I'm trying to come up with a microprocessor solution to replace the traditional analog synth patch cords, using an analog crosspoint switch.
The idea is to replicate the mechanical matrix used in this synth, but under microprocessor control:
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Google yielded some crosspoint video (analog) switch chips with memory (latching). I came across some chips like the old
74HC22106 and the newer ADG2188 etc.
These almost solve the issue except that there is no 10k isolation resistor (say) at the crosspoints. You could add 10k resistors to the inputs (say columns) but if one input was patched to more than one output line, then those two outputs would be only isolated by about 50 ohms. So cross talk would be a problem.
So my question is this- are there any crosspoint switch solutions that include an isolation resistor at each crosspoint?
The isolation resistor at each crosspoint allows each patch to automatically be summed (mixed) like the typical summing opamp circuit. It will also provide some level of isolation between each line.
On a sunny day (Tue, 29 Nov 2011 15:27:11 +0000 (UTC)) it happened Charmed Snark wrote in :
I did one in the eighties with 8 x 8 or something like that, controlled by a ZX81 Sinclair computer. But I do not understand your problem. Also I think I used opamps in the drive lines and read lines. One thing I found out that to reduce crosstalk you need to switch unused input to ground. To reduce losses you need to terminate with a high impedance. these switches like the CD220106? (reminds me of my old RCA CMOS (15V) databook, have a up to a few hundred ohm internal resitance, so you need to terminate with infinity. But I do not remember all the details apart from that it had a lot of logic level shifters to drive those chips from 5V logic.
These day may be digitise everything do it in software? Sources will be mostly digital anyways these days. I remember the ZX81 crashed one evening and caused a hell of a lot of noise in the auditorium where it was used.. The program was written in BASIC. Also reloading the program from tape took some time... before it was online again LOL. But is was sort of a first :-) I used it to switch in filters and stuff. Adventures times... I have had compliments for the sound quality too.
Can you sketch up a schematic? If the input goes to two outputs, why would they need to be isolated if they now carry the same signal?
I've done several such muxes, all in the frequency range up to 15MHz. Up there isolation is much more difficult. The standard trick is this: Run anything across the X-point chips strictly current mode. No voltage summing, always currents.
Michael A. Terrell expounded in news:U42dnQSjw4PNtUjTnZ2dnUVZ snipped-for-privacy@earthlink.com:
I have looked at the MAX4456, but I suspect that is very old now (hard to get at the moment). I'll go back and look at their line up. I just googled for "crosspoint switch(es)" when I came across MAX4456.
If they have "studio routers", that may be what I'm looking for.
Joerg expounded in news: snipped-for-privacy@mid.individual.net:
In analog synths, you want the signals to "mix" (not take over). But if one signal is going to two destinations (or more), you don't want the two different destinations coupling tightly (the 2nd output may have other signals mixed in).
Imagine a row column matrix. In the mechanical plug version of this, you plug in a plug that adds a 10k resistor between that column Y5 and row X3. This replaces the need for a patch cord between Y5 and X3.
If you then add another plug from col Y5 (input) to row X4 (output) there is now only a 20k coupling between X3 and X4, while Y5 connects to both.
Further complicate this by adding a column Y6 input to be mixed in with X4 (only). Y6 and Y5 are also now 20k isolated, yet X4 can sum both inputs Y5 and Y6. (picture your typical summer opamp circuit, except that the input resistors are added at the matrix crosspoints)
In the ss chipped version, there is a connection made with a very low (50 ohmish) resistance connection between row and column (vs the 10k in the plug matrix version). This is ok, when the column is going to _one_ destination (video), but not when multiple destinations are involved.
The same scenario in the chip would result in only about 100 ohms of isolation between X3 and X4 (when Y5 was enabled to both X3 and X4). This is almost equivalent to soldering Y5 directly with both outputs. Add another connection from Y6 to X4 (only), and you might as well solder all connections Y4, Y5, X3 and X4 together (which is highly suboptimal).
Yet in the mechanical plug version, X3 and X4 have some level of isolation (20k) to allow for different output signals. Y5 and Y6 also keep at least 20k worth of isolation from each other. This is an oversimplification with a complex network, but you get the idea hopefully.
Just scribble out what I've described (just the matrix) and I think you'll get what I'm after (when a crosspoint is enabled, it is connected by a 10k resistor). The basic problem is that the video switch is probably intended to not mix, but simply to "route" from one input to one output.
But here, I'm looking for a way to route and mix, albeit with limited isolation.
I've used the CD22M3494 with good results. It is really a video switch, and I have no idea whether there might be audible transients when the switch settings are changed, but it worked well in the application we used it in.
If you drive all the inputs with a low enough impedance, and load the outputs with a high enough impedance, then there shouldn't be a big difference if one or more outputs are switched to the same input. That's what we did in our signal switching application.
