8-bit Computer TV Channel Use

David Brownlee abs at absd.org
Fri May 29 08:41:34 CDT 2015

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...

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