Hey guys,
I'll start with a little background. I'm a helicopter pilot by trade and one of my day to day jobs is flying digital surveying flights. Essentially flying a grid pattern over specified areas for up to 8 hours at a time. I adore my job, but there is no doubt it gets a little tedious without some tunes or an audiobook to focus on. Recently I dropped some considerable cash on an in-line Bluetooth adapter for my helmet. However, the audio quality is dismal. Well suited to phone calls, but that's about it.
I've decided to try and build my own Bluetooth receiver. I've been experimenting with an ESP-32 and an I2S DAC acting as the audio source. This is working well. However, I'm a little bit stumped on how to feed my amplified music into my headset without backfeeding the helicopters intercom system.
The basic requirement is theoretically simple I think: I need to overlay my amplified music on top of the amplified signal from the helicopter without cross talk. Additionally The helicopter audio needs to pass through even if my Bluetooth system is not powered.
I'm not entirely sure how to approach this problem.
I figure I have two options: I can mix the two signals, or I can switch between the two signals.
I think mixing is the more technically correct solution. If something goes wrong with my hardware it's less likely to mute the helicopters intercom system. But switching between the amplified sources may be the more robust solution, however it would have to be carefully designed to automatically switch back to the helicopters audio when it detects audio on the intercom or if the Bluetooth battery dies.
Anyway, I'll cut to the point:
I have two amplifiers and one headset. What is the best for me to connect both amplifiers to the single headset without damaging anything? Is it better to switch back and forth, or is it possible to mix both amplified sources? What ICs or circuitry should I be looking at?
Thanks a ton for letting me pick your brains, guys. I really appreciate it!
I'll start with a little background. I'm a helicopter pilot by trade and one of my day to day jobs is flying digital surveying flights. Essentially flying a grid pattern over specified areas for up to 8 hours at a time. I adore my job, but there is no doubt it gets a little tedious without some tunes or an audiobook to focus on. Recently I dropped some considerable cash on an in-line Bluetooth adapter for my helmet. However, the audio quality is dismal. Well suited to phone calls, but that's about it.
I've decided to try and build my own Bluetooth receiver. I've been experimenting with an ESP-32 and an I2S DAC acting as the audio source. This is working well. However, I'm a little bit stumped on how to feed my amplified music into my headset without backfeeding the helicopters intercom system.
The basic requirement is theoretically simple I think: I need to overlay my amplified music on top of the amplified signal from the helicopter without cross talk. Additionally The helicopter audio needs to pass through even if my Bluetooth system is not powered.
I'm not entirely sure how to approach this problem.
I figure I have two options: I can mix the two signals, or I can switch between the two signals.
I think mixing is the more technically correct solution. If something goes wrong with my hardware it's less likely to mute the helicopters intercom system. But switching between the amplified sources may be the more robust solution, however it would have to be carefully designed to automatically switch back to the helicopters audio when it detects audio on the intercom or if the Bluetooth battery dies.
Anyway, I'll cut to the point:
I have two amplifiers and one headset. What is the best for me to connect both amplifiers to the single headset without damaging anything? Is it better to switch back and forth, or is it possible to mix both amplified sources? What ICs or circuitry should I be looking at?
Thanks a ton for letting me pick your brains, guys. I really appreciate it!
Welcome to diyAudio 
I don't know if your setup lends itself to this but the best way to mix two signals is for it to be done before the main headphone amplifier.
Combining two (essentially) power amplifier outputs is not a good way to go due to impedances and power levels. Mixing the signals before is easy.
Have a look at 'virtual earth mixers'. Figure 4 here would be a preferred method and with no interactions between sources. Exactly how such a scheme is implemented depends on the supply voltage (s) you have available but it should lend itself to a single rail with a few tweaks.
Audio Signal Mixing.
I don't know if your setup lends itself to this but the best way to mix two signals is for it to be done before the main headphone amplifier.
Combining two (essentially) power amplifier outputs is not a good way to go due to impedances and power levels. Mixing the signals before is easy.
