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DIGITAL VIDEO – Digital Signal Transmission

Signal formats, audio, video, data, digital, analog, computers and networks. Fiber optic cables, twisted pair, category 5, category 6. 220, 221, whatever it takes!

Who can keep everything straight and sort through the confusion? It can be especially difficult when trying to figure out what signal format is appropriate, or which signals might be compatible.

“Digital Video.” It’s everywhere we turn. Digital video comes in many forms and applications ranging from broadcast television transmitted over the airwaves to streaming video over the Internet. We see digital video on large screen LED billboards as we travel down the highway, in movie theaters and on the small screens of our iPods and cell phones. Every media industry is turning to digital video and producing different ways of delivering their digital content. All kinds of organizations have been created to try to standardize the sea of digital content and their delivery methods. We are in the middle of a transition period from analog to digital video and unfortunately many of the formats do not play well together.

In the worship environment, we usually try to mix video cameras and computers. Cameras give us our Image Magnification (IMAG) source and the ability to archive, stream or podcast the worship service. Computers provide PowerPoint slides or other messaging content. They may also playback our video clips. We also may have to playback a DVD from a traditional DVD player or video content from our Digital Video (DV) cameras. So how does this all fit together? When it comes to the worship environment, what are the accepted digital technologies that provide the best overall picture and sound?

Digital Characteristics
Although well designed and accurately aligned analog systems are capable of high resolution video delivery, digital provides some advantages.
• Resolution: Today’s projectors and flat panel displays use digital panels comprised of millions of pixels. These pixels determine the native resolution of the projector or the display. When the content resolution matches the display’s native resolution, high quality, high resolution video is easily attainable.
• Automatic Alignment: Some digital formats ask the display for its native resolution and will negotiate setup of the source and the display for accurate pixel by pixel alignment. This provides the sharpest pictures possible – automatically.
• Accurate: The digital signal is 100% repeatable. The content will not degrade in quality when transmitted over long distances. As long as the receiver can resolve the digital signal, the content will be accurately displayed. Also, there is no generation loss when duplicating digital content over multiple generations of media.
• Here are some areas of concerns with Digital video.
• All or None: Either you’ll get a picture, or you won’t. If a digital receiver cannot resolve the digital stream, then there is no output.
• Latency: When mixing, scaling or converting digital video signals, processing time is required. This processing may cause your video to be out of sync or delayed with respect to the audio. These “lip sync” issues can add up fast in an integrated system.
• Standardization/Compatibility: Chances are you’ll have to work with several different digital formats. Integrating those formats into one elegant system will be difficult and probably expensive. Digital format conversion can only be done through specialty devices, if you can find them.
• Of the many digital video signals that are available, we will discuss the common digital transport mediums you’ll find in most worship environments.

SDI (Serial Digital Interface)
The Serial Digital Interface was introduced in the late 1980s as a means of transmitting uncompressed standard definition video digitally over a single coax cable in a single stream. SDI is defined by the Society of Motion Picture and Television Engineers (SMPTE) in their 259M standard (see www.smpte.org). The most commonly used format in the United States is the 270 megabit per second (Mbit/s) data rate. This format digitizes the component signals of a source and transmits a digital signal in an “I” pattern over a high quality coax cable with BNC connectors. The signal can be transmitted up to 100 meters reliably.

The most common use of SDI signals in the worship video environment is with camera and switching/mixing systems. Several camera manufacturers support the SDI transmission format, either directly, or through optional add-on devices. The broadcast industry has embraced SDI and there are several switching/mixing systems. The advantages of SDI are: High quality video, long transmission distance (1000 ft) and a large number of manufacturers who support this format with their products.

HD-SDI (High Definition Serial Digital Interface)
This is the high resolution version of SDI and comes in a handful of standards. Based on SMPTE 292M, this format allows for transmission speeds up to 1.5 Gigabits (Gbits) per second over a single high quality coax cable.

Distances are limited to 100 meters. This standard will easily transport 720p (1280×768) and 1080i (1920×1080) HDTV resolutions. Also available is SMPTE 424M, which provides transmission speeds up to 2.97 Gbits/second. This is enough to push 1080p @ 60Hz refresh rate, also over a single high quality coax cable up to 100 meters. HD-SDI is the backbone standard for HDTV television production and provides uncompressed high definition video in an easy to deliver format. In the worship environment, this format is used to transport digital signals from HDTV cameras and HD video switching/mixing systems. Some projectors and flat panel displays have optional HD-SDI inputs, as well, but these devices can be quite expensive. A handful of small affordable Pan-tilt-zoom cameras are now offering HD-SDI as optional output formats. HD-SDI is becoming more universally used as HDTV gains acceptance with the general public.

DVI / HDMI
DVI and HDMI are the current standards for computer and some consumer products. The DVI (Digital Visual Interface) standard was developed by the Digital Display Working Group and released in 1999. See www.ddwg.org Originally designed to connect computers directly to a single digital flat panel display in desktop applications, this format provides backward compatibility by offering both analog and digital outputs from computer video graphics cards. DVI-I is the integrated analog and digital format while DVI-D is the digital only format (See figure 1).