Jon Elson expounded in news:XdadnRKwGtQH20jTnZ2dnUVZ snipped-for-privacy@giganews.com:
It has the same limitation I mentioned earlier, but--
It does have the advantage of low price and _wider_ width (16x8). I could adapt something like this to work with some limitations.
Using only one chip, I could pair inputs with 10k resistors going _in_ for each leg of the pair (sig + 10k into X0, and same sig + 10k going into X1 for example). That way any pair of the same signals would still wind up with 20k ohm isolation between two selected outputs.
Given the low cost (and I like that they have DIPs), I could add another, allowing 32x8 => one input could fan out through
4 different X0 to X3, each with their own isolating 10k resistors. In this manner, I could mix up to 4 of the same inputs to 4 different destinations, with 20k isolation.
Thanks for the chip reference. I'm going to look at this possibility.
Ok ... but ... unless I am having a brain freeze here:
Suppose you send something from Y5 and Y6 into X4. Y5 also goes to X3, which happens to also get a feed from some other place, and so on. If you feed the mux strictly current mode and make sure you have current receivers on all X ports then the impedance at all Y ports is very high (ideally infinity). Yet at X3, X4 and the others it is zero. Like a dead short. So how could there be any continuing leakage for example from whatever else feeds into X3 via Y5 and then into X4?
The zero ohms receiver is often also called a "transimpedance amplifier" or TIA. The "economy class version" of that would be the emitter of a common-base transistor stage. What you do have to adjust for can be done on the drive side: If you feed two X-outputs from the same Y-input then the current splits up 50:50. So does the amplitude. If you feed three then 33%, and so on. So you'd have to up the drive signal accordingly.
Don't know if it helps, but look at AD75019 - it's still available and is a IIRC +/-12V 16x16 analog crosspoint. Annoying part is serial addressing, but a slave PIC would do this easily.
The part number rings a bell. Just beware of the very odd numbering of the rows and columns.
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
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Joerg expounded in news: snipped-for-privacy@mid.individual.net:
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Ok, using the Y5, Y6, X3 and X4 example, let's look at this from the mechanical plug board pov first, that I am trying to replicate in silicon. Remember each plug has a resistor in it for isolation.
Y5 is connected to X3 and X4 Y6 is connected to X4 only
The plugs are put in at the crosspoints, each of them containing a resistor (I've assumed 10k ohms) :
(switch to fixed fonts here)
Y5 Y6 | | | |
--|-----+--|-------- X3 +R=10k+ | | | | |
--|-----+--|-----+-- X4 +R=10k+ +R=10k+ | | | |
This small matrix patch configuration is equivalent to:
From this, it is very easy to see that the isolation from X3 and X4 is
20k and also that Y5 and Y6 is also isolated by 20k. Y6 to X3 isolation isn't great, but it is 30k ohms.
But when you use the silicon matrix solutions, the R is lowered to roughly 50 ohms (or less). So then X3 and X4 is only isolated by 100 ohms and likewise Y5 and Y6 is also only isolated by 100 ohms (like I said earlier). This is completely unsuitable for voltage based signals (which these are). Further Y6 is only 150 ohms away from X3.
What I think you're suggesting is to convert from a voltage based signal and work with currents instead. The problem is that it must also accept DC control signals (inputs are not always strictly ac signal). To apply two conversions (voltage -> current and back) would introduce too much DC error. Pitch control for example, is very fussy.
Jan Panteltje expounded in news:jb2v0a$kcp$ snipped-for-privacy@news.datemas.de:
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I finally got rid of a pair of those I had in my basement a couple of years ago on ebay. Not sure what the buyer planned to use them for.
I'm not sweating those details at this point in time. The idea is to be able to replace the rats nest of patch cords, and make them restorable.
The newer chips do all that level shifting for you.
This is for an analog synth. If I was to digitize the signal, then I might as well forget about the analog synth too - where is the fun in that? :)
The British group Ultravox used to get really annoyed with the joint's tech people who would turn off and then on the breakers to show them where the breakers were. This of course forced them to have to reload all of their digital effects from tape again.
Ultravox btw, is finally coming out with a new album 24 years after their last one. They've all gone bald too .
Syd Rumpo expounded in news:jb3p48$kck$ snipped-for-privacy@dont-email.me:
Yes, the +/-12V is great, & matrix size too. The serial addressing is ok with me for simpler AVR wiring. This part has 200 ohm "on" resistance, which is a bit higher than the typical 50, but it otherwise has the same issue. However, with the larger matrix size it might work if I fan out the inputs to multiple lines.
Yup, that's pretty much how I understood your explanation in the previous post.
Depends on how much DC error can be tolerated. If you are talking low microvolts, yeah, that'll get ugly. Then you can't use one of the telco-style crosspoint ICs, that only works current mode. Problem is, the old ones have mostly vanished. Even the ones from my days (22106 and such) are gone.
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