Have a look at 'virtual earth mixers'. Figure 4 here would be a preferred method and with no interactions between sources. Exactly how such a scheme is implemented depends on the supply voltage (s) you have available but it should lend itself to a single rail with a few tweaks.
Audio Signal Mixing.
Thanks for your reply!
I'll dig into the links you provided!
Unfortunately the earliest I can get my hands on the audio from the aircraft is right at the amplified headphone output. Everything before that is buried well beyond the reach of my grubby fingers.
Im thinking the best solution then is to figure out a switching system to toggle between the two amps. Does anyone know of a switching IC that can still pass audio without a power supply?
I'll dig into the links you provided!
Unfortunately the earliest I can get my hands on the audio from the aircraft is right at the amplified headphone output. Everything before that is buried well beyond the reach of my grubby fingers
.Im thinking the best solution then is to figure out a switching system to toggle between the two amps. Does anyone know of a switching IC that can still pass audio without a power supply?
OK, so if you toggle between two amps then what is the control signal to tell them to change over?
There are such things as solid state relays which we usually make up from two power FET's and a 'Photo Voltaic Coupler' which is like an opto isolator but one that generates a DC voltage at its output. That can turn the FET relay on.
The relays are (but I can't begin to think of a part number) available as low power versions in IC packages (8 pin I think).
So a control signal (logic 1, logic 0) could be used... and could it done without a power supply... hmmm maybe depending on what the actual signals are. I'm thinking you could use the relay to 'short' one of the feeds... obviously pad it out with a resistor so as not to short the amplifier totally.
Do-able perhaps.
There are such things as solid state relays which we usually make up from two power FET's and a 'Photo Voltaic Coupler' which is like an opto isolator but one that generates a DC voltage at its output. That can turn the FET relay on.
The relays are (but I can't begin to think of a part number) available as low power versions in IC packages (8 pin I think).
So a control signal (logic 1, logic 0) could be used... and could it done without a power supply... hmmm maybe depending on what the actual signals are. I'm thinking you could use the relay to 'short' one of the feeds... obviously pad it out with a resistor so as not to short the amplifier totally.
Do-able perhaps.
Mooly,
Yeah, that's the trick. Im picturing a solution that's basically the digital equivalent of a momentary A/B selector switch. Have the aircraft's audio on the 'normally closed' connection, and the Bluetooth audio on the normally open connection.
When the Bluetooth is active have the microcontroller pull the switch high so it passes the music to the headset, when it's low, it passes the aircraft audio. Use a peak detector or some other circuit to automatically pull the switch low when it detects audio from the intercom. In theory, if I can find the right switching IC that should be relatively fool proof once it's tested. But it's going to depend on getting the right switch/relay.
PRR,
Absolutely, that's why I decided to come ask the experts rather than fool around with it myself it's important to get it right and really test it before flying with it.
That in mind, I was planning on adding an 'absolute' bypass switch to it. A DPDT type toggle switch that would just pass the aircrafts audio directly without any of my (potentially unreliable) circuitry in the way. That way if something goes wrong a flip of a switch removes all my gear from the audio path. That's even a step above the commercial solution I already have.
Thanks a ton guys!
Yeah, that's the trick. Im picturing a solution that's basically the digital equivalent of a momentary A/B selector switch. Have the aircraft's audio on the 'normally closed' connection, and the Bluetooth audio on the normally open connection.
When the Bluetooth is active have the microcontroller pull the switch high so it passes the music to the headset, when it's low, it passes the aircraft audio. Use a peak detector or some other circuit to automatically pull the switch low when it detects audio from the intercom. In theory, if I can find the right switching IC that should be relatively fool proof once it's tested. But it's going to depend on getting the right switch/relay.
PRR,
Absolutely, that's why I decided to come ask the experts rather than fool around with it myself
it's important to get it right and really test it before flying with it.That in mind, I was planning on adding an 'absolute' bypass switch to it. A DPDT type toggle switch that would just pass the aircrafts audio directly without any of my (potentially unreliable) circuitry in the way. That way if something goes wrong a flip of a switch removes all my gear from the audio path. That's even a step above the commercial solution I already have.
