Broadcast News

Bookmark and Share
23/05/2016

Going The Distance With SDI

News Image
The Serial Digital Interface (SDI) standard was first introduced back in 1989 and quickly became established as the digital video interface of choice for broadcast and professional use.

One of the main reasons for the success of SDI was the fact that it used the same coax cable and BNC connectors as the analog video standard that had preceded it. This meant that broadcasters could make the upgrade to digital video without having to completely rewire their facilities or replace all their cables and patch panels, thus reducing the cost of entry.

The original SDI standard has since been joined by HD-SDI and 3G-SDI, which add HD capability, and more recently 6G-SDI and 12G-SDI, which add Ultra HD capability. Each of these new standards builds on the last while retaining the same BNC connectors and coaxial cable.

It would be easy to fall into the trap of thinking we could simply replace our SD equipment with HD or Ultra HD equivalents and keep all our cabling infrastructure the same, but in practice it is not always so simple. As video resolution has increased from SD to HD to UltraHD, the SDI bit rate has increased dramatically from 270 Mb/s to 11.88 Gb/s, which in turn places increased demands on the cabling infrastructure that we use. In practical terms this greatly reduces the cable lengths that can be used, and some older or lower cost cables may not be usable at all.

If we take a look at the technical specifications for a typical piece of broadcast equipment, we will usually find a table indicating the maximum length of cable that can be used for each supported video standard. For example:

Max. Cable Length (Belden 1694A)Bit Rate
m 250 – Mbit/s 270
m 140 – Gbit/s 1.5
m 80 – Gbit/s 3

The table lists the performance that can be expected with a particular type of cable, in this case Belden 1694A. However, even these figures do not tell the whole story and practical cable runs are often significantly shorter.

The actual length of cable over which SDI can be successfully sent and recovered depends on a number of factors including:

• Type and quality of the cable
• Number of joins in the cable run
• Kinks or damage to the cable
• Quality of the BNC terminations
• Quality of the equipment at each end of the cable

Any of these items can cause signal degradation and in combination they can cause a significant reduction in the usable cable length. There are two underlying issues at play here: cable losses and impedance mismatch.

Cable Losses

An ideal video cable would transfer all of the SDI signal power from the source equipment to the destination with no losses, but in the real world, DC resistance and parasitic capacitance act together like a low pass filter. As the cable length increases, the signal amplitude is reduced and the higher frequencies roll off, causing significant degradation to the video signal. Because high frequencies are attenuated the most, HD and UltraHD signals are affected much more than SD. Cable quality really does make a difference here so high quality, low loss cables should always be used for HD and Ultra HD unless you only plan to go a few meters.

Impedance Mismatches

To efficiently transfer the SDI signal from one end of the cable run to the other also requires a uniform 75 ohm impedance along the cable length. Any variations in impedance result in a "mismatch" that causes some of the signal power to be reflected back down the cable in the opposite direction. This reflected signal power does not reach the destination equipment, so the usable cable length is reduced. Impedance mismatches can be caused by poorly designed equipment, badly terminated connectors or even a crushed, kinked or damaged cable.

Maximizing Usable Cable Length

Follow these simple rules of thumb to maximize usable cable length:
• Use good quality low loss cable
• Use good quality connectors
• Repair or discard poorly terminated, kinked and crushed cables
• Avoid joins in the cable
• Limit the use of patch panels

Extending SDI Transmission Distance

In some applications an SDI video signal just cannot reach far enough. You might have an Ultra HD camera on the far side of a sports field, or video monitors in far flung halls of a convention centre. In these instances another solution is required.

The range of an SDI signal can be doubled by inserting a good quality reclocking distribution amplifier (DA) in the middle of the cable run. The DA includes an equalizer, which compensates for cable losses at the input, and a reclocker which regenerates the video clock to remove jitter from the video signal. The output of the DA is a fully restored version of the degraded signal at its input.

For even longer distances the SDI signal can be transmitted over fiber optic cable. A small SDI to optical fiber media converter is used to convert from electrical to optical format without altering the video information in any way, and at the other end of the fiber another media converter is used to convert back to coaxial cable. The optical signal can be sent over several kilometres of fiber with no loss of quality.

