BNC (75 Ω) pinout
BNC / SDI Connector Reference
A BNC carries SDI as an unbalanced 75 ohm transmission line: the center pin is the signal, the shell is ground and shield. Every element in the path (cable, connectors, barrels, patch panels) must be 75 ohm parts; 50 ohm BNCs mate mechanically and degrade the signal.
| Pin | Signal | Notes |
|---|---|---|
| Center | Signal | The serial digital stream; a single conductor carrying up to 12 Gb/s. |
| Shell | Ground / shield | Return path and shield; the bayonet shell. |
What it’s used for
SDI remains the backbone of broadcast-style show video: cameras to switcher, switcher to processors, distribution amps everywhere. It runs long distances on ordinary coax, locks positively, and carries embedded audio and metadata alongside the picture.
The same BNC also carries reference (black burst or tri-level sync), analog video, and word clock in audio land, at different impedances and levels; label the looms.
Wiring & termination notes
- Use true 75 ohm BNCs and 75 ohm coax rated for the bandwidth (12G-rated cable and connectors for 12G paths); 50 ohm parts cause return loss that shows up as sparkles and dropouts at the receiver cliff edge.
- Distance falls as rates rise: the same premium coax that carries SD for hundreds of meters may only be trusted for 60 to 70 m at 12G. Re-clock or convert to fiber past the spec.
- SDI fails hard, not gracefully: the picture is perfect until it is gone. A path that "works" with sparkles is at the cliff; shorten, re-terminate, or re-clock.
- Compression-style F-to-BNC adapters and rusty patch bays are the two most common mystery-failure sources in inherited racks.
Frequently asked questions
What are the SDI data rates?
SD-SDI 270 Mb/s (SMPTE 259M), HD-SDI 1.485 Gb/s (SMPTE 292M), 3G-SDI 2.97 Gb/s (SMPTE 424M) for 1080p60, 6G and 12G-SDI (SMPTE ST 2081/2082) for 2160p30 and 2160p60 respectively.
How far can SDI run on coax?
Cable dependent: with premium RG6-class coax, roughly 300 m at SD, 150 to 200 m at HD, 100 to 140 m at 3G, and 60 to 80 m at 12G are typical planning figures. Manufacturer charts for the exact cable govern.
Does it matter if a BNC is 50 ohm?
Yes for digital video. 50 ohm BNCs (common on RF and lab gear) mate with 75 ohm ports but create impedance discontinuities; at HD rates and above they eat your margin.