ZX81CCP video output for ZX81
Posted: Mon Apr 23, 2012 5:48 pm
Well - not a new video output again, I can hear your voice.
I know, there have been many over the years, most of them have the same problem (transistor discoupling for example). The ULA delivers a disturbing noise in the white background which is caused through the ULA itself and even condensators could not stable the internal voltage drop downs when other signals/pins of the ULA working. This normally doesnt disturb the function of logic circuits as all above 2.0 V is treated as high (TTL logic) regardless any peaks on it with some more voltage. But they used the ULA as video output which is a anlog signal and not just interpreted digitally. As the norm uses 1V pp even very low 30mV peaks are visible in the white background as it corresponds to about 5% white level more or less. Can see with your eyes.
Based on a proposal from Bodo Wenzel (which was based on a publication of Chip computer magazine) I developed now an improoved module with open collector logic gates 74LS06 which scan the video output of ULA and recreate the video signal new when digitized first. This will eliminate the noise from the ULA. Additional a back porch is added to the signal regardless the ULA provide one or not (the older one does not). Missing backporch will give a picture with poor contrast and brightness and some of the newer LCD or plasma TV even do not accept those signals (show black screen).
As some people like, I added a dip switch for choosing either normal video mode (black characters on white screen) or inverted (white characters on a black screen). Here is the schematic.
The top 2 inverters are used for the video content (pixel data).
The middle 2 inverters are generating sync signal.
The last 2 inverters are for creating the back porch with a simple r/c combination, triggered from sync signal.
It adds a quite good picture on all tested screens.
You can see more sample pics of video here:
http://forum.tlienhard.com/phpBB3/viewt ... ?f=2&t=651
Integration can be made very easy due to a small SMD board (onesided) which can be fixed in the case with just a small tape: And the good news:
It''s now available for sale on www.sellmyretro.com for GBP 9.90 completely soldered, tested and pre-adjusted.
Have to be re-adjusted for newer ULA / issue 3 board.
I know, there have been many over the years, most of them have the same problem (transistor discoupling for example). The ULA delivers a disturbing noise in the white background which is caused through the ULA itself and even condensators could not stable the internal voltage drop downs when other signals/pins of the ULA working. This normally doesnt disturb the function of logic circuits as all above 2.0 V is treated as high (TTL logic) regardless any peaks on it with some more voltage. But they used the ULA as video output which is a anlog signal and not just interpreted digitally. As the norm uses 1V pp even very low 30mV peaks are visible in the white background as it corresponds to about 5% white level more or less. Can see with your eyes.
Based on a proposal from Bodo Wenzel (which was based on a publication of Chip computer magazine) I developed now an improoved module with open collector logic gates 74LS06 which scan the video output of ULA and recreate the video signal new when digitized first. This will eliminate the noise from the ULA. Additional a back porch is added to the signal regardless the ULA provide one or not (the older one does not). Missing backporch will give a picture with poor contrast and brightness and some of the newer LCD or plasma TV even do not accept those signals (show black screen).
As some people like, I added a dip switch for choosing either normal video mode (black characters on white screen) or inverted (white characters on a black screen). Here is the schematic.
The top 2 inverters are used for the video content (pixel data).
The middle 2 inverters are generating sync signal.
The last 2 inverters are for creating the back porch with a simple r/c combination, triggered from sync signal.
It adds a quite good picture on all tested screens.
You can see more sample pics of video here:
http://forum.tlienhard.com/phpBB3/viewt ... ?f=2&t=651
Integration can be made very easy due to a small SMD board (onesided) which can be fixed in the case with just a small tape: And the good news:
It''s now available for sale on www.sellmyretro.com for GBP 9.90 completely soldered, tested and pre-adjusted.
Have to be re-adjusted for newer ULA / issue 3 board.