voltage on soundcard inputs

[haydxn]

okay, i've been thinking about making a really basic device that works with a custom VST (SE) plugin to convert low voltages into midi data.

i was considering making a simple circuit;

a mono jack plug (to connect to a line input)
on one wire, a protective resistor comes right after the plug, and then a string of resistors with breakoff wires between each one would follow.

from the other wire, there would be a small battery and then a load of flying leads from the same point.

the idea being that if any one of the resistor-side wires are connected with the battery side wires, a low voltage (determined by the number of resistors it has to go thru including the large protective one) would be sent to the soundcard input; this would be quantized to a digital value and then processed by a VST plugin designed to respond to different voltage ranges by producing MIDI output.

my intention was to set up a 'marble run' with broken foil contacts on either side of the track for each resistance value, which would then be activated by a steel marble rolling past, acting kindof like a 'mechanical sequencer'.


however, as soundcard inputs deal with audio (which is usually oscillating around 0, therefore averaging at 0v input), is it safe to apply just a voltage to the input? obviously a large enough resistor could be put in so that it's not possible to draw out more than a 'safe' voltage, and a 1.5v battery isn't likely to do a whole load of damage if there's enough precaution... what would be a 'safe' voltage?

just wondering what anyone thought!

[C00kie]

I don't think the audio input of a soundcard is suitable for your application. If I would design an audio input I would take some measures to avoid DC offset. A simple capacitator would kill the DC and only let AC pass through. Effectively you would only measure a static pulse when connections are made/broken in your circuit.

However, the Joystick input port may be what you're looking for! Physically a PC joystick consists of two potmeters (variable resistors) and a set of buttons. The driver handles conversion of the current resistance to a digital value. You only need the resistors, no battery! The port supplies 5V on a dedicated pin.

You can find some essential connection diagrams at http://www.epanorama.net/documents/joys ... cuits.html

There are a couple of programs that do convert joystick data to midi CC messages. One of them is TobyBear's Midi Control Centre, but there are more. I haven't found time yet to evaluate them. Anybody else did??

[annode]

Really not a safe way to go about that...really.
First...a battery,(direct current),is not safe as an input source.
....I`m at a loss for more to say.

Sounds like a cool electro/mechanical/art sorta contraption,
but there are much simpler ways to get there.

(Stay away from your soundcard input with any batteries.)

You will find lots of home brew audio circuit projects to screw with here;
http://www.discovercircuits.com/
Aside from the midi...you will find a way to make your idea happen from some of these circuit combinations.

Have fun!

[haydxn]

yeah i kinda figured connecting a battery into it would be a nasty type of affair, which is why i didn't dare try it without at least some comments (other than the voices in my head!)

i'm definitely going to check out the joystick port stuff tho... i think a journey to maplins is in order!

[haydxn]

this is excellent. exactly what i was after...

if i manage to get this working then i think i'll probably post the plugin i make to interpret the system along with a guide to make the interface.

for anyone else crazy enough to want to do this...

[whyterabbyt]

Probably a bit expensive, but did you know about this:

http://www.doepfer.de/ctm.htm

[haydxn]

well of course, that'd be super!
mr rabbit you always have links to some of the most unexpectedly remarkable gadgets!

it'd be great to have some real tangible piece of kit that i could build and use at it was to make midi messages, but seeing as i can do all the programming and computation side of things in synthedit, and make it work in my vst host, the only thing i need to do is find a way of getting simple values into the computer.

and i believe the joystick port is my new friend!

[whyterabbyt]

I find all sorts of stuff

And DIY is always more rewarding

Tell me, was it Guitar Rig that made you think of using audio inputs as a control system in the first place?

[haydxn]

guitar rig entered my mind a little after i got the idea, as i thought "yeah i guess control signals can go thru audio inputs", but i also was entirely aware that guitar rig uses infinitely more sophisticated circuitry that the kind i was planning, and is also most likely 'digital' in some way.. i.e. not just silly barely calibrated resistance values...

i did think about the joystick port too, but thought it'd be too complex. i remembered those breakout boxes we had in school connected to the BBC micro, which we'd plug switches and motors and things into... if i could make a simple box that just reported back resistance values depending on which 'switches' were made, then i could achieve my goals of having something that is probably next to useless tho mildly intriguing.



joystick port looks like the way then.

i'm gonna nip to maplins and get some bits and bobs and a box to put em in with a plug on the end! a configurable plugin to interpret the messages will finish the package, and then i can get midi signals from anything i can dream up... (marble run first tho... that should be fun, giving me a slightly different perspective on things and just seeing what the heck happens)

[whyterabbyt]

Well good luck with it. I love the marble run idea.

'Nother thought for you, something that came up on the circuit benders list; a grid of reed switches, and magnets in the fingers of a glove...