HDMI, SDI, and NDI, Oh My!

Updated: Aug 26, 2019

In the world of video production, there are three primary interconnect and data transmission standards used today:


1. High-Definition Multimedia Interface (HDMI)

2. Serial Digital Interface (SDI)

3. Network Digital Interface (NDI)


The Consumer Standard

High-Definitation Multimedia Interface, or HDMI, is the most common form of digital video connection standard in the consumer and prosumer market. It transmits both audio and video, and depending on the version, can support 4k as well as HD video. You're probably already familiar with this interface, since it is on nearly every television, DVD, Blu-Ray, and set-top device on the market today. In addition, many computer displays also support HDMI, and of course consumer and prosumer camcorders use HDMI so they can be easily connected televisions or other displays. HDMI cables are used to connect devices, with common cable lengths ranging between 3 feet (~1m) and 50 feet (15m). In addition, standard HDMI cables also do not have a locking connector.


If you are working with a professional audio / visual or live production company, chances are they will not be using HDMI as their primary interface for distributing video signals because of the short cable runs and the fact that, without a locking connector, the cables can be disconnected easily.


The Professional Standard

Serial Digital Interface, or SDI, is the standard for video connectivity in the professional broadcast market. SDI is most commonly transmitted over durable, 75ohm coaxial cable with BNC locking cable connectors. SDI comes in a variety of flavors, each doubling the data throughput and increasing the video resolution it supports. The performance rating of each SDI variant is listed below:


1. HD-SDI supports video resolutions up to 1920x1080p at a frame rate up to 30fps and a data transfer rate of approximately 1.5Gbps

2. 3G-SDI supports video resolutions up to 1920x1080p at frame rates up to 60fps and a data transfer rate of approximately 3Gbps

3. 6G-SDI supports video resolutions up to 3840x2160p at frame rates up to 30fps and a data transfer rate of approximately 6Gbps

4. 12G-SDI supports video resolutions up to 3840x2160p at frame rates up to 60fps and a data transfer rate of approximately 12Gbps


SDI is a tried-and-true, nearly bullet-proof standard designed to hold up under the rigors of professional broadcast production. Cable lengths run up to about 328 feet (or 100m) without needing a repeater, making it ideal for most cable runs. When interfacing with professional video broadcast equipment, you'll need to make sure that your stream encoding hardware supports video input via SDI. If you're using a software encoder, you'll need to have a hardware interface (such as Blackmagic Design Ultrastudio Mini Monitor or the new Ultrastudio 4k Mini) that converts your SDI signal into a video signal that your computer streaming software (such as Wirecast or OBS) understands.


In addition to the common standards above, a new standard for 8k SDI video transmission is currently being developed and, yes, you guessed it... its 24G-SDI which will allow for 8k video transmission at frame rates up to 60fps with a data transfer rate of 24Gbps.


The Emerging Standard

Network Digital Interface, or NDI, is a standard created by NewTek and published freely for any hardware or software vendor to adopt. NDI is a leading protocol for IP-based video distribution and delivery, and leverages standard ethernet networking infrastructure to transmit audio and video signals.


Presumably, if you're working with a professional live production company, they will have their own ethernet infrastructure for their production. However, in some cases, there may be a requirement to transmit video streams over the organization's network, so it's important to know what potential impact that may have. Here are the two primary flavors of NDI and their bandwidth requirements:


NDI - a single 1920x1080p stream at 30fps needs at least 125Mbps of dedicated bandwidth


NDI|HX - a single 1920x1080p stream at 30fps needs between 8 and 20Mbps of dedicated bandwidth.


Obviously, NDI|HX is a much more compressed version of the NDI standard designed to be far more network-friendly.


The biggest advantage of NDI is that, as an IP-based network protocol, you can now leverage traditional ethernet infrastructure to transmit audio and video between devices, route video over LANs and WANs, or even have software applications directly input or output a video signal. However, that also means that your time-sensitive video signal is also subject to the same congestion and network issues that can impact any network traffic. However, with careful planning, traffic shaping, and proper QOS configuration, NDI may be an effective approach to a variety of corporate video needs.


It is also important to note that, while NDI is a video transmission standard, its NOT the same standard commonly used in live stream transmission. That standard is H.264 compressed video (with H.265 as the emerging new standard) transmitted via RTMP, RTMPS, RTSP, or SRT protocols. So you'll need to plan your stream transmission and distribution separately from the video sources being used to create said stream.


Conclusion

As an IT professional, being familiar with these video technologies is important because, whether you're working with a hardware or software stream encoder, it will need to receive its video input either via HDMI, SDI, or NDI. For hardware encoders, this means having an HDMI or SDI hardware interface and likely converting an NDI signal to one of these interface standards. For software encoders, this means selecting one that can receive a video input directly via NDI, or purchasing an HDMI or SDI computer hardware interface such one of the Ultrastudio products from Blackmagic Design.


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