(1) See 
software-defined infrastructure, 
Single Document Interface, 
Strategic Defense Initiative and 
Switched Digital International.
(2) (
Serial 
Digital 
Interface)  SDI is a unidirectional video interface.  Standardized by SMPTE in the late 1980s, SDI has been the primary interface for transmitting uncompressed digital video signals.  SDI will eventually be replaced by IP-based NDI, and the many cables used in broadcasting facilities will be replaced with an Ethernet topography.  See 
NDI.
SDI uses a high grade of coaxial cable and a BNC connector with Teflon insulation.  It also supports optical fibers with ST, SC and FC connectors.
  Following are the common formats transmitted over SDI.  See 
ITU-R BT.601, 
BNC connector, 
fiber-optic connectors and 
serial interface.
 SD-SDI                  Aspect  Transfer
 (Standard Definition)   Ratio   Rate
 Digitized NTSC (4FSC)    4:3    143 Mbps
 Digitized PAL (4FSC)     4:3    177 Mbps
 Component ITU-R BT.601   4:3    270 Mbps
 Component ITU-R BT.601  16:9    360 Mbps
 HD-SDI (High Definition)  See HD-SDI.
 Component ITU-R BT.709  16:9  1,485 Mbps
 Dual-link (two cables)        2,970 Mbps
 3G-SDI (3 Gbps)
 One-cable version of HD-SDI   2,970 Mbps
Blackmagic SDI Capture Board
Two sockets are input, and two are output.  At 4:2:2 color sampling, SDI uses one line for in or out.  However, HD-SDI at 4:4:4 requires two lines.  See 
chroma subsampling.  
(Image courtesy of Blackmagic Design Pty. Ltd.)