Professional broadcast equipment uses 75-ohm coaxial connections for SDI video, genlock, black burst, tri-level sync, and other reference signals.

Because many of these connections use the same familiar BNC connector, it is easy to assume they all require the same termination method.

However, that assumption can create problems.

Some equipment inputs already contain an internal 75-ohm termination. Other connections, especially passive loop-through reference inputs, may require an external 75-ohm BNC terminator.

Installing a terminator where one is not needed—or failing to install one where it is required—can affect signal levels and system operation.

In this guide, we'll explain what a BNC terminator does, when it is needed, how SDI termination differs from genlock termination, and what to check when troubleshooting professional broadcast connections.

WHAT IS A 75-OHM BNC TERMINATOR?

A 75-ohm BNC terminator is a connector-mounted electrical load designed to terminate a compatible 75-ohm coaxial transmission line.

Professional video coaxial systems are designed around a nominal characteristic impedance of 75 ohms.

When a transmission line is properly terminated, the receiving end presents the intended load to the signal.

This helps reduce reflections that can occur when a signal encounters an impedance mismatch.

Not every BNC port needs a separate terminator. The correct method depends on the equipment's internal design.

DO STANDARD SDI INPUTS NEED EXTERNAL TERMINATORS?

Usually, no.

Most conventional professional SDI receiving inputs are designed with an internal 75-ohm termination.

For example, a camera's SDI output may connect directly to a compatible monitor, recorder, switcher, or router input using a professional 75-ohm coaxial cable.

In this example, the monitor's SDI input provides the intended receiving termination.

An additional external terminator is not required at the occupied input.

In fact, adding an extra parallel 75-ohm load can reduce the effective load impedance and cause signal-level problems.

Always confirm the input termination specifications in the equipment documentation, particularly when working with unusual or configurable interfaces.

WHEN IS AN EXTERNAL BNC TERMINATOR REQUIRED?

One common application is a passive loop-through video reference connection.

Some professional broadcast equipment provides two BNC connectors for an external reference signal.

These may be labeled:

• REF IN / REF LOOP
• GENLOCK IN / LOOP
• REFERENCE A / B
• BLACK BURST IN / LOOP

In a passive loop-through arrangement, the reference signal enters one BNC port and can continue through the other BNC port to additional equipment.

The final end of the reference line must be properly terminated according to the equipment manufacturer's instructions.

Example: One Device with a Passive Reference Loop

For equipment that requires external termination of a passive reference loop, the unused loop port is fitted with a suitable 75-ohm terminator.

This completes the reference signal path with the intended load.

However, some devices offer internal or switchable termination. Do not add an external terminator without checking the manufacturer's instructions.

SDI VS. GENLOCK: TERMINATION REQUIREMENTS

SDI video and analog reference signals may use similar 75-ohm coaxial cables and BNC connectors, but their termination arrangements can differ.

These are general guidelines. The equipment manufacturer's termination instructions take priority.

HOW TO TERMINATE A MULTI-DEVICE GENLOCK LOOP

Some broadcast facilities distribute analog reference signals through multiple pieces of equipment using passive loop-through connections.

For example, a reference signal may travel from a sync generator through several compatible video devices.

In this arrangement, the reference line is terminated at the final end of the passive loop.

Adding a separate 75-ohm terminator at every intermediate device would create additional loads and could reduce the signal level.

The equipment must support the intended passive loop arrangement, and the complete distribution path must remain within the reference generator's specifications.

For larger installations, a dedicated reference distribution amplifier may provide a more appropriate way to feed multiple devices.

PASSIVE LOOP-THROUGH VS. ACTIVE LOOP OUTPUT

Not every port labeled LOOP behaves the same way.

Passive Loop-Through

A passive loop-through provides a continuation of the incoming coaxial reference path.

Depending on the device design, an external terminator may be required when the loop is not connected to another device.

Active or Buffered Loop Output

An active loop output uses electronic circuitry to provide a separate output signal.

It may include signal buffering or reclocking, depending on the equipment.

An active SDI loop output should not automatically be treated as an unterminated passive reference loop.

Its loading and termination requirements must be confirmed in the equipment manual.

WHAT HAPPENS IF A REFERENCE LINE IS NOT TERMINATED?

When a transmission line is left improperly terminated, the impedance mismatch can cause part of the signal energy to reflect.

In an analog reference system, improper termination may contribute to signal-level errors, waveform distortion, or synchronization problems.

Possible symptoms can include:

• Unstable reference locking
• Unexpected reference signal amplitude
• Synchronization warnings
• Intermittent equipment reference detection
• Video timing instability
• Problems that appear after adding another device to a reference loop

These symptoms do not automatically prove that termination is the cause.

The reference generator, cabling, equipment settings, connector condition, and signal format must also be considered.

CAN YOU OVER-TERMINATE A VIDEO SIGNAL?

Yes.

Adding more than the intended load can reduce the effective impedance seen by the signal source.

For example, two ideal 75-ohm resistive loads connected in parallel create an equivalent resistance of approximately 37.5 ohms.

