SDI and BNC Cable
Overview
SDI cables and BNC cables are both used for video signal transmission, but their similar names often cause confusion regarding their roles and differences.
In professional video systems that handle high-quality uncompressed digital signals, selecting the correct standards and connectors is essential for stable performance.
SDI (Serial Digital Interface) is a high-speed digital video transmission standard widely used in broadcasting and professional AV equipment.
What is Serial Digital Interface (SDI)?
What is Serial Digital Interface (SDI)?
SDI stands for "Serial Digital Interface." It is a high-speed serial interface standard mainly used in the broadcasting industry and professional video equipment to transmit uncompressed digital video signals.
Its greatest feature is the ability to transmit high-quality digital video signals over long distances through a single cable without signal degradation.
Other main features of SDI include the following:
Advantages of Uncompressed Transmission
Since the video signals are not compressed, there is minimal image quality degradation. This makes SDI ideal for professional environments where accurate handling of color information and resolution is essential.
Evolution of Standards
With technological advancements, both transmission speed and supported resolutions have improved. The original SD-SDI (approx. 270 Mbps) has been succeeded by HD-SDI (approx. 1.485 Gbps), 3G-SDI (approx. 2.97 Gbps), and high-speed standards such as 6G-SDI and 12G-SDI, which support 4K/Ultra HD.
What is BNC Connectors and BNC Cables?
BNC stands for "Bayonet Neill-Concelman" and refers to a type of connector commonly used at the ends of coaxial cables. BNC connectors are specialized for connecting high-frequency signals such as video signals (SDI).
Other main features of BNC include the following:
Bayonet Lock Mechanism
BNC connectors are equipped with a “bayonet lock” mechanism, which allows for secure locking (fixation) simply by inserting and twisting the connector about 90 degrees. This mechanism prevents cables from becoming detached due to vibration or impact, ensuring a highly reliable connection.
Impedance
For SDI signal transmission, BNC connectors with a standardized characteristic impedance of 75Ω (ohms) are used according to video signal standards. While 50Ω BNC connectors also exist, they are intended for different applications and are not typically used for video transmission.
In practice, because BNC connectors are synonymous with high-frequency signal cables, coaxial cables with BNC connectors on both ends are often referred to as "BNC cables."
Relationship Between SDI, BNC, and Coaxial Cable
SDI, BNC, and coaxial cable together form an inseparable trio in high-quality digital video transmission systems.
SDI
A specification (set of rules) for transmitting video signals.
Coaxial Cable
The physical medium (path) that carries SDI signals.
BNC Connector
The component (interface) that securely connects devices and cables.
To ensure stable transmission of digital video in accordance with SDI standards, it is necessary to use 75Ω coaxial cables that comply with the specifications, with 75Ω BNC connectors attached at the ends.
Unifying the impedance of the entire system at 75Ω is the most important point for preventing signal reflection and maintaining video quality.
Differences and Selection Points to Keep in Mind for SDI and Coaxial Cables (BNC)
As the SDI standard continues to advance toward higher speeds, the most important factors for technical personnel to consider when selecting cables are the following three elements:
Impedance
The most important factor that determines transmission quality is the consistency of impedance.
All SDI-standard systems are designed with a characteristic impedance of 75Ω. This means that all cables, connectors, and connected devices (such as transmitters and receivers) must be 75Ω.
If a 50Ω cable or connector is mixed in at any point, an impedance mismatch will occur, resulting in signal reflections. As a result, video signals may become distorted, or in the worst case, the video transmission may be interrupted, causing transmission failures.
It is essential to check that cables and connectors are clearly marked "75Ω" and to avoid mixing them with 50Ω products.
Signal Standard, Transmission Quality, and Transmission Distance
The faster the SDI signal, the greater the signal attenuation (drop in signal strength) within the cable.
For example, compared to SD-SDI at approximately 270 Mbps, the maximum transmission distance using the same cable is significantly shorter for 12G-SDI, which operates at around 12 Gbps.
To ensure sufficient transmission distance, it is necessary to select high-grade coaxial cables with "low attenuation characteristics," which are determined by factors such as the thickness of the conductor and the quality of the dielectric material.
It is crucial to clearly define which SDI standard you will use (such as 3G-SDI or 12G-SDI) and the required transmission distance. Then, select a cable that meets the required attenuation level for that distance. This is the key to stable operation.
Usage Scenarios and Selection Differences
When selecting cables, it is necessary to consider not only signal quality but also the physical conditions of the operating environment.
For fixed installations where the wiring will not be moved once laid, cables that prioritize signal characteristics are sufficient. However, in environments where cables are frequently set up and removed—such as at relay sites or event venues—the cables are subject to bending and pulling forces. In these cases, a cable structure offering high flexibility and durability is required.
When selecting BNC connectors, it is important to choose products that match the outer diameter and conductor size of the cable, and to attach them correctly using dedicated crimping tools or proper soldering techniques. The quality of this assembly process is vital for preventing signal reflections and maintaining reliable connections over time.
For more detailed information on how to attach BNC connectors, please refer to the following article.
Related Products
Understand the Differences Between SDI, BNC, and Coaxial Cable to Choose the Optimal Cable
SDI is a "standard," BNC is a "connector," and coaxial cable is the "medium." While each plays a different role, high-quality and stable digital video transmission can only be achieved when all components are unified under a characteristic impedance of 75Ω.
When building or operating an SDI-based system, especially with high-speed standards (such as 3G, 6G, or 12G-SDI), it is essential for technical staff to prioritize checking both the low attenuation characteristics of the cable and the consistency of impedance (75Ω).
By selecting appropriate 75Ω coaxial cables and BNC connectors that meet the required standards—and considering flexibility and durability to suit the operating environment—you can prevent signal reflection and noise-related video issues. This ensures the foundation for a highly reliable video transmission system.
Purchase Guide
Tyclon coaxial connectors and processed coaxial cable products can be purchased directly online using a variety of credit cards.
