Definition:

Serial digital interface (SDI) is a family of digital video interfaces first standardized by SMPTE (The Society of Motion Picture and Television Engineers) in 1989. 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.

Dual link HD-SDI consists of a pair of SMPTE 292M links, standardized by SMPTE 372M in 1998; 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. 3G-SDI (standardized in SMPTE 424M) consists of a single 2.970 Gbit/s serial link that allows replacing dual link HD-SDI. 6G-SDI and 12G-SDI standards were published on March 19, 2015. 

Difference between 3G-SDI and HD-SDI:

SMPTE 424M also known as 3G-SDI is a standard published by SMPTE which expands upon SMPTE 259M, SMPTE 344M, and SMPTE 292M allowing for bit-rates of 2.970 Gbit/s and 2.970/1.001 Gbit/s over a single-link coaxial cable. These bit-rates are sufficient for 1080p video at 50 or 60 frames per second. The initial 424M standard was published in 2006.

The HD-SDI standard is a nominally 1.5 Gbit/s interface. Two exact bitrates are defined; 1.485 Gbit/s, and 1.485/1.001 Gbit/s. The factor of 1/1.001 is provided to allow SMPTE 292 to support video formats with frame rates of 59.94 Hz, 29.97 Hz, and 23.98 Hz, in order to be upwards 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.

Conclusion:

The SDI standard uses data words that are 8 or 10 bits in length. Signals are uncompressed and are self-synchronizing between the source (transmitter) and destination (receiver). Most errors caused by noise or interference can be detected, and the lost data recovered by means of a specialized code called the Hamming code. A signal in SDI can contain up to four independent digital audio signals along with the video signal.

This standard SMPTE 292 is usually referred to as HD-SDI; it is part of a family of standards that define a Serial Digital Interface based on a coaxial cable, intended to be used for transport of uncompressed digital video and audio in a television studio environment. The standard also defines nominal bitrates of 3 Gbit/s, for 50/60 frame per second 1080P applications. This version of the interface is not used (and has not been commercially implemented); instead, either a dual-link extension of SMPTE 292M known as SMPTE 372 or a version running twice as fast known as SMPTE 424 is used for e.g. 1080p60 applications.