SDI, HD-SDI, 3G-SDI, 6G-SDI, 12G and 24G-SDI

Updated on December 20, 2023

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Serial Digital Interface, short for SDI, is widely used in SDI encoders, SDI converters, and other equipment, including the scenarios of radio and television fields and security monitoring. We have witnessed the development of the video standard with ultra-high definition standards from SD-SDI and HD-SDI to 3G-SDI, 6G-SDI, 12G-SDI, and 24G-SDI. This article will focus on this topic and make a definitive guide.

Table of Contents

What is the serial digital interface (SDI)?

Serial Digital Interface (SDI) is a family of digital video interfaces first standardized by SMPTE (Society of Motion Picture and Television Engineers) in 1989. It is a standard for transmitting uncompressed digital video and audio using coaxial or fiber-optic cables.

The SDI interface can not directly transmit the compressed digital signal. Therefore, the system must decompress the compressed signal recorded by digital video recorders, hard disks, and other equipment and then enter the SDI system to travel.

However, repeated decompression and compression will cause image quality degradation and increased latency. Therefore, various formats of digital video recorders and non-linear editing systems have provisions of their own for the direct transmission of the compressed digital signal interface.

The table below shows the history of the evolution of the standards.

Standard Name Introduced Bitrates Example video formats SMPTE 259MSD-SDI1989270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s480i, 576i SMPTE 344MED-SDI2000540 Mbit/s480p, 576p SMPTE 292MHD-SDI19981.485 Gbit/s, and 1.485/1.001 Gbit/s720p, 1080i SMPTE 372MDual Link HD-SDI20022.970 Gbit/s, and 2.970/1.001 Gbit/s1080p60 SMPTE 424M3G-SDI20062.970 Gbit/s, and 2.970/1.001 Gbit/s1080p60 SMPTE ST-20816G-SDI20156 Gbit/s2160p30 SMPTE ST-208212G-SDI201512 Gbit/s2160p60 SMPTE ST-208324G-SDI202024 Gbit/s2160p/4k@120,8k@60 48G-SDI48 Gbit/s4320p/8k@120,8k@60 Table 1: SDI Standards List

SD-SDI

The SD-SDI standard supports the 270 Mb/s bit rate. SD-SDI transmits low-resolution PAL-compatible video 720 * 576 @ 25fps and uses a clock rate of 27 MHz. In 1994, ITU-R (formerly International Radio Consultative Committee) released Recommendation BT.656-2, incorporating the new serial digital interface defined in EBU Tech.3267 and SMPTE 259M, which uses 10-bit transmission and non-Zero Reverse (NRZI) encoding. The clock rate was 270 Mb/s when transmitting a 4: 2: 2 level signal from ITU-R BT. 601 (part A), the SD-SDI standard was defined as today’s serial digital interface.

HD-SDI

The HD Serial Digital Interface (HD-SDI) standard is standardized in SMPTE 292M. Although this standard is known as the 1.5 Gb/s interface, the bit rates supported by HD-SDI are 1.485 Gb/s and 1.485 / 1.001 Gb/s.

HD-SDI is widely used in the field of high-definition surveillance. It is unique in its ability to provide high-definition video at a resolution of 1080p at a frame rate of 25 or 30 while retaining all the video details with a latency almost equal to that of analog systems.

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3G-SDI

This standard is the 3 Gb/s interface, but the actual bit rates are 2.97 Gb/s and 2.97 / 1.001 Gb/s. 3G-SDI supports several mapping levels, as described in the SMPTE ST425-1 standard. These levels are called A, B-DL, and B-DS. Like the HD-SDI standard, the 3G-SDI supports 3G CRC generation, checking, line number insertion, and capture.

The 3G-SDI standard has been widely used in the broadcasting industry, and many manufacturers in the market can provide related products. At the same time, as the security industry continues to develop, the advantages of high-speed, uncompressed digital are gradually being explored.