Thanks a ton guys!
What You need is a ducking / talk-over circuit like those built in DJ mixers or supermarkets for announcing promotions.
I presume You can steal power from the aircraft.
This will give You some ideas.
Project 183
There are also automotive head-units which feature inputs for connecting phones & voice navigation systems.
There is usually a pin on the connector activated by some kind of voltage.
I presume You can steal power from the aircraft.
This will give You some ideas.
Project 183
There are also automotive head-units which feature inputs for connecting phones & voice navigation systems.
There is usually a pin on the connector activated by some kind of voltage.
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I would be surprised if the helicopter didn't have a suitable audio mixing unit fitted as standard equipment, as this is what they are designed for.
I suggest you speak to a avionics technician who is familiar with the helicopters audio mixing system. You may find there is a audio input suitable for your bluetooth device. This is the same method to route and mix audio from multiple radio communication transceivers for the aircrew.
I suggest you speak to a avionics technician who is familiar with the helicopters audio mixing system. You may find there is a audio input suitable for your bluetooth device. This is the same method to route and mix audio from multiple radio communication transceivers for the aircrew.
Indigo, PRR,
You're both entirely correct, the intercom systems DO have auxiliary inputs for additional radios and receivers. They even make aviation grade Bluetooth receivers and radios that can be installed in the panel. The problem is the price tag. For something like that to be permanently mounted inside the aircraft its a non-trivial expense. You're likely talking about a low five figure price tag per aircraft once you factor in aircraft down time and AME salaries. Dropping that kind of cash so a bored pilot can listen to an audio book while he flies is unfortunately out of the question.
My goal was to create a personal solution. Something that's integrated directly into my gear, only I can hear, and works no matter what aircraft I hop into.
Being essentially ignorant of audio electronics, I was curious if there was some elegant way to mix amplified signals without creating a mess. It doesn't really sound like that's possible in a simple, reliable way. There are some commercial solutions, but as with most things in aviation, you really end up throwing a lot of cash at simple problems. I already put a ton of money into one commercial solution and it really doesn't stand up.
MAAC0,
Ducking circuit! Cool, thanks for the link I'll do some research and see if I can find a circuit that works in my application!
You're both entirely correct, the intercom systems DO have auxiliary inputs for additional radios and receivers. They even make aviation grade Bluetooth receivers and radios that can be installed in the panel. The problem is the price tag. For something like that to be permanently mounted inside the aircraft its a non-trivial expense. You're likely talking about a low five figure price tag per aircraft once you factor in aircraft down time and AME salaries. Dropping that kind of cash so a bored pilot can listen to an audio book while he flies is unfortunately out of the question.
My goal was to create a personal solution. Something that's integrated directly into my gear, only I can hear, and works no matter what aircraft I hop into.
Being essentially ignorant of audio electronics, I was curious if there was some elegant way to mix amplified signals without creating a mess. It doesn't really sound like that's possible in a simple, reliable way. There are some commercial solutions, but as with most things in aviation, you really end up throwing a lot of cash at simple problems. I already put a ton of money into one commercial solution and it really doesn't stand up.
MAAC0,
Ducking circuit! Cool, thanks for the link I'll do some research and see if I can find a circuit that works in my application!
I suppose you know there is a thing already:
Pilot BluLink (Helicopter Plug)
Choice of (costly) plugs. More buttons than you need for eBook.
The rig must fail-safe and preferably no-hands. The BlueTooth receiver probably has more power than the headset needs (I've driven dozens of David Clarks on a part-Watt amplifier). If we impress eBook audio across say a 10 Ohm series resistor, about the worst can happen is a constant 1dB loss of volume (which I assume can be made-up in the aircraft radio). Even this can be taken-out by shorting it. Another 10r protects the eBook source.
That's not the book in my eReader today, but I like this cover better.
Pilot BluLink (Helicopter Plug)
Choice of (costly) plugs. More buttons than you need for eBook.
