SDI — Serial Digital Interface
Serial Digital Interface (SMPTE 259M / 292M / 424M / 2081 / 2082)
For full SDI technical coverage, see signal-types/sdi.
SDI (Serial Digital Interface) is the professional broadcast standard for transmitting uncompressed digital video over 75 Ω coaxial cable. Unlike HDMI, SDI has no content protection (no HDCP), supports very long cable runs (up to 100+ m at HD rates on standard RG-6), uses robust locking BNC connectors, and can embed up to 16 channels of AES audio within the video signal. SDI is the standard for broadcast studios, live production, video surveillance, and professional camera systems. In installed AV, SDI appears in broadcast-connected conference rooms, video walls fed from cable/broadcast, and production facilities.
SDI Standards and Data Rates
| Standard | SMPTE | Data Rate | Supports |
|---|---|---|---|
| SD-SDI | SMPTE 259M | 270 Mbps | 480i, 576i (standard definition) |
| HD-SDI | SMPTE 292M | 1.485 Gbps | 720p, 1080i, 1080p30 |
| 3G-SDI | SMPTE 424M | 2.97 Gbps | 1080p60 (full raster) |
| 6G-SDI | SMPTE 2081 | 5.94 Gbps | 2160p30 (4K) |
| 12G-SDI | SMPTE 2082 | 11.88 Gbps | 2160p60 (4K60) |
| UHD-2 / 24G-SDI | SMPTE 2083 | 23.76 Gbps | 8K (experimental) |
For most installed AV requiring 1080p60, 3G-SDI is the standard. 12G-SDI is used for 4K60 single-cable transmission in broadcast and high-end production.
Cable and Distance
SDI uses 75 Ω coaxial cable (Belden 1694A or equivalent broadcast-grade). Cable distance limits by signal rate:
| Standard | Belden 1694A | Notes |
|---|---|---|
| SD-SDI | 300+ m | Essentially unlimited with good cable |
| HD-SDI | 100–150 m | Dominant installed limit |
| 3G-SDI | 80–100 m | Recommended: < 100 m with 1694A |
| 6G-SDI | 40–60 m | Higher-grade cable (Belden 4694R) recommended |
| 12G-SDI | 20–50 m | Cable quality critical; use 4855R or 4K-rated cable |
SDI over fiber optic: fiber SDI extenders (Decimator DMON, Blackmagic Micro Converter, or rack-mount SDI-over-fiber) can extend SDI over fiber to kilometer+ distances. Used in stadium, campus, and broadcast trunk applications.
Embedded Audio in SDI
SDI carries embedded audio in the ancillary data space — up to 16 channels (8 stereo pairs) in HD-SDI and 3G-SDI. This eliminates separate audio runs for camera feeds and broadcast sources. Audio embedders insert AES3 audio into the SDI stream; de-embedders extract it to AES3 or analog outputs. Most broadcast cameras output embedded audio from their microphone inputs directly in the SDI stream.
In a production workflow, the audio mixer's stereo mix is typically embedded into the SDI program output at the vision mixer (video switcher) for downstream distribution. DSPs extract this audio at the destination for local audio distribution.
SDI vs. HDMI in AV
| Factor | SDI | HDMI |
|---|---|---|
| Content protection | None (no HDCP) | HDCP required for protected content |
| Cable distance | 100 m+ (3G-SDI on 1694A) | 5–15 m passive; active to 30 m |
| Connector | Locking BNC (professional) | Push-fit (fragile in installed use) |
| Consumer equipment | Not supported | Universal |
| Audio channels | Up to 16 embedded | Up to 32 (HDMI 2.0+) |
| 4K support | 12G-SDI single cable | HDMI 2.0/2.1 |
| Cost | Higher (professional equipment) | Lower (consumer-grade available) |
SDI is preferred for: camera feeds, broadcast sources, long runs, production environments, any application where HDCP is unacceptable (lecture capture, content moderation). HDMI is preferred for: laptop/PC sources, consumer displays, short runs, and any situation requiring HDCP-encrypted content delivery.
Common Pitfalls
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Using standard RG-59 coax instead of broadcast-grade 75 Ω SDI cable. Generic RG-59 has higher attenuation at SDI frequencies, reducing effective distance by 50% or more. Fix: specify Belden 1694A (HD/3G-SDI) or 4694R (6G/12G-SDI); never substitute consumer coax for SDI.
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Confusing 3G-SDI Level A and Level B. SMPTE 424M defines two mappings: Level A (single link, direct sample mapping) and Level B (dual link mapping in one cable). Some equipment is Level A only; connecting to a Level B source produces no picture or a scrambled image. Fix: match Level A/B settings between source and destination; most modern equipment defaults to Level A.
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Not terminating unused SDI outputs. SDI requires 75 Ω termination at the end of each cable. An unterminated SDI output causes reflections that can interfere with the signal, particularly at 6G/12G-SDI rates. Fix: install 75 Ω BNC termination caps on unused outputs of SDI distribution amplifiers and routers.
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Attempting to pass SDI through an HDMI extender. SDI and HDMI are electrically incompatible — you cannot adapt SDI to an HDMI cable without a converter (Blackmagic Micro Converter SDI to HDMI, Decimator MD-LX). Fix: use a proper SDI-to-HDMI converter; never use passive adapters between SDI and HDMI.