Difference between revisions of "Minimig Video output"
(Resistor ladder D/A math and electrical bugs) |
m (Minimig S-Video moved to Minimig Video output: There might be a more general discussion on this) |
(No difference)
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Revision as of 07:54, 31 July 2007
My suggestion is to add copper traces to allow simple soldering of circuit to enable this.
By not assamble this by default some money can be saved.
A500 & A1000 to TV S-Video input using Analog Devices AD724:
http://eab.abime.net/showthread.php?t=30752
freqmax (070731):
The minimig only have a physical VGA socket for video.
4bit/color resistor ladder D/A:
Resistor values 4k, 2k, 1k, 560 ohm. Method1 - Adding resistors:
- R=1/(1/4000+1/2000+1/1000+1/560) = 282 ohm
- U=3.3 Volt
- i=U/(R+75) (impedance of VGA is defined as 75 ohm)
- Output voltage max: 75*i = 0,69 Volt.
Method2 - Using Kirchhoff's law:
- (3.3/(4000+75)+3.3/(2000+75)+3.3/(1000+75)+3.3/(560+75))*75 = 0,8 V (weird..)
This is all fine provided that impedance is exactly 75 ohm. However reality may not be perfect. So maybe the output should have some circuitry to handle monitors/cables that doesn't follow the 75 ohm impedance strictly?
Also the 560 ohm value seems nonlinear. See also Wikipedia.org Preferred number.
