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AD725 picture quality issues from a MVS arcade board.

Question asked by carlosandrade on Sep 14, 2017
Latest reply on Sep 18, 2017 by GuenterL

First, sorry about my english skills

 

I recently built a video encoder with AD725 for my Neo Geo MVS board, using the application circuit from datasheet (NTSC mode, 4FSC oscilator at 14.31818 mHz) on a breadboard for testing purpouses before design and etch a pcb, but there are some strange issues with it :

 

- Very innaccurate colors (like reddish blue and brownish black backgrouds). Solved this with a 40pf trimmer between
oscilator output to AD725 pin 3 and GND.

 

- Barely visible dark green horizontal stripes pattern on black backgrounds (when i raise the TV brightness control)
moving from bottom to top, both in composite and s-video outputs. If i disconnect crhoma using s-video output, i get a good and solid monochrome picture with no stripes.

 

- Only in s-video output, when most of the screen area is white, other parts like text (E.g. "Insert Coin"), sprites and
characters lost color accuracy and above described horizontal stripes becomes worst (in composite output, colors are stable in any situation). Again with crhoma disconnected, no stripes and good monochrome
picture.

 

- Moving "dot crawl like" artifacts in composite output visible on edges of static texts or other objects (E.g. score info, gauges, energy bars, etc.) in screen. In s-video is much, much less noticeable, but it is already there.

 

- Using the recommended luma trap (68 uh inductor with 27pc capacitor in series) in composite causes all kinds of
ghosting on picture. Better without it.


Tried another AD725 ic with exactly same results (i ordered 2).

 

MVS Board outputs R, G, B and 15 Khz composite Sync, connected to HSYNC of AD725 while vsync is always high (VCC). Video output is not interlaced, but only one field of 240 lines (AKA "240p" like a SNES or Sega Genesis console)

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