Trying to understand video DAC termination...
David Cuthbert
Posts: 12
I'm trying to make sense of the comments on the demo board schematic on the video DACs:
"These three resistors form a 1-volt, 75-ohm, 3-bit DAC..." and
"These three sets of two resistors form a 1-volt, 75-ohm, 2-bit DACs..."
Specifically, by my calculations, they're actually forming 150-ish-ohm terminators. The 3-bit DAC uses 1.1k, 560, and 270 ohm resistors; in parallel, that's 156 ohms. The 2-bit DAC uses 240 and 470 ohms, or 159 ohms.
If this is correct, there's an impedance mismatch. Of course, this makes sense if you're trying to divide the 3v output down to 1v -- 75 ohms on the remote end yields a voltage divider: (75 / (75 + 150)) * 3 = 1 volt p-p. In the grand scheme of things, this would be a fairly minor sin (after all, we're not expecting any incident waves on the Propeller to reflect back).
Am I missing anything here, or is my analysis correct?
BTW, this is one of the friendliest chips I've ever had a chance to play with.
"These three resistors form a 1-volt, 75-ohm, 3-bit DAC..." and
"These three sets of two resistors form a 1-volt, 75-ohm, 2-bit DACs..."
Specifically, by my calculations, they're actually forming 150-ish-ohm terminators. The 3-bit DAC uses 1.1k, 560, and 270 ohm resistors; in parallel, that's 156 ohms. The 2-bit DAC uses 240 and 470 ohms, or 159 ohms.
If this is correct, there's an impedance mismatch. Of course, this makes sense if you're trying to divide the 3v output down to 1v -- 75 ohms on the remote end yields a voltage divider: (75 / (75 + 150)) * 3 = 1 volt p-p. In the grand scheme of things, this would be a fairly minor sin (after all, we're not expecting any incident waves on the Propeller to reflect back).
Am I missing anything here, or is my analysis correct?
BTW, this is one of the friendliest chips I've ever had a chance to play with.
Comments
-Phil
Leon
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Any quality differences have been too slight to notice so far. But the longest cable I've used is about two meters long. I suspect that with a longer cable there would be a discernable contrast in image quality.
Leon:
All the resistors shown are standard 1% values, obtainable in either through-hole (metal film) or SMD chip form. The cost penalty over using 5% resistors is insignificant.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 11/26/2007 8:06:18 PM GMT
Are you asking about quality because you are using PAL?
I have found the prop able to do NTSC very well but struggles with PAL (lots of dot crawl) no matter how close the crystal is to the PAL carrier frequency of 4.433618 MHz..
With my PropGFX card I fitted an AD724 and used the Prop to generate the subcarrier frequency with very satisfactory results as good as a dedicated oscillator.
Is it that the video serializer in the Prop is tweaked for NTSC perhaps.....?
I wonder if other users have found this too?
Regards,
Coley
Maybe the Prop II will do PAL better ?
Perry
Just so there's no misunderstanding its purpose, the 191-ohm resistor to ground in my schematic is not a load resistor. It's part of an impedance-setting and attenuation network that provides a 2V P-P signal with a 75-ohm source impedance. The resistor values were obtained by solving a set of simultaneous equations that satisfied the following conditions:
- The parallel combination of all four resistors had to equal 75 ohms.
- The voltage at the output terminal had to be 2.0V when all of the input resistors were excited by 3.3V.
- The three input resistor values had to have the ratio 4:2:1.
When terminated at the other end of the cable with a 75-ohm load, the signal strength is cut in half to 1V P-P. Matching the source, cable, and termination impedance is important for eliminating reflections in the cable which, over long cable runs, can show up as ghosting or other weirdness on signal transistions.-Phil
(2) The main factor for good quailty is that your video and memory settings nicely match the natural number of pixels on your screen.
(3) The video DAC is a current DAC, i.d. it can best be understood in that it adds (or subtracts) current flowing through the 3 resistors. This is "reconverted" to voltage at the 75 Ohms resistor inside the monitor. So the source impedance is tendencially higher than in the case of R-2R ladders where you generally buffer the voltage to get a low impedance signal.
(4) As I do not use standard boards (except the hydra from time to time) all my video interfaces use 220, 470 1k
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The more I know, the more I know I don't know.· Is this what they call Wisdom?
Right you are, and there are definite advantages in doing so. But you do need to make sure the source impedance is still 75 ohms. In the attached circuit, an NPN transistor, wired as a follower, buffers the DAC output. This allows the use of higher-value input resistors that load the Propeller pins less. It also breaks the dependency between the load impedance and the output voltage. In the circuit, the collector resistor sets the source impedance but, because the NPN is being used as a follower, the output voltage depends only on the base voltage and not on the load current. For this reason the input and bias resistors should be calculated to produce a voltage on the base that ranges from 0.7V (all inputs low) to 1.7V (all inputs high). This will produce a 1V P-P output on the emitter that ranges, ideally, between 0.1 and 1.1V.
-Phil
On a more technical note hope to be able to have a look at the video output on a Vectorscope over the Festive break and produce some results on what the PAL output is really doing. I feel sure this has already been done, but I have not been able to find any answers whilst searching the Forum.
Even with the video output problems, is still an amazing device and a total credit to all the guys that concepted this as a consumer product.
With Regards
Mike.