TV & VGA color using less pins...
Cluso99
Posts: 18,069
In a quest for using less pins to generate both TV (composite video NTSC in color) and VGA I decided to do some tests to validate some of my theories as to how this works.
Composite Video (TV)
I already have 1pin TV mono working (derived from Eric's and Jonathan's works), but this is really using the VGA video counters. From my re-reading of the NTSC specification, chrominance is modulated onto the grey scale signal, so I thought it may be possible to generate limited colors using a single pin. Here are my results...
Note that I used potatotext012 from the obex for testing. I get some shimmering on my TV with the standard setup (3 resistors on P12, P13, P14 and no adjustments). The result is a mid grey background with colored test. The border is near black.
1pin: The only color that works (sort of as some sync is lost) is when the 560R is connected via P13. The border is a light grey and the background is black, and characters are in color. Perhaps by fiddling with the timing and the resistor value and maybe a ground resistor (as per Phil's prior discussions about 75 ohm termination) it may be possible to get this running.
2pin: This works reasonably. P12 & P13 yields black background and colored text and looks the best for text. P13 & P14 yields a darker grey background and light blues become washed out towards grey/white.
The above tests seem to imply the chrominance is added on P13 and luminance is added on P14. P13 is a required pin. (Pins are numbered according to the normal usage with P12 as the first of the set).
NOTE: I did not disable the pin(s) in the VCFG, but merely disconnected them at the resistors.
VGA
2pin: I previously published (in another thread) untested circuits to share the HS & VS with two RGB output pins. It requires some simple gates (ICs). A palette of 4 colors including black should be able to be achieved by using selected resistors and buffers.
Without resorting to adding logic chips, 3 pins is the minimum.
3pin: My testing confirms that only HS and VS and a color output pin is required for a single color (R or G or
plus black. By using a combination of resistors to the RGB pins no doubt any single color could be set.
More pins can be added to increase the colors.
Composite Video (TV)
I already have 1pin TV mono working (derived from Eric's and Jonathan's works), but this is really using the VGA video counters. From my re-reading of the NTSC specification, chrominance is modulated onto the grey scale signal, so I thought it may be possible to generate limited colors using a single pin. Here are my results...
Note that I used potatotext012 from the obex for testing. I get some shimmering on my TV with the standard setup (3 resistors on P12, P13, P14 and no adjustments). The result is a mid grey background with colored test. The border is near black.
1pin: The only color that works (sort of as some sync is lost) is when the 560R is connected via P13. The border is a light grey and the background is black, and characters are in color. Perhaps by fiddling with the timing and the resistor value and maybe a ground resistor (as per Phil's prior discussions about 75 ohm termination) it may be possible to get this running.
2pin: This works reasonably. P12 & P13 yields black background and colored text and looks the best for text. P13 & P14 yields a darker grey background and light blues become washed out towards grey/white.
The above tests seem to imply the chrominance is added on P13 and luminance is added on P14. P13 is a required pin. (Pins are numbered according to the normal usage with P12 as the first of the set).
NOTE: I did not disable the pin(s) in the VCFG, but merely disconnected them at the resistors.
VGA
2pin: I previously published (in another thread) untested circuits to share the HS & VS with two RGB output pins. It requires some simple gates (ICs). A palette of 4 colors including black should be able to be achieved by using selected resistors and buffers.
Without resorting to adding logic chips, 3 pins is the minimum.
3pin: My testing confirms that only HS and VS and a color output pin is required for a single color (R or G or
plus black. By using a combination of resistors to the RGB pins no doubt any single color could be set.More pins can be added to increase the colors.
Comments
I never got around to try and save any HUB ram that became unused.
(I am hoping to get a go at modifying the single pin composite today, I wanted to get 16 rows x 48 columns so double height could be invoked. I just hope that it's a quiet day)