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Thread: Adding S-Video to TG-16/PCE Duo...

  1. #21
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    Nowhere do I see where he said he used a scope (nor is that applicable), I suspect he measured the resistors with a DMM (and they are not accurate in-circuit). You aren't understanding me, the circuit he captured DOES NOT TIE LINES TOGETHER. Each output and resistor pair is a Thevenin source, and in parallel you get weighted-averaged MIXING of the sources. This is completely different than tying signals together, and no offense to tomaitheous but tying signals does not sum them nor is it smart nor is summing even what you want to do here since the composite circuit does not sum. Summing requires precision mixing (identical Thevenin resistances for an exact midpoint between the voltages) followed by precision voltage amplification.

    I also don't see an analysis anywhere of the circuit but I can give a poor one. Y, I and Q ("R-Y" and "B-Y") are voltage mixed then further mixed with colorburst, in this order seemingly for economical purposes. This node is buffered with a common-collector amp and attenuated, then buffered with a complementary common-collector pair. I think the PNP is to negate the Vbe drop of the final NPN CC which sets the output impedance. Because the 75 R is shunt and not in series, and there is no voltage amplification, and there are no shunt resistors for the DAC output it can be assumed that A) the DAC is voltage-driven and B) the signals are directly ~1 Vpp. This still doesn't make it clear which values to use. Analysis of the DAC and transistors is difficult so it's likely even Hudson adjusted the attenuation for example with a scope.
    Last edited by Calpis; 11-26-2011 at 04:48 AM.

  2. #22
    Well the analysis by Tomaitheous wasn't in this thread, of course. Tomaitheous appears to have spent a lot of time looking at the video encoder below. How would you be able to identify the colorburst signal, phase offsets, etc without a scope?

    http://pcedev.wordpress.com/2011/01/11/s-video-mod/
    http://pcedev.wordpress.com/2011/01/...nal-analyzing/
    http://pcedev.wordpress.com/2011/01/...hannel-update/

    So there are two opinions here that don't agree with each other...that's fine. I'm just trying to figure it all out. That's fine, though. My hope was to get more info to see if this is doable. Thanks for your input. For now, I'll hold off on going any further with this.

  3. #23
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    See, this is exactly why I put that disclaimer in there.

    Quote Originally Posted by Calpis View Post
    This is completely different than tying signals together, and no offense to tomaitheous but tying signals does not sum them nor is it smart nor is summing even what you want to do here since the composite circuit does not sum.
    I thought it sounded fishy but I couldn't quite figure out why. 99% chance it's due to a total lack of formal EE education. Being self-taught is fun but you tend to break things along the way.
    http://www.assemblergames.com/forums...ad.php?t=31524
    My feedback thread, since it seems somewhat difficult for people to find.

  4. #24
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    @Yoder: with a service manual? I dunno.

    @APE: Well for future reference there are very few circumstances where you want to tie outputs, the reason for this is that typically outputs are buffered by current amplifiers which provide the circuit following it with a very low output resistance in order to effectively transfer a signal to a load (which is hopefully high resistance for maximum transfer). If you make a low resistance path between two voltages current flows from the high voltage to the low voltage, only restrained by the resistances and the current-sourcing abilities of the high voltage and current-sinking abilities of the low voltage. In most cases there is no protection against shorts, and transistors will cook themselves if given the opportunity.


    Also something I didn't notice for the analysis is that two cascaded common-collectors (PNP, NPN) also probably drastically reduces the NPN's output resistance so the display only sees the 75 R.
    Last edited by Calpis; 11-26-2011 at 04:59 PM.

  5. #25
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    Friend of mine did try to tie his 5v and 12v lines together on an ATX power supply to charge some batteries for his R/C car. Thankfully it was a quality power supply and it shut itself down before any damage went down.

    Since that would be running the 5v and 12v lines in parallel it wouldn't have the voltage required to charge his 17v batteries. Even if it could and you put in series a diode (probably really stupid) wouldn't you need a diode capable of handling X watts? At that point you might as well just build a proper circuit.
    http://www.assemblergames.com/forums...ad.php?t=31524
    My feedback thread, since it seems somewhat difficult for people to find.

  6. #26
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    Quote Originally Posted by APE View Post
    Since that would be running the 5v and 12v lines in parallel it wouldn't have the voltage required to charge his 17v batteries. Even if it could and you put in series a diode (probably really stupid) wouldn't you need a diode capable of handling X watts? At that point you might as well just build a proper circuit.
    Even joining two diode-protected supplies at different potentials does not sum the potentials/voltages. If only it were that easy... (And that is not stupid, it's actually a common thing to do in battery-backup systems.) When you put diode-protected 12V and 5V in parallel, 12V wins out, and since current doesn't flow from a lower potential to a higher potential, 5V provides 0 current.

  7. #27
    Someone did a quick svideo mod by directly tapping the 6260 pins: http://www.pcenginefx.com/forums/ind...=10850.new#new

  8. #28
    Thanks for the update! So it turned out to be fairly basic after all! Just as you suspected! I'll have to give this a shot when the semester is over.

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