An SDI video connection should be simple: connect the source to the destination with a 75-ohm coaxial cable and the video appears.

But when an SDI signal starts dropping, flickering, producing intermittent video or disappearing completely, finding the cause can be frustrating.

The problem becomes even more noticeable with higher-bandwidth formats such as 6G-SDI and 12G-SDI, where cable quality, transmission distance and connector performance become increasingly important.

Before replacing equipment, check these nine common causes of SDI signal problems.

1. THE SDI CABLE RUN IS TOO LONG

Transmission distance is one of the first things to check when troubleshooting an unreliable SDI signal.

Coaxial cable introduces signal loss, or attenuation, and that loss increases with both frequency and cable length.

This means a cable that successfully carries HD-SDI or 3G-SDI does not automatically provide the same transmission distance at 12G-SDI.

A system operating close to the cable's practical distance limit may appear to work normally and then experience intermittent dropouts as equipment, connectors or installation conditions change.

If you're troubleshooting a long 4K run, start with our 12G-SDI cable distance guide.

2. YOU'RE USING THE WRONG COAXIAL CABLE

Not every 75-ohm video cable has the same transmission characteristics.

Professional SDI systems commonly use several coaxial cable sizes, including Mini RG59, RG59 and RG6.

Smaller coax can be extremely useful in racks, fly packs and other space-constrained environments because it is lightweight and easy to route.

Larger coaxial cable generally provides lower attenuation and can be advantageous as transmission distances increase.

If you're deciding which cable construction fits the application, see our Mini RG59 vs RG59 vs RG6 comparison.

3. THE BNC CONNECTOR ISN'T DESIGNED FOR THE APPLICATION

The coaxial cable is only part of the signal path.

The BNC connectors and their termination also affect overall system performance.

Professional SDI systems use true 75-ohm connectors designed to maintain the characteristic impedance of the video signal path.

Using an inappropriate connector, poorly installed connector or damaged termination can introduce impedance discontinuities and degrade signal integrity.

This is especially important with higher-bandwidth 6G-SDI and 12G-SDI signals.

4. THE BNC CONNECTOR IS LOOSE OR DAMAGED

Sometimes the problem is mechanical rather than the cable specification.

Inspect both ends of the cable for:

• Loose BNC connectors
• Damaged bayonet locking tabs
• Bent or damaged center contacts
• Loose cable-to-connector termination
• Visible corrosion or contamination
• Connectors that do not lock securely

A cable that works when positioned one way but loses video when the connector or cable is moved deserves immediate inspection.

5. THE COAXIAL CABLE HAS BEEN DAMAGED

Broadcast cable assemblies often live difficult lives.

They may be repeatedly deployed, coiled, transported, pulled through racks or used in temporary production environments.

Excessive crushing, severe kinking, repeated abuse or an extremely tight bend can damage the physical structure of coaxial cable.

Damage is not always visible from the outside.

If an SDI problem follows one particular cable from system to system, substitute a known-good cable of similar length and specification. If the problem disappears, the original cable becomes the primary suspect.

6. A CABLE WORKS AT 3G-SDI BUT FAILS AT 12G-SDI

This is an important troubleshooting clue.

A cable successfully carrying a lower-data-rate SDI signal does not prove that it will reliably carry 12G-SDI.

12G-SDI places significantly greater demands on the entire signal path.

A marginal cable assembly may therefore work perfectly with an HD or 3G signal but become intermittent when the equipment is switched to a higher-bandwidth 4K format.

If changing the source to a lower SDI data rate restores reliable video, investigate cable type, cable length, connectors and termination before assuming the source or display has failed.

For professional 4K applications, see our 12G-SDI BNC cable solutions.

7. TOO MANY CONNECTION POINTS ARE IN THE SIGNAL PATH

A production signal may travel through more than one cable.

The complete path could include:

• Patch panels
• Barrel adapters
• Bulkhead connectors
• Routers
• Converters
• Patch cables
• Multiple longer cable runs

Every additional interface becomes another potential troubleshooting point.

When diagnosing intermittent video, simplify the signal path whenever possible.

Temporarily connect the source directly to the destination using a known-good cable. If the direct connection works reliably, add the intermediate components back one at a time until the problem returns.

8. A COMPACT CONNECTOR ADAPTER OR PATCH CABLE IS CAUSING THE PROBLEM

High-density broadcast equipment increasingly uses compact interfaces such as HD-BNC and DIN 1.0/2.3.

These connector formats are useful when equipment manufacturers need more video connections in a limited panel area.

But troubleshooting should include every component in the path—not just the longest BNC cable.

If a system uses a BNC-to-HD-BNC or BNC-to-DIN cable, test that assembly just as you would the primary coaxial cable.

For more information on compact broadcast connectors, read our HD-BNC vs BNC guide.

9. THE PROBLEM ISN'T THE CABLE

It is easy to blame the cable whenever video disappears, but the cable should be tested as part of the complete system.

Other potential causes include:

• Incorrect source output format
• Unsupported resolution or frame rate
• Equipment configuration problems
• Faulty source output
• Faulty destination input
• Converter or router problems
• Firmware or compatibility issues

Testing with a short, known-good SDI cable is one of the fastest ways to separate a transmission problem from an equipment problem.

A SIMPLE SDI TROUBLESHOOTING PROCESS

When an SDI signal is dropping, avoid changing several things at once. A systematic process makes it easier to identify the actual failure.

1. Check the connectors. Make sure every BNC connection is fully seated and locked.
2. Substitute a known-good cable. Preferably use a short cable first.
3. Connect source directly to destination. Temporarily bypass patch panels, adapters and converters.
4. Verify the video format. Confirm that the destination supports the source resolution, frame rate and SDI format.
5. Reduce the data rate if possible. If 3G works but 12G does not, investigate transmission margin.
6. Check total cable distance. Include patch cables and other coaxial sections in the signal path.
7. Add components back individually. Identify exactly where the failure returns.

WHY 12G-SDI REQUIRES MORE ATTENTION

Moving from HD-SDI or 3G-SDI to 12G-SDI allows professional video systems to transport high-bandwidth 4K UHD signals over a single coaxial connection.

The tradeoff is that transmission margin becomes increasingly important.

Cable type, cable length, connector quality, termination and every connection in the signal path can affect performance.

This is why a professional 12G installation should be designed as a complete signal path rather than simply selecting a cable because it has BNC connectors on both ends.

PREVENTING SDI SIGNAL PROBLEMS BEFORE THEY START

Many intermittent SDI problems can be avoided during system design.

Choose cable based on the required SDI format and transmission distance, use appropriate 75-ohm connectors, avoid unnecessary adapters, protect cables from physical damage and leave reasonable transmission margin instead of designing every run at its practical limit.

For permanent installations and production systems, selecting the correct cable construction from the beginning can also make future troubleshooting considerably easier.

PROFESSIONAL SDI CABLE ASSEMBLIES FROM AV-CABLES

AV-Cables manufactures professional SDI BNC cable assemblies for broadcast facilities, studios, live production, corporate AV, houses of worship, government, OEM and systems integration applications.

Solutions include Mini RG59, RG59 and RG6 SDI assemblies, HD-BNC cables, DIN 1.0/2.3 cables and multi-channel SDI snake cables.

AV-Cables professional cable assemblies are hand assembled in the USA and individually tested before shipment.

AV-Cables — Professional Broadcast Connectivity Solutions