You need a longer SDI cable, but the production is already underway.

There are two shorter BNC cables nearby and a BNC barrel coupler in the toolbox.

Can you simply connect the cables together?

In many situations, an appropriate coupler may work. But every additional connector, adapter and cable section becomes part of the SDI transmission path—and that becomes increasingly important with higher-bandwidth signals such as 6G-SDI and 12G-SDI.

Understanding what happens when you add a BNC coupler or adapter to an SDI connection can help you decide when it is a practical solution and when a single properly sized SDI cable assembly is the better approach.

WHAT IS A BNC COUPLER?

A BNC coupler—often called a BNC barrel—allows two BNC cable assemblies to be connected together.

A common configuration has a female BNC connection on each side.

That allows a signal path such as:

Camera → SDI Cable → BNC Coupler → SDI Cable → Switcher

Mechanically, this is an easy way to extend an existing cable.

Electrically, however, you have created a longer transmission path containing additional connection points.

DOES A BNC COUPLER AUTOMATICALLY CAUSE SDI SIGNAL LOSS?

No.

A properly designed coupler used within an appropriate SDI signal path does not automatically cause the video to fail.

Professional systems routinely contain multiple connection points, including equipment connectors, patch panels and routing systems.

The more useful question is:

How much transmission margin does the complete SDI path have?

If a system already operates close to its reliable transmission limit, adding another connection can reduce that margin further.

EVERY CONNECTION BECOMES PART OF THE SIGNAL PATH

An SDI system should be evaluated as a complete transmission path rather than as a collection of independent pieces.

Consider this example:

50 ft Cable → Coupler → 25 ft Cable

The relevant cable distance is not simply 50 feet or 25 feet.

The signal travels through approximately 75 feet of coaxial cable plus the connectors and coupler between the source and destination.

If the signal also travels through a patch panel, router or additional adapter, those interfaces become part of the path as well.

WHY 75-OHM IMPEDANCE MATTERS

Professional SDI video uses a nominal 75-ohm transmission system.

The coaxial cable and connectors should therefore be appropriate for professional 75-ohm video applications.

A coupler should not be selected simply because a BNC connector physically fits.

BNC interfaces are also used in other applications, including systems designed around different characteristic impedances.

For SDI, use components intended for the 75-ohm video signal path.

WHY 12G-SDI MAKES THIS MORE IMPORTANT

A connection that works reliably at a lower SDI data rate does not necessarily have the same transmission margin at 12G-SDI.

As data rate increases, the performance requirements placed on the coaxial cable, connectors and overall signal path increase.

This can create a familiar troubleshooting scenario:

The coupler is not necessarily the only problem. The complete combination of cable type, total distance, connectors and equipment determines whether enough signal margin remains.

See our 3G-SDI vs 6G-SDI vs 12G-SDI guide for more information about the differences between these interfaces.

TWO SHORT CABLES DON'T BECOME ONE SHORT CABLE

This sounds obvious, but it matters when troubleshooting.

If you connect a 50-foot SDI cable to another 50-foot cable, you have created approximately a 100-foot coaxial run plus the additional connection between them.

The transmission capability therefore needs to be evaluated against the combined distance.

This is particularly important when using compact coaxial cable such as Mini RG59 for applications approaching the cable's practical transmission limits.

For cable construction differences, read our Mini RG59 vs RG59 vs RG6 comparison.

WHEN IS A BNC COUPLER USEFUL?

BNC couplers can be extremely useful tools in professional production.

Common applications include:

• Temporary production setups
• Emergency cable extensions
• Testing and troubleshooting
• Changing equipment locations
• Temporary camera runs
• Adding a short equipment patch cable

For a temporary live-production requirement, joining two appropriate cables may be far more practical than stopping production because the exact cable length is unavailable.

WHEN SHOULD YOU USE ONE LONGER SDI CABLE INSTEAD?

For permanent or frequently deployed connections, a single cable assembly of the appropriate length is often preferable.

One continuous assembly can:

• Reduce the number of connection points
• Reduce opportunities for accidental disconnection
• Simplify troubleshooting
• Improve cable management
• Eliminate an intermediate barrel hanging in the cable path
• Create a cleaner permanent installation

AV-Cables offers standard and custom-length SDI cable assemblies so installations do not have to be designed around whichever cable lengths happen to be available.

WHAT ABOUT BNC ADAPTERS?

An adapter performs a different job from a simple barrel coupler.

Adapters may be used to change connector formats or accommodate equipment interfaces.

In professional broadcast systems, equipment may use:

• Standard BNC
• HD-BNC
• DIN 1.0/2.3

An adapter may solve an immediate compatibility problem, but another option is using a cable assembly terminated with the correct connector on each end.

For example:

BNC → HD-BNC

or

BNC → DIN 1.0/2.3

This can eliminate the need for a separate adapter in permanent or regularly used installations.

BNC TO HD-BNC CABLE VS USING AN ADAPTER

Modern routers, converters, monitors and other high-density equipment increasingly use compact HD-BNC connectors.

If one device uses standard BNC and another uses HD-BNC, there are two basic approaches:

Option 1: BNC cable + separate adapter

Option 2: Purpose-built BNC-to-HD-BNC cable assembly

For a permanent system, the purpose-built assembly can reduce component count and simplify the connection.

For more information about the connectors themselves, see HD-BNC vs BNC: What's the Difference?

THE SAME PRINCIPLE APPLIES TO DIN 1.0/2.3

DIN 1.0/2.3 is another compact coaxial connector used on professional broadcast equipment.

If equipment requires DIN 1.0/2.3, a purpose-built BNC-to-DIN or DIN-to-DIN cable assembly can provide the required connector configuration without adding a separate inline adapter.

This can be particularly useful in racks, fly packs and other high-density systems where reducing unnecessary components helps keep the installation organized.

DON'T FORGET MECHANICAL RELIABILITY

Signal performance is not the only consideration.

Every additional inline connection creates another mechanical connection that can potentially become loose, damaged or accidentally disconnected.

This matters in environments such as:

• Live production
• Stage installations
• Mobile production trucks
• Fly packs
• Camera positions
• Frequently reconfigured systems

A coupler lying on a studio floor or hanging behind equipment may experience very different mechanical conditions from a connector secured directly to equipment.

WHAT IF THE SDI SIGNAL STARTS DROPPING AFTER ADDING A COUPLER?

If a previously reliable system becomes intermittent after adding an extension or adapter, simplify the signal path.

Try:

1. Remove the coupler or adapter.
2. Connect the source directly to the destination using a known-good cable.
3. Verify the required SDI format.
4. Inspect each connector.
5. Confirm the total cable distance.
6. Verify that the cable construction is appropriate for that distance and data rate.

If the direct connection works reliably, add components back one at a time.

This makes it easier to identify where the problem enters the signal path.

Our SDI troubleshooting guide covers additional causes of intermittent SDI video.

CAN CONTINUITY TESTING FIND A BAD COUPLER?

A continuity test can identify obvious electrical failures such as an open connection or direct short.

However, just as with an SDI cable, continuity alone does not prove high-frequency signal performance.

A connection can be electrically continuous and still be marginal when transporting a demanding SDI signal.

For more detail, read our guide to how to test an SDI cable and why continuity differs from signal-performance testing.

A SIMPLE RULE FOR SDI COUPLERS AND ADAPTERS

PROFESSIONAL SDI CABLE ASSEMBLIES FROM AV-CABLES

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

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

Standard and custom cable lengths can help eliminate unnecessary inline connections when building permanent or frequently deployed production systems.

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

AV-Cables — Professional Broadcast Connectivity Solutions