Avionics intercom mic signal

Thread Starter

barneyward

Joined Apr 29, 2019
16
Hi there. I'm looking to inject an audio signal from a hobby project into the intercom system of a light aircraft using one of the spare mic sockets. The project produces audible warnings, and has an output which can be used to drive earbud type headphones. I had thought (incorrectly!) that I could simply drop this level down to mic level (ie. a few mV or so) and connect via one of the microphone sockets, but all I get is white noise. I don't know much about audio, but I'm guessing there's some DC voltage passed with the mic signal to bias the microphone element...
Any suggestions on what I need to do to the signal to present it in a way the mic socket will accept would be appreciated. The mic sockets are supposed to work with almost any amplified dynamic or electret microphone.

Regards,

B
 

Dodgydave

Joined Jun 22, 2012
9,935
Aviation mics use two together, the second mic picks up the background noise and is inverted and added to the mouth piece mic to cancel the background noise.
 
Last edited:

Yaakov

Joined Jan 27, 2019
2,880
Avionics headsets get microphone power from the connection to the aircraft. Using a DC blocking capacitor on the output of your project might fix your problem. Having said that, you really need to provide more information about the device you are trying to connect.
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
Ok, the device connected is a DFPlayerMini MP3 player, and I'm using one digital audio output channel as the source. There's nothing in the data sheet about the level produced, but it runs small headphones pretty well and is probably a couple of volts peak to peak. I had simply tried reducing this down with a potential divider and throwing the resultant signal down the mic plug.
I will certainly try a capacitor to block the DC component. The other thing I have just realised is that the MP3 player is battery powered, and there is no common earth between the player and the aircraft intercom. I'd better sort that too.circuit.png
 

Yaakov

Joined Jan 27, 2019
2,880
In theory, you could use the microphone supply voltage to power the device. On the other hand, I am already hinky about connecting a homemade device to an avionics system.

Erring on the side of safety, I would probably use acoustical coupling through a proper headset to do this, in spite of its apparent inelegance.
 
Last edited:

Thread Starter

barneyward

Joined Apr 29, 2019
16
Oooh - that's a cunning plan!

I'll look into that - it'd be much better than relying on a battery which would end up flat anyways!

Thanks
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
You're probably right - I don't think I have the electronics to safely pull power from the intercom without frying the system!

So by 'acoustical coupling' are you suggesting I should just play the sound into the mic of an existing headset?
 

Yaakov

Joined Jan 27, 2019
2,880
You're probably right - I don't think I have the electronics to safely pull power from the intercom without frying the system!

So by 'acoustical coupling' are you suggesting I should just play the sound into the mic of an existing headset?
Precisely. That allows it to be galvanically isolated and should it malfunction, it can't touch the avionics and can be easily removed.
 

MisterBill2

Joined Jan 23, 2018
8,405
You could also get a replacement microphone cartridge for an avionics headset and use that in front of whatever earphone or small speaker would match it. But I thought that those intercom sets were PTT (push to talk) setups, and I am not sure how to handle that part with whatever your sound source is.
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
You could also get a replacement microphone cartridge for an avionics headset and use that in front of whatever earphone or small speaker would match it. But I thought that those intercom sets were PTT (push to talk) setups, and I am not sure how to handle that part with whatever your sound source is.
Thanks, that’s an option too, I suppose. On aircraft the push-to-talk button is just for communicating with air traffic control - when it’s not pushed all coms are internal.
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
Thanks Ken - ultimately I do want to get a grip on how the whole intercom system works, and that should help a lot.
Regards,

Barney
 

Wolframore

Joined Jan 21, 2019
2,249
I don’t like messing with avionics. Especially something vital as communications. Might be able to do a device that plugs in between headset and radio. Since it’s low voltage battery I would keep them isolated and couple audio using transformer. Just get a good metal case for shielding and ground it. Put a mix knob so you can lower it when you need to concentrate. With a module, if there’s an issue you can unplug it and hide the evidence. :)

Not sure about injecting it directly into the mic input. Because you don’t want that transmitting to everyone just for you and passenger.

