The Serial Digital Interface (SDI) is used for transmitting uncompressed and unencrypted video data.

SDI refers to a range of video interfaces used to deliver high quality video in the production and broadcast domain.  

In production SDI interfaces are most commonly connected via  coaxial cable and BNC connectors. With ever increasing data-rates the maximum length of coaxial cables is reducing faster than cable manufacturers can develop better cables. As a result fiber optic SDI connections have become popular in larger installations. There are different SDI interfaces, supporting different resolutions and bandwidth. These are described by SMPTE as

• Dual Link HD-SDI
• 3G-SDI
• 6G-SDI
• 12G-SDI

In addition to uncompressed video additional data can be embedded into the signal:

• Up to 16 audio channels (32 via dual link)
• Time code data (TC, VITC, LTC)
• Additional data (Ancillary Data)
• EDH packets for error detection

Unlike the analog production signals; Composite (CVBS) and Component, where audio tracks and timecode need to be transferred separately, with SDI, all data and video can be combined into one transmission.

It is essential that cable lengths are actively managed as the maximum cable length for SDI signals is limted. In SD-SDI, a maximum cable length of 300 meters is possible and in HD-SDI lengths of about 75 to 100 meters are currently possible. Each connector in the path will reduce the maximum signal path substantially.

SDI Overview

• SD-SDI (SMPTE 259M) 270 Mbit / s
HD-SDI (SMPTE 292M) 1.485 Gbit / s
• Dual Link HD-SDI (SMPTE 372M) effective 2.970 Gbit / s
• 3G-SDI (SMPTE 425M) 2.970 Gbit/ s
• 6G-SDI (SMPTE ST 2081) 6 Gbit/s
• 12G-SDI (SMPTE ST-2082) 12 Gbits/s

The 3Gb/s interface with a transfer rate of 2.97 Gbit/s  was developed to enable the use of a single cable, rather than the dual link approach, which requires two separate feeds linked together.

It’s important to match the entire signal chain to the desired interface as not all mixers, non-linear editors and effects keyers support all of the SDI standards.

The Future of SDI

SDI currently continues to play an important role in live production and the standards are being evolved from 6G-SDI  to 12G-SDI and soon even more. However the importance of SDI is expected to reduce over time as it is replaced by IP technologies. In large scale live-production environments SDI is already being replaced with IP.