Alright, I neglected to respond to this earlier, but felt like doing so now. Sorry to get testy, but it's warranted.
Pasky wrote:Hmmm maybe its possible the luma ouput is different per snes revision.
Or maybe people are creating broken "amps" due to minimal understanding of video signals, termination or transistor amplifier topologies.
Pasky's desire to slightly attenuate the native S-Video-prepared, ready-to-go Luma/Y for use with a YPbPr output mod had NOTHING to do with my Red-Y/Blue-Y amplifier circuit or ANYBODY else's that you wanna throw stones at! NOTHING! If you or ANYBODY else produced the perfect Red-Y/Blue-Y amplifier circuit, he would still be complaining that the picture seems a little bright and I believe I understand why now but that's besides the point.
kyuusaku wrote:You're missing the point; of course the encoder's output is "compliant" with a DC offset. Luma is already prepped for series source termination so it's irrelevant.
MikeMoffitt didn't miss any point. You had none, and simply wanted to reiterate that nobody here has done a good enough job of producing a Red-Y/Blue-Y amplifier circuit to your satisfaction!
kyuusaku wrote:Wiring component's Y in parallel with S-video's Y does mean however that you can't leave S-video and component connected at the same time because there will be dual-loads. A simple remedy with a single resistor...
Funny thing, you could've actually been helpful here with this alleged "simple remedy" using a single resistor, but you didn't bother to actually tell us how!
kyuusaku wrote:What's really wrong is how Pb and Pr are handled, when you connect them directly a compliant signal is not what's delivered to your TV over the transmission line.
Yeah, no shit! That's what this thread is all about or should be, to come up with the best Red-Y/Blue-Y amplifier circuit. Did we really need you to reiterate that ?? How about instead you FINALLY just crack open your own SNES and dazzle us with what you could come up with ?? I can't be the only that thinks it's time for you to put up or shut up! Nobody, to my knowledge, has yet to claim that they came up with the perfect amp... People are giving it a shot. I'm sorry that they're not trained engineers for you, but tough noogies!
kyuusaku wrote:As for the Pb/Pr amp above, here's why it doesn't make any sense.
REALLY ?? No sense at all, huh ?? Just a weird buffer that serves no purpose here ?? Well, let's see about that and how much sense you actually made...
kyuusaku wrote:1) The video chip doesn't need protecting in a common-collector amplifier. The resistor actually hinders the feedback.
With his response here, he demonstrated no intellectual curiosity whatsoever! Instead of asking what this "protection" was all about, what my thinking was there (since I had arrived in the thread), he simply threw a "Hey, look at me, I say your circuit makes no sense!
" type response without any kind of inquiry. I saw ego and arrogance there, didn't call him on it earlier, but am doing so now!
So, since you didn't bother to ask, how about I let you and anybody else know the thinking behind it ? Gee, what could it be ?? Well, it's not actually something that I came up with you'll be happy to know, I, not being someone with an electrical engineering background that is. Somebody WITH a background in EE actually DID, though! That would be thesteve, right here, who came up with a YPbPr circuit
for NEC systems. See, there's a particular way in which a transistor breaks down. It can short, and if the collector to base junction shorts, then that would wind up applying a genuine +5 DC voltage back to what's connected to the base which is...either a Red-Y or Blue-Y pin!!! I'd rather a diode facing outward to fully block that or a 1K resistor be there should something like that ever happen!
So, I'm supposed to take your word that if that happened, the S-ENC video chip could likely handle a genuine +5 VDC being applied back to it ?? It doesn't "need" protecting ? It's about safety, and yes, perhaps overly paranoid as far as the odds of that Radioshack 2N3904 transistor of yours breaking down that way, but like you originally said on the 1st page of this thread, you care when mods involve the alteration of remaining video game stock. So, it's hilarious that you would list an attempt to protect the S-ENC chip on a system no longer manufactured as one of 4 reasons why my circuit "doesn't make any sense..."
You also complain that the 1K resistor would "hinder feedback," but ROHM engineers used 470 Ohms for the Luma, Chroma, Composite circuits where I used 1K for my simple Red-Y/Blue-Y circuit. Your shit doesn't seem to hold up very well. I tried 470 Ohms, I tried 1K, I got the same shit color-wise, only a very slight attenuation with 1K and since I STILL need output attenuation before going to the jack on to the TV, why the hell not have a bigger resistor (twice as big in this case) in place there so hardly any current could move back to the video chip if a transistor short occurred ?? If I lose a transistor, I lose the colors next time I turn on my SNES, I fetch another 2n3904, no big deal, but what I WON'T lose is my S-ENC chip!!! Yeah, doesn't make ANY sense at all, does it, eh chief ??
kyuusaku wrote:2) 80 ohms is unnecessarily low so it leads to lots of current waste. 80 ohms also isn't a preferred value. If it's dropped to 75 ohms then it also serves as a parallel source termination.
