By Garrett Seeley
Serial Digital Interface, or SDI, is a high-definition (HD) video connection that originated in the professional television video camera industry. It is widely used for video devices, including gastrointestinal (GI) and surgical video systems, because of its superior image quality and extended transmission distance. Since it comes from the television broadcast industry, this standard is made for high resolution and multiple devices into a standard mixing or control board. It is one directional in nature but can use two conductors for bidirectional control.
TECHNICAL SPECS
SDI operates on a dedicated unidirectional digital line using 75-ohm coaxial cabling, like the cabling used in a 10Base5 network. The video transmitted through SDI is digital, uncompressed and unencrypted. In a classic SDI video system, the camera output is sent directly to a mixing board, which acts as a receiver for processing the video output stream. Data rates can exceed 12 Gbps in full HD video range. Typically, several cameras are connected to a professional grade mixing board, which selects the appropriate camera feed for main output. There is no control required because a camera operator is on the other end, with a headset for communication. This is how a TV setup works at a large, live event, such as a football game.
Modern SDI systems can incorporate computer-generated effects, such as multi-screen or picture-in-picture options. USB connections facilitate timing for capture, processing and storage, enabling full digital storage and editing of the output. This setup is often used by modern online video streamers to integrate multiple cameras into one output. A popular manufacturer of this equipment is Blackmagic, and the mixing boards are known as ATEM Control Panels, although they are also referred to as mix boards or switchers.
CABLING & CONVERSION
SDI cabling can be converted to various formats, including HDMI, DVI, Coaxial BNC connectors or Fiber links. The unidirectional nature of the SDI signal means that two coaxial cables or two fiber optic cables are necessary for longer-range controlled connections. BNC connectors are the most used in medical applications, with one input line and another separate output line needed for bidirectional communication, often referred to as Dual Link HD-SDI.
APPLICATION IN GI VIDEO SYSTEMS
In gastrointestinal video applications, such as endoscopy towers, the endoscopy camera is controlled directly by a Camera Controller, which serves as the image source. The Camera Controller typically has a separate control line, often marked as Y/C, that is distinct from the video signal. This line refers to the two data streams in the Dual Link HD-SDI protocol, enabling bidirectional connections between system components. The SDI video signal can be an SDI BNC connection or an RGB connection that runs through an SDI converter. From the image source, the output BNC connects to the input BNC at the destination. It’s important to note that this video signal is typically one output to one input without using a splitter, converter or duplicator.
APPLICATION IN RADIOLOGY
In radiology applications, such as room camera systems, most cameras are unidirectional and do not require a control SDI cable. These cameras’ BNC output lines connect to the control panel’s BNC input. For long distances, a fiber converter can be used. If controllable devices are involved, input and output cables run between the control panel and the device. A fiber optic patch cable can be adapted for long, bidirectional digital runs. For systems with multiple devices, the control panel often connects to a converter that functions as a hub, managing communication with each device, which is typically assigned to a control ID using setup software.
CONCLUSION
SDI communication standards offer a fast, fully digital HD video solution for medical applications. It provides longevity to the older BNC ThickNet cabling, which was always designed to maximize range. SDI communication surpasses HDMI or DVI in capability for these specialized applications, exceeding the typical 50-foot range of home use HD cabling. This is the primary takeaway, that of range and a native dataflow of one direction. Since the HD SDI standard is unidirectional, requiring two connections for bidirectional communication. Be careful to hook it up with an input receiving from an output. The ports are ideally labeled, and the OEM documentation should be consulted as the ultimate guide. Also remember that although BNC connectors are the preferred choice in medical systems, manufacturers may use other connections for proprietary applications. Let’s just say that the documentation is of key importance in SDI setups.
