View Full Version : TV Composite DAC Values

01-03-2009, 12:44 PM
Straight to the point - How important are the DAC Resistor values for the composite TV output? I don't have any "normal" resistor values on hand, so the closest multiples I could find are 124, 301, and 680. I'm getting a gray screen whenever I try my TV output on my board... Could these values be why? I know that ideally, the resistors would be multiples of each other... 270, 560, and 1.1k seem to be the most common as these are the closest values to 150 when in parallel. Any info would be great.

Mike Green
01-03-2009, 12:55 PM
The ratios of the resistors are important for providing enough evenly spaced voltage values as the bits change between 3.3V and 0V. Also, the impedance of the video line is 75 Ohms and the 150 Ohm value in series with the 75 Ohm value creates a voltage divider with the voltage on the video line coming out to roughly 1V maximum. With your lower resistors, you're going to get nearly twice that voltage which is probably why the screen is gray.

Peter Jakacki
01-03-2009, 01:04 PM
Mike Green, where do you get the time for all these posts mate?

Phildapill, you could add a load resistor to drop the output voltage as Mike mentioned that the load is normally 75 Ohms as part of a voltage divider so you could add another 75R to 100R from the signal to ground.

That should work.


01-03-2009, 01:04 PM
Great! That response is much better than the alternative that I thought might have happened... But never mind that. I'll find some better resistor values and get this fixed. As always, thank you Mike "lifesaver" Green.

Phil Pilgrim (PhiPi)
01-03-2009, 01:07 PM
There are three constraints that should be observed:

1. The resistance ratio should be 1:2:4.

2. The peak-to-peak output voltage into a 75-ohm load should be 1.0 V.

3. The source impedance of the signal should be 75 ohms.

#1 and #2 are a minimum requirement. Constraint #3 should be observed for longer cable lengths. The circuits that I've presented here (http://forums.parallax.com/showthread.php?p=760599) satisfy #1, #2, and #3. The standard Parallax circuit satisfies #1 and #2. Your values make a faint acknowledgement of #1 and actually meet #3 (78 ohms) but provide a peak-to-peak voltage of 1.65V, so miss constraint #2 by a lot. You can try combining the resistors you have on hand in various series and parallel combinations to get closer to meeting #1 and #2.


Post Edited (Phil Pilgrim (PhiPi)) : 1/3/2009 6:14:01 AM GMT

01-03-2009, 01:34 PM
Yippy-I-O-Kiyay! Sorry - nerdy play on words. Thank you so much, guys. I've never been so relieved to see an error message on my TV before! You'll said the signal be an absolute peak of 1V, so I figured it would be ok to use higher value resistors than what is recommended. I'm now using 301, 680, and 1.23k. The TV output looks just fine.

Oh, and by the way, Phil, I'm using your improved windowed TV_text. Just wanted to thank you again for the flicker free screen! :)