Hybrid CCTV systems.
Analog and digital
Specialists in television surveillance systems have long encountered the concept of a «hybrid CCTV system» in their practice. Subconsciously believing that this is, without a doubt, very good, many do not fully understand what it is, why it arose and why, in fact, it is needed.
By typing hybrid CCTV into a search engine, a curious individual will receive surprisingly diverse interpretations of this combination. There are numerous references to video recording devices that work with both analog and IP cameras, and even systems powered by non-traditional energy sources (wind generator and solar battery) are mentioned.
Trying to figure out where the truth is, the author turned to encyclopedias for the exact meaning of the word «hybrid».
Hybrid(from Latin hibrida, hybrida – crossbreed), the offspring of crossing two genotypically different organisms. [GSE]
In relation to electronic devices, we usually talk about two “genotypes” – analog and digital.
In CCTV, the concepts of “analog” and “digital” are traditionally interpreted in the following not entirely usual way:
“Analog” components are those that produce and receive images in the form of an analog video signal that meets the well-known EIA/NTSC and CCIR/PAL standards.
«Digital» components deal with images transmitted in the form of packets of information over data networks.
This distinction reflects differences in the structure of the CCTV system and has little in common with the classical definition of analog and digital (discrete) signals in electronics.
The scene filmed by the camera, as well as the final image seen by the observer, is essentially a stream of light with spatially different intensity and spectrum, changing in time. They have an analog (continuous) nature.
The author will not be discovering America by noting that from the time of the advent of television broadcasting to the present day, there has been no purely analog transmission of images from the scene to the observer. The image is divided into lines, which is spatial discretization, and the lines form half-frames and frames transmitted one after the other in a certain sequence — i.e., there is also temporal discretization. The use of CCD matrices as a light-sensitive element added the division of lines into pixels.
Thus, of all the parameters of the transmitted image — geometry, intensity, color and time — only two were analog.
In addition to the discrete properties of the video signal, there is another reason to consider the above division conditional. The fact is that since the late 80s, analog components of television surveillance systems began to appear, inside which the signal underwent digital processing.
The first and most common example of such a device is the quadrator. Watching four pictures on one screen simultaneously required the use of video memory, where four original images were recorded in digital form, and then the complete picture was converted back into a video signal. Such a device did not violate the classification order too much. But further — more.
Multiplexers could not only divide the screen into parts, but also «cut» a sequence of half-frames from several input signals, not synchronized with each other, for recording them on a single tape recorder, and all this thanks to the digital filling.
In TV cameras themselves (primarily color ones), digital signal processing (DSP) has become the norm. Many functions of modern TV cameras (backlight compensation, noise reduction, slow shutter, etc.), which have become familiar, are impossible without it.
Even video recorders with digital recording of images on a hard disk, which replaced tape special video recorders, in our terminology remain analog as long as the input and output of images from them is carried out by means of a video signal.
Thus, we classify devices that are “through and through” digital as analog! The division of CCTV system components into analog and digital is determined only by the method of transmitting images between them and does not depend on what happens inside the device.
Structure
Having determined the classification of “genotypes”, we will move on to “crossbreeding”.
The CCTV system consists of television cameras (devices that form an image), image processing equipment (switching, storage, video analysis, etc.) and display equipment. These components are interconnected by communication lines and, as we have agreed, can be analog or digital.
Let's consider the evolution of a television surveillance system belonging to the medium or high price category.
Figure 1 shows a typical structure of a CCTV system used in the early stages. It did not have components with digital video signal processing, so there was no multi-screen display, and recording was done selectively (usually, when an alarm signal was received, the corresponding TV camera was switched to the VCR input). Communication lines were coaxial cable. Amplifiers were used for extended lines. Cameras on monitors were selected using an analog matrix switch. The simple structure and minimum video signal conversion ensured its high quality. Systems of this type are used in some places to this day, for example in casinos, where quality and speed are of paramount importance.
The system in Fig. 2 is more modern, it contains components with digital signal processing (TV cameras with DSP, multiplexers, video motion detectors). Transmission lines are more diverse — along with coaxial, twisted pair and fiber optics were used (fiber optic channels also often used transmission in digital form, but there was a video signal at the input and output of the channel). Such systems became widespread in the second half of the 90s of the last century. The possibility of multi-screen display, round-the-clock recording of all TV cameras became generally accepted requirements for analog CCTV, although these functions, as we know, are impossible without «digital».
