Extron Electronics VTG 400D_400 DVI User Guide - Manual (Page 83)

A-19 VTG 400D/400 DVI • Appendix
26. Contrast Transfer Function (CTF)
The Contrast Transfer Function (patent pending), or CTF, provides an
indication of overall system high frequency response including the
display’s optical path. While most high frequency test patterns
provide electrical bandwidth performance information when used
with appropriate test instruments, evaluation of overall system response, including
the optical pathway, is virtually nonexistent. Good high frequency performance
translates to the perception of contrast and detail in the image. As high frequency
performance degrades, contrast ratio between high and low image transitions
approach middle gray and appear muddy or non-distinct. Should high frequency
transitions become fully attenuated, the contrast falls to zero.
The CTF pattern contains bursts of high frequency alternating pixels and bursts of
low frequency lines. Both appear as vertical bands on the screen. In the default
mode, the alternating pixel bursts flash On and Off at a rate of 0.5 second, or 2 Hz.
Pushing the soft key on the VTG’s display screen defeats the flashing effect, if
desired.
The low frequency line bursts will pass through a display system with less relative
attenuation than the high frequency bursts. Since the high frequency bursts will
appear to approach middle gray and lose detail when system response is degraded,
the user may adjust the level of the low frequency bursts up or down via the soft
keys on the VTG control display to match the perceived brightness of the high
frequency bursts. When the point is reached where the perceived brightness of all
the bursts is equal, the VTG provides a percentage number on its display. This
percentage number is the approximate percentage of contrast performance in the
system, including electrical and optical pathways. It may be used as a relative
index of performance for the system’s bandwidth.
27. H Pattern
The H pattern represents simple text that can be used to evaluate
image sharpness or overall response quality using symbology that
anyone can understand. The invert soft key on the VTG 400 reverses
the text from white text on a black background to black text on a white background.
Use the H pattern for high frequency response evaluation where text legibility is the
most critical application. This pattern may also be used to evaluate transient
response, focus, lens distortions, video clamping stability, and image sharpness.