ISF Wire Specifications for Video Coaxial Cables

Introduction

The purpose of video cable is to accurately convey electronic signals from the source to their destination. Since the impedance of video source and load devices is 75 ohms, the cable has to be 75 ohms in order to properly convey the signal.

Video cable is not perfect, it will introduce a loss to the signal along its path. In specifying cable it is necessary to determine how much loss is acceptable. The ISF believes that it should be lower than what can easily be seen in the picture. If the signal were reduced by 1 or 2 dB at all frequencies, the Contrast control could be used to compensate; up to the point of adding noise to the picture. Unfortunately signal attenuation is usually accompanied by an unequal loss at various frequencies, causing picture detail to be sacrificed. Our rather tight specification of 1 dB at 5.5 MHz is designed to preserve picture detail even though a larger loss could be tolerated if all frequencies were passed equally.

Cable loss is cumulative. If you start at the video source device, routing the signal through a switcher, then on to a processor; such as a line doubler, or directly to the display device, you must be concerned with the loss over the total length. In looking at a lot of installation we’ve determined that it is necessary to specify video cable for two run lengths. The first is a total path length of 25 feet, which will fit most systems, and the second is 100 feet, which should fit systems within a single room. Since we’re concerned about picture quality, the parameters for acceptable loss are the same for both 25 and 100 foot lengths. If the total path for composite video is under 25 feet then one type of cable will work just fine. If the total length is larger, you’ll need a cable that has far less loss per foot.

As we’ve mentioned, cable loss is usually frequency dependent. We are therefore specifying two types of cable, one that works well in standard video applications and another that will handle both standard video and higher frequency applications. Those higher frequency applications would include video processing; line multiplication devices, and any of the advanced television systems. We specify this second category to 50 MHz, but at a slightly higher loss than we’ve specified for the standard video cable at 5.5 MHz. There are two reasons for this. We don’t expect the majority of consumer to be using analog video information much beyond 30 MHz and believe that serial digital signals going out to 50 MHz, or above, can deal with the 1.7 dB loss at 50 MHz.

In measuring path length, you’ll want to consider the distance in which the signal remains in tact. The complete NTSC or S-Video path as an example. A break in the path would come at a line multiplier, processor, where the type of signal is transformed into much higher frequencies. The NTSC or S-Video path will require a standard resolution cable and the line processed video will require a high resolution cable. You could, of course, use high resolution cable in both applications as long as adapters were provided for the RCA type video connectors on most consumer equipment.

Our longest cable length is specified at 100 feet. In multi-room applications you often need to route video over far longer lengths. This is usually done with high quality cable and video equalizing distribution amplifiers. These devices can pre-emphasize the amplitude and high frequency information at the input to the cable, compensating for the cable’s loss. This allows the signal at the distant end to meet or exceed ISF specifications for path loss.

In measuring cables for certification, the ISF will provide manufacturers with much more information about the quality of the cable than just the ISF specifications.

Summary of ISF Specification:

ISF certified video cable must have a characteristic impedance of 75 ohms, as measured by the Time Domain Reflectometer, (TDR).

There will be two categories of certification based on conveying NTSC or PAL composite video or applications requiring extended resolution and/or higher scan rates. The categories are SR for Standard Resolution composite video and HR for Higher Resolution signals.

Two run lengths will be specified within each category. A cable will be certified for use up to 25 feet or up to 100 feet of total length from initial source to final load. Cables will therefore fall into the SR-25, SR-100, HR-25 or HR-100 categories.

The SR composite cables can be terminated with high quality RCA connectors or 50 or 75 ohm BNC connectors, as long as the connectors do not cause the cable’s frequency response or delay performance to fall outside of the SR specification. Cables with RCA connectors on one end and either type of BNC on the other also qualify for the SR category. An HR cable will be terminated on each end with a 75 ohm BNC connector.

Beyond the type of connector used on the cable and a constant impedance of 75 ohms over the entire length of the cable, the total amount of signal loss and frequency response will be the primary factors in determining the category of the cable. Other important specifications include frequency dependent delay, measured as chroma to luminance delay, and group delay, using Sin x/x and 2T Pulse measurements.

Additional measurements will be made to assist in analyzing the cable characteristics.

SR Specification:

1. Low frequency luminance loss from 30 Hz to 1.0 MHz of no more than 0.5 dB. A 100 IRE input would come out as 94.4 IRE at 0.5 dB loss.
2. Overall frequency loss from 30 Hz to 5.5 MHz of no more than 1.0 dB at any point in the spectrum. A 60 IRE, reduced amplitude multiburst or video sweep would produce nothing lower than 53.4 IRE at the output, at any frequency.
3. Chroma delay can be no greater than ± 10 nanoseconds over the usable length of the cable. You’ll see that as a shift in space, to the left or right, of the color relative to the black and white in a composite video signal.

HR Specifications:

1. Low frequency luminance loss from 30 Hz to 5.0 MHz of no more than 0.5 dB at any point in this spectrum. A 100 IRE input would come out as 94.4 IRE at this 0.5 dB loss.
2. Overall frequency loss from 30 Hz to 50 MHz of no more than 1.7 dB at any point in the spectrum. The loss characteristics should not have any bumps in this spectrum.
3. Chroma delay can be no greater than ± 2 nanoseconds over the usable length of the cable. This is particularly important as we move into the serial digital video domain. In the analog world it will delay picture detail, putting it in the wrong place in the picture.

Reference Cable, Beldon 8281B:

1. Low frequency luminance loss from 30 Hz to 5.0 MHz is about 0.33 dB at 100 feet.
2. Overall frequency loss from 30 Hz to 10 MHz, as specified by Beldon, to be no more than 0.78 dB per 100 feet with no more than 1.7 dB at 50 MHz in 100 feet.
3. Chroma delay no greater than ± 1.0 nanosecond over a 100 foot length of the cable.

Cable Test Report Form

Date

Reference Cable: Beldon 8281b Terminated in 75 W BNC’s

Cable Being Tested:

Summary:

Brief description of what we found, including a pass category or report of failure.

Test Results: (parameters which will be reported)

Details of the cable parameters:

Extended description of test results.

Description of Testing:

Test signal: Measured by: Function:

Test Equipment used:

© 1997, Imaging Science Foundation.