Propeller to multiple monitors of same ntsc source
OakGraphics
Posts: 202
Howdy gang!
Say - I am looking at hooking up my propeller project to multiple NTSC Monitors for the same display.
(one for a small LCD, and then one for a display that will be about 100 feet away.
I would also like to perhaps hook up more then 1 additional monitor.
Does anybody know a circuit that can take the 1v ptp NTSC signal and output the same signal on 2 or more pairs?
I think there might be a chip for this type of thing, but I haven't found it yet.
Any ideas?
I thought of doubling the pins, but then that doubles the cogs too.
Thanks in advance,
Daniel
Oh yeah - I am also looking for a good source for a cheap NTSC LCD. 3, 5 or 7" would be fun.
Say - I am looking at hooking up my propeller project to multiple NTSC Monitors for the same display.
(one for a small LCD, and then one for a display that will be about 100 feet away.
I would also like to perhaps hook up more then 1 additional monitor.
Does anybody know a circuit that can take the 1v ptp NTSC signal and output the same signal on 2 or more pairs?
I think there might be a chip for this type of thing, but I haven't found it yet.
Any ideas?
I thought of doubling the pins, but then that doubles the cogs too.
Thanks in advance,
Daniel
Oh yeah - I am also looking for a good source for a cheap NTSC LCD. 3, 5 or 7" would be fun.
Comments
I'm not entirely sure how well this will work with a typical Propeller NTSC output, though, since the standard DAC resistors don't provide the correct source impedance. I have a feeling it won't be a prblem, as long as the total length of your daisy chain is fairly short.
-Phil
So OakGraphics ought to be able to daisy-chain any number of monitors on any length of line, just as you suggest -- as long as only the last one terminates the line, as you also mention.· The farther down the line the signal goes, the more it will be attenuated by line loss, of course.· Somewhere in there the signal will be too weak, but a·hundred feet ought to be feasible, especially if low-loss line like RG-6 is used.· With lossy line like RG-59 it'd fade more quickly with distance.· Distance matters, but the number of nonterminating monitors doesn't matter at all, with the condition that they're well designed and really don't load the line.
But the original idea of feeding several pairs in parallel is a poor one, because the Propeller would then see an impedance much lower than the 75 ohms we know it can feed.
Having said all that, one must notice that, while studio monitors always (well, nearly always) have switchable terminators and daisy-chain connectors, equipment intended for consumer use typically has the termination resistors hard-wired.· OakGraphics would have to disable the terminations in all the monitors but the last one.
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· -- Carl, nn5i@arrl.net
Daniel,
If you're using a long feedline and need to eliminate the impedance mismatch, use either of these DAC outputs, rather than the one recommended by Parallax:
-Phil
If you're unable to switch out the termination resistors in all but the last monitor, you can use a video distribution amplifier, like this one
····www.radioshack.com/product/index.jsp?productId=2103065&tab=techSpecs
and run a separate cable to each monitor.
-Phil
That's a simple solution - I like it!
Yeah - I can not control the monitors termination on the far-side, but maybe I can on the LCD screen I hope to embed into the project case. I will look at the spec sheet I have and see if it has anything.
Of course I am still looking for a cheap NTSC LCD monitor. I found a 3" lcd for 50$ that seems pretty good, but would prefer something more around 5 or 7"
The consumer ones don't look very hack-able.
Thanks for these mods! I will incorporate them into my circuit. I appreciate that.
Over a long distance of 100 feet the signal would degrade even if the source was a perfect 75 Ohms (right across the frequency range) but should give an useable picture. An amplifier would give a better output match and probably some tweeks for lenth losses. The video signal is not intended to be of broadcast standards to start with and so the simpler methods should be ok.
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Style and grace : Nil point
(1)· All energy going away from us (away from the Propeller) wil be absorbed at the far end, and none will be reflected, if the line is properly terminated in a resistance equal to its characteristic impedance.· The characteristic impedance, incidentally, never has a reactive component and is always a pure resistance.
