SDI - Serial Digital Interface


SDI signals are transmitted as unbalanced signals on 75 Ohms coax cable and 75 Ohms BNC connectors. Transmission and reception involves differential amplifiers that format and detect both data phases. The signal amplitude at the source is 800 mV (10%) peak-to-peak. Transmitted are digital component signals (YCbCr), 10bit.

The first standardized digital component signal was defined in 1987 with the creation of the D1 format and the standard SMPTE 125M. It was an 8/10 bit parallel system used for connections between digital tape recorders. D1 was distributed over 8 to 10 twisted pairs and the interface was a 25-pin D-sub miniature connector.

Distributing digital video as a single bit-stream on a coax cable has significant advantages, even with much higher bit rates. It's much easier to route and switch one cable than a parallel system of cables and there will be no issues with clock and data synchronization between different signal paths.
The SDI format utilizes a differential signaling technique and NRZI (non-return to zero inverted) coding. Differential reception creates additional headroom and robustness in signal-to-noise performance. Pseudo-randomizing the data bits and use of NRZI coding increases channel transmission reliability and NRZI coding is desirable because its operation is independent of signal polarity.
High and low levels do not communicate as data 1s or 0s. High and low states are detected simply by the change from one level to another (a zero means that the transmission level stays the same, while a one is transmitted if the level transition from one level to the other occurs.

Data Transmission

SD-SDI data structure
HD-SDI data structure

Signal Structure

The SDI signal is transmitted by using the NRZI method ("non return to zero, inverted"). The binary signal is mapped to a physical signal for transmission. The logical "1" is represented by a transition of the physical level, the logical "0" is represented by NO transition. The signal level has a nominal value of 800mV.


The SDI data stream carries:
- video data
- 16 audio channels in groups of four
- time code information
- ancillary data
- EDH packets

Formats and Frame Rates

See: Video Formats and Frame Rates


Standards are defined by the Society of Motion Picture and Television Engineers (SMPTE) for serial digital video transmissions (SDI), the standards define the coding and decoding of the SDI single bit-stream.

ITU-R BT.601
international standard for digital component PAL transmissions with a data rate of 177 Mb/s, 4:2:2
standard definition SD, 576 interlaced lines, 25 full images (frames) mit each two half images (fields)

SD-SDI (standard definition), interlaced, with a bit rate of 143/270/360 Mbps (composite NTSC 143 Mbps (Level A) and PAL 177 Mbps (Level B), 525/625 lines component transmissions of 270 Mb/s (Level C) and 360 Mbps (Level D), 10bit

ED-SDI, component widescreen transmissions of 540 Mb/s

HD-SDI (high definition), interlaced or progressive, transport over one coax line, bit rate of 1.485 Gbps

Dual-Link HD-SDI (high definition), interlaced or progressive, transport over one coax line, bit rate of 2,970 Gbps

3G-SDI (high definition), progressive, transport over one coax line or one glass fiber path, bit rate of 2.970 Gbps, supports 1080p50 and 1080p60, 4K

6G-SDI, defines resolution with bit rates of 6 Gbit/s: 4K

12G-SDI, defines resolution with bit rates of 12 Gbit/s: 4K

Aspect Ratio and Pixel Count

Aspect Ratio Scanning Type Number of Pixels V Number of Pixels H Number of Pixels V Total Number of Pixels
per Frame to be displayed
480i 4:3 interlaced 480 640 307,200
480p 4:3 progressive 480 640 307,200
480p 16:9 progressive 483 720 347,760
720p 16:9 progressive 720 1280 921,600
1080i 16:9 interlaced 1080 1920 2,073,600
1080p 16:9 progressive 1080 1920 2,073,600

Signal Quality, Eye Pattern, Re-Clocking, Insertion Loss

Embedded Audio: Standard SMPTE 299M

AES digital audio signals, 24bit, can be inserted into the 1.485 Gb/s SMPTE 292M bitstream. 16 channels, SMPTE 299M allocates four groups of four audio channels that can be embedded. One group consists of two AES channels (each two audio channels L+R).

SMPTE 259M (SD Video) is also defined with four groups of four audio channels, but these are not defined as perfectly phase coherent and should not be used for surround sound transmission.
With SMPTE 292M (HD Video) and SMPTE 299M phase coherence between the audio channel groups is higher.