DVI uses Transition-Minimized Differential Signaling protocol (TMDS), which provides three digital signals for video and a separate channel for timing data or “clock”. It also includes provisions for Extended Display Identification Data (EDID) for “Plug and Play” functionality with the data display or projector. Essentially, the DVI format allows the computer and display to negotiate with one another so the source device will provide the native resolution video to the display device. This provides the maximum video resolution and sharpness at the display device “automatically”. See figure 2 for an overview.

Because DVI provides three data channels for video, it is able to reach a bandwidth of 165 MHz providing transport of computer resolutions up to 1600×1200@60 Hz and HDTV rates up to 1920×1080. A second DVI “link” (six data channels) can be added to push the bandwidth up to 340 MHz and computer resolutions up to 2048×1536, although these “dual link” systems are not very common.

The single link DVI cable is relatively large and comprised of 24 pins for DVI and 29 pins for DVI-I. The original specification limited the cable length to 5 meters (15 feet). With advancements in cable construction technology, high quality cables can support the DVI format in lengths up to 50 or 75 feet. Some extender products can drive DVI transmission lengths in excess of 200 feet at medium computer resolutions (1024×768@60Hz). Fiber extenders can be used to extend the distances well beyond hundreds of feet to several miles at the higher resolutions, but at a significant cost increase.

Here are some issues to be aware of when working with DVI signals. Because DVI was originally designed to be a desktop solution from one computer to one display, it does not integrate well in larger systems. Once a source determines the resolution of a display, the source is locked into that resolution until it re-boots. Switching between multiple displays with different resolutions becomes problematic. The result of all this is inconsistent performance and reliability in integrated systems. Transmission distances are limited without investing in extender products. Product offerings for mixing and routing DVI signals are limited, although more products are being introduced as DVI gains popularity. DVI is a great digital transmission format for a single source to single display environments and manufacturers are working to provide solutions to extend its use into integrated systems.

The HDMI (High Definition Multimedia Interface) transmission standard also uses the TMDS protocol. The HDMI standard was released in 2002 by www.hdmi.org. It has recently been updated to version 1.3a, which adds substantially more bandwidth for higher resolutions and color depth. Many consumer products are adopting HDMI as the de-facto video and audio transport. The popularity of HDMI in the consumer market is driving its use into commercial and professional arenas. HDMI is commonly found in consumer electronics like Blu-ray high definition DVD players, cable and satellite receivers and Macintosh laptops. It has also become the common audio/video connector for HD video on most consumer flat panel displays.

HDMI is similar in structure to DVI, but has these added benefits. Bandwidth has been increased from 165 MHz to 340 MHz over a single link or cable. This equates to a bit rate of 10.2 Gigabits per second. That’s fast! The standard supports much higher resolutions, refresh rates and color depth bit rates up to 48 bits for richer, deeper smoother colors. HDMI includes up to eight channels of digital audio in its format and has provisions for automatic lip sync compensation. That’s also big! The HDMI cable is smaller than the DVI cable. Cable length limitations are not specified in the standard, but distances are similar to DVI using high quality cables. HDMI extenders are also available to drive medium resolution signals up to 200 feet.

HDMI suffers from many of the issues related to DVI, however. It also falls short when there is a need to drive video to multiple displays with different resolutions. We are starting to see switching systems, however, HDMI seamless video mixing systems have yet to be developed. For more detailed information on DVI and HDMI, see the Extron.com website for this and other great articles by Steve Somers: http://www.extron.com/company/article.aspx?id=dvihdmi_ts
One other consideration with HDMI and DVI is High-Bandwidth Digital Content Protection or HDCP. This is a high level content protection system that is similar in performance to TCP/IP SSL used in Internet security. HDCP can be used to protect content from being digitally copied and/or viewed on multiple displays. Part of the protocol also prevents conversion of the digital signal to analog video formats. This can become a big issue in an integrated system where multiple displays and conversion to/from other formats are the norm. Protected content may not play through these systems and appears as an intermittent system problem if you’re not aware of HDCP encoding.

Troubleshooting these problems can be quite difficult.

In-House Digital Distribution Systems
With all these digital formats, how do we then distribute these signals around our campus? Which formats do we choose? Fiber optic transmission systems will provide the distance and bandwidth for uncompressed video and audio, but at a very high cost. In house Digital Cable TV systems can also be implemented, but compress the signal and can also be costly. There are also several systems and formats for streaming video over a local area network, wide area network and the Internet. These network delivery systems are more affordable, but compress the video to the max. As you can see, delivery of content digitally involves a whole other discussion that covers a wide spectrum of delivery methods and standards.

Transmission Transition
I’ve barely scratched the surface in this article when it comes to details of digital video transport methods. There’s a tremendous amount of information available by searching the web. Fortunately, the transition from analog to digital is evolutionary, not just revolutionary. Next year’s National Association of Broadcasters convention will still have analog video products in their exhibit halls. But, new digital content and delivery strategies are being invented every day. Many of them do not play well together without one or two converter products. Over time, however, different digital delivery methods will all work together seamlessly.

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