What is HD-SDI?

HD-SDI stands for High-Definition Serial Digital Interface. It is the common term for a high-definition digital video transmission standard known as SMPTE 292M.

The SMPTE 292M standard is part of a family of Serial Digital Interface standards that use a coaxial cable to transport uncompressed digital video – currently still predominantly used in professional TV studios.

In short, HD-SDI is a standard for transmitting uncompressed broadcast-grade HD video signals over short distances (although longer distances are possible with fibre optic cables).

More than 400 interface standards are published by the Society of Motion Picture and Television Engineers (SMPTE), an internationally recognised industry organisation that develops common standards, practices and guidelines mostly for television and cinema. See video standards.


Serial digital interface (SDI)

refers to a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video. A related standard, known as high-definition serial digital interface (HD-SDI), is standardized in SMPTE 292M; this provides a nominal data rate of 1.485 Gbit/s.

An emerging interface, commonly known in the industry as dual link HD-SDI and consisting essentially of a pair of SMPTE 292M links, is standardized in SMPTE 372M; this provides a nominal 2.970 Gbit/s interface used in applications (such as digital cinema or HDTV 1080P) that require greater fidelity and resolution than standard HDTV can provide.

A more recent interface, 3G-SDI, consisting of a single 2.970 Gbit/s serial link, is standardized in SMPTE 424M that will replace the dual link HD-SDI.

These standards are used for transmission of uncompressed, unencrypted digital video signals (optionally including embedded Audio and/or Time Code) within television facilities; they can also be used for packetized data. They are designed for operation over short distances (less than 300m. with coaxial cable); due to their high bitrates they are inappropriate for long-distance transmission. SDI and HD-SDI are currently only available in professional video equipment; various licensing agreements, restricting the use of unencrypted digital interfaces to professional equipment, prohibit their use in consumer equipment[citation needed]. There are various mod kits for existing DVD players and other devices, which allow a user to add a serial digital interface to these devices.

The various serial digital interface standards all use (one or more) coaxial cables with BNC connectors, with a nominal impedance of 75 ohms. This is the same type of cable used in analog video setups, which potentially makes for easier upgrades (though higher quality cables may be necessary for long runs at the higher bitrates). The specified signal amplitude at the source is 800 mV (±10%) peak-to-peak; far lower voltages may be measured at the receiver owing to attenuation. Using equalisation at the receiver, it is possible to send 270 Mbit/s SDI over 300 metres without use of repeaters[citation needed], but shorter lengths are preferred. The HD bitrates have a shorter maximum run length, typically 100 meters.

Uncompressed digital component signals are transmitted. Data is encoded in NRZI format, and a linear feedback shift register is used to scramble the data to reduce the likelihood that long strings of zeroes or ones will be present on the interface. The interface is self-synchronizing and self-clocking. Framing is done by detection of a special synchronization pattern, which appears on the (unscrambled) serial digital signal to be a sequence of ten ones followed by twenty zeroes (twenty ones followed by forty zeroes in HD); this bit pattern is not legal anywhere else within the data payload.