In its most simple and common deployments today, SDI is a 1.5Gb/s uncompressed and unencrypted data stream, and 3Gb/s is becoming common.  It is normally transported on coaxial cable but can also be delivered in a standardized format on fiber.  Field terminations are relatively easy and make up the vast majority of connections.

The SDI format can carry a SUBSET of the framerates and resolutions supported by HDMI, but the vast majority of native SDI devices can be switched to one of several formats to be accommodated across all SDI devices.  SDI supports several bit rates  including 143 and 177 Mb/s (standard-definition COMPOSITE – defunct), 270 Mb/s SD component, 360 Gb/s widescreen enhanced SD, 540Mb/s EDTV, 1.5Gb/s for HD at 720p and 1080i, 3Gb/s for 1080p and other signals supporting various iterations of interconnects for 4K and WCG/HDR variants up to 12G-SDI.

 Notice that these standards DO NOT include many of the “computer” resolutions and frame rates supported by HDMI and HDBaseT.  Conversion from those resolutions and rates to a valid SDI standard requires the use of a scaling converter such as Decimator Design MD-HX, Grass Valley ADVC-G1A, Datavideo DAC-70, Lumantek ez-MD+, etc.  Keep in mind that scaling converters by nature will add a frame or two of latency to the converted video which can become an issue if live in-service video must also go through this conversion for iMag purposes.

 SDI has a pair of “Ancillary Data Spaces” used for varying purposes.  The Horizontal Ancillary Data space (“HANC”) is customarily set aside for discrete audio pairs, totaling 8 pairs (16 “tracks”) and the Vertical Ancillary Data space (“VANC”) is customarily used for various types of time-code, product-specific signaling and triggers, both standardized to the market and application.

 The telling difference between SDI and HDMI or HDBaseT is that the SDI interface exists only on professional production and display equipment.  Because of security for motion pictures and other licensing agreements, an unencrypted interface cannot be included on consumer equipment.

 Should you find yourself having to send HDCP encrypted content (via HDMI-to-SDI conversion) to your projectors, this question must be considered: is the content legal for display in your venue?

 If so, oftentimes the rights holder will make the portion that you need for supporting the service or message available without encryption, often with a “courtesy of…” semi-transparent “bug” over the corner of some portion of the clip.

 Otherwise, you are left to your own devices and legal risk if you want to circumvent HDCP encrypted content to pass it through an SDI infrastructure.

SDI – The Conclusion

If you are looking for the most robust single-purpose application of transporting video to your projectors and screens, SDI fits that bill with elegance. Here’s why:

1. SDI is a robust, unencrypted format with relatively inexpensive copper and optical interfaces capable of ~100M distances on copper and much longer over optical interfaces.  All SDI compliant copper<>optical interfaces will properly interoperate across all brands provided they are operating on a matched set of standardized optical wavelengths. 
2. SDI copper connectors are robust and have definitive “locks” requiring a “twist-off” action to loosen a connection before it can be removed.  SDI connections cannot be removed by simply pulling on the cable.
3. As an unencrypted format, there are no restrictions for distribution.  You will never have to depend on sometimes shaky EDID/HDCP workarounds for supporting multiple monitors and displays.