HD-SDI is regularly transmitted over low-loss digital video grade RG6-style coaxial cable up to a nominal maximum distance of about 100 meters. However, when a lesser diameter cable like the RG179 is used, higher loss is encountered. This is primarily because of the size (cross-section) of the center conductor. Electrical signals travel through a wire by an effect known as the ’skin effect’ and the lesser the area available, the more the attenuation. Using RG179 for transmitting HD-SDI signals is good to about 100 feet. Another important factor is to use cable sweep tested for the highest possible frequency -bandwidth. Although the Mil-SPEC RG179 cable has a very strong outer jacket, the RG179 MIL-C-17/94F is rated to carry a maximum of 3GHz, with “attenuation rated at 21dB per 100 feet, maximum, at 0.4 GHz”. In comparison the Belden RG179DT (Digital Truck) cable carries signals to 4.5GHz. The ultimate distance limitation occurs for any digital signal when the perceived signal jitter component seen by the receiver impairs its ability to recognize and reconstruct bit transitions.
The range of operation for an HD-SDI receiver is specified in SMPTE 292M to at least -20 dB at one-half the data clock rate, or about 743 MHz. Therefore, a standard level 0.800 volt peak-to-peak digital transmission may be attenuated to as low as 0.080 volt, or 80 millivolts, while performing reliably. A very high-grade receiver may recover the HD-SDI signal at a level as low as -30 dB, or 70 mV.
To perform a cable loss calculation, the designer should look for the attenuation in dB at 743 MHz, or a frequency very close to that value, on the cable specification loss chart. Cable loss is based on a 100 foot length or a 100 meter length depending on the chart column used. Divide the cable run distance by 100 and then multiply by the dB value to attain the total attenuation in dB for that run. Refer to manufacturer tables. The SMPTE recommends the designer factor in about 10% less cable than the calculated run length so as to build in a safety margin for reliable operation.
Coax is not the only medium available. Serial digital video may be routed through fiber optic cable for essentially unlimited distances depending on the system configuration. A convertor is used to convert the electrical signals to light signal, and then sent along the fiber optic cable whereas the other end would have the decoder to convert the light signals back to electrical signals. If your application requires long runs of cable, do not hesitate to contact us for suggestions and solutions.
