In this guide: Short answer · What HDMI is · HDMI problems on site · What SDI is · Generations and rates · Cable distance · Coax and BNC · Side by side · Conversion · Twisted pair and fibre · What to spec · Mistakes · FAQ

Walk behind any live production video position and almost nothing is connected with HDMI. Screens, cameras, switchers and recorders are wired with thin black coax and metal BNC connectors that twist and lock, and HDMI appears only in short stubs at the ends of the chain. That is not habit. These are two interfaces designed for completely different jobs, one for a living room and one for a broadcast facility.

The short answer

Use HDMI at the ends of the chain, over short distances only: laptop to a scaler, switcher output to a nearby screen, camera to a monitor on the same stand. Use SDI for anything that leaves the desk: camera runs across a stage, a feed to a projector at the back of a hall, anything that gets walked on and coiled every night. Converters are small and cheap, so the practical rule is convert to SDI as early as you can and back to HDMI as late as you can.

What HDMI is, and where it belongs

HDMI carries video, multichannel audio, control and content protection over one cable, negotiated between source and display. That negotiation causes most of the trouble on a show floor: the source asks the display what it can do, the display answers with an EDID data block, they agree a format, and if the content is protected they authenticate before anything appears.

Bandwidth has climbed steadily. HDMI 2.0 (2013) raised the ceiling to 18 Gbps for 4K at 50 and 60 frames per second plus 32 audio channels, and deliberately defined no new cables or connectors. HDMI 2.1 introduced the Ultra High Speed HDMI Cable for up to 48 Gbps, and HDMI 2.2 reaches 96 Gbps with the Ultra96 cable. Certified cables are rated by bandwidth: High Speed 10.2 Gbps, Premium High Speed 18 Gbps, Ultra High Speed 48 Gbps. HDMI 1.4 added the HDMI Ethernet Channel at up to 100 Mb/s full duplex, which is why leads are sold as “HDMI with Ethernet”. Within its envelope HDMI is excellent, and it is the only connector on a great many sources and destinations. You cannot avoid it, but you can avoid asking it to do things it was never designed for.

The practical HDMI problems at an event

No locking connector

The standard Type A connector is a friction fit: no thread, no bayonet, no latch. On a desk that is fine. On a stage floor it is a single point of failure any passing foot can trigger, and because HDMI renegotiates on reconnection you do not get a glitch, you get a black screen while the handshake rebuilds. Locking shells and clamps help, but they patch a connector never intended for this.

EDID and the handshake

Because HDMI negotiates, it can fail to negotiate. A laptop reads the EDID of whatever is plugged into it and picks a format to match, so once a splitter, extender or switcher is in the middle, the EDID the laptop sees may not be the EDID of the screen at the far end. Symptoms: a resolution the projector cannot display, an image on one splitter output and not another, or everything reshuffling when someone unplugs a screen. Any decent splitter or scaler lets you manage EDID, and on a job with mixed presenter laptops, showing every one of them the same known-good EDID is the difference between a smooth changeover and thirty seconds of panic.

HDCP on protected sources

HDCP is content protection, licensed separately across HDMI, DVI, HDBaseT, DisplayPort and other consumer interfaces, currently at version 2.3. If a source asserts HDCP, every device in the chain must authenticate or you get a black screen. Presentations and video files are normally unprotected; streaming apps, set-top boxes and consoles are not. SDI is not an HDCP-protected interface, so encrypted content cannot legitimately pass through an HDMI to SDI converter, and compliant converters block it.

Reliable passive run length is short

The detail that catches people out: the HDMI specification does not define a maximum cable length, only the performance a cable must meet. There is no number to quote, just a probability, and the higher the bandwidth the steeper the curve. Cables have passed compliance testing at up to 10 m without a repeater, but at older, lower data rates. At 18 Gbps the failure mode is sparkles, intermittent black or nothing at all. Treat passive HDMI beyond about 5 m as needing proof by testing, and beyond 10 m as a job for an active cable, an extender or a conversion.

What SDI is, and why live production runs on it

SDI, the Serial Digital Interface, is a family of SMPTE standards for sending uncompressed digital video down a single 75 ohm coaxial cable terminated in BNC connectors. Its design assumptions are the opposite of HDMI’s in every way that matters on site.

• The connector locks. BNC is a bayonet: push, twist a quarter turn, mechanically captive.
• No negotiation. SDI is unidirectional and point to point: the source transmits regardless of what is at the far end, and a receiver either locks or does not. No EDID, no handshake to fail, no renegotiation delay.
• No content protection. Nothing to authenticate, which is a feature in production and a limitation if you wanted protected material on screen.
• Audio travels embedded. Under SMPTE ST 299-1, HD and above carry up to 16 channels of 24-bit audio in the ancillary data space, in groups of four. SD-SDI under SMPTE 272M is more limited.
• Long runs on cheap cable. Coax is thin, robust, repairable and coils happily onto a drum.

The trade-off is that SDI carries video and embedded audio and little else: no control channel, no power, no return path. For live production, the right compromises.