(2)· Energy coming toward us·from the far end (some of it)·would be reflected back into the line upon arrival at the Propeller with its 150 ohm source impedance.· But there isn't any energy coming toward us from the far end because there's no source there, and no energy can be reflected back from the far end because of the perfect match at that end.
(3)· Some energy would be reflected to the Propeller from the near end of the cable, but that energy would not enter the cable.· This effect would be the same no matter what the length of the cable, because the impedance at our end is always the same, no matter the length.· If the (75 ohm, in this case) cable is terminated in its characteristic impedance at the far end, it always presents 75 ohms pure resistance·at the near end, and in fact no physical test made at the near end can detect its length or anything else about it, except the impedance it represents.· You can't measure its length with a reflectometer, for example, because that requires that a signal be reflected back to us -- which won't occur.
(4)· So, no matter the length, the Propeller will always see the same load, and consequently will always behave exactly as if it were connected, not to the transmission line, but to a 75 ohm resistor.· No matter the length.
You speak of·"the reflections from a long transmission line that can result in visible ghosting" -- but if the line is properly terminated there can be no reflections and there will be no ghosting.
Now, having said all that, if the non-terminating monitors spaced along the line don't maintain 75-ohm characteristic impedance at the point from which they sample the video, or if they absorb any energy, there may be reflections from the taps, resulting in ghosting.· But well-designed monitors don't do that.· I have three Panasonic NTSC studio monitors with switchable termination·on a line that goes about 70 feet in my house, and no ghosting.
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· -- Carl, nn5i@arrl.net
Post Edited (Carl Hayes) : 9/25/2009 4:28:57 AM GMT
monitors. The Propeller video pins would each go to 2 pins of each 4050, and the resistors would be on the outputs.
I suggest searching for another fast buffer chip since the 4050 pin order is weird. Perhaps 74HC244 would make
more sense if it is cheaper than $1 as I expect, and has 2 extra buffers per chip so 2 chips might be able to drive
5 monitors, but check the data sheet. 74HC244s can be found for free in the junk bins inside old PCs that don't
run the latest Windows and they can be coaxed out with a heat gun and a chip puller.
Thanks for your clarification. You are correct of course, and you make a compelling argument for cases where a perfect transmission line is properly terminated. In such cases, I have to agree that source impedance matching is a non-issue. However, in cases where the line is not properly terminated, or where there are impedance "bumps" along the way (e.g. from less than perfectly designed pass-through connections, it's still advantageous to match the source impedance to that of the transmission line, so that any reflections that might occur as a result don't get re-reflected at the source end, causing ringing and worse ghosting than might otherwise occur.
So, given the choice, I would still recommend the output circuits I presented above, but maybe a little less stridently than before.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 9/25/2009 8:01:56 AM GMT
Actually I guess I druther [noparse][[/noparse]as we say in the Old South] do it your way too, if I hadn't had the experience of knowing that my particular monitors are designed specifically for daisy-chaining, and have very good pass-through characteristics.· Your way will work with any monitors, even poor ones.· The disadvantage is that you've got to buy lots more cable, and string lots more cable around the building.
Bummer.· Stringing cable is almost as hard·on the knees as katame-no-kata, and my knees will soon be 70 years old.
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· -- Carl, nn5i@arrl.net
Post Edited (Carl Hayes) : 9/25/2009 9:29:40 AM GMT
To do it properly you'll need to incorporate gain, EQ and clamping circuits.
Without the video DA you'll notice that the gain will be reduced and video quality will suffer.
Let's not even talk about the effects of ground loops.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 9/26/2009 4:24:42 PM GMT
The signal is not that wonderful to start with, when compared to broadcast specs.I would have been repremanded for sending a Video signal down a cable that was longer or thiner than another, and that was with the luxury of opto-coupled or differential inputs, multi band eq DAs. But that was for network feed, and not a monitor on the tail end of a microcontroller. If you have a DA use it, if not workaround it. A decent one would not be cheap.
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Style and grace : Nil point