Jailbars

Animation showing a clean video output, and one with a simulated jailbars effect (with magnified zone).

The "jailbar effect" is an analog video perturbation caused by some versions of the AES (console) boards. Maybe some MVS boards also ?

Symptom

The picture is overlaid with faint, thin static vertical "jailbars" of alternating luminosity. Some players who are used to see the output of their own system might not notice the defect, but when comparing with a cleaner output from another board version, the difference is clearly noticed.

Note that severely marked colored vertical lines are more probably a graphic glitch caused by board damage or bad contacts.

Cause

The cause is a PCB routing oversight from SNK theirselves. The unamplified analog RGB video traces cross the ones from the color burst oscillator which are on the opposite layer, causing the 3.57954MHz wave to mix with the video signals by proximity coupling.

The jailbar appearance comes from the relation between the NeoGeo pixel clock (6MHz) and the 3.57954MHz signal, causing an "one-out-of-two lines" looking effect.

Fix

Only 3 trace cuts. See http://www.mmmonkey.co.uk/snk-neo-geo-aes-rgb-neo-aes3-6-vertical-lines/.