SDI vs HDMI – What’s the difference?

Everyone who has owned an xbox (or is old enough to remember what a DVD player is) has encountered HDMI. It’s cheap, it’s easy, and it’s reliable. You plug it in and it … just works. Well, at least it’s supposed to. So why do so many people experience problems when running HDMI to their projectors and other screens in their churches, schools and businesses? 

What is HDMI? 

Born in 2002, HDMI was the digital successor to the common RCA cable (Those of you who remember what a VHS player is will recall the red, white and yellow connectors of old). HDMI ushered in the new digital age, transmitting 1’s and 0’s rather than the old analogue wave modulations of composite video and those yellow RCA connections. The HDMI 1.0 standard was revolutionary – all of a sudden a single cable would pass crisp, clear video with high fidelity audio. Based on the DVI standard, the audio data would be passed in the ‘gaps’ between video frames. A true technological marvel, and an invention that home theater lovers quickly adopted. Although the supported resolutions were small, they quickly grew with 4K (that’s 4096 x 2160) support added in 2009.  HDMI was quickly adopted by the content studios and distributors because of its ability to enforce stricter copyright rules – commonly called HDCP (more on that later). 

What’s inside an HDMI cable? HDMI Connection with Pins

An HDMI cable consists of four shielded twisted pairs – not unlike the ones you’d find inside a CAT5 ethernet cable, with the addition of seven other separate conductors. If you were to take out a magnifying glass and look at the connector, you’d notice 19 separate pins, each with their own specific use. HDMI cables come in all shapes and sizes. You can pick a cheap one up from amazon for a couple of dollars, and if you’re as rich as Elon Musk you might buy one of the more expensive $1299 models with gold plating and unicorn dust cores that advertise “low distortion” and “colorless sonic presentation”. (Don’t buy these – they’re a scam). 

What happens when you plug an HDMI cable into your xbox?

The first thing that happens is pin 19, the ‘hot pin’ will become active – telling the device on the receiving end “Hey, I’m a new video device and I’d like to send you some video”. Your TV (the receiving device) will then enter a ‘handshake’ mode – where the two devices will exchange basic information – supported resolutions, bitrates, frame rates, color spaces, and other additional features like ethernet.  Once the devices agree on the necessary details, the handshake is complete and the TV can now see the video from the xbox. This all happens in the background with no input from the user, meaning that even your 90 year old grandmother can connect a new playstation to her tv without needing the geek squad. Before granny can unwrap a Werthers butterscotch, she’s ready to frag some n00bs in COD. Nice. 

HDMI for professional AV equipment. 

HDMI Splitter
An ‘affordable’ HDMI Splitter
The popularity of HDMI quickly spread from consumer focussed products to those in the AV industry, most notably projectors. Almost every projector you buy today will have an HDMI input, and that’s a good thing – it provides a cheap and easy way to connect almost any video device to these high end video displays. Quickly, it was discovered that sometimes a video signal needs to be sent to multiple destinations – like a left and right projector, or TV’s in a foyer. Devices called “HDMI Splitters” became readily available, and at the time of writing you can pick one up for just a couple of dollars on Alibaba

And this is where the problems usually start. 

HDMI was never intended to be ‘split’ into multiple signals. In fact, when it was designed, this was one of its selling points to the content studios; one DVD = one DVD player = one HDMI cable = one TV. If you wanted to play the same content on multiple screens, then you’d need to buy another copy of the DVD. Easy money for the studios. So how do HDMI splitters work? They use something called EDID emulation to trick the source into thinking it’s a real physical display, then the splitter sends this data out to multiple displays. Each display will handshake with the splitter rather than the source device. They’re completely legal, and are widely used.  These signals are often sent by very long HDMI cables to their destinations. Often an HDMI splitter is coupled with an HDMI extender, which converts the data from the 19 HDMI pins to the 8 you’d find on a CAT5 cable, and then back again. The device on the receiving end still needs to handshake, and all this happens over some very long distances and over multiple protocols.  One of the issues we often encounter when we overhaul an AV system is the use of HDMI splitters and HDMI extenders. A common story we hear is “they used to work, but all of a sudden they stopped working” or “they flicker and we don’t know why”. The answer is a simple one: the HDMI protocol was not designed to be used like that. HDMI is a great conduit for the ‘first and last hop’ in a system, but not for transmitting over long distances. I can hear you asking, “If I’m not supposed to use HDMI for these long runs – what should I use?”. Enter SDI.

What is SDI?

BNC Connector
A BNC Connector commonly used for SDI
SDI stands for ‘Serial Digital Interface’. It’s a protocol that has been used in the broadcast video world since its own transition away from analog. Unlike HDMI and RCA, SDI uses the same BNC connector that its old analogue counterpart utilized. But that’s where the similarities end. SDI is capable of transmitting 4K video at an incredibly high bitrate – up to 12G (which stands for 12 gigabits per second – or roughly the capacity of 12 of those blue CAT6 cables you have running your IT equipment). 

What’s inside an SDI cable that makes it so special? 

Inside one of those coaxial cables you’ll find four simple elements: a copper ‘conductor’ in the center, a foil shield, and a braided shield. There is also a thick ‘dielectric’ jacket around the conductor, and a thinner but rugged outer jacket for protection. That’s it. Compared to the 19 conductors of an HDMI cable, it is deceptively simple. 

So how does SDI work?