Seems simple but there are some challenges. One is level and impedance matching.
 

MisterBill2

Joined Jan 23, 2018
8,405
If you have an extra microphone input on the intercom system then a small audio transformer with an isolated winding could deliver the signal and if the amplitude is correct everything should work out quite well. But now here is a question regarding operating the radio transmitter: When one person operates their PTT button to transmit, do other channels also have their microphone signals transmitted? That could be an issue if the device sounds off while the pilot is talking to somebody. So I am wondering about how that part functions. It may also be that whichever headset keys the transmitter, the other mic inputs are disabled.
And one more question is are the headphones stereo? And does communication come from both phones? I am guessing that the whole setup is a lot different from what was in military aircraft 50 years ago.
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
If you have an extra microphone input on the intercom system then a small audio transformer with an isolated winding could deliver the signal and if the amplitude is correct everything should work out quite well. But now here is a question regarding operating the radio transmitter: When one person operates their PTT button to transmit, do other channels also have their microphone signals transmitted? That could be an issue if the device sounds off while the pilot is talking to somebody. So I am wondering about how that part functions. It may also be that whichever headset keys the transmitter, the other mic inputs are disabled.
And one more question is are the headphones stereo? And does communication come from both phones? I am guessing that the whole setup is a lot different from what was in military aircraft 50 years ago.
That’s helpful, thanks! Yes, when one PTT switch is keyed, all mics are live to broadcast .
To be honest, the device I’m building is triggered by either the stall warning tone, or the landing-without-the-wheels-down tone (different tones, both of which sound over a loudspeaker, and which modern noise cancelling headphones are very good at cancelling out!). If the stall or gear not down alarm goes off, I don’t care if it gets broadcast, and I don’t want it reduced in volume - I need to know about it!
Hopefully it won’t go off (except maybe the stall alarm just on touchdown - but no one will be keying their mic at that point anyway!!)
I’ve done all the circuitry to distinguish between the two different alarms, and trigger the right audible warning for each, it just needs to be able to be heard on the intercom!
Thanks again,

B
 

Yaakov

Joined Jan 27, 2019
2,880
I hate to second guess your work, but...

Couldn't you use the tones, injecting them into the headsets and disabling the injection on PTT?
 

Thread Starter

barneyward

Joined Apr 29, 2019
16
I hate to second guess your work, but...

Couldn't you use the tones, injecting them into the headsets and disabling the injection on PTT?
Ah, but where’s the fun in that?! I want a synthesised voice telling me what the problem is!
Seriously tho’ - there’s a great video on YouTube of someone doing a wheels up landing at Megeve, and the landing gear warning tone is sounding for ages and ages. When you are completely focussed on one task - like landing - a tone in your ear sometimes is not enough. That’s why 747’s have aural warning systems such as ‘Terrain, Terrain’ etc.
 

Yaakov

Joined Jan 27, 2019
2,880
Ah, but where’s the fun in that?! I want a synthesised voice telling me what the problem is!
Seriously tho’ - there’s a great video on YouTube of someone doing a wheels up landing at Megeve, and the landing gear warning tone is sounding for ages and ages. When you are completely focussed on one task - like landing - a tone in your ear sometimes is not enough. That’s why 747’s have aural warning systems such as ‘Terrain, Terrain’ etc.
You need a robotic arm to slap you in the head at the same time.
 

KMoffett

Joined Dec 19, 2007
2,918
"If the stall or gear not down alarm goes off, I don’t care if it gets broadcast"
Even if it blocks the frequency for other pilots? Although broadcasting "Stall Stall Stall" or "Landing Gear Landing Gear Landing Gear" on the approach frequency might act as a pseudo CVR to tell everyone why you crashed. ;)

Ken
 
Top