AN427 Video Sample Design for VGA

[Altera_Forum]

I am new to the world of FPGA and have bought the Bitect Video Dev Kit (Cyclone III etc). I managed to get the Altera Video Up Conversion Sample Design (AN427) working easily enough but have so far failed to convert it from DVI (as supplied) to analog VGA (using the output on the quad video card). 

 Basically I have access to a DVI monitor (for testing) but ultimately want to deploy on an old-fashioned analog VGA screen.   I have modified the reference design to accommodate the smaller image size and lower video clock rate (I have used 640x480 throughout and 25MHz for the clocked output), and have fed the existing (24-bit) clocked output through an altddio_out function to get the desired 12-bit output for the Bitec quad video card.   All compiles and runs seemingly OK, but no image displayed.   I am not sure what parameter settings to use to get VGA out of the clocked output function (eg front/rear porch etc).   I am not sure how to go about debugging. Any pointers to help get me started much appreciated.

[Altera_Forum]

Did you have a look at this tread? 

 http://www.alteraforum.com/forum/showthread.php?t=3500

[Altera_Forum]

First, thanks for your prompt response to my first ever forum posting. 

 I'm using the Cyclone III Video Kit (from BItec), which expects 12-bit multiplexed RGB and which seems to use both edges of the video clock to drive its video output chip. I've tried to come up with a hybrid sytem based on the Altera Video reference design (which has DVI o/p) but but adapted to VGA by looking at the reference design that comes with the Bitec Kit (which doesn't use the Avalon clocked output component, just loads of VHDL).   The suggested thread is helpful to give me increased confidence in getting the right parameter settings for the clocked output, but there are still too many unknowns and I still can't quite get it working (eg required clock rate - I'm using 25MHz). I think I might have to refer this to the Bitec people.