The suppliers have launched many 3G-SDI series products, including optical transceivers, conversion equipment, digital switching matrix equipment, and splitters. These devices adopt 3G-SDI signals and are also backward compatible with 1.5G signals for long-distance transmission to meet users’ diversified needs.

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6G-SDI

6G-SDI standard defines a bit-serial data structure, electrical signal, and coaxial cable interface for transporting signals with a total payload of 5.940 Gb/s or 5.940/1.001 Gb/s. This standard also specifies the electrical and physical characteristics of coaxial cables and connectors.

This standard defines several mapping modes for the carriage of 2160-line and 1080-line image formats and associated ancillary data into a Single-link 6 Gb/s [nominal] SDI bit-serial interface.

12G-SDI

The 12G-SDI is a serial digital interface standard developed to support higher resolution, frame rate, and color fidelity. It provides four times the bandwidth of 3G-SDI, carrying 12Gbps, making it ideal for the 4K 60p format. This is not new. UG has been developed 6G / 12G since 2012 but has not been approved by the standard governing body SMPTE (Society of Motion Picture and Television Engineers) under the SMPTE ST-2082 draft name.

ShieldRock Quad 12G-SDI Optical Extender (Image source: B&H)

Most 4K professional cameras, medical endoscopes, and monitors use four BNC connectors simultaneously transmitting 12G-SDI signals. Some cameras, such as the Sony PXW-Z280 handheld all-in-one camera, now include 12G-SDI output connectors as technology advances; nevertheless, only one SDI cable meets the 12G transmission standard and can directly transport 4K 60p signals.

Some video transceiver suppliers also launched 12G-SDI transceiver to meet the higher resolution television and broadcast market usage.

24G-SDI

24G-SDI also known as UHD-2 or 24G UHD-SDI, defined in SMPTE ST-2083, this standard supports 8k 120p resolution. It is the latest generation of serial digital interfaces for targeted UHDTV real-time streaming media interface applications. 24G-SDI uses eight lines of SMPTE ST 2083, allowing video signals to be transferred at speeds of up to 24 Gbps.

ShieldRock Quad 12G-SDI Optical Extender (Image source: B&H)

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48G-SDI

48G-SDI is a new term some industry leaders propose but has not been defined in SMPTE. By mixing four independent 12G-SDI channels, 48G-SDI supports an 8K (48G) signal over fiber cable. It is ideal for transmitting multiple uncompressed SDI streams or 8K broadcast-grade UHD signals.

History of serial digital interface (SDI)

1982

In 1982, the former CCIR issued CCIR 601 based on the institutional proposals of the European Broadcasting Union (EBU) and the American Society of Motion Picture and Television Engineers (SMPTE) at a sampling frequency of 13.5 MHz and 8 Bit quantification and 4: 2: 2 chrominance sub-sampling unify the digital parameters of both 525/60 and 625/50 television scanning systems.

1983

In 1983, CCIR became the International Telecommunication Union Radio Telecommunication Sector (ITU-R).

1986

In 1986, CCIR released CCIR 656, based on the EBU Tech.3246 and SMPTE 125 standards, and proposed a parallel interface that transmits CCIR 601 specifications using 11 twisted pairs and 25-pin D-type connectors.

Some early digital devices used this interface. Still, because of the short transmission distance, the connection is complicated and, for other reasons, not suitable for large-scale use.

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The CCIR 656 also includes the EBU Tech.3247 serial digital interface standard proposed by EBU in 1983 and uses 8/9 block coding at a bit rate of 243 Mb / s but only supports 8-bit quantization and is not easy to design a stable cheap interface chip.

1994

In 1994, ITU-R released Recommendation BT.656-2, incorporating a new serial digital interface defined in EBU Tech.3267 and SMPTE 259M, which uses a 10-bit transmission and NRZI encoding. The clock rate is 270 Mb / s when transmitting a 4: 2: 2 level signal from ITU-R BT—601 (part A), now the famous SDI.

Using a 75-ohm coaxial cable and 75-ohm BNC connector (IEC 60169-8) enabled the reuse of many existing cabling inside the station in a digitized system, and SDI became standard on digital devices. Based on the final realization of the studio, master, broadcast control system of digital.

