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Altera_Forum
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14 years ago

flicker in CVO -> CVI -> MIXER -> CVO to output screen

hi, In my VIP chain I'm mixing two video streams (one 800x600 and one 640x480) and then clocking out the mixer output to screen. The two streams going into the mixer are setup such that...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

Hello,

If I get this right, there are two CVOs in this system.

The first one after the deinterlacer is configured for 640x480 @ 60Hz with a 25.175MHz clock.

The second one for the output screen configured for 800x600 @ 60Hz with a 40 MHz clock.

There is no apparent mechanism in the second path (cvo -> cvi -> mixer_din_2 -> cvo) to do frame rate conversion. This means that you will get some flicker if the two clocks (25.175MHz and 40 MHz) are independent. If these clocks are not synchronized they will drift apart and you will get an overflow or an underflow at some point. Assuming this is the issue, it can be fixed by adding a triple buffer in front of mixer_din_2. Longer FIFOs will just delay the problem since you really need something that will drop or repeat a frame when the two clocks have diverged too much.

Hopefully the frame buffer in front of mixer_din_1 will not be needed in your final design so making this change will not be too expensive in terms of resources and memory bandwidth usage.

vgs