29 May
2015
29 May
'15
6:41 a.m.
On 23 May 2015 at 16:35, Chris Osborn <fozztexx at fozztexx.com> wrote:
Are you on the CoCo mailing list? Have you seen the RGB2VGA by Luis Antoniosi
(CoCoDemus)? I know at one point he had been tinkering with making it support composite
from the Apple II. It?s semi open-source, I think there are 2 versions and the latest
version is currently all closed source.
https://sites.google.com/site/tandycocoloco/rgb2vga
Is it me or was there a trick missed in the ADC section?
To measure a voltage signal you need to perform
successive comparisons against different
voltage levels until you find which voltage is higher. For 3-bit ADC we need to perform
8
comparisons. We start with the DAC in 000 (the lowest voltage) and this will return 0 or
1
according to the signal voltage. We concatenate all the subsequent tests to create a
thermometer for the signal.
[...]
This will gives us the voltage of the signal, but there are errors due to noise and
voltage
fluctuation. We will never be free of them though one way to reduce the problem is to
perform
more tests, adding more bits to the DAC. Unfortunately we are on the limit of the FPGA
clock speed.
If you have 8 comparisons to spend I wonder if a modified binary chop
would have been more efficient? A standard binary chop would take
three comparisons, but I would have thought if you repeated each stage
twice (and a third time for up to two of them if they differed), that
would give you a more robust output...