Extron Electronics VTG 400D_400 DVI User Guide - Manual (Page 81)

A-17 VTG 400D/400 DVI • Appendix
24. Alternating Pixels (2-dimensional)
This pattern is similar to the alternating pixel pattern discussed
earlier; however, while it consists of one-On, one-Off alternating pixels
on each horizontal line, the phase is shifted 180 degrees for every
other line thereby creating a minute checkerboard effect. Because of
the line-to-line phase shift, this pattern is useful for critically evaluating pixel timing
and phase in a graphics display system. The pattern is produced at the highest
clock rate required for the selected image resolution. Incorrect pixel digitization
within a display manifests on the screen as regions of “dancing pixels”, instability,
or noise in the test pattern. Pixel timing and phase may be adjusted in most fixed
pixel displays until any abnormal noise effects are nullified.
Multipulse
The multipulse pattern is uniquely applied to evaluation of group
delay in television signal distribution systems. Individual frequencies
propagate along cables, such as coaxial cables, at different speeds.
High frequencies are affected more than lower frequencies relative to
one another. The rate at which signals of differing frequencies propagate manifests
as time of one signal component to the other. This phenomenon is referred to as
group delay.
In the case of NTSC or PAL television signals which contain many frequency
components that must maintain strict timing relationships, the chroma signal
typically becomes misaligned in time (delayed) compared to the luminance
transitions. This results in poor edge quality and fidelity for areas containing color
information since the color information is not only delayed, but suffers amplitude
loss as well.
The multipulse consists of a low frequency level or “white bar” which indicates
maximum video signal level. This bar is followed by a series of pulses. The first
pulse is a 2T pulse without any modulation. The 2T pulse represents the highest
frequency component in the television luminance channel. When its amplitude is
maintained at the same level with the bar throughout the system distribution, the
low to high frequency response is considered satisfactory.
NTSC Multipulse
The 2T pulse is followed by a series of modulated pulses: First, a 20T pulse
encompassing a 1 MHz sine wave burst, followed by a 12.5T pulse containing a 2
MHz sine wave burst, followed by a 12.5T pulse containing a 3 MHz sine wave
burst, followed by a 12.5T pulse containing a 3.58 MHz sine wave burst, followed
by a 12.5T pulse containing a 4.2 MHz sine wave burst. Much like the multiburst
signal discussed earlier, each of these frequencies provides an intermediate
bandwidth point for consideration, including the color subcarrier and a frequency
burst at the bandwidth limit of the system.
The frequency burst inside each pulse is generated to fit within the pulse
symmetrically and have an amplitude that matches the pulse height and duration
without extending past its areal limits. Group delay in a television distribution
system causes relative time shift between the imposed burst and the encompassing
pulse. Amplitude disturbance of the burst within each pulse manifests as an
upward or downward level shift, which appears as concaved upward above black
level for low level to convexed downward beyond black for excessive level.