Internet Technology

Serial Digital Interface: What is SDI?

Serial Digital Interface- What is SDI?
Written by prodigitalweb

Regarding the transmission of digital audio & video signals, you can find different transmission standards, and SDI is one of them. This article lets you know what SDI is, what SDI stands for, what SDI video is, etc.

What is an SDI Connection?

SDI or Serial Digital Interface is a digital standard for transmitting video and audio signals over coaxial or fiber optic cabling.  Hence, the range of speed is between 270 Mbps and 12 Gbps. In this regard, you should know that the current standard was released in 2015.

It uses data words of 8 or 10 bits in length. Besides, these signals are uncompressed. In addition, these are self-synchronizing between the transmitter and receiver. Therefore, you can detect most errors caused by noise or interference. Moreover, you can use the Hamming code to recover lost data. These signals can contain up to four audio signals with a video signal.

Brief History of Serial Digital Interface:

In 1989, this video standard was first developed by the Society of Motion Picture and Television Engineers (SMPTE). Since then, we have gotten six different standards, one under development.

Understanding Bitrates:

Before knowing about different serial digital interface standards, you should know about this crucial parameter of the standard. Bitrate means bandwidth. It refers to the rate of bits being transferred from one source to another. Besides, it means how much data you can get in a given time. So you should measure it in bps, kbps, Mbps, Gbps, etc.

Serial Digital Interface Benefits:

There are many advantages of this digital standard.

  • It is an uncompressed signal. Therefore, it can easily maintain the highest video quality. Besides, it is crucial for professional applications.
  • Finally, it is because the picture must be accurate and pristine in such cases.
  • It is also a unidirectional signal, meaning you see signal degradation rare than HDMI. As a result, it becomes more reliable than the high-definition multimedia interface. However, it can experience issues while transmitted over long distances.
  • These cables are less susceptible to electromagnetic interference than high-definition cables. While high-definition cables send audio & video signals down over a single cable, these can send them via different cables.
  • It is less costly than HDMI. Therefore, it has become a good option for professional video production companies who want to buy many cables simultaneously.


  • Most consumer-level devices do not support this standard. Therefore, it is only suitable for pro-level camcorders and other video recording equipment. However, you might find it annoying when your equipment cannot connect your device to a non-HDMI device.
  • In addition, these are harder to twist than HDMI wires. Thus, it has become more challenging to manage.
  • These wires are very thick.

Ideal Environment:

You can use it for concerts, sporting events, and multiple active foot traffic venues. Besides, these connections are suitable for wildlife places or harsh weather. In addition, if you don’t want any interruption during streaming, you should use this interface.

It is available in broadcasts like emergency declarations. But most professionals don’t want to take risks. Remember, one technical issue can lead the viewers to find new content. But this connection reduces the chance.

SDI Data format:

The serial data format for SD and ED applications is 10 bits wide. On the other hand, this interface is twenty bits wide in HD applications. It is categorized into two parallel 10-bit datastreams called Y and C. You can see the SD datastream arranged like this:

Cb Y Cr Y’ Cb Y Cr Y’

While the HD data streams are arranged like this:


Y Y’ Y Y’ Y Y’ Y Y’


Cb Cr Cb Cr Cb Cr Cb Cr

4:2:2 YCbCr format is the native color encoding for all interfaces. ‘Y’ represents the luminance channel encoded at full bandwidth (13.5 MHz in 270 Mbit/s SD, ~75 MHz in HD). There are two chrominance channels which are as follows: Cb and Cr. These are subsampled horizontally and then encoded at half bandwidth (6.75 MHz or 37.5 MHz). In this case, you should know that the Y, Cr, and Cb samples are co-sited. While Y means luminance samples, C indicates chrominance samples. Cr and Cb indicate the red and blue “color difference” channels.

There are other color encodings whose interface you can treat as a generic 10-bit data channel. Video and ancillary data payload can use 10-bit words in the range of 4 to 1,019 (00416 to 3FB16). However, the values 0–3 and 1,020–1,023 (3FC16–3FF16) are reserved. Therefore, these won’t appear in the payload. These are especially for Synchronization packets and Ancillary data headers.

