Runco REFLECTION CL-510 Bedienungsanleitung PDF herunterladen (Seite 13)

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Fall 2003 ExtroNews 14.3 13

maximum rate, which visually demonstrates its

high frequency response.

The vertical lines should be crisp and distinct.

As system bandwidth declines, the definition of

the alternating lines becomes less distinct until

the pattern blends to middle gray. At full level,

the “modulation depth”, or peak transition be-

tween black and white, of the image is 100%.

Modulation depth decreases (whether display or

entire display system) with declining system

bandwidth. On a scope, the peaks between

white and black begin to descend toward one

another until they meet half-way; thus present-

ing a screen seen as middle gray. In display

devices, the pixel rate where the modulation

depth reaches only 10% of the full level char-

acterizes its usable resolution.

Split Gray-Scale

Gray-scale patterns tell us about the linearity of

the visual presentation. Each intensity step of the

16-level scale is a distinct, equal-size change in

luminance value. Dividing a 0.70 volt video signal by

(note that one level of the 16 is black) provides

47 millivolts per step (about 7%). Viewing both

excursions of the split gray-scale yields a bisected

stairstep pattern (see

Figure 1

). This feature enhances

evaluation of system linearity performance.

H Pattern

The full ﬁeld H pattern provides several pieces

of useful information about a display system. The

letter ‘H’ is relatively simple to construct electron-

ically and provides a serious test of system high

frequency versus low frequency performance.

The white character on a black surround requires

the display system response to be optimum in

order to maintain brightness level between

the vertical legs (high frequency content) versus

the horizontal connecting bar (low frequency

content). As high frequency performance

diminishes, the vertical legs become dimmer than

the bar. On the scope presentation, the vertical

legs represent single pixel events which can be

used to measure system rise and fall time

effectively. On the display screen, the white ‘H’

provides an effective means to evaluate image

center focus versus corner focus. For digital pro-

jectors, the black surround can be used to

subjectively evaluate black level performance

against the peak level excursions of the text. This

also provides more of a real-world perception of

usable contrast ratio.

Crosshatch 32x24 and 32x18

One of the oldest and most used test patterns

for convergence alignment of CRT displays, the

crosshatch, in its many variations, still provides

significant support for visual evaluations and

testing. Two versions are provided in the VTG 300.

The 32x24 version supports tests in the 4:3

aspect ratio (32 divided by 24 = 1.333…) while

the 32x18 version (32 divided by 18 = 1.777…)

supports the widescreen aspect ratio and, very

importantly, sets the absolute outside boundary of

the display’s active region with a white line border.

Why use a crosshatch for digital projectors?

Several reasons. If you are feeding an analog

signal to a digital projector, there is still analog

video processing which may affect frequency

response. Like the ‘H’ pattern, the vertical lines

represent high frequency one pixel events and

should be as bright as the horizontal lines. The

squares created by the intersecting lines, as well

as the overall rectilinear presentation of the

pattern, tell us about geometric distortion such

as keystone, the most common distortion for

digital projectors.

But, here are a couple surprise uses for the old

crosshatch pattern. Those technicians working

with video over CAT 5 wiring will ﬁnd it perfect

for evaluating and adjusting time delay among

the cable pairs. Time delay error created by these

UTP cable transmission schemes appears as a

misconvergence effect on the screen. But, that’s

not all. Today, low-cost digital projectors may

trade off optical path quality, comprising several

components including the projection lens.

Though the lens resolving capability may exceed

the resolution of the imaging device, what about

image distortion? Short focal length (wide angle)

lenses, in particular, must have better optical

design specs for color correction and other

aberrations than its longer focal length

counterpart. Use the crosshatch to evaluate

corner color separation and elliptical distortion

that easily occurs with cheaper lenses. With

short throw distance, the peripheral resolution

and correction quality of the lens is very

important.

Do you have concerns about ﬁnding raster

edges during setup with some patterns? In the

Video Setup Menu, a selection may be made to

“turn on” a raster border. This one-pixel border

will outline the active image area extremes to

ensure proper image positioning.

Split-Field Color Bars

The split-ﬁeld color bars are enabled when the

VTG 300 is in one of the 31 computer graphics scan

rates. Color bars support display system testing

with the primary colors (red, green, and blue) as

well as the secondary colors (cyan, magenta, and

yellow) plus a white and black reference. This

pattern exercises fully saturated colors which can

be measured with appropriate colorimetric

equipment to determine color gamut of the display

system, among other aspects. The split field

arrangement allows viewing of each primary and

secondary color against its complement (see

Figure 2).

Figure 1: Graphics/PC split gray-scale waveform.

VTG 300 Video and Audio Test Signals — As Good As It Gets (continued)

Figure 2: Monitor screen shot of graphics

color bars.

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