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Appendix C – About SDI Video Formats
Serial Digital Interface (SDI) video takes many forms. A brief overview of some of
its history can help to understand these different formats.
Origins, SD and HD, 1.5G
SDI was originally designed for Standard Definition (SD) and High Definition (HD)
4:2:2 color depth video, and was able to carry 270 Mbits (SD) or 1.485 Gbits (HD, or
1.5G) of data per second.
Because it is digital, SDI can convey other types of data, so embedded digital
audio (8 channels/4 pairs for SD-SDI, 16 channels/8 pairs for HD-SDI) and other
types of ancillary data were added to the specification.
Evolution, 3G
When higher frame rate (1080p above 30fps) and increased color depth video
was developed, the existing 1.5G data rate was exceeded. To get around that
limitation, Dual Link 1.5G SDI video was defined that conveyed this data over two
separate physical cables (links). Digital data can be organized in different ways for
transport (data packing). For higher frame rate video with 4:2:2 color, complete
video images are transmitted over both links and combined to create the higher
frame rate video. A complete image (but at a lower resolution) can be monitored
on either of these links. For lower frame rate video with increased color depth,
the first link carries the complete image as a 4:2:2 color signal, and the second link
carries the extra color information. Combining these signals creates a complete
4:4:4 video signal. In this second case, if the color is YCbCr, it is possible to monitor
the entire image on just the first link (but at a lower color depth). And because
two 1.5G signals are used, the total number of embedded audio channels
supported grows to 32 (16 pairs).
The 3G SDI standard was subsequently created with increased carrying capacity,
making it possible to convey a higher frame rate or greater color depth SDI
signal over a single physical cable. The two-link data organization was retained,
however, even though only one cable was used. Two of the data packing forms
of 3G SDI video data organization are Level A, intended for higher frame rate
video, and Level B for increased color depth video, as explained above. Another
3G SDI data packing specification exists, Dual Stream, designed to carry two
independent video signals, or streams, on a single link.
Further Evolution, 6G, 4K/UltraHD
When larger image raster sizes were developed, a means to transport data at
even higher rates became necessary. These larger image sizes include Ultra High
Definition (UltraHD) video (3840x2160 pixels developed by the video community),
and the slightly wider 4K video (4096 x 2160 pixels developed by the cinema
community). Each of these image rasters is four times the size of a previous
standard (HD, 2K), and as with all SDI specifications several different frame rates
and color depths are defined for these formats.
The method used to transport this even higher data rate is similar to the above,
with one approach specifying four 1.5Gb links that require four separate cables
(Quad Link 1.5G SDI). These four links are called SDI1-SDI4. With the 3G transport