SDI (Serial Digital Interface)

showstack / protocols / sdi

SDI (Serial Digital Interface)

Also called SDI, HD-SDI, 3G-SDI, 6G-SDI, 12G-SDI, SMPTE 259M, SMPTE 292.

SMPTE's uncompressed baseband digital video interface, carried on 75 ohm coax and BNC from 270 Mb/s standard definition up to 11.88 Gb/s 12G. It is still the backbone of most live video: point to point, no negotiation, no network, and it either locks or it does not.

Video transportOpenSMPTEVerified

Stewarded by SMPTE (Society of Motion Picture and Television Engineers).

Media Large, continuous, on time and in order

Continuous streams that are large enough to be sized properly, and that must arrive in order and on time.

What kills it. Anything that starves the link — usually a management transfer nobody scheduled — and jitter that outruns the receive buffer.

The four flows, and why the model is worth having →

What it carries

Sample rates: 48 kHz
Audio bit depth: 24-bit
Audio channels: up to 16
Audio encoding: Embedded AES3 in the ancillary data space
Resolution: 3840x2160
Frame rate: up to 60 Hz
Colour sampling: 4:2:2, 4:4:4
Video bit depth: 10-bit, 12-bit
Video compression: None — uncompressed

Audio: Sixteen channels in four groups, riding in the horizontal blanking.

Video: 3840x2160 needs 12G-SDI on a single link; above that you are on two or four. The rate tiers are SD 270 Mbit/s, HD 1.485, 3G 2.97, 6G 5.94 and 12G 11.88. Cable reach falls as the rate rises, because coax loss goes with the square root of frequency and the frequency that matters is half the bit rate.

What goes wrong

The bit rate is fixed by the standard, not by the picture. ST 259 is 143/270/360 Mb/s, ST 292-1 is 1.485 Gb/s (or 1.485/1.001), ST 2082-1 is 11.88 Gb/s. A "12G" run is really an 11.88 Gb/s run.

The connector is 75 ohm BNC per IEC 61169-8. 50 ohm BNCs are mechanically identical, will fit, will pass SD happily, and will then fail at 3G and 12G in a way that looks like a bad cable everywhere else in the chain.

Cable loss is specified at half the clock frequency, around 20 dB for ST 292-1. The same drum of cable that carries HD across a stadium will not carry 12G, so re-derate every existing run when you upgrade rate.

There is no negotiation, no handshake and no error correction. A marginal link produces sparkles or black frames, and the receiver reports lock rather than quality, so "it says it is locked" proves very little.

Audio, timecode and captions ride as ancillary data in the blanking intervals. Some converters, re-clockers and scalers pass video cleanly and drop ANC, so timecode and captions vanish with no error anywhere.

Return loss is specified at 15 dB and rise time at 270 ps for HD, 45 ps for 12G. Cheap patch panels and barrel connectors are the usual reason a 12G path that works on the bench fails in the rack.

What speaks SDI (Serial Digital Interface) (8)

Product	Direction	Notes
Millumin Anomes (Philippe Chaurand)	bidirectional	Via Blackmagic (Intensity, UltraStudio, DeckLink) and AJA cards, used both for capture and as outputs; Epiphan, Magewell, Inogeni and GigE cameras are also listed for capture.
Ventuz Ventuz Technology	bidirectional	SDI in and out with suitable capture hardware.
Analog Way Aquilon RS4 Analog Way	bidirectional	12G and 3G-SDI on the input and output cards.
Barco Event Master E2 Barco	bidirectional	3G-SDI inputs and outputs on the standard cards.
Blackmagic ATEM 4 M/E Constellation HD Blackmagic Design	bidirectional	3G-SDI in and out across all ports.
Blackmagic Smart Videohub 40x40 Blackmagic Design	bidirectional	12G-SDI routing across all ports.
Brompton Tessera SX40 Brompton Technology	in	12G / quad 3G-SDI input depending on configuration.
NovaStar MX40 Pro NovaStar	in	3G-SDI input.

Typical use

Camera to switcher and switcher to projector or LED processor
Distribution amplifiers and routers in a control room or OB truck
Feeding media servers, recorders and multiviewers
Long runs on fibre via SDI to fibre converters (SMPTE ST 297)

Sources

SMPTE ST 292-1:2018 — 1.5 Gb/s Signal/Data Serial Interface — SMPTE primary
SMPTE ST 259:2008 — SDTV Digital Signal/Data, Serial Digital Interface — SMPTE primary
SMPTE ST 2082-1:2023 — 12 Gb/s Signal/Data Serial Interface — SMPTE primary
SMPTE EG 2111-1:2021 — SD-SDI and HD-SDI Standards Roadmap — SMPTE primary

Understand the mechanism

Show networks Node graphs and game engines

Something wrong, or missing?

Known gaps on this entry: default_ports, first_published, implementations. If you can source one of them, that is a ten minute pull request.

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