This is where YOU actually make NO sense! SNES engineers chose 78 Ohms as the pull down resistance after the amplified transistor output when it comes to their Luma circuit (though technically ROHM made that decision, you see it in their datasheet for the video chip
So, for my solid 80 Ohm resistor versus the 78 Ohm total in the native SNES Luma amplifier, my circuit "stopped making sense" partly cause of that ?? REAALLY ?? For 2 f--king Ohms ??
So, "lots of current waste" with that 78 Ohms by SNES/ROHM engineers then, right ?? How about all the circuits that are even lower with 75 Ohms ?? I've seen plenty of that after a transistor! So everybody (and their professional circuit designs that I've seen) has been getting it wrong according to you, huh ??
Just how much voltage do you think there is on the emitter output of a transistor being powered by the +5 VDC on its collector end ?? My DMM read 2-3 V before the use of a pull down resistor and part of that is the DC offset of the video signal, not even genuine! How is one going to see "lots of current waste" with a 7X Ohm pull down resistor ?? Pfffft. You got a 470 Ohm resistor on their designs, so hardly NOTHING "current" wise would ever move through the base, only what comes out of the transistor's emitter in relation to it being powered would matter, and I think that's about 1 V. Whatever, take it up with ROHM/SNES engineers and a whole host of others, not me!!!
kyuusaku wrote:3) 470 uF is a good value for composite video which has low frequency components for sync, Pb and Pr don't have that, so it's unnecessarily high.
Well, this is debatable and I actually did switch back to 220 uF in my newer circuit which was RIGHT IN YOUR FACE, but you STILL went back to criticize the choice of 470 uF in the old one... That was curious! Nonetheless, why did I choose 470 uF originally ?? There's an answer for that, not that you cared at the time, because obviously you know better than SNES engineers and you ALSO know better than RCA engineers! Voila:
I took this photo just for you! See, I figure that you think that I pulled that 470 uF cap choice out of my rear end, so I thought that I'd actually show that that wasn't the case... Well, hard to believe, huh ? That is an Audio/Video Output module from a basic RCA DVD player! And guess what ?? You see those three 470 uF capacitors right there in your face ?? All 3 of them, their negative terminals, go right straight to the RCA outputs for Luma/Y, Red-Y, and Blue-Y, etc. That's right, that's what the RCA engineers did for this particular model! There is no 75 Ohm resistor in between, in series! That's right, as I said, the negative lead of those 3 capacitors that you see there, go straight to the RCA output jacks. Indeed, I used my DMM and did a continuity test. Only about 2-3 Ohms of resistance results from the trace path in between the RCA jacks and the negative lead of the 470 uF capacitors.
So, there ya go, mystery solved! That's why I chose 470 uF originally! Does that make sense now for ya ? I saw that RCA engineers did it for this DVD player and thought that it was a standard design choice for YPbPr output. Sue me! I then also asked my engineer friend, thesteve, and he didn't have a problem with it either...
Now, why did I go back to 220 uF anyway ? Cause it's a cheaper capacitor, SNES engineers stuck with it, I looked at another DVD output module which used 220 uF as well, so if it's cheaper, smaller and still good enough to do the job of DC offset filtering, I guess, why not ?? So that's the story of how I went from 470 uF back to 220 uF!
Anyhow, to list 1 of 4 reasons that my old circuit "doesn't make any sense" because of the use of 470 uF capacitors for Red-Y and Blue-Y when RCA engineers did it for a PROFESSIONAL DVD player shows how much sense YOU make! Take your problem up with RCA engineers since you know better than them!!
kyuusaku wrote:4) 15 ohms R at the output serves no purpose but to marginally attenuate an already "compliant" 80 IRE peak-to-peak signal. A proper series 75 ohm termination would attenuate the level to 40 IRE which would really break things.
THIS made no sense whatsoever! You only had 3 genuine points to make, but in order to add a 4th one against my basic transistor amplifier circuit for JUST Red-Y and Blue-Y signals (I advocated against attenuating the native Luma from the VERY beginning and you NEVER ONCE saw ANY of my schematics indicate otherwise!
), you threw in Pasky's desire to add about 15 Ohms of resistance to the native Luma output, yet his desire to do so, once again, has NOTHING to do with a competent, Red-Y/Blue-Y amplifier circuit!
Even if there was a perfect Red-Y/Blue-Y amplifier circuit, if the Luma is still coming in a little hot as he appears to believe, he'd STILL be complaining and wanting to add a few Ohms of resistance REGARDLESS of how good OR bad the Red-Y/Blue-Y amplifier circuit design is! Also, your apparent desire to see 75 Ohms everywhere (series-wise, before the output
) doesn't hold up when looking at the official Luma, Chroma & Composite amplifier circuits in my previous post.
Finally, Pasky NEVER even used my amplifier circuit to begin with, so even if you were trying to blame his complaints of Luma on it, it STILL had nothing to do with it! He used Ziggy's earlier one! You simply don't need Red-Y/Blue-Y connected to know that you have a problem with Luma! He can plug in Luma by itself and feel that it's too bright, coming in a little too hot, etc.! Adding a perfect Red-Y/Blue-Y would only make the picture a little brighter if you ask me, I can't see how it'd make it darker, so his complaint wouldn't change with the perfect circuit if one is trying to make use of the native Luma!