Catalyst Video Distribution Primer

An Introduction to Video Distribution

Video distribution can become a complex problem depending on application requirements, budget and available video equipment. This primer describes video distribution products and tradeoffs for using them as components of your video distribution system.

Catalyst is based on the Macintosh G5 platform outputting to two displays from its ATI X1900XT graphics card. Mac compatible graphics cards generally have two DVI outputs with DVI to VGA adapters.

VGA, DVI and VGA, DVI and ADC cables have distance limitations based on resolution and sync frequency; and the connectors are generally not robust enough for live environments. VGA can be run a maximum of 10m with the finest quality cable, depending on resolution and sync frequency. DVI can be run a similar distance with high quality cables. However, this distance drops dramatically as resolution goes up to as little as a meter with very high resolutions. ADC is Apple Computer's own version of DVI for use with Apple Studio Display monitors. ADC cables carry USB and power as well as DVI. Because ADC carries high current, low voltage power, it is viable for only short distances.

When running a computer video signal any significant distance, it is advisable to convert VGA, DVI or ADC into component RGBHV. RGBHV has separate cables for red, green and blue, horizontal sync, and vertical sync. VGA to RGBHV cables are available from most computer retailers. DVI and ADC are both digital signals, however, they both carry an analog VGA signal as well. Converting ADC and DVI to VGA is a matter of using the correct adapter.

A VGA Splitter to split the VGA signal into two or more signals at unity gain is an essential Catalyst accessory.

When running a computer video signal any distance, it is advisable to convert VGA, DVI or ADC into component RGBHV. RGBHV has separate cables for red, green and blue, horizontal sync, and vertical sync. VGA to RGBHV cables are available from most computer retailers. DVI and ADC are both digital signals, however, they both carry an analog VGA signal as well. Converting ADC and DVI to VGA is a matter of using the correct adapter.

A line amplifier boosts a VGA or RGBHV signal to send it successfully over long cable runs (typically 100m/330ft) without the signal level dropping below an acceptable limit. Line Amplifiers often feature multiple outputs, with separate gain and equalization controls for each output.

A simple example of a line amplifier is available from Kramer:

One way to avoid signal loss, interference, expense and weight of good quality RGBHV cable is to send VGA over CAT5 Ethernet cable. CAT5 can support high resolutions, is inexpensive and ubiquitous.

To send VGA over Ethernet you need converters, a VGA to CAT5 transmitter, and a CAT5 to VGA receiver at the other end. Some systems also allow RS232 serial strings to be transmitted over the same length of CAT 5.

Examples of these systems include:

One popular method of distributing Catalyst outputs is by using a scan converter to convert to composite or SDI video. A scan converter is a device that converts computer video signals into NTSC or PAL video signals. There are several NTSC/PAL video signal formats: S-video (Y/C), Composite, Component and SDI. Composite and SDI signals use a single coaxial cable to transmit the video signal, and are used most commonly for distributing video in installations or in live environments, as they are generally the easiest to distribute.

NTSC and PAL video signals are very different from computer video signals. Video signals use a color space system called YUV, and is divided in to luma and two chrominance channels: red – luma (R –Y), and blue – luma (B-Y). YUV has a more limited color gamut than RGB color space, and is used by all broadcasters, because it has a low much lower bandwidth than RGB color space When it was designed, when color television was in it's infancy, there was a need to have a color space system that backwards compatible with black and white television sets.

A detailed discussion of video color space can be found at:

Composite is an analog signal, and as such is susceptible to signal loss and interference. It also sends luma and chrominance down a single cable, so the signal quality isn't always optimum. The quality level depends on the source, the length of the cable run, and the quality of the cable itself. However, composite is the easiest video format to distribute. The maximum length of the cable run is generally 30m to 50m depending on source and the quality of the cable. It is possible to run composite further, but you may need isolation transformers (aka humbuckers) at each end of the cable run to eliminate hum and interference on the cable. It may also be necessary to boost the signal.

S-video is better quality than composite, as luma and chrominance are sent through separate cables. Typically S-video cables have both cables in a thin multicore. However, S-video can be more difficult to distribute than composite, and sometimes it is necessary to split S-video into separate Luma and Chrominance cables for easier distribution. S-Video suffers from many of the same interference and cable problems as composite.

Both Composite and S-video are used widely in consumer A/V equipment. Composite cables in consumer equipment use RCA phono connecters, whilst S-Video uses a special S-video connector. Composite video is still used in professional video equipment, but has been superseded by component video and SDI.

Component video has three channels: Luma (Y) and two Chrominance channels: Red (Cr) and Blue (Cb). Component Video is used on professional digital video decks and is digital. Analog component video is known as YPrPb and is used on older professional analog video. Since the three component parts of the video signal are sent down separate cables, both kinds of component video are much better quality than either S-video or composite video.

SDI stands for serial digital interface, and can carry luma and both chrominance signals down one coaxial cable. SDI is used widely on broadcast and professional video equipment. It has the advantages of being very good quality, digital, immune to interference, and it can be run up to 100m or so without needing to be “reclocked”. The downside is that SDI equipment tends to be expensive. High definition SDI equipment is extremely expensive.

Currently most scan converters send video signals in PAL or NTSC standard definition resolutions. PAL is 720 x 576 at 25 frames per second, NTSC is 720 x 480 at 29.97 frames per second. High-end scan converters and other high-end video equipment now support high definition resolutions, which are significantly higher resolution than standard definition. However, high definition video equipment is very expensive at the moment.

Scan converters commonly used with Catalyst include:

Many LED screen processors and projectors now support DVI. DVI is a very high quality digital computer video signal standard supported by most modern graphics cards and monitors. Apple Mac OEM graphics cards (i.e. the one included with the computer) typically have one DVI output, and one ADC output. The retail graphics cards often have one DVI output, a VGA output and an S-video output. ADC can be converted to DVI by using an adapter. Dr Bott's DVIator is ideal:

Short for generator locking device, a Genlock is a system that allows synchronization of two or more video signals. Typically, cameras in a broadcast studio are genlocked together to ensure smooth transitions from one shot to another. Genlock is achieved by connecting a special genlock input on the video equipment to a “black burst” generator, This provides a special signal that allows the video equipment to synchronize horizontal, vertical, frame, field and color information.

Many broadcasters will insist that computer video sources, like Catalyst media servers, are synchronized with the rest of the broadcast equipment in the studio. To do this, you need the genlock facility on a scan converter.