How Far Can I Run Digital on Coax?
Understanding Coaxial Cable for SDI Transmission
In the late 1980s, the Society of Motion Picture and Television Engineers (SMPTE) established a standard for transmitting serial digital interface (SDI) video signals. These uncompressed, unencrypted digital signals continue to be widely used in broadcast applications today.
Choosing the Right Cable for SDI
When SMPTE developed the SDI standard, 75-ohm coaxial cable was the preferred medium for signal transmission. While fiber optics is an option, coaxial cable has been a staple in broadcast environments due to its reliability, ease of termination, and strong electrical performance.
As SDI technology evolved, the market saw the introduction of various types of coaxial cables designed to meet different needs—riser, plenum, LSZH, high-flex, and gel-blocked outdoor versions—available in sizes ranging from miniature RG-59 to RG-11. With so many choices, broadcast engineers needed a straightforward way to determine the maximum transmission distances for each cable type.
Understanding Distance Limitations
To assist with system design, tables have been developed based on signal loss recommendations for different video formats as outlined by SMPTE. These tables are regularly updated to include new signal formats and cable variations as technology progresses.
However, it’s important to recognize a key consideration when using these tables: "The bit error rate (BER) can vary significantly as transmission distances approach the calculated limits. BER is influenced by receiver design and the specific characteristics of the coaxial cable used. It’s always advisable to consult with equipment manufacturers to confirm their recommended maximum transmission distances."
Some engineers design systems that push coaxial cable to its maximum distance limitations, while others incorporate a safety margin to ensure long-term reliability.
Equipment Considerations for 12G-SDI
Performance also depends on the active equipment used. Not all 12G-SDI receivers are designed to function at the highest possible signal loss levels, potentially reducing the maximum transmission distance by up to 50%. This variation is acknowledged in the SMPTE 2082-1 Standard, which states: “… however, receivers designed to work with greater or lesser signal attenuation are acceptable.”
The distance tables typically reference signal loss at half the signal frequency, as defined by SMPTE and based on signal type. However, these values apply only to the cable itself and do not account for losses introduced by connectors, bulkheads, or other components in the signal path. Since transmission equipment can vary significantly, direct testing of the full signal chain is often the most reliable method to determine actual performance.
Planning for 12G-SDI and Beyond
As the industry continues its shift toward 12 Gb/s single-link transmission under SMPTE 2082-1, verifying distance capabilities is more important than ever. To ensure a system meets the required performance, broadcast professionals should check with equipment manufacturers to confirm compatibility with specific cable types.
Another recommended approach is to set up a test system using the intended equipment and cabling to evaluate the signal output at the required transmission distance. Some system designers opt for fiber optics to extend transmission distances, but coaxial cable remains a strong and viable solution for many applications.
With the right cable selection and proper planning, broadcast engineers can achieve high-performance signal transmission with minimal distortion, downtime, or interference—ensuring seamless video and audio delivery across any production environment.