Screen Display Video Generator to reduce the effects of finite switching time on the
insertion of graphics onto a video signal.
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.
crosspoint switch with integrated fast MUX for OSD insertion.
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.
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.
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.