Not Your Grandfathers SDI
Published 12/1/2021
With the advent of Ultra-High-Definition (UHD) TV, picture quality has improved dramatically, reaching resolutions once only possible with film and digital cinema production. But with UHDs 3840 x 2160 picture resolution, the familiar Serial Digital Interface (SDI) standard has had to evolve to carry UHDs heavier data loads throughout the broadcast workflow, including over-the-air ATSC 3.0 transmission.
While this subject can quickly get very deep and technical in scope, in this blog we hope to provide an overview of SDIs evolution and a few of the most common formats and approaches for managing and transmitting 4K/UHD signals.
SDI panel of CU-HD1300-S1 CCU with 12G and 3G 4K outputs
It all began in 1989 when SD-SDI was standardized as SMPTE 259M, with a bit rate of 270Mbps, capable of carrying 480i standard resolution NTSC video. A decade later, HD-SDI appeared, and was standardized as SMPTE 292M. HD-SDI can carry 1.5Gbps (1.485Gbps), where Gbps stands for Gigabits per second. Thats sufficient for 720p/1080i HDTV video.
In 2006, the 3G SDI format was standardized as SMPTE 424M, and with its roughly 3Gbps bit rate, (actually 2.97Gpbs) it could support a 1080/60p full HD (ATSC 3.0) video signal, which travelled over a single coax SDI cable.
But by 2015, the landscape became more complex as the industry explored ways of dealing with UHDs higher resolution picture and High Dynamic Range (HDR). Today, three of the most widely used approaches include: Quad-link HD-SDI, Quad Link 3G-SDI, and 12G-SDI.
Quad-Link HD-SDI involves the transmission of four separate 1080p or 2K signals, and it supports 10-bit 4:2:2 at up to 30 frames per second (fps). This format was mostly used for 24p UHD. As the name implies, the UHD picture is split four ways for transmission over four separate SDI coax cables.
Quad-Link 3G-SDI also requires the use of four separate SDI coax cables as the picture information is also split four ways for transmission. However, when the four separate 3G-SDI signals are combined, together they deliver a 12Gbps signal. This extra bandwidth afforded by 3G-SDI signals enables the support of 10 bit 4:2:2 4K at 60p or 12-bit 4:4:4 at 24p or 30p.
A single 12G-SDI cable takes a completely different and less cumbersome approach. With 12G-SDI, four 3G-SDI signals are multiplexed and transmitted over a single SDI coax cable. With a bit rate of 12Gbps, 12G-SDI supports the transmission of full 4K/60p UHD video and is backwards-compatible with all previous SDI standards.
The broadcast industry is used to having video formats change at a rapid pace as video bandwidth and/or digital video recording requirements grow. Since the early 1990s, digital video tape recording ushered in new formats, such as D-1, D-2, D-5, D-6, DVCAM, and DVCPRO.
Early D-1 and D-2 HD video tape recorders used large, 25-pin D type parallel interconnection signal cables. Later, parallel to serial and serial to parallel converters allowed the use of a single SDI coax cable. Eventually, all future VTRs came standard with SDI Input/Output (I/O) capability. While todays SDI interface is unique to the professional television industry, its established, well understood, widely available, and cost effective.
Count on Hitachi Kokusai to keep pace with the latest industry advancements. Check out our top-of-the-line camera systems, such as the Hitachi SK-HD1800, at our website.