SDI generations, data rates and what each carries

Each generation is a faster version of the same electrical idea, so cable and connector never change, only the bit rate and the kit at each end. Two figures are quoted for several rates: the integer rate, and the same rate divided by 1.001 for 59.94 and 29.97 frame rates.

Two more sit at the edges: ST 344 defined a 540 Mbit/s enhanced-definition rate you will rarely meet, and ST 2083 defines 24G-SDI at 23.760 Gbit/s. Before 12G, UHD travelled as quad-link 3G-SDI under SMPTE ST 425-5, splitting a 2160-line image across four 3G links on four coaxes, which is why older 4K kit has BNCs in groups of four. Receivers are generally compatible downwards, so a 12G input will normally lock to 6G, 3G, HD and often SD. The reverse never works: feed 12G into a 3G-only input and you get no picture at all.

How far will it really go?

Coax attenuation rises with frequency, so as the bit rate climbs the same cable delivers a usable signal over a shorter distance. The standards set the cable loss a compliant receiver’s equaliser must handle, measured at half the clock frequency: 30 dB for SD under ST 259, 20 dB for HD under ST 292 and 3G under ST 424, and a more generous 40 dB for 6G and 12G under ST 2081 and ST 2082. Published distances are calculated against those budgets. Representative figures for common 75 ohm coax classes, to design inside rather than aim at:

Three things there surprise people. The ranges within each class are real: solid-core installation cable goes further than the flexible stranded-core cable used on road-going drums, sometimes by a fifth or more. 12G on thin flexible cable is not a supported combination at all. And counter-intuitively, the 6G column is often longer than the 3G column, because 6G and 12G receivers must equalise twice the cable loss 3G receivers do. Beyond what the cable will do, add a reclocking distribution amplifier partway along, or go to fibre.

Coax quality and BNC care

SDI is a 75 ohm system end to end: cable, connectors and panel sockets. Mismatches cause reflections, reflections close the eye pattern, and a closed eye pattern is a black screen. The first common mismatch is 50 ohm BNC, which looks identical and is everywhere in test gear, RF and DMX-over-BNC equipment. It mates happily, passes SD well enough, then fails at HD or 3G on a longer run.

The second is using audio cable for video. Microphone cable is not video coax. A balanced twisted pair has no controlled characteristic impedance at gigahertz frequencies and no proper coaxial return path. It will pass a picture over a metre or two by accident and fail unpredictably once the run gets real. For the audio side of the same rig, see our audio connectors guide.

Connector grade matters as the rate rises. A UHD-rated BNC such as the Neutrik NBNC75BTU11X is specified at 75 ohm with return loss better than 30 dB to 6 GHz, 24 dB to 12 GHz and 20 dB to 18 GHz, on the standard bayonet lock plus Neutrik’s rearTWIST assembly. Connectors are also matched to one cable geometry (that one to 7.3 mm outer diameter), and the wrong dielectric diameter spoils the impedance even when it physically assembles. Then care: coil in loose over-under loops, cap unused ends so centre pins do not bend, colour-code with boots, and retire anything crushed, because a flattened dielectric is a permanent fault you cannot see.

HDMI and SDI side by side

Converting both ways

Straight converters versus scalers

A straight converter changes the electrical format and nothing else: HDMI in, the same format out as SDI, or the reverse. 1080p60 in gives 1080p60 out. They are small, cheap, low latency and often bidirectional, which is the sensible thing to keep in a flight case. A scaler converts format as well as connector, taking whatever the source offers and outputting one format you chose. That is what belongs in front of a presenter’s laptop: the laptop can output what it likes while the switcher always receives the same thing. Frame rate conversion is strictly a scaler job, since no straight converter turns 60 Hz into 50 Hz.

The audio path

Both formats embed audio, so in principle sound travels with the picture. In practice this is where shows go wrong: some converters embed and de-embed, some pass embedded audio untouched, and some quietly discard it. If sound must reach a mixer rather than a screen you want a de-embedder that pulls audio out to analogue or AES, and you want to know it is fitted before load-in.

Latency

Straight conversion adds very little delay. Scaling, frame rate conversion and anything that buffers a whole frame adds at least one frame, and frames stack through a chain. That rarely matters for a projected slide. It matters when the same person is heard through the PA and seen on a screen, because a few frames of video delay against no audio delay is visible lip-sync error.

Extension alternatives: twisted pair and fibre

HDBaseT is the established way to send HDMI over structured cabling. Its 5Play feature set carries uncompressed video, USB 2.0, 100 Mb/s Ethernet, control and up to 100 W of power over a single category cable at up to 100 m, and HDBaseT 3.0 supports fully uncompressed 4K60 4:4:4 over that distance. The alliance’s own guidance is that Cat6 is best practice, so specify network cable accordingly. It suits fixed installs better than touring. Usefully, HDCP is licensed over HDBaseT, so protected sources pass a compliant link where an SDI conversion would block them.

