The Format Hierarchy
Signal formats, audio, video, data, digital, analog, computers and networks. Fiber optic cables, twisted pair, category 5, category 6. 220, 221, whatever it takes!
Who can keep everything straight and sort through the confusion? It can be especially difficult when trying to figure out what signal format is appropriate, or which signals might be compatible.
Short history of the analog television signal
Have you recently watched a rerun of the TV show I Love Lucy – or better yet, The Honeymooners? Now switch to Discovery HD or ESPN-HD. What you’re seeing is the bookends of the evolution of the television industry, supported by its various and different signal formats.
I Love Lucy and The Honeymooners remind us of the original television format – one that is still with us today. It was designed around light emitting phosphors and electron beams that needed to be steered onto a cathode ray tube (CRT) using synchronizing signals. To support those technologies, the first analog video signal was designed as a composite of picture information and synchronizing signals conveniently transported on a single cable. Did I mention the system was only capable of black and white images?
A color system was the next challenge. Several systems were proposed, but the requirement for the new color system to be backward compatible, limited what could be accomplished. Development of the color system resulted in the color information being added on top of the luminance picture information creating today’s color composite (luminance, color and sync) analog signal.
Over time, the need for television to match the quality and resolution of the film theater experience began driving new standards and video formats. Over the years, the effort has been to move away from the limited composite video format and to develop transport mediums that are capable of providing higher resolution video for the home theater experience.
The computer industry has been a major driver in the development of higher resolution images, as well. Computers internally generate video digitally and convert the signal to an analog output. The quality is defined by the number of picture elements in a row (Pixels) and the number of lines in the image. The first popular resolution was 640 pixels across by 480 lines down. This resolution became known as the “VGA” standard and closely approximates the television image in picture size and quality. The VGA connector is found on almost all computers to this day, regardless of the higher resolution capabilities of today’s computer’s graphics cards.
Over time, increased resolutions were developed, starting with 800×600 and increasing to – well pick your number. We are seeing common resolutions today up to 4096x 2048 and development of 10k and possibly 20k resolutions. The most common resolution, however, is still 1024×768.
Analog Video Hierarchy
I have described the composite video signal and the “VGA” computer signal. These are the two extremes of what we call the video hierarchy. This is a ranking of signal transport methods based on their ability to provide the best overall analog signal performance. Here is a listing of the various analog signal transport types from highest quality to lowest quality.
This is the most common format for transporting computer and high resolution video signals. This format contains three separate video signals (red, green, blue) and separate horizontal and vertical synchronizing signals. It is essentially a “five wire” analog video system most commonly transported through an HD15 pin “VGA” connector and “VGA” cable.
The standard “VGA” cable contains five separate small coax cables. For convenience, this cable is usually fairly small. In larger commercial systems, the RGBHV signal is transported over a cable made up of five larger coax cables, bundled together inside an outer protective jacket. These RGBHV cables come in three sizes, or diameters: mini, hi-res (RG59) and super hi-res (RG6). The largest cable is the size of a garden hose and can be difficult to work with and install. The larger the cable size, however, the better the performance. BNC connectors are used on the end of each individual coax cable and provide a secure and high quality electronic connection.
Separating the color and horizontal/vertical sync signals in this RGBHV five wire system provides a transport medium that can deliver very high resolutions. In this system, it is easy to accurately control luminance, color, picture position and picture size because of the independent nature of the format throughout the entire signal chain.
The RGBHV signal format is the highest quality analog video transport format for computer and other high resolution devices.
This is a computer video format with Red, Green, Blue and separate composite (combined horizontal and vertical) sync. This is a four wire system and is generally used in very large systems where removing one wire of the five wire RGBHV signal system will lead to substantial savings in cable, hardware and installation labor.
This three wire format has the familiar Red, Green and Blue signals. The unique aspect of this format is that the composite sync is combined with the Green channel (GS). This combining, however, limits the ability to control some aspects of the signal and this format is not seen very often any more.
Although the above signal formats are technically classified as component signals, the common term “component” usually refers to a group of signal formats related to DVD players, satellite receivers, cable set top boxes and other specialized equipment. It is also the format for analog HDTV signals.
Component video generally refers to a signal format comprised of a luminance channel (Y) and two color difference channels. It is derived by mathematically subtracting the luminance information from two axis of the color information (Blue-minus-luminance and Red-minus-luminance). The luminance signal also contains the composite synchronizing signals. This format is usually referred to as the Y/Pb/Pr signal format and can be found on green, blue and red RCA connectors on devices supporting component analog signals. Several different television resolutions can be supported from standard definition television rates to HDTV rates up to 1080i and 1080p.
This format (also known as Y/C) is designed to increase the resolution and color performance of standard composite video by separating the luminance and chrominance (brightness and color) signals. This separation results in “sharper” images with less color artifacts or crawling color dots. S-Video only supports standard definition resolutions.
When the luminance, color and synchronizing signals are all combined on one signal, the receiver has to split apart (or decode) the composite signal in order to display the image. The process of decoding is not perfect and there can be many artifacts. Composite video is also limited to standard definition resolutions.
Composite video is probably the most popular video format available. It is a convenient single wire system that is easy to install. It has been around for many years and is a familiar format. However, it is the lowest quality video signal in the video hierarchy.
Analog video characteristics
Analog signals have many common characteristics. In general, video voltage is related to image content and levels range from 0 Volts for black to 0.7 Volts for maximum white. Sync voltage levels for separate sync signals (RGBHV, RGBS) will range from 3-5 Volts. For Sync signals that are part of a video signal (RGSB, component, S-video and composite) the voltage level for the sync portion of the signal will be at around 0.3 Volts. For this sync format, the combined video and sync voltage should not exceed 1 Volt.
As the analog signal travels through cable and electronic equipment, it will naturally begin to degrade. It will lose voltage level (brightness) and sharpness (resolution). By far the biggest contributor of signal degradation is the choice of cable and the distance we are sending the signal. The resolution of the video signal will play a factor in determining the amount of degradation, as well. The higher the resolution, the quicker it will degrade when traveling down a cable. Degradation can be calculated by knowing the cable attenuation specification, the resolution of the signal and the distance the signal needs to travel.
At standard computer resolutions (1024×768), the degradation can be noticeable through 25-30 feet of cable. At that distance, a small VGA cable may degrade the signal significantly while a medium size RGBHV cable will pass the signal with little, if any degradation.
RGBHV with its separate video signals and its 5 Volt sync level is very robust compared to the RGSB format that has a sync signal of only 0.3 volts. The RGBHV format will be more reliable through a system because of the separate video signals, especially at higher resolutions.
Fortunately, we can compensate for degradation or attenuation of the analog signal by using standard electronic devices. Several manufacturers provide interfaces, equalizing distribution amplifiers (DAs), line drivers and signal extenders to combat the affects of cable attenuation and signal degradation.
It is common to have a mix of standard and high resolution analog sources in today’s presentation systems. Contrary to popular belief, a well designed and properly setup analog system will perform equally with today’s digital technologies. Digital signals do provide some operational conveniences, but analog systems are just as capable of providing the highest resolutions.
Stakeholders need to decide on the value of high resolution systems and compare cost to quality. Converting your PowerPoint slides to your existing composite, single wire video backbone is much more affordable than investing in a high resolution system where high definition (RGBHV) sources are the standard. The higher the resolution, the bigger the budget need. The good news is that transporting high resolution images with analog technologies can be easily accomplished.