Transmission of video: How to choose among the options
Jan 1, 2001 12:00 PM
CHARLIE R. PIERCE
With all the changes in cameras, multiplexers, digital management systems and other equipment in the CCTV industry, the various methods of transmitting the video signal from point A to B can go unnoticed. It's a subject that's worth a closer look.
So where do we start? We will list the various methods of transmitting the video signal and then take a look at the technology, advantages and disadvantages of each.
Video transmission methods include (in order of popularity in the U.S.): coaxial cable, fiber optics, two-wire, telephone line (POTS, ISDN, Internet), microwave, wireless, twisted-pair and infrared. From the large choice of transmission methods available come questions. The first: With so many choices, which one should I use? The second: Can I use more than one format of transmission within the same system? And also: Why are there so many different forms of video transmissions? All three questions have fairly easy answers. First, you use the method of transmission that best suits the application. Second, you can use as many different modes of video transmission as is necessary within the same system. And third, as the technology progresses, so do the various methods of working the video system.
Technology is a word that can send some people reeling. In the end, however, if you don't understand the outer limits of the technology, you cannot properly apply or use the best there is to offer.
Coaxial cable Let's start with coaxial cable, which was originally designed to transmit radio signals from one point to another without outside interference or detection. Among the large choice of types and styles of cable, for CCTV, only one cable is designed to carry the video signal from the camera to the controller - one with a 100 per cent copper shield, 75 ohm dielectric and a 100 percent copper center core. Anything less will cause a whole slew of problems from reflection of signal (ghosting), to noise interference (snowy or unclear images), to signal loss (poor images). Consequently, the entire question of which coaxial cable you should use in your video system boils down to three choices: RG59/U, RG6/U, or RG11/U. The real choice is based solely on the distance you plan to run the cable from the camera to the first point of connection. From zero to 750 feet, you would use an RG59/U; for zero to 1500 feet, use an RG6/U; and for distances up to 2500 feet, use an RG11/U. Implied here is the first major downfall of coaxial cable: It is distance-limited. Although not a problem in most applications, distance can be the first and most challenging problem we encounter. A second factor is that coaxial cable is not the best or most economical choice available today. It is sensitive to outside interferences (RF, ground loops, EMI) and requires studious care when planning where to install. Coaxial cable has been a long and dear friend to the video industry. However, it has come to a point of outliving it's usefulness. There have been no major changes to the design of coaxial cable in the last 30 years. Consequently, there are better methods and choices available today to move the fragile video signal along for processing.
Fiber optics Fiber optics is an excellent second or even first choice for system cabling. Although widely misunderstood and intimidating, this carrier has been with us since the days of Alexander Graham Bell. The simplest explanation is that we take the electronic signal, turn it into a modulated or pulsed light, inject that light into a piece of pure glass or plastic and then turn it back into an electronic signal. Past that, we end up with a medium that will carry the signal for up to five miles or more with little or no signal loss. Add to that the fact that glass or plastic cannot carry or induce electrical or radio frequencies, and you have a very solid format of video transmission without the fears of RF interferences, ground loops, lightning, or surge protection.
Additionally, because fiber optics offer extreme control and versatility, we can transmit a whole contingency of signals simultaneously through a single fiber. The first and only major disadvantage of fiber optics is cost. There will be a slight increase of cost in materials (due to the need for transmitters and receivers) over coaxial cable. This increase of cost, however, quickly diminishes with the size of the video system. The larger the system, the lower the cost of fiber optics versus coaxial cable.
Two-wire systems Two-wire systems are fast becoming a more popular mode of transmitting a video signal. This transmission mode is so popular, it is fairly safe to predict that it will take the place of coaxial cable in most applications before the end of 2005. Not to be confused with their father, the twisted-pair, two-wire systems can be used easily and economically in most applications without the need of a special set of tools or processes. Given a choice, the average designer, installer, or service person would rather work with a pair of speaker wires or telephone wires over coaxial cable. Like fiber optics, two wire systems require a solid-state transmitter and receiver. Like coaxial cable, two-wire systems are limited by distance of transmission, so planning is necessary to ensure proper operation. Because of their design, two-wire systems have little or no interference from outside sources such as RF or ground loops.