China also formulated the corresponding national standard BG / T 17953 concerning the above standards. EBU Tech.3268, SMPTE RP145 and ITU-R BT .799 propose dual-link to meet the demand of high-quality program production for 4: 4: 4 level image and chroma key of ITU-R BT.601 (Part A) Concept that transmits both the R’G’B ‘/ 4: 4: 4 image and the other broadband signal over two SDI channels at the same time.

In 1990, with the release of Recommendation ITU-R BT.709, the accelerated development of high-definition television technology, the use of a serial digital interface to transmit high-definition signals has been agreed upon in the industry, for which SMPTE defined in the 292M standard clock frequency up to 1 The serial digital interface of 5 Gb / s level, the corresponding international standard is ITU-R BT. 1120, this is the well-known HD-SDI.

2005

In 2005, the ITU-R specified a 2.97 Gb / s serial interface in BT.1120-6, which still uses the 75-ohm coaxial cable and the IEC 60169-8 standard connector. In addition, SMPTE 424M also gives a similar definition of a 3 Gb / s level interface. The advent of the 3 Gb / s serial interface solves the previous need for dual-link HD-SDI, such as 4: 4: 4 / 12bit or 1080p50 / 59.94 format programming.

Manufacturers have introduced 3 Gb / s serial interface chip products. In some occasions that require long-distance transmission, such as the connection of two distant studios, copper is a bit powerless.

Currently, optical fiber cable has become a natural substitute for copper. ITU-R BT.1367 and SMPTE 297M are standards for transmitting serial digital signals over optical fiber cables. Take ITU-R BT.1367 as an example. Only single-mode optical fibers and corresponding optical connectors are allowed when transmitting high-definition signals – Electrical, electrical-optical conversion by the appropriate optical receiver and optical transmitter to complete.

SD-SDI vs. HD-SDI vs. 3G-SDI, what is the difference?

The primary electrical specifications of HD-SDI and SD-SDI are the same, but the transmission bit rate is much higher than that of SD-SDI. Since the ITU-R BT.1120-2 specifies that the luminance sampling frequency of high-definition video signals is 74.25 MHz and the sampling frequency of two color difference signals is 37.125 MHz respectively, the primary bit rate of HD-SDI reaches 1.485 Gb/s. Considering that the distribution of high-frequency transmission cable parameters affects the transmission of high-definition video signals, the cable length will be significantly reduced.

The data transmission format of HD-SDI is the same as the transmission format of SD-SDI, and the luminance signal Y and the color difference signals Cb and Cr subjected to time-division multiplexing are handled as 20-bit words. Each 20-bit word corresponds to a color difference sample and a luminance sample. The multiplexing modes are (Cb1Y1), (Cr1Y2), (Cb3Y3), and (Cr3, Y4).

With the advent of high-definition (HD) video standards such as 1080i and 720P, interfaces have been adapted to handle higher 1.485Gbps data rates. The 1.485-Gbps serial interface, commonly called the HD-SDI interface, is defined by the SMPTE292M.

It uses the same 75-ohm coaxial cable. SMPTE approved the new standard, SMPTE424M, which doubles the SDI data rate to 2.97Gbps over the same 75-ohm coaxial cable and supports higher-resolution images such as 1080P and digital cinema.

3G-SDI is an upgraded version of HD-SDI. The system supports SMPTE424M, SMPTE292M, SMPTE259M, SMPTE297M, SMPTE305M, and SMPTE310M standards.

What are the SDI Advantages?

• High Reliability: SDI transmits video data without compression, resulting in a high-definition multimedia interface.
• Latency-free: SDI signal is transported through coaxial cables and clear HD images with no delay and uncompressed digital signals. It’s crucial to ensure real-time signal transport, like monitoring and live broadcast.
• Long-distance: compared with traditional technology, the serial digital interface supports tens of transmission kilometers over fiber cable without degrading signal quality.
• Maximum cabling: most SDI utilize