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HDMI vs. SDI vs. NDI: What Are The Differences?

Intro

HDMI versus SDI versus NDI for streaming. What’s the difference? Whether you’re streaming a gaming session, a live event or a webinar, the quality of your stream depends heavily on the equipment you use. If you do a lot of live streaming, you’re probably familiar with both HDMI and SDI. These are two well-known ways to connect video production gear. However, if you’re new to streaming, you may be wondering what the difference is between HDMI and SDI and what the heck is NDI and which one is better for your needs.

What’s HDMI

Let’s get right into it. HDMI stands for High Definition Multimedia Interface. This connection is used to transmit high quality audio and video signals between devices. HDMI connections can be found on most consumer electronics such as TVs, DSLRs, gaming consoles, and laptops. The HDMI 2.1 specification, established back in 2018, is designed to support up to 8K and higher resolutions.

Generally, most people use it for 4K and primarily for HD, which is plenty for most consumer grade applications. HDMI cables are also capable of transmitting audio signals, so you don’t need a separate cable for audio. HDMI devices can also digitally communicate back and forth across that same HDMI cable. A digital handshake, so to speak. Not only does your laptop communicate with your monitor to know what resolution it can handle, your Roku can tell your TV to change inputs without you needing to pick up the TV remote. A downside to this handshake is that there’s a copyright protocol called HDCP for high bandwidth digital content protection. This will prevent you from getting an image out of a DVD or Blu-ray player into a video mixer. Some DSLRs also seem to look for HDCP approved devices and won’t feed their video out to a video mixer, so be aware of this.

HDMI Connection

There are three sizes of HDMI connectors. Standard HDMI is the largest, and it’s the one you typically see on larger devices like TVs, laptops, computers, video mixers, and other devices. Mini HDMI is considerably smaller, but it has a similar shape as standard HDMI. It is commonly used on smaller devices such as camcorders, DSLR cameras, and tablets. Lastly, there’s micro HDMI. This is the smallest version of the HDMI connector, and it is used on very small devices where space is at an absolute premium like on a GoPro action camera, a tablet monitor, output, or other mobile devices.

Despite the size, the number of wires in the connection remains the same, and so does the functionality. So a micro HDMI port can deliver the same 4K just as well as a standard size HDMI. You can find cables and adapters to convert between all three sizes without affecting your video.

Initially, HDMI cables were limited in length because of the sensitivity of the 19 little wires in that cable. The only way to go longer than the previous standard of about three meters was to use a heavy duty cable with thicker wires. These are tough to use because they don’t flex easily at all, and they also put a lot of strain on the HDMI connector of your device. Because of the stiffness and weight of the cable. That changed when fiber optic HDMI cables came around.

You see, there’s a small bit of power coming out of the HDMI port, and that can power the light needed to transmit the signal along a fiber optic cable. Suddenly, instead of just being limited to three meters or needing a heavy and thick cable for longer distance, you could easily have a lightweight and flexible cable that could deliver a pristine image over 30m or more. Typically, these cables are directional, so you need to pay attention to which end sends and which end goes to the TV. And they cost more than a basic cable, but they are a great way to deliver HDMI over long distances.

What’s SDI

SDI stands for Serial Digital Interface. It’s a standard used to transmit high quality digital video and audio signals between professional video equipment such as cameras, switchers and monitors. A key advantage of SDI over HDMI is the twist locking bayonet connector that holds the cable to the device. It can’t jiggle loose or get bumped and come out. It has to be pushed in and rotated before it comes off. This makes it a very secure and reliable professional connection. Also, the SDI jack is often part of a metal backplane and is more rugged than the HDMI connector, which usually is just soldered onto the motherboard. One bad bump and suddenly the HDMI port doesn’t work anymore. SDI was originally created for standard definition, so there’s the original SDI. Then in 1998, HD-SDI was established for 720p and 1080p 30 high definition signals. When the HD standards were increased to include 1080p 60, a new SDI standard was established in 2006, called 3G-SDI for the third generation.

And of course we keep going because when 4K hit the scene, it was four times the size of HD. At first, the only way to carry the signal over SDI was a clunky quad link SDI that took four SDI cables to carry the four quadrants of the image. In 2015, 6G- SDI was standardized to carry 4K over one cable, and 12-SDI was designed to carry double the bandwidth for higher frame rates or greater bit depth. But don’t hold your breath because in 2020, 24-SDI was established to handle 4K at 120 frames per second and 8K at 60 frames a second. SDI also carries up to 16 channels of digital audio.

