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PostPosted: Tue Dec 11, 2018 8:18 pm 
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Hope I'm not bothering anyone from obsessing about this now. :lol: I had showed a video a while back ago where a CRT TV was modded to accept RGB input by tapping into the lines going to the jungle IC, but would have the side effect of blending the onscreen display with the image rather than overwriting it. Mods showcased in other videos I've seen didn't even do that; they just used a switch to select whether information from either the microcontroller or the RGB input went to the jungle chip.

While definitely not the most practical solution because it requires a lot of extra parts, I was wondering if you could have the onscreen display "overlay" the RGB input by using MOSFETs, where whether or not the microcontroller is generating a blanking signal determines whether the signals from either the microcontroller or the RGB input are sent to the jungle chip.

Here's the awful schematic I made; if anything it at least better represents what I'm trying to say, because I know it's riddled with flaws. Basically, there's a transistor for each color element for both the microcontroller and the RGB input. That chip in the middle is a hex inverter that will invert whatever the blanking signal outputted from the microcontroller is for the RGB input transistors so that if that if microcontroller blanking is on, microcontroller RGB is passes through while RGB input does not and vice versa. The switch determines both whether blanking is on permanently or not as well as if the hex inverter gets power, the later of which I just now realized shouldn't really make a difference and that the hex inverter VCC wire could be put before the switch and the second diode removed... I guess you can save whatever electricity it takes to power the IC when not in RGB input mode :lol:

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I'll be honest, I didn't really know what I was doing with the resistors but thought I might as well leave them in... A big problem I have is that I don't understand what the three lines going to ground were for in this:

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The explanation in the video is that they're for lowering the voltage, but the three resistors from the microcontroller should have done that anyway.


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PostPosted: Tue Dec 11, 2018 9:11 pm 
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You're basically describing an analog multiplexer. Yes, that works.... but....

Your control signal has some problems. Does BLANKING actually indicate what you want it to? What pulls BLANKING down after your diode OR?


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PostPosted: Tue Dec 11, 2018 9:46 pm 
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If the switch is open, for blanking, the jungle chip would pull it down, and if the switch is closed, the hex inverter would pull it down (now I remembered why I wired the hex inverter the way I did..). Or at least that's what I expect / hope...


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PostPosted: Tue Dec 11, 2018 10:12 pm 
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Can't leave the inverter unpowered... you'll end up powering it via its overvoltage protection diodes if you do.

Right now, the overlay can pull the jungle chip signal high via D1, and the switch can pull the jungle chip signal high via D2, but what pulls the signal low?


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PostPosted: Tue Dec 11, 2018 10:28 pm 
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I'm not sure I get the question, because I'm not sure I know what "pulling low" means... If you mean when does the jungle chip get logic 0, it would be the same as normal for when the switch is off, and never when the switch is on.


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PostPosted: Tue Dec 11, 2018 10:38 pm 
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ok, let's just say: the circuit you have right now will cause the jungle chip to see logic high (+5v, "1") whenever the switch is closed OR the microcontroller drives BLANKING high.

But when the switch is open and the microcontroller drives BLANKING low ... the jungle chip will see any random thing because nothing causes the voltage on its input to drop.*


* Yes, technically diodes are very large resistors when reverse-biased, but the amount of time it will take for that to happen will be annoyingly long


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PostPosted: Tue Dec 11, 2018 10:56 pm 
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Wait, I think I get it? Does no current actually go through blanking on the jungle chip?


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PostPosted: Wed Dec 12, 2018 12:00 am 
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Yeah, everything after ... eh, 1990? is purely voltage-based logic, almost never current-based.

edit: ... But even when it was current-based, your diodes are pointing the wrong direction. In 74LS logic, you get diode ANDs for free, not diode ORs.


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PostPosted: Mon Dec 17, 2018 10:40 am 
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After going back at this with more knowledge, I'm pretty sure that initial schematic is wrong in about 100 different ways. What determines if current flows through the NMOS transistor is not just voltage is applied at the gate, but that the gate voltage is higher than the source, which is why it's now connected to ground. If there's gate voltage (blanking logic 1), then the output voltage is 0V, which is the opposite of what's wanted, so the microcontroller RGB needs to be inverted instead of the input RGB. Testing this in Multisim shows that everything checks out (the switch controls is blanking is always on, the blanking voltage determines whether the output voltage is from the microcontroller or RGB input) but I'm pretty sure that there's no such thing as a giga Ohm resistor for something like this... :lol: The higher the resistor value though, the closer the output is to the source voltage. Multisim also won't even try and emulate things like insane current blowing up the circuit or anything like that.

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PostPosted: Mon Dec 17, 2018 11:19 am 
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Really, you just want an analog multiplexer. Something like a CD4053 will be more than adequate. Use it to select between the analog external input, or the voltage divider from the TV's OSD outputs (which are almost certainly 5V digital RGB)


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PostPosted: Mon Dec 17, 2018 12:45 pm 
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Wow, you're right; it even has the exact number of inputs and outputs too. I'm surprised seemingly no one has done this given the IC is priced at less than a dollar.


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PostPosted: Mon Dec 17, 2018 2:16 pm 
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I mean, the only instance we've seen widely of this hack is "The 8 bit Guy"'s video, right? There's too small of a sample size to say anything about what do people actually do.

Plus even if it's a $1 IC, it's a $1 IC that you probably don't already have on hand.


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PostPosted: Mon Dec 17, 2018 2:52 pm 
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I saw about 4 other significantly less popular videos that either did the same thing or just forgot about mixing the signal altogether and removed the OSD. Obviously it doesn't matter much either way.


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PostPosted: Tue Dec 18, 2018 12:57 pm 
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Oh, here's another possible reason: The CRT in my house has only two RGB stages: 8Vpp and 80Vpp. Can't trivially sub in 0.7Vpp in without adding a lot of circuitry.


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PostPosted: Tue Dec 18, 2018 2:58 pm 
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This CD4053B data sheet seems to suggest that it will take analog signals of up to 20V though: http://www.ti.com/product/CD4053B


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