The actual electrical behavior depends on the equipment and circuit configuration, but the practical rule remains the same: use the termination arrangement specified by the manufacturer.

DOES 12G-SDI REQUIRE A SPECIAL TERMINATOR?

For a normal 12G-SDI connection between a compatible transmitter and a conventional internally terminated receiver, a separate external terminator is not required.

However, if specialized equipment requires an external termination at high frequencies, the terminator must be suitable for the actual signal bandwidth and return-loss requirements.

Do not assume that a generic terminator intended for low-frequency analog video is appropriate for a specialized 12G-SDI application.

Also, a terminator does not compensate for excessive cable attenuation or a poorly constructed cable assembly.

Reliable 12G-SDI transmission still requires the appropriate 75-ohm coaxial cable, connectors, equipment, and total signal-path length.

Explore Professional 12G-SDI BNC Cable Assemblies

BNC CONNECTOR TERMINATION VS. 75-OHM LINE TERMINATION

Another common source of confusion is the word termination itself.

In professional cable manufacturing, terminating a BNC connector means attaching the connector to the coaxial cable using the correct preparation and assembly method.

For example, a professional crimp connector must be matched to the specific cable construction.

Mini RG59, full-size RG59, and RG6 cables may require different connectors and installation tooling.

This is different from installing a 75-ohm resistive terminator at the end of a transmission line.

Connector termination: The process of assembling a connector onto the cable.

Line termination: Providing the required electrical load at the end of the signal path.

Both matter, but they solve different problems.

Explore Professional 75-Ohm BNC Connectors and Adapters

CHOOSING THE RIGHT COAXIAL CABLE FOR SDI AND REFERENCE SIGNALS

Professional SDI and compatible analog reference connections commonly use 75-ohm coaxial cable.

However, the appropriate cable construction depends on the signal format, length, and installation environment.

Belden 4855R Mini RG59

Belden 4855R is a compact 75-ohm professional coaxial cable designed for compatible 12G-SDI applications.

Its small diameter makes it useful in high-density broadcast racks and production equipment.

Shop Belden 4855R 12G-SDI Cables

Belden 1855A Mini RG59

Belden 1855A is another compact professional 75-ohm cable used for compatible SDI applications through 6G-SDI.

It is useful for shorter equipment connections where compact cable routing is important.

Explore Professional 6G-SDI Cables

Full-Size RG59 and RG6

Larger 75-ohm coaxial constructions can be appropriate for longer video connections where lower attenuation is needed.

Examples include professional RG59 and RG6 cable families used in broadcast installations.

For analog reference signals, confirm that the cable and equipment support the required reference format and distance.

The correct cable selection should be based on the complete signal path rather than connector appearance alone.

BNC TERMINATION TROUBLESHOOTING CHECKLIST

Do not change termination arrangements on critical production equipment without first confirming the correct configuration.

FREQUENTLY ASKED QUESTIONS

What does a 75-ohm BNC terminator do?

It provides a suitable 75-ohm load at the end of a compatible coaxial transmission line to help prevent unwanted signal reflections caused by an improper load.

Do SDI inputs need BNC terminators?

Most conventional professional SDI inputs are internally terminated at 75 ohms and do not need a separate external terminator. Verify the specifications for the exact equipment.

Does genlock need a 75-ohm terminator?

Passive loop-through genlock connections commonly require termination at the end of the reference line. Internally terminated reference inputs may not require an external terminator.

Does black burst require termination?

Yes, black burst reference signals require the correct receiving load. Whether that load is provided internally or by an external terminator depends on the equipment.

Does tri-level sync require termination?

Tri-level sync systems also require the proper reference input load. Check whether the receiving equipment provides internal termination or uses a passive loop-through arrangement.

Should I terminate every unused BNC port?

No. Unused BNC ports do not all have the same termination requirements. Follow the manufacturer's instructions for the specific port and signal type.

Can I use a 50-ohm BNC terminator for SDI or genlock?

Do not substitute a 50-ohm terminator where a 75-ohm video load is specified. The impedance mismatch can affect signal performance.

Is a BNC coupler the same as a BNC terminator?

No. A BNC coupler joins compatible coaxial connections. A terminator provides an electrical load. They perform different functions.

Can a missing terminator cause video problems?

Yes, in a system that requires external termination, a missing load can contribute to reflections, reference instability, or other signal problems.

Does a 12G-SDI cable need a terminator on both ends?

No. A normal 12G-SDI connection uses a compatible transmitter, a suitable 75-ohm cable assembly, and a properly terminated receiving input. External terminators are not added to both ends of an ordinary active SDI connection.

THE BOTTOM LINE

A 75-ohm BNC terminator is an important component in certain professional broadcast signal paths, particularly passive analog reference loop-through connections.

However, most standard SDI receiving inputs already provide the required termination internally.

The correct approach is not to add a terminator to every BNC connection.

Instead, identify the signal type, understand the equipment's input and loop-through design, and follow the manufacturer's termination instructions.

Whether you're working with 6G-SDI, 12G-SDI, genlock, black burst, or tri-level sync, maintaining the proper 75-ohm signal path is an important part of professional broadcast system reliability.

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