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What is Serial Digital Interface (SDI)? Quick Explanation of the Difference Between SDI and HDMI

Serial Digital Interface Explained

SDI stands for Serial Digital Interface. It’s used to transmit and receive digital audio and video. Beyond video and audio, SDI also transmits timecode and metadata.

Video is transmitted through a premium-grade coaxial cable and BNC connectors with Teflon insulation.

For optical fiber cables, ST, SC, and FC connectors may also be used.

An SDI signal may carry up to 4 separate audio signals alongside a video signal through a standard 75-ohm coaxial cable.

SDI History and Development

The SDI video standard was first introduced by the Society of Motion Picture and Television Engineers (SMPTE) in 1989.

Since then there has been a steady development of the standard as follows:

Standard Definition SDI

SD-SDI is the first standard introduced in 1981 under SMPTE 259M. Under this standard, video and embedded audio are sent and received at bitrates ranging from 177 Mbps and 270 Mbps. It’s popularly employed for 480i and 576i videos.

High Definition SDI

The HD SDI was introduced in 1998 under the SMPTE 292M standard. The video data rate supported is 1.5 Gbps. HD-SDI is often used with 720p and 1080i video.

Enhanced Definition SDI

This SMPTE 344M standard SDI is known as ED-SDI. It was introduced in 2000. Supporting a bitrate of 540 Mbps, it works well with 480p and 576p video formats,

Dual-Link HD-SDI

Released under the SMPTE 372M in 2002, this standard delivers video at a data speed of dual 1.5 Gbps links, when combined offers 3 Gbps. It is mainly used for the 1080p video format.

3 Gbps SDI

As its name suggests, this standard delivers video at a speed of 3 Gbps. It was introduced in 2006 under the SMPTE 424M standard and is used with 1080p video.

6 Gbps SDI

This SDI standard was introduced in 2015 under SMPTE ST 2081. Data speed is 6Gbps. Two video resolutions suit this standard – 1080p at 60fps and 2160p at 30fps.

12 Gbps SDI 

Like the 6Gbps version, this standard was introduced in 2015. Its standard is laid down in MPTE ST 2082. It supports video transmission at 12Gbps and is used for 2160p video at 60fps.

What’s the Difference Between SDI and HDMI?

SDI is often compared with HDMI because both are capable of transmitting high-quality video and audio (uncompressed and compressed) through a single cable.

But then there are differences between the two cables.

An HDMI connection, as you may be well aware, is found in consumer-level devices – HDTV, Blu-ray players, game consoles, TV boxes, and even computers.

SDI, on the other hand, is common in professional setups like broadcast studios and sound studios.

Data Type

Different types of video data are transmitted by SDI and HDMI.

SDI transfers raw data whereas HDMI transfers picture or image data.

What do you mean by raw data? Raw data means the signal is transferred on 1s and 0s. This data is then sent to the recording monitor. The recorder then encodes it into an image that will be displayed on the monitor screen.

Cost

HDMI is cheaper to set up compared to SDI which requires expensive equipment. Also, coaxial cables are more expensive.

Distance

An HDMI signal is effective up to about 50 feet. Beyond that, you would need an amplification device to boost the signal.

SDI can run up to 100 feet and can be extended to 1000 feet if fiber optic is used.

Connectors

HDMI uses a standard 19-pin connector, although there are other types like mini/micro-HDMI. Different HDMI versions support different video resolutions and frame rates.

For SDI with coaxial cable, the BNC (Bayonet Neill-Concelman) connector is used.

Why is SDI Preferred in Professional Setups Instead of HDMI?

Apart from the edge in terms of signal strength to cable length (useful for outdoor production), most pro-grade equipment use SDI as standard connections.

The BNC connector used by SDI has a physically secure connection. It has a tighter grip compared to HDMI.

So, there’s less risk of someone tripping over the cable and disconnecting it or someone accidentally pulling it out.

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