Ripcord Designs is a UK-based manufacturer of high quality broadcast equipment for the budget-conscious AV professional. Through our own web shop we offer a range of broadcast converters that can extend the reach of SDI signals from SD to Ultra HD.

Our line-up of reclocking distribution amplifiers includes the MC-DA-3G which works with all SDI formats up to 3G-SDI, and the MC-DA-12G which adds 6G and 12G-SDI capability. When longer distances must be covered we offer a range of media converters capable of transporting SD, HD and 3G SDI over 10 kilometers of single mode fiber.

In addition to manufacturing our own products we also provide our design services to other broadcast manufacturers. From contract electronic and software work to full turnkey product design, we have it covered.


www.ripcorddesigns.com

(JP/LM)
Solidmate Ltd Memory Card Hire London

Top Related Stories
Click here for the latest broadcast news stories.

17/04/2023
Globecast Launches First Video Public Cloud Video Headend
Globecast has developed and launched the first video public cloud video headend, powered by AWS, for a major European Telco. Globecast joined the AWS
15/02/2024
LiveU Delivers The Power Of IP-Video
This year promises to be a monumental one for live news and sports with major events including the US Presidential Election and the Summer Games in Pa
15/02/2024
Fuse Technical Group Selects Matrox Video
Matrox® Video today announced that multimedia system designer and integrator Fuse Technical Group chose Matrox Video's technology to drive SMPTE ST 21
08/04/2005
Gefen unveils HD SDI connectivity and conversion solutions for AV and broadcast sectors
New HD serial data interface (HD SDI) equipment on display at NAB from Gefen include a 4x2 HD SDI Switcher, 1:4 HD SDI Splitter, HD SDI to DVI Convert
12/09/2023
LiveU Releases Single 4K/Quad HD Video Receiver
LiveU has leveraged its leading IP-video experience to release a single 4K/Quad HD video receiver for ST 2110 broadcast facilities. With this latest a
21/04/2023
FOR-A To Showcase NDI®-Compatible Live Video Production Ecosystem
FOR-A is showcasing a fully NDI®-compatible live video production ecosystem at CABSAT 2023 (stand S1-M10, Dubai World Trade Centre, 16 – 18 May). The
20/04/2023
DTEN's D7x Video & Collaboration Solution Now Certified For Microsoft Teams
DTEN has announced that its DTEN D7X video conferencing & collaboration solution is now certified for Microsoft Teams. DTEN, in collaboration with Mic
19/04/2023
Zero Density Displays Virtual Video Bundle At NAB 2023
Zero Density has announced ZD | Virtual Studio, a new software bundle that lowers the barrier of entry into virtual production. ZD | Virtual Studio is
18/04/2023
Ross Video Teams Up With DreamWall
Ross Video has announced a new partnership with DreamWall, a leading provider of augmented reality and virtual studio services based in Charleroi, Bel
12/01/2009
Blackmagic Design Announces DeckLink SDI
Blackmagic Design has announced DeckLink SDI, a new model of capture card which combines highest quality 10 bit SD/HD-SDI capture and playback with an
23/04/2009
Vaddio Expands ProductionVIEW Line With New HD-SDI Camera Control Console
Vaddio, the leading manufacturer and OEM distributor of specialty PTZ cameras and high-end camera control systems, introduces the ProductionVIEW HD-SD
15/12/2023
Ross Video To Exit LED Display Business
Ross Video has announced plans to exit the LED display business and is now considering strategic opportunities for D3LED. While this process takes pla
14/12/2023
Broadpeak To Highlight Video Streaming Solutions At CES 2024
At CES 2024, Broadpeak® is to highlight innovative video streaming solutions that ensure an outstanding quality of experience (QoE) for live sports, b
14/12/2023
Brompton Brings Forward-Thinking LED Video Processing Technology
Earlier this year, Melbourne welcomed the inaugural Now or Never festival – a 16-day celebration of digital art and future thinking. The festival used
13/12/2023
Black Box To Feature IP-Based KVM And Video Wall Solutions
At ISE 2024 on Booth 5E330, Black Box® will feature IP-based KVM and video wall solutions that enhance control room workflows and empower operators by