Last Updated 2 days ago
Filmmaking Dictionary: Number 7 / 75-Ohm Video Cable
Definition
75-ohm video cable is coaxial cable designed with a nominal characteristic impedance of 75 ohms. It is widely used for professional video signals, including SDI. Characteristic impedance describes how a changing signal travels through the cable and interacts with the connected equipment.
It is not the same as the cable’s ordinary end-to-end DC resistance. Measuring a loose cable with a simple resistance meter does not establish that it is a suitable 75-ohm transmission line.
Why Matching Matters
A video signal path is designed around compatible source, cable, connector and receiving impedances. Mismatches can create signal reflections and degrade the connection. At high data rates, small imperfections in connectors or cable construction can become significant.
BNC connectors are common on SDI equipment, but the connector shape alone does not prove compatibility. BNC hardware also exists for other impedance systems. A cable that physically connects may still be the wrong choice for the signal.
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What to Check
- Use coax and connectors specified for the intended SDI data rate.
- Check the manufacturer’s distance guidance for that cable and format.
- Inspect bends, crushed sections, damaged connectors and unreliable adapters.
- Confirm all intermediate converters and routing equipment support the chosen signal.
Practical Example
A long cable run works with an HD signal but drops out after the camera switches to a higher-rate UHD output. That does not mean the camera has become faulty. The cable, connectors or receiving equipment may no longer provide enough signal margin at the new rate.
Substituting a known suitable shorter cable helps isolate the problem. Cable length alone is not the full specification: different constructions have different attenuation and performance. An arbitrary maximum distance quoted without a cable type and signal format is not dependable.
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Frequently Asked Questions
Is every 75-ohm cable suitable for every SDI format?
No. Characteristic impedance is only one requirement. Bandwidth, attenuation, connectors and run length also matter.
Should an extra terminator be added to every SDI input?
No. Many receiving inputs provide their required termination internally. Follow the equipment’s design rather than adding an external load indiscriminately.
Is 75-ohm video cable the same as DMX cable?
No. They are designed for different signal systems and use different transmission arrangements.
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