US How a BNC connector works
A BNC connector is a coaxial connector that locks with a quarter turn. The name stands for Bayonet Neill–Concelman: the bayonet coupling, plus Paul Neill of Bell Labs and Carl Concelman of Amphenol, who designed it. It dates from the 1940s, when it was introduced on military radio equipment, and it is still the standard video connector on broadcast and industrial capture equipment.
The male plug has a centre pin and a rotating collar with two slots. The female jack has two lugs on its outer shell. Push the plug on, turn the collar a quarter turn and the lugs seat in the slots, so the connection cannot vibrate loose or be pulled out by a snagged cable. The centre pin carries the signal and the outer shell carries the shield, which keeps the cable’s impedance constant through the joint.
Quick, positive locking and a controlled impedance are why BNC has outlived most connectors of its age. A BNC cable is simply coax with a BNC plug fitted to each end.
Types of BNC connector
BNC connectors are grouped by impedance, by size and by how they attach to the cable. The type matters more than it looks, because two connectors that mate perfectly can still be wrong for the signal.
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75 ohm BNC. The video version, used for SDI, analog composite video and HD-TVI CCTV. The insulation inside the connector is sized to match 75 ohm cable.
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50 ohm BNC. Used for RF work, oscilloscopes, signal generators and timing inputs such as 10 MHz references and 1PPS. It was also the connector on legacy 10BASE2 thin Ethernet.
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Mini-BNC and HD-BNC. Smaller high-density versions that fit more channels on a card bracket or router panel, usually with breakout cables to full-size BNC.
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Panel-mount and bulkhead jacks. The female socket fitted to capture cards, monitors and enclosures. Bulkhead versions pass the signal through a panel wall.
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Crimp, compression and twist-on plugs. Crimp and compression plugs hold impedance and shielding at SDI data rates. Twist-on plugs are a field fix best kept to analog CCTV.
50 ohm vs 75 ohm BNC
The two impedances look almost identical and will mate with each other, so they are easy to mix up. On analog video a mismatch makes little visible difference. At SDI data rates it does: each mismatch reflects part of the signal back down the cable, which shows up as shorter reach or dropped frames, and it is worst at 6G and 12G-SDI.
For video, keep connectors, cable and patch leads 75 ohm from end to end. For 12G-SDI, choose connectors and cable specified for 12G, typically rated to 12 GHz, because a standard BNC is usually only characterized to about 4 GHz.
How far SDI runs over BNC and coax
Serial digital interface (SDI) video is the main reason BNC is still fitted to new equipment. Each SDI generation doubles the data rate, and the higher the rate, the shorter the cable run.
| SDI standard | SMPTE | Data rate | Highest format | Typical run on RG-6 |
|---|---|---|---|---|
| SD-SDI | ST 259 | 270 Mb/s | 576i / 480i | Around 300 m |
| HD-SDI | ST 292-1 | 1.485 Gb/s | 1080i / 720p | Around 100 to 140 m |
| 3G-SDI | ST 424 | 2.97 Gb/s | 1080p60 | Around 80 to 100 m |
| 6G-SDI | ST 2081 | 5.94 Gb/s | 2160p30 (4K) | Around 60 to 80 m |
| 12G-SDI | ST 2082 | 11.88 Gb/s | 2160p60 (4K) | Around 40 to 70 m |
The runs are typical for good-quality RG-6 class 75 ohm cable. They depend on the cable grade and on the receiver’s cable equalisation, and thinner RG-59 cable shortens them, most of all at 6G and 12G. Check the card and cable datasheets before committing to a long run.
Our SDI capture cards take SDI on BNC, from single-input cards to 2- and 4-channel boards, with 12G-SDI capture cards for 4K60 sources. Our guide to SDI capture for edge AI covers how those inputs feed an inference pipeline.
Which cable a BNC connector uses
BNC plugs are made for specific coax sizes, so the plug has to match the cable diameter as well as the impedance. When buying a BNC cable for video, check three things: that it is 75 ohm, which SDI rate it is rated for, and its length. A lead rated for 3G-SDI may not carry 12G-SDI reliably over the same distance.
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RG-59 (75 ohm). Thinner and more flexible. Fine for analog CCTV and short SD or HD-SDI links.
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RG-6 (75 ohm). A thicker centre conductor and lower loss at high frequencies, so it is the usual choice for 3G, 6G and 12G-SDI and for longer runs.
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RG-58 (50 ohm). The common 50 ohm cable for RF, test leads and timing signals. Not suitable for SDI.
Where BNC is used in industrial systems
Outside broadcast studios, BNC turns up wherever a video or timing signal has to travel further than HDMI or USB allow, or has to stay connected on a vibrating machine.
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Live production and broadcast capture. SDI cameras and switchers feed capture cards in encoders, replay and graphics systems.
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Machine vision and medical imaging. SDI cameras on inspection lines, and the SDI outputs of many surgical and endoscopy systems, are captured for recording or AI analysis.
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Analog HD CCTV. HD-TVI, AHD and HD-CVI cameras send HD video over existing coax and BNC, so HD-TVI capture cards can record them without re-cabling the site.
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Monitoring at the point of use. Rack and panel SDI monitors show a camera or switcher feed where the operator works.
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Test, timing and RF. Oscilloscopes, signal generators and 10 MHz or 1PPS timing references use 50 ohm BNC.
Quick answers on BNC
BNC connectors: common questions
What does BNC stand for?
Bayonet Neill–Concelman. Bayonet describes the quarter-turn lock, and Neill and Concelman are the two engineers who designed the connector.
Can you convert BNC to HDMI?
Yes, but only with an active converter. BNC carries SDI or analog video and HDMI carries a different digital signal, so a passive adapter will not work. Use an SDI-to-HDMI converter for SDI sources and a scaler for composite or HD-TVI. To bring the feed into a computer, an SDI capture card removes the extra box.
Are BNC connectors male or female?
Both. The plug, normally on the cable, is male, with a centre pin and a rotating collar. The jack, normally on the equipment, is female, with two bayonet lugs. A female-to-female barrel joins two cables.
Is BNC analog or digital?
Either. The connector sets the mechanical fit and the impedance, not the signal. The same 75 ohm BNC socket can carry composite video, HD-TVI or a 12G-SDI stream, although 12G needs connectors rated for it.
Is BNC the same as SMA?
No. SMA is a smaller, threaded 50 ohm connector used for antennas and RF at much higher frequencies. BNC uses a bayonet lock and is the usual video connector. The two only connect through an adapter.
We fit SDI capture cards in industrial and edge AI computers as standard, install the drivers and validate each input before the system ships. Send us your sources, formats and cable runs, and we will specify the card, cable and connectors to match.