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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

Artifacts

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.