# Switching Audio Signals

Please bare with me here, you will quickly see that I know just enough on this subject to be dangerous. I'm trying to switch two audio signals on and off into one Aux audio input of my motorcycles stereo system. I have a Satellite Radio that I want to hear most of the time. I also have a GPS that when I'm fellowing a route, at times will give voice directions. both of these signals will be going into the same Aux input of the stereo. I need a way to switch from the Satellite radio to the GPS when a signal from the GPS is received. will a simple relay do the job? there isn't much voltage coming from the GPS to the speakers to use as a signal for switching. I put a volt meter across the speaker wires and as best as I can tell there is less than a volt there. please try to condescend down to my level in your response that I might understand how this can be done. thank you for your help and your patience.

## Answers and Replies

berkeman
Mentor
Instead of switching, can you just combine or mix them? You might be able to just resistively add them together, and put that sum signal into your stereo amp.

not sure about mixing the two audios together. how could I hear just the GPS giving directions if at the same time the Satellite radio is still playing? thanks for your reply.

chroot
Staff Emeritus
Gold Member
Just use a double-pole, single-throw switch. Or, if the signals are stereo, a double-pole, double-throw switch. You can probably find something appropriate at your local Radio Shack or other electronics hobby store.

- Warren

berkeman
Mentor
Just use a double-pole, single-throw switch. Or, if the signals are stereo, a double-pole, double-throw switch.

I think he's looking for a way that it can be automatically switched over, when the audio voice starts coming out of the GPS output, and then it goes back to radio when the voice stops talking. (If I'm understanding the question)

Well, you can do that, but it would take a powered circuit with some opamps at least, and either a relay or opamp routing of the two audio sources. A simpler way might be to have a mechanical switch that lets you either have stereo radio (the normal way that you would keep it), or switch into a mode where the radio is mono and comes out of the right speaker, and the GPS output voice signal comes out of the left speaker. You can turn up the volume on the left channel so the voice will be heard over the music. You only get mono music when you are in this mode, but the spatial separation will help you to be able to hear the voice better.

That's the kind of setup that we use in our EOCs (emergency operation centers) and command vehicles -- we separate the Resource channel (the command channel) and the Tactical channel spatially. Either with two speakers spaced on either side of you, or with headphones with one channel in each ear. It takes some practice, but you get good at being able to listen to two things at once, and separate them out in your mind.

that would work for sure using a manual switch. I was wanting it to switch automatically when a audio signal from the GPS comes on. a normal relay would be nice but not enough voltage is present in the GPS audio signal to operate a relay (I think?). what about using a transistor to operate the relay? would there be enough voltage in the audio signal from the GPS ( a little less than a volt,I think) to turn a transistor on? if so couldn't I have 12 vdc on the other side of the transistor so when it turns on it would operate a 12V relay to switch the audio signals. that way I would have only one audio signal going the Aux input of my stereo at any time. or can't a transistor be use as a amplifier? if so could I use a transistor to amplify the GPS voltage up to a point that it would be enough to operate a relay? thanks for your patience.

berkeman
Mentor
Yeah, the simplest way would be as you say, to do the amplification and rectification and RC hold time with just a couple transistors, and have the output of that turn on a 12V relay (DPDT as chroot suggested) when there is an AC audio output from the GPS unit. Using opamps would give you more flexibility, but a couple-three transistors should be enough to make it work as well.

chroot
Staff Emeritus
Gold Member
I find it hard to believe there aren't off-the-shelf black boxes to do this sort of thing. I might have to poke around a Musician's Friend catalog for voice-over boxes.

- Warren

thanks for your help. I don't know much about this sort of thing, as you can see, but will I'm going to give it a shot. there are companies that build a device for motorcycles that will do just what I'm trying to do here. BUT, they want $250 or more of it. I don't see it to be worth that at all. I thank you all for your input. chroot Staff Emeritus Science Advisor Gold Member You might not even need to use fancy analog electronics at all. Portable, battery-powered devices are going to turn off their output drivers when they're not actually making noise, and you can probably quite easily open the device, find the control signals that are turning the drivers on and off, solder a lead onto those traces, and bring it outside the box. You might need to buffer it with a simple TTL inverter, and then feed it to your relay which does the switching. You might also want to look at the output of your GPS on an oscilloscope (DC coupled). It's possible that it produces a common-mode voltage when it's making noise, and might be able to just detect that with a comparator. - Warren NoTime Science Advisor Homework Helper What you want is often called a VOX switch. Most of the commercial ones are set up to run with microphone level input. In this case you would eliminate the microphone preamp and feed your line level signal to where the mic preamp output goes. Here is one kit version, but you can probably just buy a VOX Microphone and cut out the unwanted bits for$10 or so.

The basic circuit is simple. It's just a diode and capacitor (peak detector) with a resistor to discharge the cap. An op-amp set up as a level detector drives the relay to switch your signals.

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man this GREAT, you guys ( or gals ) have given me a world of good ideas. if anyone else wants to add anything, by all means please do. again thanks to all for taking the time to respond.

Ouabache
Homework Helper
What you want is often called a VOX switch.
Here is one kit version,

That's funny, they have a layout diagram but no schematic. Don't they realize hobbyists also want to understand how their kits work, to be able to troubleshoot it, if they make a mistake during assembly, or if the kit stops working at some point.

VOX (voice activated transmission) is useful, but not instantaneous. All the ones I've used (on commercial products), clip off a couple seconds of audio. I suppose that occurs due to the time delay between voice activation of the circuit and the time it takes the relay to switch over the audio output to the speakers. That's the trade-off in using VOX.

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NoTime
Homework Helper
Yea, I noticed the lack of schematic myself, but they might include it with the shipment. I suppose you could ask them.

Some of the microphone VOX delay is on purpose.
You don't want the transmitter (the common use) turning on with noise spikes.
A line level output won't have that problem.
So those parts can be eliminated as well.

you bet, that looks like something I could use alright. these all good ideas. now I just have to do crash course in electronics to see how to put all this together. what a circuit board to mount all this on. I don't understand how I'm going to get the traces right to hook all this up. is there a way for me to put the traces on a blank board so they go where they need too. I told you know just enough to be dangerous. thanks for leading this blind guy around.

chroot
Staff Emeritus