Is Plug-and-Play functionality affected by video extension?

Plug-and-play is a hardware/software feature that allows for automatic discovery, installation, and configuration of hardware components in a PC. With regard to PC video, the foundation of this process is a data structure referred to as Extended Display Information Data (EDID). EDID information is stored in non-volatile memory on a display device and is a readable by a graphics card via a serial bus called the I2C bus. Together, this functionality is referred to as the Display Data Channel, or DDC.

EDID information is used by the video driver and configuration utilities to allow supported modes of the display to be utilized and to disallow unsupported modes from being selected. The following is a subset of information provided by an EDID viewer on my own PC:

Power Management and Features:
Standby : Supported
Suspend : Not Supported
ActiveOff : Supported

Established Timings :
800 x 600 @ 60Hz (VESA)
640 x 480 @ 75Hz (VESA)
640 x 480 @ 67Hz (Apple, Mac II)
640 x 480 @ 60Hz (IBM, VGA)
1024 x 768 @ 60Hz (VESA)
832 x 624 @ 75Hz (Apple, Mac II)
800 x 600 @ 75Hz (VESA)

Preferred Detailed Timing:
Input Type : Digital Separate
Interlaced : False

Stereo Display: Stereo Display : Normal display (no stereo)

Digital and analog video extenders, rarely provide direct connectivity of the DDC channel from the source to display device. Some analog Cat5 transmitters, for example, provide ‘canned’ EDID information to the source device. Others offer no EDID information, in which case the OS assumes that a non plug-and-play device is attached. Yet other analog extenders provide EDID pass through from a display that is locally-attached to the transmitter. Some extenders are able to cache EDID information on the transmitter, thereby allowing the locally-attached display to be disconnected. Digital extension products may actually read EDID information from the display and pass it back to the transmitter. The transmitter may edit the information to remove modes that it does not support before passing the information to the source PC.

Implementations that rely upon canned or no EDID information may prevent selection of video modes that are supported by your display. Likewise, canned information may allow a user to select a mode which the display does not support. In the case of my sample EDID above, my laptop might not be able to go into Standby if the canned EDID did not indicate this support. Conversely, if allowed for in a canned EDID, a user might be able to select a 16:9 resolution such as 720p or select interlaced video even though these are not supported by the display. Many HDMI displays support embedded audio. However, if this was not enabled via a canned EDID, then the speakers would be rendered useless.

When selecting a video extension product, let your supplier know what aspects of your display device are important to you. For example, you may require: a certain resolution, digital audio, HDCP, interlaced video, power management, the flexiblity to change displays from time to time, the ability to restrict modes that a user can select, or a specific aspect ratio. For multi-point extension, you’ll want to ask how to select a particular display device as the EDID source, especially if video is being extended to a variety of monitors.

Don’t panic if a extension product results in a plug-and-play limitation. Most qualified vendors and integrators should be able to offer a software or hardware work around. Or, if you wish, feel free to reply to this post for suggestions.

Video compression: Is it good or bad?

Compression involves reducing the amount of data required to display a stream of digital video (HDMI/DVI-D) frames. It is a tradeoff between image acuity and bandwidth. A stream of uncompressed HDMI video, for example, requires up to 10.2Gbps of bandwidth - greater than the combined capacity of 100 100Mbps Ethernet networks. The disk storage capacity of uncompressed video is equally eye-opening.

For these reasons, compression techniques are widely used to enable cost-effective transmission and storage of digital video. Video encoders compress a video stream down to a video bit rate that is compatible with commodity transmission and storage media. The following chart illustrates standard bit rates in Mbps for various video offerings.

At 40Mbps, the ultimate in video quality, Blu-ray, is highly compressed when compared to 10.2Gbps.

What implications does this have on video extension?

There are two types of digital video extenders on the market. Some encode digital video and audio data for transmission over commodity IP networks while others transfer digital signals in an uncompressed manner over point-to-point dedicated cabling. So, which is better, and why?

As always, the answer depends upon your needs. From the chart above, we can deduce that compression is not a critical factor unless you’re involved in video post production. As long as the video bit rate is 20Mbps (HDTV) or greater, the quality should be adequate for most normal needs. Bit rates of 50 (more than Blu-ray) or greater are quite acceptable for most critical viewing needs. If compression is not the primary deciding factor, then what is?

Consider factors such as the following when making your decision:

• Is your customer able to leverage an existing IP network infrastructure?

• IP networks can send data around the globe, whereas point-to-point extenders offer quite respectable but limited transmission distances.

• Is HDCP required for display of protected content? Few, if any, non-IP extension products are able to support one-to-many extension of protected content.

• Point-to-point extenders cascade from one receiver to another in serial manner. Thus, break in the cable will affect all downstream displays. IP networks support a host of matrix and redundancy options.

• Ethernet networks are designed to span across buildings with dissimilar ground planes. Thus, extender burnout is not an issue.

• Other features to look into involve bi-directional serial and IR data transfers, analog audio for DVI-D connections, and central management features.

The above list is intended to introduce topics for discussion in the weeks to come rather than an exhaustive treatment of this topic. Stay tuned for a deeper dive into these emerging video extension techniques.

AV Technology's AudioVisual Untethered Article Provides a Closer Look Into Wireless

AV Technology's April 2009 issue includes an informative article by writer Denise Harrison on wireless AV technology and raises the question: Is it the right time to add a wireless network to your system? It briefly covers the history of wireless networks, wireless alternatives, and challenges and trade-offs. Read the full article.

Under what conditions can wireless connectivity be more cost-effective than analog Cat5 extension?

I’ve been asked this question a number of times and, as you might expect, there are a variety of conditions that call for wireless connectivity. The most novel condition arose from an installation in Asia. When I asked why he insisted upon wireless extension, the integrator replied that wired connectivity was simply not reliable because “rodents love to nibble through the wires but they can’t bite into wireless”. Although this ranks among the most novel answers that I’ve ever heard, it’s not the most relevant to all readers.

Ground loop current is much more relevant. Unintended ground loop current occurs when two devices connected to different ground planes are interconnected via a data cable. Ground plane voltage differences act like poles of a battery and cause current to flow through the data cable. These damaging currents cause UTP extenders to run hot and burn out over time. This condition is most problematic when UTP cabling is used to interconnect extenders between buildings or across long distances within a building. A spark when connecting the UTP cable to the extender or the extender to the AC mains may be a symptom of ground loop current.

If you suspect that two extenders are connected to different ground planes, then wireless connectivity is an excellent alternative that allows you to avoid the high costs resulting from business down time, extender replacements and field service calls. As with rodents, ground loop current can’t bite into a wireless connection. Tune in next week for Q&A on related connectivity topics.

- Chuck Pheterson, VP, product marketing, Connectivity and Control Division of Avocent.