Electrical interface

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, 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 specialsynchronization 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.

Standards

Standard Name Introduced Bitrates Example video formats
SMPTE 259M SD-SDI 1989[2] 270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s 480i, 576i
SMPTE 344M ED-SDI   540 Mbit/s 480p, 576p
SMPTE 292M HD-SDI 1998[2] 1.485 Gbit/s, and 1.485/1.001 Gbit/s 720p, 1080i
SMPTE 372M Dual Link HD-SDI 2002[2] 2.970 Gbit/s, and 2.970/1.001 Gbit/s 1080p
SMPTE 424M 3G-SDI 2006[2] 2.970 Gbit/s, and 2.970/1.001 Gbit/s 1080p
SMPTE ST-2081* 6G UHD-SDI   6 Gbit/s 4Kp30
SMPTE ST-2082* 12G UHD-SDI   12 Gbit/s 4Kp60

* SMPTE standards for ultra-HD links are still being drafted, as of August 2014. Working group 32NF-70 is in the process of developing standards ST-2081, ST-2082, and ST-2083 for 6Gbit/s, 12Gbit/s and 24Gbit/s SDI respectively.

Bit rates

Several bit rates are used in serial digital video Signal:

  • For standard definition applications, as defined by SMPTE 259M, the possible bit rates are 270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s. 270 Mbit/s is by far the most commonly used; though the 360 Mbit/s interface (used for widescreen standard definition) is sometimes encountered. The 143 and 177 Mbit/s interfaces were intended for transmission of composite-encoded (NTSC or PAL) video digitally, and are now considered obsolete.
  • For enhanced definition applications (mainly 525P), there are several 540 Mbit/s interfaces defined, as well as an interface standard for a dual-link 270 Mbit/s interface. These are rarely encountered.
  • For HDTV applications, the serial digital interface is defined by SMPTE 292M. Two bit rates are defined, 1.485 Gbit/s, and 1.485/1.001 Gbit/s. The factor of 1/1.001 is provided to allow SMPTE 292M to support video formats with frame rates of 59.94 Hz, 29.97 Hz, and 23.98 Hz, in order to be compatible with existing NTSC systems. The 1.485 Gbit/s version of the standard supports other frame rates in widespread use, including 60 Hz, 50 Hz, 30 Hz, 25 Hz, and 24 Hz. It is common to collectively refer to both standards as using a nominal bit rate of 1.5 Gbit/s.
  • For very high-definition applications, requiring greater resolution, frame rate, or color fidelity than the HD-SDI interface can provide, the SMPTE 372M standard defines thedual link interface. As the name suggests, this interface consists of two SMPTE 292M interconnects operating in parallel. In particular, the dual link interface supports 10-bit, 4:2:2, 1080P formats at frame rates of 60 Hz, 59.94 Hz, and 50 Hz, as well as 12-bit color depth, RGB encoding, and 4:4:4 colour sampling.
  • A nominal 3 Gbit/s interface (more accurately, 2.97 Gbit/s, but commonly referred to as "3 gig") was standardized by SMPTE as 424M in 2006. Revised in 2012 as SMPTE ST 424:2012, it supports all of the features supported by the dual 1.485 Gbit/s interface, but requires only one cable rather than two.

Other interfaces

SMPTE 297-2006 defines an optical interface for SDI. An 8-bit parallel digital interface is defined by ITU-R Rec. 601; this is obsolete (however, many clauses in the various standards accommodate the possibility of an 8-bit interface).