Analogue video formats and interfaces.
Analogue video formats and interfaces.
Analogue video formats fall into two broad categories: those designed for broadcast video/television systems and those designed for computer graphics.
The design requirements for these categories differ. Broadcast and television video formats must limit the transmission bandwidth required for the signal. Computer graphics formats, in contrast, are far less restricted in bandwidth, but they must deliver a picture suitable for viewing from very short distances.
We’ll explain many of today’s different video formats that fall into these two categories.
Broadcast/television video formats
Composite video is very familiar to most of us. It’s the analogue television signal before modulation onto an RF carrier and is the standard that connects most consumer video equipment, including VCRs, camcorders, security cameras and video CD players.
As its name suggests, Composite video has the luminance (black and white), chrominance (colour) and sync pulses combined in one signal. When developed, Composite video was designed to work with both colour and black-and-white TV signals. This backwards compatibility ensured a smooth transition between the two formats in the 1950s. Black-and-white TV sets were able to ignore the colour component while newer sets separated it out and displayed it with the luminance information.
Although this format solved the problem of backward compatibility at the time, by today’s standards, Composite video doesn’t project a very sharp picture. Because all the video components are transmitted together, they can interact with each other and cause picture defects like dot crawl and colour smear.
Y/C video (also often called S-Video) was introduced in the 1980s to overcome some of the shortfalls associated with Composite video. It’s a less encoded video format. Colour (C) and luminance (Y) information are transmitted separately to produce a sharper picture image on the display device.
Most video equipment with an S-Video connector also has a Composite video connector. When connecting devices together that support both interfaces, it’s best to use the S-Video connector because it will generally give you a sharper picture.
Component video (YCbCr) separates the signal to an even greater extent than S-Video, further reducing the chance of interference and, as a result, improving picture quality. Component video separates colour information into two colour difference signals: B-Y (Blue minus luminance, also called Cb or Pb) and R-Y (Red minus luminance, also called Cr or Pr). These along with Y (luminance) result in a total of three signals.
You can find Component video on the latest DVD players and TV receivers, displaying the very high quality images permitted by DVDs to their best advantage.
Broadcast video signal standards
In addition to the video formats discussed above, there have also been more than a dozen different broadcast standards in use at different times throughout the world. Today, most countries use one of three standards. These standards aren’t compatible, which means that when connecting video equipment, you not only need them to support the same video format but also the same broadcast signal standard.
Here’s a brief summary of the standards:
NTSC (National Television Systems Committee) standard systems are used primarily in North and Central American countries, most South America countries and Japan.
The technical format of NTSC is 525 lines per frame with a 30 frames per second refresh rate. The 30 frames consist of 60 fields, the timing of which is based on the 60-Hz electrical system used in these countries. One field is one-half (every other line) of the interlaced frame. Other countries use a 50-Hz electrical system, which means their television systems are based on 50 fields per second rates.
PAL (Phase Alternation Line), developed in Germany, is the European equivalent of NTSC and offers 625 lines per frame. The refresh rate is 25 frames per second based on 50 fields per second because it uses the European 50-Hz electrical system. Compared to NTSC, PAL has a greater number of lines. This adds detail to the picture, but PAL’s 50 fields per second rate (when compared with NTSC’s 60 fields per second rate), means a greater chance of noticeable flicker.
SECAM (Systeme Electronique Couleur Avec Memoire) is very similar to PAL with the same number of lines and same frame rate. It was developed in France and is also used in Russia, parts of Africa and Eastern Europe. Despite the similarities of the two standards, SECAM is not compatible with PAL because, unlike PAL, the chrominance is FM modulated.
HDTV (High-Definition Television), a very high quality digital broadcast television standard, is the long-awaited, next-generation solution to replace analogue TV formats like NTSC and PAL. HDTV services already exist in several places, including Japan, Canada and the U.S., but Europe has some catching up to do.
HDTV delivers much clearer, sharper images with 1050, 1125 or even 1920 lines of resolution compared to the 625 lines of PAL, but requires the use of new HD compatible television sets and receivers—and these are still comparatively expensive.
Computer graphics video formats Television video signals, as we have seen, are typically combined together into a lower-bandwidth encoded signal like Composite video. In contrast, computer graphics signals don’t have the same bandwidth restrictions and, therefore, keep the red, green and blue colour signals separate to allow higher-resolution pictures that are suitable for viewing from short distances.
There are many different analogue graphics video formats, all based on separate RGB signals but differing in the connector style used, how the sync information is transmitted, and what resolutions and refresh rates are supported. Care must be taken to select the correct display hardware for a particular video interface because differing formats are often incompatible and need active converters if they are to be interconnected. Some new display technologies, such as DVI, provide digital video connectivity. This enables enhanced video quality but requires the use of digital display devices to realise this.
Here’s an overview of computer graphic video formats, both old and new:
CGA and EGA
CGA (Colour Graphics Adaptor) was the original colour graphics adaptor from IBM® and used digital TTL signals for both the video signal and the sync signal. It uses a DB9 connector and was capable of a resolution of 320 x 200 with 16 colours or 640 x 200 with 2 colours.
CGA was a very limited format and had associated display problems like flicker and snow, but it remained common for some time after its introduction.
EGA (Enhanced Graphics Adaptor), introduced in 1984, was very similar to CGA with the same connector style and TTL signals. EGA was capable of 64 colours and was backwards compatible with both CGA and monochrome video.
VGA based formats
The VGA (Video Graphics Array) graphics card from IBM, introduced in 1987, represented a huge improvement over EGA. With a horizontal scan rate of 31.5 KHz (from 24.1 KHz), VGA supports resolutions up to 640 x 480 with 256 colours. The video signal is analogue RGB with separate horizontal and vertical sync signals presented on an HD15 connector.
SVGA (Super Video Graphics Array), XGA (Extended Graphics Array) and later formats have continued the drive to provide ever sharper images and greater colour depth. Meanwhile, VESA (Video Electronics Standards Association) standards have brought structure and interoperability to a market that was becoming a mixture of competing and often incompatible SVGA graphics cards.
Here‘s a comparison of maximum resolutions with the various VGA formats:
Graphics cards supporting the SVGA standard and higher are usually capable of supporting a range of resolutions, allowing backwards compatibility with lower-resolution display technologies.
Connectors for Sun® and Mac® systems
New Sun computers often have VGA graphics adaptors for interoperability with PC hardware, but older Sun machines have 13W3 video connectors that carry analogue red, green and blue signals with composite sync (unlike VGA which uses separate sync signals). These graphics adaptors are often capable of very high resolutions, with 1152 x 900 at 76 Hz being common. Multiple resolutions are supported and the resolution used is determined by ID bits that signal the monitor’s capabilities to the interface.
Several other computer types also use 13W3 video connectors including IBM Power PCs and Silicon Graphics® computers, but the pinning of these connectors and the way the sync signals are transmitted differs between manufacturers.
The Apple® Mac® II, Quadra® and Power Mac series, popular with trainers and presentation professionals, all use a 15-pin connector. It’s unusual in that unlike the VGA and Sun 13W3 connectors, it can carry separate or combined sync signals, and some Mac graphics cards can support BGBS, RGBHV and RGsB. Apple graphics cards are capable of displaying a range of resolutions and refresh rates depending on the monitor used.