Fig. 3 is a direct predecessor of the hybrid system. The difference from the previous scheme is in the digital video recorders. While performing almost the same functions as the multiplexers and video recorders they replaced, they also marked an important difference. They stored images as information files, potentially suitable for further transmission and use in digital form (when exporting to a removable media, such a transfer actually took place). A small modification — adding a network interface to the video recorder — turned this system into a hybrid (Fig. 4).
Systems like the one shown in Fig. 4 gave the user direct access to the information contained in the video recorder via a digital network. Network workstations (computers) were used for this purpose. The software played the role of a switch, establishing a connection with the required recorder and video channel.
Thus, the system had two independent channels – analog (via the matrix switch and video monitors) and digital (via the network and workstation). Each of them could be used independently to solve television surveillance problems, but each had its own shortcomings at that stage. The analog channel provided the best quality and real time, but did not provide access to video recording. On the contrary, the digital channel allowed video recording to be played back, but the possibilities for real-time surveillance were limited by relatively low quality and large delays.
Over time, a new generation of real-time video recorders has proven that quality, transmission speed, and delays can be improved to the point that a separate analog channel is no longer necessary. Moreover, digital components can provide characteristics that are unachievable in analog systems (for example, in the case of megapixel IP cameras).
The software has improved and become a universal means of accessing live and archived video information, with advanced search tools, video analysis, and various service functions.
An increasing number of systems are beginning to be built in accordance with the model in Fig. 5. The video recorder remains a hybrid component that provides communication between analog cameras and digital display devices. And with the advent of IP cameras, the hybrid system becomes completely digital.
Finally, in the system in Fig. 6, all the main components are digital, all video information is transmitted via the network. This structure is distinguished by the fact that the functions of control, processing and storage of video information are divided between different devices: NVR — network video recorder (receiving information from IP cameras, processing/video analysis, storage, issuing to users); NAS — network storage (storage); Management server — control server (control of information transmission in the network, ensuring security, access control).
Switching in the system is carried out programmatically by establishing IP connections between the corresponding network devices.
Video sources are also usually digital (IP cameras). To ensure compatibility with analog systems, encoders and decoders are provided that convert the analog video signal into a data stream transmitted via an IP connection, and vice versa. The latter give us some grounds to call such a system hybrid, although, strictly speaking, their presence is not mandatory.
Analytics functions are mainly transferred to the image source side, to the IP camera or encoder. There are two reasons for this: firstly, uncompressed images of the highest quality can be obtained there and, secondly, the power of digital resources of TV cameras and encoders has become sufficient for video analytics.
Prospects
Fig. 7 shows the migration process of CCTV components from analog to digital. As we can see, there is currently a digital embodiment for each component. However, only for video recorders and video detectors has the analog version become irrelevant. For other components, both options continue to coexist, determining the future of hybrid CCTV systems.
The popularity of analog cameras is explained by their wider choice on the equipment market and a significantly lower price compared to IP cameras.
Analog signal lines on twisted pair and coaxial cable are often less expensive than building a new data network for an equivalent distance.
Matrix switches and analog monitors are popular in systems where the emphasis is on real-time monitoring, where they provide the best quality at the lowest cost.
The above mentioned section illustrates how the CCTV system is being transformed into a hybrid. The hybrid structure was not specially developed by engineers, but is a consequence of the desire to combine the achievements of CCTV technology in the most advantageous way, and in the course of further development hybrids will inevitably be replaced by digital systems with greater functional potential. But for now this is hindered by the following factors:
The combined use of analog and digital components has advantages over purely analog and purely digital structures primarily due to the wide range of equipment in terms of price/quality ratio.
The introduction of fully digital systems is constrained by the presence of internal problems, namely: insufficient choice of IP equipment at a high price, incompatibility of digital transmission protocols from different manufacturers, and the discrepancy between existing transmission networks and the requirements of digital CCTV systems.
The transition to fully digital systems cannot be pushed or artificially accelerated. It will happen when existing analog technologies become a brake on new product introduction and prevent manufacturers from making profits.