ZX-Gate Revisited

[1024MAK]

The "back porch" is not an extension of the sync pulse as such. The time for the back porch is subtracted from the length of the video line before it reaches the visible section of the screen. It allows colour display systems to help define the black level of the video.

Mark

[PokeMon]

Yes - just keep the video output at low/black level for additional 16 clock cylces after end of hsync. The length of one line must be still the same (207 cycles) - these are not additional cycles. Backgound: The TV measures the voltage of the video signal in this timing window and assumes this voltage level as black. If the signal is high/white than this would result in a dark picture with less contrast and brightness.

[1024MAK]

Mark

[blittled]

Thanks for all the info . I almost have it working. The screen is nice and bright but at the top the screen bends to the right. Also when I played Mazogs the screen rolls horizontally so the top half of the screen is on the bottom. The timing of hsync is probably off a little.

Rather than hold things up the following link https://www.dropbox.com/s/qpx83ovesjg71 ... o.zip?dl=0 is to my current code without the back porch and 16K internal ram. The Quartus project is in the zxgate_de0_nano directory and ZXGate and T80 are the newest sources. Maybe someone else can figure out the back porch.

I used the following schematic to interface the TV, keyboard and tape in http://zxgate.sourceforge.net/pics/zx01io.png

[PokeMon]

How do you internally create the VIDEO signal ?
As this should be the already mixed composite video you need 3 voltage levels (sync level, black level, white level). So how is this signal created ? This can not be a plain digital signal as you can not get three levels from a single digital signal - maybe that is the problem with syncing (depending on display type).

[Andy Rea]

Well you can get 3 levels from a single pin Hi, Lo, and Tri-state, which in turn can have a weak voltage divider to create the third (Tri-state) voltage. Feed that into an emitter follower and jobs a good un

Andy