A/D and D/A CONVERSION/SAMPLING CIRCUITS VIDEO CIRCUITS
Application Note 1943: Mar 21, 2003
Improving the Performance of the MAX4455 by Reducing On-Screen Display
This article describes an applications circuit for the MAX4455 Arbitrary Graphics On-
Screen Display Video Generator to reduce the effects of finite switching time on the
insertion of graphics onto a video signal.
This application note describes a circuit modification to the MAX4455 EV board that will
improve the performance of the MAX4455/MAX4358 chip set by reducing on-screen display
(OSD) artifacts caused by finite switch time effects. This modification can be included in any
similar MAX4455/MAX4358 application circuit yielding the same performance improvements.
The MAX4455 is an 8-channel arbitrary graphics OSD generator and the MAX4358 is a 32x16
crosspoint switch with integrated fast MUX for OSD insertion.
Explanation of Finite Switch Time Effect
In the MAX4455 there are two outputs involved in the generation of the OSD graphics: the first
output, the OSDFILL, is the analog representation of the video fill signal, and the second
output, the OSDKEY, is a digital signal that controls the turn on and turn off of the fast MUX
switch. They are synchronized to within a nanosecond or so at the output of the MAX4455.
Due to the finite switching time of the fast MUX in the MAX4358, the OSDFILL signal
emerges slightly delayed from the OSDKEY signal. This results in a slightly delayed OSDFILL
graphic with a trailing black level (0 IRE) equal in width to the switch delay time (as shown in
Figure 1). This effect may be objectionable in applications that display all white characters, due
to the high contrast between the white character and the trailing black edge. This is even more
obvious on a light background due to the light dark light pattern.
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Figure 1. Finite switch time effect
With the circuit modification shown in Figure 2, the trailing black level will be replaced with a
trailing white level. If this white level equals the OSDFILL level then the trail effectively
disappears, eliminating the finite switch time effect. This is shown in Figure 3. The resistor
divider formed by R14 and R sets the brightness of the trailing white level, which can be set to
equal the brightness of OSDFILL. When R equals 453 ohms, then the trailing white level is
nominally 100 IRE. Figure 2 shows a modification to only one of eight channels. Four
MAX4616s can be used to improve all eight channels of the MAX4455.
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