Synchronization packets:

A synchronization packet is called the timing reference signal or TRS. It mainly happens before the first active sample on every line and after the last active sample. It comes with four 10-bit words. While the first three are the same—0x3FF, 0, 0; the fourth one comes with three flag bits with an error-correcting code. Therefore, eight different synchronization packets are possible in total.

The packets co-occur in the HD-SDI and dual link interfaces in the Y and C datastreams. But you can see only one datastream in SD-Serial digital interface and enhanced definition interfaces. As a result, you can get one synchronization packet at a time. The format is similar to all Serial digital interface versions.

There are flag bits called H, F, and V. Hence, the H bit means the beginning of the horizontal blank. In addition, it represents the synchronization bits that have H set to one. A few packets are known as End of Active Video or EAV packets. Similarly, the packet which appears just before the start of the active video has H set to 0. It is the Start of an Active Video or SAV packet.

The V bit indicates the Start of the vertical blanking region. Besides, an EAV packet with V=1 means that the following line is part of the vertical interval. In addition, an EAV packet with V=0 implies the line is part of the active picture.

The F bit lets you know if the line comes from the first or second field in interlaced and segmented-frame formats. Regarding progressive scan formats, this bit is always set to zero.

Line counter and CRC:

Extra check words are offered in the high-definition serial digital interface and dual-link HD to boost the interface’s robustness. The four samples contain a cyclic redundancy check field and a line count indicator in these formats. Hence, the CRC field offers a CRC of the preceding line. You can use this to detect bit errors in the interface. The line count field refers to the line number of the current line. You can not get CRC and line counts in the SD and ED interfaces. Instead, you can use a special ancillary data packet called an EDH packet to offer a CRC check on the data.

Line and sample numbering:

Every sample is assigned a unique line and a sample number. The first sample is assigned sample number 0. Besides the next one is sample 1. Regarding SD interfaces, the Cb sample is the 0th sample, while the Y sample is the 1st sample. In addition, the Cr sample is the 2nd sample, and the ‘Y’ sample is the third one. After that, the pattern repeats.

Regarding HD interfaces, you will see every data stream with its sample numbering. Therefore, the Y sample is the 0th sample of the Y datastream, while the Y’ sample is the next one. Similarly, the CB is the first sample in the C datastream, followed by Cr and Cb.

Link numbering:

It is only a problem in multi-link interfaces. While the first link is assigned a link number of 1, subsequent links are assigned to boost the link numbers. For example, link 2 is the second link in a dual-link system. A VPID packet is the link number of a given interface.

The primary purpose of the data layout design in the dual link is to feed the primary link into a single-link interface. Besides, the secondary link contains extra LSBs (in 12-bit formats), non-cosited samples in 4:4:4 sampled video, and alpha or data channels.

Different Versions of SDI:

SD-SDI (SMPTE 259M): Published in 1989, it is the first version of this digital standard. The SD version is compatible with resolutions up to 576i. Besides, NTSC and PAL support resolutions at 25 fps. In addition it supports four different bitrates including 270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s.

But do you know what is SDI rate exactly? People mostly use the 270 Mbit/s as the PAL/NTSC standard. However, the widescreen version can support bandwidths up to 360 Mbit/s.

ED-SDI (Enhanced Definition, SMPTE 344M):

It is compatible with progressive resolutions (576p). Besides, this version can enable you to bitrate up to 540 Mbit/s and a dual 270 Mbit/s implementation.


In 1998, this version came on the market following the introduction of HD resolution (720p and 1080i). Besides, it supports bitrates up to 1,5 Gbit/s (1.485 Gbit/s and 1.485/1.001 Gbit/s). In addition, it is compatible with video formats with 59.94 Hz, 29.97 Hz, and 23.98 Hz FPS.

