kiphartzell
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Help! RGB Color weirdness - Apple IIGS/Pacman/Pat9000
All- I could use some help understanding this RGB signal distortion issue.
Background: I have a working Pacman board running through my PAT9000 with a Apple IIGS (A2M6014) monitor attached as the Raster display. I am seeing distortion/weirdness with the displayed colors using this testbench setup. I traced the problem to a specific circuit on the Video Interface Card within the PAT9000 and would like some help with what the intent of this circuit.
When I drive the Apple monitor directly from the Pacman board (not through the PAT9000), the colors are crisp and correct (as seen in the first to pictures attached). Also, when I attach a Arcadeshop Programmable Jamma card in this setup using the PAT9000, the colors are correct so I know this general setup through the PAT9000 (and the Video Interface card) does work for some boards.
This problem seems to be something with how Pacman gnerates/drives RGB and how this mystery circuit (diagram attached) in the PAT9000 affects these RGB signals. The Circuit in question looks like it might be a simple 2 stage emitter follower buffer circuit for re-driving the RGB signals, but I also see an increased amplitude on these output signals when scoped. I am rusty with my analog design knowledge and I haven't done much with RGB signal processing before so any help from more experienced people would be appreciated. Thanks.
One other thing, when I attach a standard 19" arcade monitor instead of the Apple monitor, colors appeared to be more accurate and generally correct. So.... it seems to be a combination of what the RGB signals are like coming from the Pacman board and the input tolerance of the RGB signals on the Apple Monitor (once conditioned by the PAT9000 mystery circuit).
First two pictures are the good colors directly from the Pacman card, the last two pictures are what I see when the RGB signals are passed through the PAT9000 Video Interface card.
All- I could use some help understanding this RGB signal distortion issue.
Background: I have a working Pacman board running through my PAT9000 with a Apple IIGS (A2M6014) monitor attached as the Raster display. I am seeing distortion/weirdness with the displayed colors using this testbench setup. I traced the problem to a specific circuit on the Video Interface Card within the PAT9000 and would like some help with what the intent of this circuit.
When I drive the Apple monitor directly from the Pacman board (not through the PAT9000), the colors are crisp and correct (as seen in the first to pictures attached). Also, when I attach a Arcadeshop Programmable Jamma card in this setup using the PAT9000, the colors are correct so I know this general setup through the PAT9000 (and the Video Interface card) does work for some boards.
This problem seems to be something with how Pacman gnerates/drives RGB and how this mystery circuit (diagram attached) in the PAT9000 affects these RGB signals. The Circuit in question looks like it might be a simple 2 stage emitter follower buffer circuit for re-driving the RGB signals, but I also see an increased amplitude on these output signals when scoped. I am rusty with my analog design knowledge and I haven't done much with RGB signal processing before so any help from more experienced people would be appreciated. Thanks.
One other thing, when I attach a standard 19" arcade monitor instead of the Apple monitor, colors appeared to be more accurate and generally correct. So.... it seems to be a combination of what the RGB signals are like coming from the Pacman board and the input tolerance of the RGB signals on the Apple Monitor (once conditioned by the PAT9000 mystery circuit).
First two pictures are the good colors directly from the Pacman card, the last two pictures are what I see when the RGB signals are passed through the PAT9000 Video Interface card.