A key advantage with SDI cables is that they’re very simple, with a center pin and a shield, so cutting and making custom length cables is actually quite easy to do. Also, if a cable gets broken passing under a door, it can be cut cleanly and two new ends put on. In contrast, HDMI is not easily repaired in the field. Also, even when HDMI was not capable of long runs, SDI was carrying HD signals 30m or more with absolute reliability.

Cable Quality

Your device will be designed to do HD 4K, whichever. Like with HDMI, you need to make sure you have a cable that adheres to the specification that matches the video you’re trying to carry. Only with the proper cable will the digital video cleanly make it between one device and the other. In other words, don’t use an old HDMI 1.3 cable for 4K, which is an HDMI 2.0 and 2.1 specification. Using an older, slower cable for 4K can deliver a glitchy image, and other artifacts like your audio may drop in and out. This is the same for HDMI and SDI. You need good cables made for the standard of video that you are trying to carry. Also, stringing together several cables through barrel connectors, as we call them to reach further, can degrade the signal as well and cause problems. It’s always best to use a single cable of the proper specification, even if it’s too long. Paying a lot for a simple wire may seem silly, but you’ll buy the cable once and use it for many, many years. Good cables are worth the investment and will help you to deliver glitch free streams, which is really what you want.

What’s NDI

NDI stands for Network Device Interface. What this really means is video over IP, which is standard computer networking. NDI is a technology developed by Newtek and it continues to be updated and refined today. NDI enables anyone to transmit high quality video and audio signals over standard Ethernet networks. It provides low latency, high quality video and audio, as well as bidirectional data communication between devices. While SDI and HDMI require a dedicated cable between each video source and destination, NDI is converting the video and audio to data that can use existing Ethernet networks. A single Ethernet cable can carry multiple streams of video and audio in different directions at the same time. For example, you can have one Ethernet cable between your mixer and the stage and use it to capture two cameras from the stage and send one video. Return back to the stage, as well as give you pan tilt zoom controls. Talley Communications, and more, all with the convenience of running just one cable.

Now, this flexibility can come with some risk, of course, because any damage to that one cable means the loss of all those connections. So be sure to carefully lay your cables at an event where they won’t get accidentally damaged. NDI also has easily integrated wireless options because it works with standard networking hardware. A Wi-Fi access point can also receive a camera roaming around in the audience. Another advantage comes from the nature of network devices. One camera can be seen and used by multiple receivers, multiple screens or switchers, but you need to watch your total bitrate going through the router or switch, because each of those HD camera streams of NDI is using about 100 megabits of data, and if it’s going to three destinations, that’s 300 megabits of total throughput. Do that with two cameras and you’re pushing 600 megabits or more and you’re nearing the upper limit of a gigabit switch.

So NDI also requires some good networking skills, understanding routers and switches, bit rates and total throughput, dynamic IP addresses and static IP addresses, and how to troubleshoot network issues sometimes, as well as a dedicated network IT professional, while also doing all the video and audio stuff too.

Which is Better for Streaming

If you are streaming from a consumer grade camera or computer in a controlled environment and your cables will be relatively short, HDMI is likely the best choice for you. HDMI cables are affordable, widely available, and easy to use. They’re also capable of transmitting pristine video and audio signals, making them a good choice for most live streaming applications.

If you are streaming from a professional grade camera and are working in a large venue, SDI is worth considering. SDI cables are more reliable than HDMI cables and can transmit signals over longer distances without problems. Understanding that advancement in fiber optic HDMI cables can be a good alternative. Also, keep in mind that HDMI and SDI signals can be converted back and forth without too much additional cost for converters. So if you have a DSLR with HDMI and that’s a long way off converting it to SDI for the long run and back to HDMI for the switcher is something people do all the time. That’s leveraging the advantages of SDI cables with HDMI gear.

If you need to transmit signals over a local area network without dedicated cabling, NDI is a great option to consider. NDI can transmit high quality video and audio with low latency. However, it requires a reliable network and it can be more complex to setup than HDMI or SDI because it’s not as simple as a camera out to switcher in.

Conclusion

In general, the choice between HDMI, SDI, and NDI depends on your specific needs of your streaming setup. Consider the length of your cable runs, the resolution of your video source, and the environment in which you’ll be streaming. With a little thought and consideration, you can choose the solution that best meets your needs and helps you produce high quality and reliable live streams.

Darren

Filmmaking tips & tricks, camera gear reviews, cinematography breakdowns, and other training-based content to help you level up your filmmaking.

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