Systems which record video signal in digital and compressed form are nowadays basic platform of image registration, offering numerous features connected with processing and transmission of information.

Advantages of digital systems:

Disadvantages of digital systems
These are mainly problems with stability (dependent on concrete system, installed components and stability of the operating system).
Some countermeasures - UPS - protects the system against most problems with power.
There are more problems with software. It is not always sensible to install the newest versions - they may be unstable.
Usually there is no problem with stability of standalone DVRs. They are equipped with their own operating systems and usually protected against power failure, saving the latest settings.

Digital registration as material evidence.
There is no strict definition, what registration form can be judicial evidence. Nowadays we have to accept that court itself evaluates value of registration supported by opinion of legal expert.

Possibilities.
Usually there is used registration of several tens of frames per second. However it is also possible to register a few hundreds of frames per second (from all cameras) nowadays, (e.g. GV1000 - 400 frames).

Obviously, larger number of recorded frames, as well as other parameters like compression factor and resolution have an effect on the size of ultimate file. For example, continuous recording with maximal number of frames per second (50 fps) for the card Hicap50 and HDD of 120 GB capacity may be performed for 72 hours, and with card Hicap200 (200 frames) for 18 hours.

It is also possible: motion detection, zone intrusion detection and detection of lighting changes. Thanks to motion detection, system can change compression factor, what enables better use of memory resources. It is also possible to program areas in which objects can move and in which they cannot (if they do, it switches alarm on). Programming of areas is made with a mouse by selection of areas on the screen.

Strong abilities of indexing during alarms registration allow to find relevant video fragment quickly.

Particularly useful feature of digital systems is their ability of simultaneous recording and playback, yet after a dozen of minutes from the start of recording. Sometimes it is also possible to search fragment of recorded video sequence with some characteristic features, e.g. with detected motion, by motion detection time, by lighting change, by zone intrusion. That feature is unavailable for analog systems. As the result of that search, we receive set of images fulfilling asked criterions, and it can be done without interrupting recording process. It is also possible to use digital zoom function to magnify image even up to 16 times. Unfortunately, it doesn't make resolution higher, anyway the use of filters reducing noise and increasing sharpness simulates resolution improvement in a sense. Next advantages of digital systems are: quick initialization of alarm mode and ability of easy setup during normal work mode.

Frequently digital system has built in Ethernet card, which can be used to connect a few devices together or for remote online monitoring and playback. There is possibility of remote control of PTZ cameras, I/O devices etc.

We should note that most of devices allow to remove HDD and install it in normal PC, where the recorded video material can be watched, processed, provided with a description and printed.
One of the methods of content protection is "watermark".
The copy of the data can be made on the built-in CD/DVD writers, or via USP port (e.g. to a flash memory).
The copied data can be played with popular multimedia players or with dedicated software.

Record parameters.
HDD is nowadays the most commonly used means of recording video data. We should remember that quality of video image transmitted to system should be good enough, because digital recording doesn't improve bad quality of input signal. 24 bits are often used for such recording, which enable to achieve about 17 millions of colors at the full screen resolution 640x480 pixels. In most of cases JPEG, MPEG or Wavelet formats or their manufacturers' modifications are employed. In opposite to analog devices, digital recording systems offer registration of color image no matter its standard, of course we have to declare what color standard we want to use before we start recording.

More about compression methods here.

Alarm inputs and outputs.
Devices usually have the same quantity of alarm inputs as video inputs. Thanks to that, it is possible to change recording rate of adequate channel after an alarm. Usually there are many methods of image recording. One of them is recording only after a motion is detected with an external sensor connected to alarm input. The most popular solution is the use of motion detection functions.

If we want to keep old record after HDD becomes full, we may configure alarm output to alarm operator about low HDD capacity. Using alarm inputs and outputs we may connect a few devices together in cascade way. Similarly to classical VCR, alarm output of first device is connected to alarm input of next device. And when first device reach its capacity limit, adequate signal will be transmitted to next device's alarm input, activating recording process.

Access lock.
There is ability to block access using password system. Additionally, each digital system is secured from recording data coming from other sources. It means that stored video data can't be modified on outer PC and then replaced in HDDs of the system like it was from cameras. This security measures ensure that only data coming from cameras is registered in system's memory. In order to increase authenticity, there are data and time added to each stored picture.

Abilities related to transition, remote control and identification will be detailed in next chapter.

Especially popular among digital systems are:

Before we decide on registration system we should take into account basic features of these devices:

Cards
offer most advanced features at lowest prices, usually use newest software and the system based on them is most flexible for configuration and future extension. The problem is right selection of hardware, software and additional security measures against unwanted modification performed by people or viruses. Such situations of "interference" into Windows system, in the case of registration systems based on cards are the cause of 60% cases of improper work of systems (the other 40% are problems with wrong hardware selection, sometimes mistakes during configuration and choice of cheapest parts - like too weak power supplies in PC housings). However it doesn't mean that cards are fighting a losing battle. Even in a case of small installations in some special circumstances they can do better than other solutions. For example: set of several separated surveillance CCTV systems scattered in a few cities and their remote control from one place. This problem can be easily solved with the use of cards, which can realize much more functions than simple stand-alone DVR. Software usually enables remote viewing from any place using standards Internet browser (IE) or additional remote client applications.

Specialized computers
Such solutions with their own operating systems don't have any disadvantages related to hardware selection or software modifications, - they are sold in the form of motherboard with operating system (mostly Linux) and require only power supply and HDD installation. They can be installed in standards PC housings, also together with parallel working computer (so we can have two computers in a single case).

Standalone DVRs
They offer simple mounting and easy operation - similar to devices of common use, stable work. However, the systems are difficult to extend and modify. For simple models, playback of stored data is possible only on recording device. It isn't possible to remove HDD and connect it directly to computer. Admittedly, devices offering copying of recorded material by some other way for example Compact Flash cards slowly occur e.g. M85032, however the number of these devices available on market is still relatively small. These abilities are available only in more expensive standalone DVRs, where customer additionally gets special software for playback of recorded data.

Video capture card - Geovision M8205

We will describe features of cards on the example of ADACS card - similar features are available in other cards. The main advantage of a card is easy installation, very good work in local area network (LAN) as well as in outer network (WAN) and simple managing by the operator of a system, together with ability of future extension of the system (adding more cards, increase of number of inputs and recording speed).

Typical functions available in software attached to video capture card:

Very important function is ability of hiding viewing of cameras. Then even operator can't see images from all cameras that may be registered.

As it was previously mentioned, cards work in network very well, anyway these abilities will be described in next chapters.

Detailed description of cards' features can be found on their catalog pages:

M8340, M8205

For the reason of simple operation and installation these devices become more and more popular. They have own operating systems; it ensures high stability of work on the one hand and gives the installer certainty that user won't damage the system on the other hand. Standalone DVR is very good solution in the case of simple CCTV installations (houses, small shops) in small objects. Installer can spend more time on service training then. In turn customer (operator) will have less problems with mastering the registration device.

Example of use of stand-alone DVR

Single channel standalone DVR.
Camera or other signal source can be directly connected to the recorder, used as digital VCR. In the case of work with more than one camera, it will be required to connect additional CCTV devices i.e. switchers, quads, multiplexers.

Multi channel standalone DVRs.
Recorders with larger quantity of inputs (4, 9, 16) usually have built in multiplexer, what eliminates necessity of installing additional devices and significantly decreases total cost of more compound system.

Standalone DVRs with network interfaces.
They enable remote surveillance, what makes them the basis for modern installations.