.

Some integrators recommend no less than 2CIF at 30 frames per second for all gaming and cash areas and no less than CIF at 4 to 15 frames per second anywhere else.

Fiction: DVRs Can Compensate for Bad Cameras

A DVR can only be as good as the entire installation of which it is part. If the cameras to which a DVR is attached are not focused properly or their view is not lit well enough, the best DVR cannot clean up the image. It’s the old computer adage – garbage in; garbage out. The principles of good photography apply to CCTV, as well.

Sometimes users mismatch camera quality and DVR capacity. High-definition IP cameras do not help picture quality if their images are archived with low quality. They also are more expensive than cameras that would capture images at roughly the same quality at which they are archived. It is best to match the quality of cameras and lenses with that of the frame rate, compression method and DVR.

Sidebar: Looking Ahead at DVR Design

In the future, all-digital, higher-resolution cameras and DVRs are expected, along with higher compression rates and less loss of image quality. The digital images from IP cameras would be transmitted over networks and sent to DVRs without conversion to analog at any step of the process. All cameras would be addressable and could be seen from any monitor in the system, even at remote locations around the continent or around the world.

Another trend continuing into the future is embedding greater intelligence into DVRs so they can be plugged into a system without PC-based network software. The dedicated software in these DVRs can sometimes run more reliably than networked systems.

However, the trade-off with these DVRs is that software on a network can be upgraded more easily and new capabilities added. So, computer-based DVRs may be more flexible. For users, the key is whether a system is relatively stand-alone or functions as part of an extensive network.

A greater number of manufacturers of DVRs and other digital security equipment is expected to emphasize full packages of equipment as total solutions – from camera to storage – instead of just producing individual pieces of equipment.

Editor’s note:

This article is based on comments from the following people: Cyndi Freschi, president of North American Video, Brick, N.J.; Paul Baran, key account manager for Bosch Security, Bensalem, Pa., and chairman of NTS for NBFAA; Howard Kohnstamm, owner of Videoguard Integrated Technologies, Nashville, Tenn.; and Charlie Pierce, president of LeapFrog Training & Consulting, Davenport, Iowa.