Understanding Digital Video Recorders

From CCTV Information
Jump to: navigation, search

The practice of recording CCTV images in digital format is now widely accepted and there are many manufacturers and resellers of what is generically known as the DVR (Digital Video Recorder).

However, whilst it is convenient to think about a “DVR” as if it is a commodity item, the truth is that these products differ wildly in their capabilities, performance, quality, reliability, support, compatibility and price.

The Technology employed in these devices has changed significantly in the last ten years, and is still evolving, so it's wise to understand what is being offered and, perhaps more importantly, what you need.

Historically, the camera signals connected to a recorder would be analogue, and usually connected with a BNC connector. These days, it is increasingly likely that the camera signal will arrive in a digital format, via a network connection. In fact, there are devices, generically known as NVR's (Network Video Recorders) which will only accept network connected cameras.

This section is specifically about what I will describe as“High end” DVR's”. These devices are usually of a hybrid design, accepting analogue and digital camera signals, and being capable of a wide range of functions. These are the products that you will usually find in larger systems and higher security applications.

High end video recorders have particular characteristics that make them suited to more demanding applications. These characteristics will usually include:

  • Higher resolution recording to give better playback quality
  • Higher capacity to allow many sources to be recorded for long periods at high frame rates and high quality
  • The ability to be integrated into large CCTV systems, often with high level interfaces to control and auditing systems
  • A robust build quality with features ensuring system integrity, even if components fail..
  • A means to analyse, copy and transmit bulk footage in the event of a major incident.
  • Analytic or advanced search facilities, allowing days weeks or months of video recording to be filtered down to the required events very quickly.

Compression methods and Picture quality

There are many different compression methods in use by different video recorders. All of these methods are capable of delivering very high picture quality, provided that the hardware or software being used has been implemented with this as an objective, and that the product in question has been programmed correctly.

Image based compression methods, such as JPEG and WAVELET, will take a digitised representation of an analogue video signal, and compress it to a lower number of bytes for easy storage and/or transmission. Each video picture is then stored as separate entity, much like your digital camera at home. This can create a very high quality recording, but will tend to use more disk space and more network bandwidth than other methods.

Conditional systems based upon JPEG and Wavelet are designed to retain the image quality of these systems, but reduce storage and transmission requirements by only storing/transmitting changes in a reference picture. You will often see this type of system referred to as MJPEG. (Motion JPEG)

Building upon this concept, we come to motion predictive methods, such as MPEG2, MPEG4 and H.264. These are the methods used in DVD's, satellite TV (SKY), cable TV (Virgin Media, Telewest etc.) and the BBC iplayer. These systems identify repeated patterns in the video data, and dynamically allocate bandwidth. This is a particularly efficient method of storing and transmitting video, and if used appropriately can provide very high quality images, whilst using less storage space or transmission bandwidth than other methods. IP cameras, and network attached devices in particular are likely to conform to these standards.

Some products will use a combination of these methods, usually to cater for picture storage and picture transmission as separate processes, the transmission process using one of the MPEG variants to reduce transmission bandwidth requirements.

The best advice I can offer to anyone in the market for a new recording system, regardless of the technology being used, is simply this, trust your own eyes! Look at the live images being produced by your cameras, and then look at some replay of the same cameras. The replayed images should look just like the live. If the fine detail in the image becomes fuzzy, or the image becomes “blocky” or “pixelated” this is usually a sign of too much compression or insufficient transmission bandwidth.

Many “off the shelf” DVR's, when used with their default programming, apply far too much compression. As a rule of thumb, less compression = better pictures, so it is worth taking the time to make sure that compression ratio can be reduced.

This is particularly true in low light conditions, when the noise generated by a camera trying to produce a usable picture will be difficult to compress. When selecting a high quality recorder you should check that it is capable of producing high quality images in all lighting conditions that your cameras experience.

Beware of anything that lowers resolution to attain recording speed. Reducing the resolution to CIF (352 pixels x 288 pixels) will seriously degrade the picture quality. For high-end applications, you should expect at least 2CIF resolution (704 x 288), or better still 4CIF (704 x 576) or D1 (720 x 576). But remember these numbers alone do NOT tell you about picture quality, because they simply refer to the number of pixels that make up the image. Since the images are compressed AFTER the pixels have been created, the pixels coming out of the recorder (the pictures you see) do not necessarily look the same as the pixels that went in! So whilst lowering the number of pixels is generally bad, a high number of pixels is not necessarily good. This is why it's important to look at the pictures being replayed.

Look at the small print

Some manufacturers will attempt to hide their shortcomings by promoting performance figures that cannot be attained in a normal installation. For example, it is quite common to see blurb such as “Real time recording” and “D1 resolution”. However, many recorders cannot record ALL cameras at this resolution AND speed. In the small print you may find that real-time recording requires a limited number of cameras or resolution, and that D1 resolution is only possible for a limited number of cameras, recorded at slower speeds, and often at very high compression (i.e. low quality). This does not signify a bad product, but it does show limitations that have been designed in, usually to meet a price. This is where you must balance your technical requirements against your budget.


Recording lots of video sources, faster (i.e. more pictures per second) and/or at higher picture quality settings will generate more data. Some video recorders cannot be fitted with enough hard disks to record at the desired quality, or for the desired duration, so it's very important to determine these factors before you choose equipment.