Additionally, these systems can be ordered with built-in lightning and surge protection. If you look around, you will find more and more video equipment (cameras, multiplexers, etc.) that are two-wire-ready, thus eliminating the need for additional equipment such as transmitters and receivers.
Telephone line transmission Telephone line transmission systems can be broken down into two categories: slow-scan and fast-scan. Slow-scan systems, for the most part, are obsolete in the United States. These systems, through the use of standard telephone lines (POTS), were the first introduction to transmitting video images around the world. The major problems arose from transmitting the huge amount of information necessary to create a single frame of video on standard telephone lines; the amount of contrast and resolution is greatly restricted. The average transmission time was eight to 20 seconds per image. Enter stage right, fast-scan systems. Through digital reproduction and quad and multiplexing techniques, and balanced telephone lines (ISDN), we are now able to send multiple images via active telephone lines to anywhere in the world. The size of image, contrast, and resolution are still stumbling blocks. However, we are able to achieve an average transmission of one to 30 video pictures per second.
Although a semi-popular mode of transmission, fast-scan systems are considered to be useful for system interrogation and/or remote control versus complete system operation. In simpler terms, we use fast-scan systems as support or enhancement versus standard transmission of the camera to first point of control. It is also highly recommended with fast-scan systems that an original point of recording or image storage be established at the system site, regardless of the final point of signal transmission.
The major disadvantage of these systems is the need for balanced telephone lines for high transmission rates. This factor adds to the up-front costs as well as the continued monthly costs of system operation, since these lines are rented or leased.
More and more of these systems are being adapted for Internet transmissions, thus deleting the need of balanced lines. However, buyer beware - these systems, regardless of the type of line used, are still limited by the size of the video signal. Manufacturers are introducing and pushing computer PC-control systems for use over telephone lines, either ISDN or POTS. They are using, in most cases, adaptive formats of fast-scan. Although impressive from the perspective of remotely controlling and viewing a CCTV system from any point in the world (via active modem), key factors for the end-user are still:
- The physical size of the image on the monitor or PC screen. Most of these systems promote an image maximum of 1/3 to 1/2 of the actual PC screen, with controls and command buttons taking up the rest.
- The number of images per second. Although very impressive in many cases, real-time (60 fields per sec NTSC or 50 fields per sec PAL) are not available at full resolution or in color.
- The amount of overall screen or image resolution at the reception point. Again, it's a very important factor since most fast-scan or Internet-based systems transmit at lower resolution to increase fields-per-seconds. Also, the type or style of digital compression used to make the video image transmittable will determine just how much loss is incurred. The average resolution of the image at the receiving end can be 250 TVL or less. In the U.S., the average television receives at 325 TVL for comparison.
- The time loss factors from point-of-control command to the point of execution. Because we are in the process of receiving video and trying to send various control points (i.e., pan/tilts), there can be as little as a few micro-seconds to as much as 2 seconds of delay from moment of command request to the moment of command response. This can be a critical design decision and should be watched carefully. Additionally, depending upon the type and design style of the system, the user may or may not be able to have real-time image results of the command given. Imagine closing your eyes and pushing the accelerator of your car for two seconds and then seeing the results.
- Last, but not least, is the ability to enter the system from points beyond. If I must open my system to telephone or Internet interrogation, regardless of point of origin, it means that I open my system, potentially, to anyone with similar technology and/or abilities. Therefore, password and priority interface are certainly important, but equally important will be signal encryption or coding during transmission.
In the end, these fast-scan systems are definitely here to stay. However, there is much to watch and learn, and the continued research and development of these systems can be expected for several years to come. Stay alert and pay attention to the details to ensure that you get the best that you deserve and can afford.
