At the Heart of Video Recording Technology
by Ron Nelson
Installing a DVR for your client doesn’t necessarily mean you’re giving them a ‘digital’ video system. Learn about the differences between DVRs and NVRs, and the extent to which each technology performs with its varied components.
In the realm of video surveillance recording, digital technology is clearly the dominant approach. Yet, if you ask 10 people what “digital” means, you are likely to get 10 different answers. The problem is that because it is a relatively new technology there are different system types each being called digital, but not all are truly digital end-to-end.
Some digital systems are basically analog systems that have the ability to digitize images for storage and retransmission; some will work with only digital components; others allow the intermingling of both analog and digital devices. All of this can make it difficult to understand how to compare systems and to design a system that is capable of utilizing the benefits that IP-based (Internet Protocol) systems offer.
Two major components of digital video systems are digital video recorders (DVRs) and network video recorders (NVRs). In its simplest form a DVR is a bridge between traditional analog systems and the latest purely digital systems. In this sense it is a replacement for the standard multiplexer and video cassette recorder (VCR) combination that comprised most CCTV systems for many years. To understand this application of a DVR we should first review the components and connections that comprise a traditional analog CCTV system.
Traditional Analog Technology
The diagram below, Analog Cameras to an Analog VCR, shows a typical analog CCTV system that uses cameras, a multiplexer, and a VCR. In this scheme the cameras transmit analog video signals continuously to a multiplexer using coaxial cable. The multiplexer digitizes the images and combines the video signals from multiple cameras into a single “multiplexed” video stream that is recorded on a tape in the VCR. In order to store long periods of time the VCR would operate in a time-lapse mode, in which it would not record images constantly, but would record an image every few seconds or longer, depending upon the desired storage time. When playback was required the recorded signal was sent from the VCR back to the multiplexer, where it was decoded and displayed on the monitor. This technique worked well, but had restrictions due to the limitations of the coaxial cable, image quality, length of the tapes, and failure of the recording heads or tape. The playback image quality was particularly low when viewing images recorded in time-lapse mode or in still-image viewing.
The First DVR
When first introduced, DVRs combined the features of the multiplexer and VCR into a single unit that used a hard disk to store the images, eliminating the problems associated with tapes. It also allowed for quicker searches and clearer images on playback. To increase the amount of images that could be stored on a given disk size the manufacturer incorporated a compression algorithm, which significantly reduced the amount of space necessary to store the video information. This compression did, however, reduce the quality of the images.
Many DVRs also included a means of remotely connecting to the unit to view live or recorded images. Initially this was a simple POTS (Plain Old Telephone Service) line and while it did work, speed and image quality were not very good due to the limitations of the telephone network. As technology improved and the market changed most manufacturers added a NIC (network interface card) to the DVR that allowed it to be connected to a network or even the Internet.
The diagram, Analog Cameras to a DVR, shows the basic components and connections for a DVR in an analog system. As you can see, the only difference between the VCR and DVR systems is that the DVR replaced the multiplexer and the VCR. A DVR can have connectors for each camera, one or more analog monitors, alarm inputs/outputs, telephone line, network, and power. Some DVRs require a computer keyboard, mouse and monitor, while others use the buttons on the front of the unit to program and set up the system and an analog monitor for viewing. Likewise, some DVRs use standard computer operating systems like Microsoft Windows, while others utilize an embedded operating system that has all of the necessary features to perform the video functions but limit user access to many system parameters and may be less prone to hacking and virus problems.
DVRs Beyond the Basic Functions
Actually, a DVR performs more tasks than simply recording the images. When recording the DVR digitizes the analog images, compresses them, performs video motion detection, and controls the recording rate/resolution based upon the initial program, time, and activity. On playback it converts the digital images to an analog output that can be displayed on a standard video monitor. All of this activity takes a considerable amount of computer processing power and requires constant disk activity. This activity can be more easily seen by looking at the diagram, Analog System Functions, which shows the various functions that are required for the system to operate and where they are performed. As you can see, the DVR is doing all of the work manipulating and recording the images which are constantly being sent by the cameras, and the only place the video images are digital is inside the DVR. The only network connection a DVR uses is for remote viewing or administration, either with a proprietary software or standard Web browser. In light of this, it can have a lower impact on the network infrastructure than an IP-based video system does.