For runs beyond copper, fibre serves both formats. SDI over fibre is standardised in SMPTE ST 297, whose current edition covers ST 259, ST 344, ST 292, ST 424 and the 6G and 12G interfaces of ST 2081-1 and ST 2082-1, with engineering guideline EG 2069 covering optical SDI network design and link budgets. Fibre swaps a copper attenuation limit for an optical link budget, so distance depends on the transceivers and fibre type rather than the video rate, and singlemode goes considerably further than multimode. Event converters are usually packaged with rugged locking optical connectors such as opticalCON. Fibre also has no earth path, which helps between separately supplied positions, and no pickup near dimmers and motors. Against that it needs clean connectors, the right mode at both ends, and no tight bends.

What to spec for common jobs

One projector from a laptop

Within a few metres, a certified HDMI cable of the right category is fine, with something mechanical stopping it being kicked out. Once the projector is across the room, stop trying to solve it with a longer HDMI cable: convert to SDI at the laptop, run coax, convert back at the projector, and carry a spare converter and a USB-C to HDMI adapter.

Multi-camera stage feed to a switcher and screens

SDI throughout, no exceptions. 3G-SDI is the sensible default for 1080p work: comfortably within coax distance on any sane stage layout, universally supported and inexpensive to cable. Specify 12G-rated cable and connectors only if you are genuinely working UHD on a single link. Use reclocking distribution amplifiers where a feed serves several destinations, convert to HDMI only at the final screens, and colour-code both ends of every run.

Conference room or boardroom install

HDBaseT territory. Cat6 to each plate and display, short HDMI stubs on panel-mounted connectors rather than dangling leads, EDID managed centrally so any laptop presents correctly, and HDCP compliance end to end so somebody’s streaming content works when they inevitably try it.

A livestream feed out of a show

Take the stream from a dedicated SDI output on the switcher rather than looping through the projector feed, so a change on stage cannot take the stream with it. If the encoder is HDMI only, use a straight converter and confirm the embedded audio survives it: streams lose audio here more often than anywhere else.

Common mistakes to avoid

• A long passive HDMI cable as the plan. The spec sets no maximum length, so a 20 m lead is sold legitimately and may still fail at your resolution.
• No spare cable. Both formats fail, usually at a connector, usually at the worst moment.
• HDCP discovered mid-show. Test the actual playback device beforehand, and remember no HDMI to SDI conversion will pass protected content.
• Running 12G on old coax. Thin flexible cable that was fine for HD is not rated for 12G at all. Check the connectors as well as the cable.
• Unlocked connectors on a stage floor. If HDMI must live where people walk it needs a locking shell, clamp or panel plate. Better still, make the floor run coax.
• Forgetting the audio path. Assuming embedded audio survives every hop is the commonest cause of a silent screen or a silent stream.
• Mixing 50 ohm and 75 ohm BNC. They mate, they look identical, and the fault only shows at higher rates on longer runs.
• Mixed laptop formats straight into a switcher. Put a scaler in front of presenter inputs and standardise on one format.

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Frequently asked questions

How long can an SDI cable be?

It depends on the data rate and the cable. On common RG-6 class 75 ohm video coax, expect roughly 320 to 385 m at SD, 85 to 109 m at HD, 57 to 75 m at 3G and 56 to 70 m at 12G. Heavier RG-11 class cable roughly doubles the higher-rate figures, and flexible stranded-core cable runs shorter than solid-core install cable.

How long can an HDMI cable be?

The specification gives no answer: HDMI defines cable performance rather than a maximum length. Cables have passed compliance testing at up to 10 m at older data rates, but at 18 Gbps and above it depends on the cable, source and resolution. Treat anything beyond about 5 m as needing a test with the actual kit.

What is the best HDMI extender for a live event?

For a one-off show, converting to SDI and running coax is the most robust and usually the cheapest route, and it gives you a locking connector on the floor. For a fixed install, HDBaseT over Cat6 to 100 m is the standard answer and keeps HDCP intact. For very long runs, or between positions on separate supplies, use fibre.

What is the difference between 3G-SDI and 12G-SDI?

3G-SDI (SMPTE ST 424) runs at a nominal 2.970 Gbit/s and carries up to 1080p60 on one link. 12G-SDI (SMPTE ST 2082) runs at a nominal 11.880 Gbit/s and carries 2160p60 UHD on one link. Same connector, same cable type, four times the data rate. Because attenuation rises with frequency, 12G travels a shorter distance and needs cable and connectors specified for it.

Is 12G-SDI backwards compatible with 3G-SDI?

A 12G-capable input will normally lock to 6G, 3G, HD and often SD, so slower sources feed faster equipment happily. The reverse does not work: a 3G-only input shows nothing at all when fed 12G. Check rate support on every device in the chain, not just the ends.

Does SDI carry audio?

Yes, embedded in the ancillary data space of the video stream. Under SMPTE ST 299-1, HD and above carry up to 16 channels of 24-bit audio in groups of four; SD-SDI under SMPTE 272M is more limited. To get audio out as analogue or AES you need a de-embedder.

Will an HDMI to SDI converter put a streaming service on the big screen?

No. Streaming apps and consumer players usually assert HDCP, and SDI is not an HDCP-protected interface, so compliant converters block protected content. Run protected material over an HDCP-compliant HDMI or HDBaseT path instead, and test it on the actual device beforehand.