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Display Capture: Trouble With Aspect Ratios and Resolutions
Last Updated: Dec 15, 2017 05:00PM EST

This article applies to:  Echo360 Admins

Summary

Echo360 often receives reports of incorrect display resolution detections in appliance captures. This is generally observed in Echoes as a geometric error in the display component: the screen capture image might be smaller than its window, stretched in the horizontal direction, or letterboxed with black bars.

Viewers and administrators commonly call these effects “distortion.” The bottom line is that the capture doesn't match up with the expected size and shape of the display image that was connected to the appliance. Our experience tells us that such problems are more often than not the result of inadequate capture testing after the appliance was installed. Since virtually every graphics adapter these days has subtle differences — many if not most of which aren't immediately apparent to the lay user — they will all behave slightly differently when captured.

Echo360 makes two important recommendations that can go a long way toward minimizing the occurrence and impact of these problems. First, always thoroughly test the graphics devices (PCs, notebooks, document cameras) that you intend to use with the appliance by running captures at a range of signal resolutions and configurations until you find what works best for your deployment. Do this before any mission-critical captures are scheduled to take place so that no unanticipated problems arise. If you later decide to connect a new kind of signal source to the appliance, repeat the tests for it, too.

Second, remember that not all resolutions are created equal and some may be completely non-standard; such “custom resolutions” are far more likely than others to cause problems. When in doubt, consult the deployment guide for your version of the EchoSystem to see what combinations of resolutions and aspect ratios have been tested and shown to work well in the most common environments.

The remainder of this article will discuss, in detail, some of the better-understood causes for display capture distortion. These paragraphs are for the more technically inclined and can offer specific insights into what to look for, and what to correct when these causes are encountered.

Causes

Display capture distortion symptoms are not the result of a software or hardware error, but rather a function of how graphics adapters and their receivers communicate with one another about the dimensions of the display signal. Under some common circumstances, the resolution presented to a display device (in this case the appliance) can be ambiguous or even incorrect. The outcome of this confusion is frequently troublesome for A/V technicians.

There are basically four categories we've identified into which causes of distortion fall:

  • Analog display signals at non-4:3 resolutions
  • Digital display signals at non-standard or undefined aspect ratios
  • Display driver (software) flat panel scaling
  • Signal changes introduced by distribution amplifiers

Mixed Standards

Ever since the introduction of digital flat-panel display devices, users have had an ever-increasing number of options for their output resolutions. Because computer graphics display technology began with CRTs — completely analog devices — backward-compatibility requirements have limited the ways in which resolution information can be encoded into the signal.

The practical result of this limitation is that digital display receivers, when given an analog RGB (VGA) signal, have to “interpret” the exact display dimensions from less precise parameters such as the number of horizontal lines, the duration of each line pulse, and so on. These parameters contain no information about digitally-relevant quantities like aspect ratio or pixel shape, since the analog standard assumed all displays would be 4:3 in ratio — which is correct for all CRTs, but not for flat-panels that can be any shape or size.

The appliance is a digital display receiver. If given an analog signal, it cannot distinguish between different resolutions that have the same number of horizontal lines because the necessary information is absent. It has to make an educated guess, using parametric lookup tables, to determine the resolution to capture. For example: resolutions like 1024x768 (4:3) and 1280x768 (15:9) look exactly the same to it in analog.

Custom Ratios

A digital display signal is encoded with up to twelve distinct parameters that define the exact dimensions of the image. Mapping these parameters to capturable resolutions requires a sort of lookup table: when the appliance receives a signal, it measures each of these parameters and looks for a match in its resolution list. Assuming a limited set of standard aspect ratios (specifically, 4:3 and 16:10), this is generally enough to accurately detect the proper resolution.

Many modern graphics adapters, however, allow the user to output display signals with a wide variety of custom resolutions and aspect ratios. Since these resolutions do not exist in any standard, their parameters can match those of standard resolutions, preventing any logical distinction between the two on a software receiver. This problem is called masking.

For example, some netbooks provide a custom, ultra-wide resolution option of 1280x600. Its signal has the same parameters as the standard 4:3 resolution of 800x600. The two are therefore indistinguishable by the lookup algorithm described above, so one is said to “mask” the other, and a decision has to be made as to which to place in the table.

Standard resolutions and aspect ratios always win over non-standard ones whenever this conflict occurs. This doesn't mean that a capture will necessarily fail if the appliance is given a non-standard resolution, but it does mean that the image will be captured at a similar resolution with a standard aspect ratio. In the example above, a 1280x600 signal would be captured at 800x600, and would look horizontally squished.

(Un)helpful Drivers

Since the introduction of flat-panel display devices, it has sometimes been necessary for the display signal to be scaled or stretched in order for the image to fill the entire screen area. Usually, the display device handles this scaling itself, assuming that the graphics adapter is actually outputting the resolution it says it is.

Some early flat-panel displays do not have this feature. This has led some graphics drivers to introduce the capability for their adapters to “help” the display device: it pre-scales the display signal by embedding one resolution inside another, often without informing the user. When it is an option, it is sometimes called “panel fit.” When it is enabled, the actual resolution of the image is not the resolution encoded into the signal's parameters.

If the appliance receives a signal like this, it has no choice but to trust the signal parameters and try to faithfully capture at the advertised resolution. The result is an image that looks compressed in one or both dimensions, and sometimes even surrounded entirely by a thick black region. The only solution here is to disable the "panel fit" feature and allow the graphics adapter to output exactly the signal it says it will.

Man-in-the-Middle

If a display signal is relayed through signal-shaping or redirecting devices like distribution amplifiers (DAs), these intermediate devices in the signal chain can alter the actual resolution so that it does not match the dimensions of the original output image.

It isn't always possible to detect these variations at the display receiver — in much the same way that flat panel scaling can confuse receiver logic — so it can result in a distorted image when captured. When introducing a distribution amplifier to a display circuit, it is very important to test the receiver so that any discrepancies in the signal are revealed before they affect a mission-critical capture.

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