Coaxial Cable Cross Section
Cross Section of a Coaxial Cable from rfcables.org
First of all, the notion of a ‘handshake’ does not exist within SDI. While an HDMI cable provides two way communication between the sending and receiving device, SDI only passes data one way. The second difference is that SDI passes a “serial” feed of data, (those of you who paid attention in your high school computer ed class will remember serial means the 1’s and 0’s are lined up and sent sequentially, where parallel means they are split up and sent concurrently).  Without the handshake involved, a receiving device simply looks at the data coming down the cable and attempts to identify the resolution, bitrate, color space, and audio channels. If the receiving device can ‘decode’ the data, it is then displayed as output.  An SDI cable can reliably be run up to 300ft, and I’ve personally seen them go further. There are multiple cable specifications – some are rated for simple 1080p while others are rated for 12G 4K. The protocol remains similar, however the cable composition is different for each ‘frequency’ of data that is transmitted.

Pairing SDI with HDMI

There is no escaping HDMI, even in the professional AV world, and nor would you want to. It’s a great protocol for the ‘first and last hop’ in your video signal chain. So how do AV professionals make HDMI and SDI play nicely with each other?  Many brands make devices that convert HDMI to SDI, and then another device that converts the SDI back to HDMI on the other end. Some converters are ‘bi-directional’ meaning you can convert both to and from SDI with the same converter (although generally not at the same time).  Blackmagic Design makes a nifty little converter for $49 (and if you want it with a power supply, that costs more, because…. Of course, it’s BMD). Our go-to tool for the job is the Decimator MD-LX bi-directional converter. We like these for two reasons; they are bi-directional meaning they can be repurposed from ‘sending’ to ‘receiving’ as needed, and we have found them to be rock solid in terms of reliability (the cheaper models seem to ‘burn out’ at a quicker rate, we don’t know why but suspect it’s to do with electrical load and power surges).

What about sending SDI to multiple destinations? Can I do that?

Blackmagic Design Smart Videohub
A Video Router from Blackmagic Design
The short answer is – yes! In fact, unlike HDMI, SDI was designed to be duplicated and sent to multiple destinations through a device called a Video Distribution Amplifier (VDA). Often an SDI source like a video switcher will provide multiple SDI outputs, eliminating the need for a VDA in small installations. In large installations, we would pair the video switcher with a video router. A video router allows for all of your inputs and outputs to be brought together in one device, and then you can dynamically assign inputs to outputs as needed. You can even have one input go to 40 or 400 outputs thanks to this flexible design.

Does SDI pass audio as well as video?

A single SDI cable can pass not only the video signal, but also up to 16 channels of audio. Thanks to the ‘serial’ transmission protocol, the audio stays in sync. Many times these additional channels are used to carry vocal stems, instrument only mixes and click/cue feeds to auxiliary venues and rooms.

Why SDI?

  • It’s super reliable.
  • It can be sent up to 300ft to a receiving destination
  • It has a locking connector, preventing the cable from being ‘bumped out’
  • It’s super low latency, thanks to the small amount of processing needed.
  • It was designed to be ‘split’ (aka distributed) via a VDA or router
  • SDI audio and video remain in sync across a system
  • SDI can be ‘clocked’ to allow the frames to be in sync with each other
  • There is no two-way handshake protocol which boosts reliability

Why HDMI?

  • It’ great for the ‘first and last hop’ in a video system
  • It provides compatibility between a range of devices
  • The cables are cheap and easy to install
 
Simple SDI HDMI Video Setup
A simple HDMI and SDI video setup.

What about that HDCP content you mentioned earlier?

HDMI has one leg up over SDI, and that’s when you need to play HDCP protected content. Because HDCP is a unique standard to HDMI, content that is HDCP protected is difficult to transmit to multiple displays. Remember how the studios want you to buy multiple copies of the media so they can make some extra dollars? They also want to make sure that tv stations don’t broadcast consumer use copies of their movies without the appropriate licenses. There’s a workaround. There is a class of device that can strip the HDCP from the HDMI signal. The HDMI signal can then be converted to SDI through traditional means. These devices are technically illegal in the USA, however you can pick them up on the grey market. (No, we can’t provide a link and we definitely can’t sell you one).

Okay, I hear you. SDI is great. All hail SDI. But I’ve seen HDMI used in professional installations at my school or university. What hardware are they using to make it work reliably?

Don’t get us wrong, there are plenty of great HDMI based pro AV devices available. Crestron and QSC make some awesome gear that allows for SDI to be sent reliably over CAT6 with no quality issues. These devices can even pass the HDCP data allowing you to watch Lord of the Rings on your universities 120” display. But these systems are not cheap. If you’re needing a system that can send HDCP content over long distances, then these are the solutions you need, but be prepared to pay handsomely for them. Can you achieve all of this with a $25 splitter from Monoprice? Sure. Will it work? Maybe, maybe not. If it works now, will it still work in 2 years? Maybe, maybe not. Will it drop out? Maybe, maybe not. We could go on, but you get the picture. Mission Critical infrastructure can’t be subjected to a “maybe it’ll work” design philosophy. When you need rock solid performance and high reliability for a mission critical production system, you should seriously evaluate the use of SDI as a backbone for your video distribution. 

Our team provides turn-key audio visual solutions for churches, businesses and universities. Our design philosophy focuses on creating systems that are reliable and easy to use.

Schedule a quick call with us to learn more.