Straight to Digital with NVRs
The NVR is strictly a digital device. It is designed to connect to other digital devices to provide a video system that offers many more benefits than traditional analog systems, without sacrificing any functionality. The diagram below, Network Camera Video Connections, shows the typical layout for an IP-based system with an NVR. At first glance you might say that there is not much difference between the NVR and the DVR configuration because the only obvious change is that the cabling used is not coaxial, but that is only the tip of the iceberg.
In a traditional analog system each camera required a home-run coaxial cable, a power cable, and an additional cable if PTZ functions were used. The IP cameras can use a single cable to provide power, using PoE (Power over Ethernet), video images, PTZ functions, and if necessary audio information, as well. These cables do not have to be home run, but can be run to a nearby network switch or hub. This can represent a considerable savings in cable and labor, although network equipment may have to be installed if not already in place. If the system included PTZ functions an analog DVR-based system also had the additional burden of matching the PTZ protocol of the camera with the protocols supported by the DVR.
A variation on the NVR is a hybrid NVR, which incorporates all of the features of a standard NVR but also has connections for analog cameras. This may be a good choice if you need to utilize existing analog cameras but want to have the ability to connect IP cameras, as well. The diagram, Hybrid Video Connections (bottom of column at left), shows a typical hybrid NVR connected to both analog and IP-based cameras. An alternative to using a hybrid NVR to accommodate existing analog cameras is to add a video server which converts the images from analog cameras to digital signals that can be sent over a network to a standard NVR. Video servers are available in single- or multiple-channel configurations; there also are video decoders that allow you to connect existing analog video monitors to the NVR.
Operationally an NVR system moves most of the image processing functions to the camera, which accounts for the higher price of an IP camera. As you can see in the diagram, IP System Functions, the IP camera not only captures the images but it also digitizes them, analyzes them for motion detection, performs the compression, and determines what frame rate and resolution to use based upon set up parameters and activity. Moving this processing to the camera can significantly reduce the required network bandwidth, because the camera can be programmed to only transmit video when necessary or transmit video at a low frame rate unless a specific event occurs.
NVR as a Black Box
NVRs are available as a black box, which combines both the software and hardware necessary to perform all desired functions in one easy-to-install box or as software only that is intended to be installed on compatible computer hardware which may already exist on a net-work. In the black box configuration the unit is dedicated to the video system, although it may utilize network resources for storage and data transmission, while in the software-only setup the computer where it resides may perform other functions, as well.
In the final analysis NVRs have some advantages over their DVR “brothers,” such as offering the promise of not being outdated because they operate on open standards and off-the-shelf network hardware which can be upgraded and improved without any negative impact on the system. Scalability is another advantage; typically, DVRs are available in fixed-camera configurations (4, 8, 16 and 32), while some NVRs allow increasing the camera capacity by software license upgrades, as long as storage requirements can be met. This can overcome the drastic cost of replacing a DVR when you need to add one more camera. NVRs also offer the ability to use standard off-the-shelf network hardware for expansion of storage requirements, while many DVR manufacturers limit the types of storage media that they support. NVRs offer shorter down time; because they use standard network hardware, replacing a defective component is easier because it can be obtained from a number of vendors, while an all-in-one DVR or black box NVR may require that you have it repaired or obtain a replacement only from the manufacturer.
Sidebar: The Choice: DVR or NVR?
The ultimate decision about whether to use a DVR or an NVR should be made only after considering all aspects of the installation and in-depth discussions with your client. When speaking with the end-user you should consider many factors, such as the:
- need to use any existing analog cameras and associated cabling.
- ability to use existing cable infrastructure, network cables and hardware.
- speed of existing network configuration.
- knowledge and skills of your technicians in networking.
- size and availability of network storage devices.
- future expansion needs.
- overall system cost.
The final and most important consideration whenever you are considering a venture into any digital video system that includes network interconnectivity is the cooperation of the IT department. You may find that some IT managers are very reluctant to add video or any software that they are not familiar with to their networks. This can frequently be overcome by having frank discussions with them before designing a system, fully explaining the system operation and soliciting their input and support. If you have a network-knowledgeable person from your staff participate in these discussions, the IT manager often will see that there is no danger to their systems and that adding video is yet another value that they bring to the table.
February 1, 2007 -- (Reprinted with permission from SDM magazine, Copyright 2007)
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