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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 1943
Maxim > Design Support > Technical Documents > Application Notes > Video Circuits > APP 1943
Keywords: MAX4455, OSD, on-screen display, video overlay, video graphics, arbritrary, arbitrary graphics
APPLICATION NOTE 1943
Improving the Performance of the MAX4455 by
Reducing On-Screen Display Artifacts
Mar 21, 2003
Abstract: 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 32 x 16
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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