Digital Video Recorders
The conventional solution for CCTV
Digital Video Recorders [DVR’s] have now become the norm for CCTV surveillance solutions even at the entry level where only two or three cameras may be required. Anyone still using a tape based system should seriously consider upgrading to a digital solution. A DVR replaces both the switch/multiplexer and tape recorder of a tape based system and offers many advantages; not least, removing the daily chore of replacing the tape.
There are a multitude of suppliers and models of DVR available on the market today and selecting the most appropriate equipment for your requirements is not as simple as it may first appear. There are many things that need to be taken into consideration when choosing a DVR:-
- Resolution – Most of the cheaper DVR’s only record at medium resolution (CIF, 352 by 288 pixels) while some record at 2CIF (704 by 288) and the better ones will record at D1 (720 by 576) or 4CIF (704 by 576). Some older models record in VGA (640 by 480) or even quarter VGA. The best models allow resolution and frame rate to be selected an a camera by camera basis.
- Frame Rate – The real time frame rate for PAL recording is 25 frames per second per camera, irrespective of the resolution. Frame rates for DVR’s are generally quoted by the device rather than per camera. A quoted frame rate of 100fps means that it is shared between all of the channels, therefore a 16 channel device would only have 6fps available per camera. Furthermore these frame rates are usually quoted for CIF resolution and will be reduced by a factor of 4 if recording at full D1 resolution is available.
- Recording Format – Over the years a number of compression algorithms have been used in DVR’s including Wavelet, MJPEG, JPG2000, and MPEG4. More recently H.264 has emerged as the most popular format. This is due to the fact that it provides high compression while maintaining good image quality. The high compression also makes it more suitable for streaming over low bandwidth internet connections.
- Storage Capacity – the amount of storage capacity required to record for a given period of time is dependant on the resolution, frame rate and the percentage of change in the image. The use of motion based recording will reduce the overall storage requirement but makes it difficult to predict the length of storage possible for a given hard drive capacity. Many of the cheaper DVR’s will only accommodate a single hard drive and the size may be limited by the embedded software. It is always better to invest in a machine that can accommodate additional storage.
- Backup – The cheapest DVR’s don’t have any real backup facility and the only real way is to play back into a VHS recorder, which is both inconvenient and time consuming. The next level up is a USB port that allows the use of a memory stick for back up. However the use of evidence taken in this way may be challenged in a court of law as the media is not secure and could have been the subject of tampering. For evidence purposes is it always better to choose a machine that utilises an internal CD or DVD for back up. The use of digital water marking is an added feature that makes tampering much more difficult.
- Replay – The most important reason for recording CCTV images is so that one can revue something that has happened in the past. The replay capabilities of a machine can make a tremendous difference in how easy it is to find a particular incident. Basic system only allow you to replay from an inputted time and date, which could mean trawling through significant footage if you don’t know the exact time an incident occurred. Better systems will provide a time line that allow you to see exactly when individual cameras recorded and can allow you to narrow down the period of search. For the best capability it is wise to choose a system that offers intelligent search capabilities. This usually involves highlighting a part of the image and then the machine will search the recorded footage for instances of movement in that area.
- User Interface – The user interface can make a large difference to the ease with which the machine can be configured and operated. The basic models use a tree structured menu system which can make it difficult to find a particular feature if you are not familiar with the machine. Better systems use a graphical user interface with tabbed menus that make navigating much easier.
- Stand-Alone or PC based – Stand alone DVR’s employ embedded firmware, normally Linux based, and come in fixed sizes, typically 4, 8, 9 or 16 channels. Performance and functionality of these units has improved over the years but ultimately they are limited by processing power and constrictions of the embedded software. PC based units generally offer a more feature rich environment with a graphical user interface. They can be easily upgraded by adding additional or new capture cards and software. Advanced features such as EPOS integration, object counting and video analytics are more prevalent in PC based systems. Models of up to 64 channels can be supplied.
The recording device is arguably the most important component in a Surveillance system and the most appropriate unit for a particular application will depend on several factors including activity, image quality, recording period, overall usability and of course budget.
We are quite happy to discuss your requirements with you and will give impartial advice on the most suitable equipment to meet your needs and available budget.