Check that the recorder you are planning to purchase has enough storage capacity to meet your objectives, or that it can be expanded to meet those objectives in a cost effective way. Many recorders have default settings that will make them appear to record for very long periods, but closer inspection will often reveal that this is at the expense of picture quality and/or the frequency at which the pictures are recorded. Be sure that that your chosen equipment meets all of your objectives SIMULTANEUOSLY. You may have a nasty surprise if you expect your recorder to playback a months worth of video at real-time rates, only to find you have recorded one picture per second at a poor quality.

Different systems also employ different storage solutions, so it's a good idea to understand the type of storage you are buying, and its expansion capabilities, fault tolerance and hardware requirements.

Most high end products offer some form of RAID functionality. This could be as simple as RAID 1 ( a basic disk mirroring arrangement) through to RAID 6, with data being distributed amongst many disks, and various combinations in-between. RAID fundamentally refers to the availability of redundant disks, rather than a particular piece of hardware or software, so it's worth checking exactly what is on offer, and how it is implemented. Some systems will require more maintenance than others, a higher level of fault tolerance, ease of disk replacement etc., so the exact configuration is important to the operational requirements of the system.

REMEMBER, every year hard disks achieve higher capacities and become (relatively) cheaper. If you can buy something that can be expanded, you may be able to take advantage of the advancements in hard disk technology, and record faster, longer or at higher resolution in the future than today. With higher resolution cameras becoming available, you may consider this important.


High security applications often involve various pieces of technology, sourced from various manufacturers, each performing different functions. It's often desirable to have these technologies “talk” to each other. This is particularly true for network attached appliances such as IP cameras, which are often made by a different company than the recorder. It is advisable to check that each element of the system is compatible and that any software required to integrate each part of the system will work with the hardware you have chosen.


High end recorders should be built to last, and to stand the knocks of a professional environment. Unlike domestic type equipment, CCTV recorders are generally used 24 hours per day, 7 days per week for many years. This type of usage demands robust build quality.

Hard disks, in particular, must be maintained at a safe working temperature and be isolated from unecessary vibration and/or shock to avoid catastrophic failure. A good quality digital recorder should monitor the condition of its hard drives and be able to report any problems to the user before a disk has failed completely. In this way, potentially valuable footage may be saved. You should expect a good quality recorder to offer ten years of service or more.

Most high end manufacturers will happily point to prior applications of their product stretching back a number of years. If you have any doubt, ask for a reference and talk to someone who has used your chosen product in a professional environment. In this way, you should determine not only how a new product performs, but often how the supplier performs in the after-sales stakes.

Power, Heat and Noise

These are often over-looked factors which will have an important bearing on the successful operation of any CCTV system.

Power consumption is directly related to the amount of heat generated , and equipment that generates heat, tends to use fans to extract it. Fans tend to be noisy, hence the relationship between these factors.

Choosing low-power equipment has beneficial effects in a number of ways. Firstly and most obviously, your electricity bills will be lower. What is perhaps not so obvious is that equipment rooms often need air conditioning to remove the heat that is generated. Using low power recorders will reduce or possibly negate the need for air-conditioning.

Equipment generating less heat, will tend to use less fans, operating at slower speeds, and hence will create less noise. This can be a major factor in a working environment. Depending upon the amount of equipment in use, this may have consequences for the positioning of the equipment in relation to where people work, and the type of cabinets used to house it.

Major Incidents

If you're unlucky enough to be involved in a major incident, you may be asked by the police for a copy of everything you've recorded. Can your chosen recorder provide this whilst still operating as normal?

In some cases, the Police will seize an entire recorder if the footage cannot be made available. Does your machine allow disks to be removed, or provide fast copying of the contents? What happens to your recorder whilst this is happening? These are all issues that should be addressed by a high-end recording system. A DVR or NVR that has to be “off-line” to provide evidence is unlikely to fulfil a high security role.

Analytics & search tools

Analytics is a term that seems to adorn just about every brochure associated with CCTV. In practice the term usually refers to a set of tools to assist with tasks that are arduous and time-consuming for human operators to achieve. This can be anything from simple detection of motion within a picture, through to more sophisticated analysis of human behaviour that may be exhibited prior to terrorist acts, violence, burglary etc.

Analytics can be used on both live and recorded video and can be employed for a multitude of tasks from security to traffic control through to marketing.

With so many hours, days and weeks of video footage being collected, looking for particular events within a long period of recording can be very challenging, and time consuming. Various tools have been developed in both hardware and software to assist with this task.

In order to succesfully employ video analytics, I would advise attempting to define what you wish to achieve operationally, and discuss this with your chosen provider, prior to purchasing equipment. Many of the tools you require may be available off-the-shelf. Number plate recognition, for example, is now used widely and is aproven example of the technology. However, if you want to know how often men wearing green jumpers turn left at the fish counter, you may require a custom designed tool.

If the function of the tools is critical to the operation of your system, don't be afraid to ask for a trial.

Summing up

There is an old adage that “you get what you pay for” and to a certain extent, this remains true for high-end video equipment. The next time you stop to marvel at the quality of the pictures on a BBC HD wildlife documentary or Wimbledon coverage, or perhaps even the FA Cup, stop to consider the quality of equipment that has been employed to make the transmission possible. These productions don't use cheap unsupported products, and if your objective is a high performance CCTV system, neither should you.

This article was supplied by Tecton CCTV who manufacture DVR's for organisations requiring superior quality recordings.

See also: Setting the Rules on Digital Evidence, Digital Video Recorders, Digital Imaging Procedures and Digital Evidence Kit.