Microwave transmission Microwave transmission technology has been with us for a long time. In the late 1970s and throughout the `80s, there was a great rush of microwave transmission and reception technology introduced and implemented in the CCTV market. In the early 1990s, we were given short-range microwave transmission systems for cable runs of under 600 feet (200 meters). However, in the mid-to-late 1990s, microwave systems, for whatever reason, resolved to ride in the trunk. It seems they are only pulled out now for large-scale road or extensive-coverage type systems. One factor is the cost of design.
It may be due to the cost of the systems from an initial set-up perspective or it may just be due to an overall lack of marketing on the part of the U.S. microwave providers. However, in the end, microwave transmission of the video signal and/or system commands is alive and well and should be considered a viable transmission source.
Wireless systems Wireless systems have been flitting around the edges of the security industry for several years. Primarily used for covert CCTV throughout the 1980s and `90s, wireless has been out of the closet for larger scale, overt projects for several years. Wireless systems convert the video signal to RF signals. Consequently, there is the need for a transmitter and receiver system. There are two simple distinctions within the wireless portion of the CCTV industry: short-range and long-range. Overall, most short-range wireless transmission systems offer good to excellent video reception at distances between 100 and 150 feet. Some such systems brag 300 to 400 feet transmission capabilities. However, buyer beware! All RF systems are limited in transmission capabilities and quality as determined by the immediate surrounding area.
If you are in an area that has a lot of metal grid work (i.e., false ceilings or steel support construction), you could find yourself in for a major disappointment regarding the actual transmission capability of the system. Consequently, transmitting between floors of a building might turn into a difficult assignment for these systems. Long-range wireless transmission systems have been available for the past 10 years or so. These units offer multi-mile transmission capabilities of control and video signals with very high quality results. The major disadvantages of these systems are:
- long distance transmissions require line of sight;
- generally perceived high cost of initial system start-up; and
- a general lack of market awareness.
Twisted pair transmission Twisted pair transmission of the video signal has been an option for at least 15 years. These systems work on the premise of sending the video signal via two wires that are twisted around each other, so many turns per inch. Although, twisted pair technology is alive and well in the European market, it pretty much fell out of popularity in the U.S. about 10 years ago because of several perceived problems. First, these systems require a 120 ohm resistive loop. Regardless of the length of the wire run, the maximum and minimum total resistance of the total loop from point A to point B and back is 120 ohms. Consequently, in a steadily heated or cooled environment, these systems would only require a tweak every six months or so.
However, in an environment where the temperature could change from warm to cold or vice versa, the system might require tuning with every drop or rise of the mercury, due to the increase in general resistance of wire when heated and the drop of resistance when the wire is cooled.
Secondly, these systems require six sensitive adjustments and an oscilloscope to do the initial set-up and then each tuning. They are technically labor-intensive. Thirdly, these systems will not work on an existing telephone network where there are push-pin splice bars. These types of systems simply add too much resistance to the overall loop. Are they still available to the U.S. market? Yes, but as usual, buyer beware! These are considered to be high-maintenance systems.
Other systems Infrared and laser transmission systems have been in use for the past 20 or more years. These units change the video signal into a modulated infrared light in the same manner as fiber optic transmitters. Two major differences:
- the light is modulated by a very intense laser light source;
- the light is transmitted through a lens into free space.
For the most part these systems have proven to be expensive and labor intensive. One major problem has been lack of resolution of the transmitted video signal. Another problem is that the slightest vibration at the point of the transmitter installation can cause signal loss.
In conclusion, the type and style of transmission you will use for your video signal or system will be determined by your application, needs and budget. The key is to completely investigate the pros and cons of any/all methods you intend to use. Don't hesitate to ask for complete demonstrations, and don't hesitate to do comparisons. The net results of your tests will only benefit your final designs.