Serial Digital Interface
SDI is a
unidirectional 270 Mbps
transport service that is offered by most local
telephone companies. It is designed to transport
NTSC System M (525 lines and 60 fields per second)
digital television signals. Compliant end equipment conforms to
SMPTE 259M, which describes 10-bit 4:2:2
component video.
The 270 Mbps data rate is derived from the standard
sampling requirements for
component video:
Luminance (Y) - 13.5 MHz
Color Difference (B-Y) - 6.75 MHz
Color Difference (R-Y) - 6.75 MHz
The total sampling
frequency is 27.0 MHz. Each sample is a 10-bit word, so the serial output is clocked at 10 times the sampling frequency -- 10 bits per Hz gives us a data rate of 270 Mbps.
The SDI service is also compatible with
DV-ASI equipment, allowing broadcasters to transport multiprogram MPEG2, and ancillary data streams.
Source: Qwest Technical Publication - Serial Digital Video Service at 270MBIT/S (SDVS-270)