Dual Link HD-SDI (SMPTE 372M):

It is the more updated version of the previous one. In addition, it needs to support higher bitrates. Besides, this version is compatible with bitrates 3 Gbit/s and up to 1080p resolution at 60 Hz, 59.94 Hz, and 50 Hz fps. Therefore, the bitrate is 2.970 Gbit/s, and 2.970/1.001 Gbit/s more precisely.

3G-SDI (SMPTE 424M, 424:2012):

It is a 3 Gbit/s interface standardized in 2006. Later, in 2012, it was revised, supporting all similar features as the Dual Link HD-SDI 1.485 Gbit/s interface.

6G-Serial digital interface (SMPTE ST-2081):

This 6Gbit/s interface is precisely 5.940 Gb/s or 5.940/1.001 Gb/s. It can support 4K 30 frames on a single link and was introduced in 2015.

12G-SDI (SMPTE ST-2082):

This version was introduced in 2015 and revised in 2018. Besides, it can cover SDR and HDR Source image formats into single or multiplex 10-bit interfaces. In addition, this version is compatible with 4K 60 from a single link and 8K 30 from a dual.

What Makes Serial Digital Interfaces Unique Compared To Other Standards?

Picture quality is always a crucial factor for video standards. Unfortunately, if you look at DVI, HDMI, and DisplayPort standards, you don’t find any image quality among them. However, the serial digital interface still has a few more benefits than others.

Serial Digital Interface Cable Lengths:

It can distribute audio & video signals over longer distances. All versions of this standard use one or more coaxial cables with BNC connectors. You can run an HD/3G signal over 100 meters based on cable quality. It is impossible for other standards like HDMI, DVI, or DisplayPort.

In this case, you should use optical fiber as a transmission cable. It helps to send a high-end 4K or 8K signal over long distances.

Optical Fiber Support:

Manufacturers have made some versions of the next generation, 12G, on the market. For instance, Blackmagic Design comes with a Teranex Mini converter containing a default optical fiber connector. As a result, it is possible to push 12G version UHD at 2160p60 up to 10,000 using fiber.

Physical Connector:

All versions use BNC connectors with coaxial cables. But what benefits can it offer? First, you should know that it is a close-fitting connection locked while attached. In addition, the male connector comes with a pin you can fit into the main cable. Finally, you can keep the cable secured with an external ring that locks in position. This feature lets you know what SDI security system is.

Uncompressed Video And Audio:

It can transmit uncompressed video like other video standards. So, it does not make a huge difference. But the serial digital interface family can offer embedded audio.

Embedded Audio:

All display stands don’t support embedded audio. For instance, DVI doesn’t support it though both HDMI and DisplayPort support the audio. Besides, HD and SD interfaces are compatible with sixteen channels of embedded audio. The SMPTE ST 299-2:2010 can extend the audio support with the new 3G version to 32 audio channels on a single link.


HDCP, or High-bandwidth Digital Content Protection, is a security protocol that stops copying digital audio and video content. Intel Corporation developed it. Besides, different connections like DisplayPort, DVI, and HDMI use it. This protection needs a handshake between the source and the output device. As a result, you can face issues like no signal, signal drops, etc. You can encounter the problem even in professional installations. Remember that the HD version doesn’t have HDCP. As it is slow, the signal can respond quicker while switching between sources.


This standard can carry both video signal and time code in one cable. In this regard, you should know that HDMI or DVI is incompatible with the time code, but DisplayPort does.


The 24G version of the serial digital interface standard is in development and is expected to be the future of this digital standard. It will support 8K, 120p resolution. Therefore, ensure that this digital standard will be around for an extended period. For instance, you will see the first 8K projector and how these ran sixteen wires into it for a single 8K feed.

Frequently Asked Questions:

  • Why is it used?

You can use this to connect different equipment pieces like recorders, monitors, PCs, and vision mixers.

  • What is the difference between SD and SDI?

While SD is a resolution of a video signal, a serial digital interface is a digital video connection.

  • What is better, SDI or HDMI?

HDMI does not feature a locking mechanism and is limited to shorter distances. But the other one locks into place. Besides, it can carry data over longer distances.

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