The rig must fail-safe and preferably no-hands. The BlueTooth receiver probably has more power than the headset needs (I've driven dozens of David Clarks on a part-Watt amplifier). If we impress eBook audio across say a 10 Ohm series resistor, about the worst can happen is a constant 1dB loss of volume (which I assume can be made-up in the aircraft radio). Even this can be taken-out by shorting it. Another 10r protects the eBook source.
That's not the book in my eReader today, but I like this cover better.
Attachments
Indeed, I bought myself a Blulink with my helmet. That's the commercial solution I've been so disappointed by and what's pushed me to explore DIY options.
I don't want to bash Blulink or pilot-USA, so let's just say the blulink is highly optimized for making phone calls, not reproducing music. After investing ~$400CAD in getting it all working I would describe the audio quality as roughly equivalent to a recordable greeting card. It does work, and it does do a good job with making phone calls. But even spoken word content is marginal for real listening.
Perhaps I should just accept that as good enough for the time being. Haha. I just know there has got to be a better solution out there.
Thanks for the circuit diagram. I'm going to order a cheap intercom unit to start testing circuits on. That way I can really test everything out before plugging it in to a real helicopter. I totally agree with you about the need for safety and simplicity. I really appreciate you taking so much time to discuss this with me, it's given me a ton of things to learn about and explore.
I don't want to bash Blulink or pilot-USA, so let's just say the blulink is highly optimized for making phone calls, not reproducing music. After investing ~$400CAD in getting it all working I would describe the audio quality as roughly equivalent to a recordable greeting card. It does work, and it does do a good job with making phone calls. But even spoken word content is marginal for real listening.
Perhaps I should just accept that as good enough for the time being. Haha. I just know there has got to be a better solution out there.
Thanks for the circuit diagram. I'm going to order a cheap intercom unit to start testing circuits on. That way I can really test everything out before plugging it in to a real helicopter. I totally agree with you about the need for safety and simplicity. I really appreciate you taking so much time to discuss this with me, it's given me a ton of things to learn about and explore.
Indiglo,
That's certainly part of the problem, But there are also some electrical oddities in the design. Issues like the right channel being mixed onto the left channel at half the volume and some sort of filtering that further degrades quality. Like I said I really don't wanna bash the blulink too hard since the company does good work. But there is certainly room for improvement.
altec9440,
It depends on the class of aircraft you fly and the kind of work you do. I won't get into the details because it's a little off-topic and the specifics are dependant on circumstance. But for us, anything that is not permanently mounted to the aircraft is up to the pilot's discretion.
That's certainly part of the problem, But there are also some electrical oddities in the design. Issues like the right channel being mixed onto the left channel at half the volume and some sort of filtering that further degrades quality. Like I said I really don't wanna bash the blulink too hard since the company does good work. But there is certainly room for improvement.
altec9440,
It depends on the class of aircraft you fly and the kind of work you do. I won't get into the details because it's a little off-topic and the specifics are dependant on circumstance. But for us, anything that is not permanently mounted to the aircraft is up to the pilot's discretion.
A rude, but dirty cheap solution is using some audio transformers:
Antique Radio Forums • View topic - Stereo to Mono Adapter
150/150ohm, half watt transformers will do the trick, but:
-They are bulky, fragile, heavy
-Not hi-fi
-Not EMC compliant
So: not so good for aviation use.
So, in my ultralight aircraft, I would add an aviation intercom, it's the perfect tool for our purpose.
Antique Radio Forums • View topic - Stereo to Mono Adapter
150/150ohm, half watt transformers will do the trick, but:
-They are bulky, fragile, heavy
-Not hi-fi
-Not EMC compliant
So: not so good for aviation use.
So, in my ultralight aircraft, I would add an aviation intercom, it's the perfect tool for our purpose.
I'm not sure how your helmet works, but is it possible to change out the headset you use with it? Or to get a new helmet to fit a different headset? Because the Bose A-20, David Clark DC One-X, and Lightspeed Zulu all seem to have the exact feature you're looking for. You can hook up your phone with bluetooth or aux, and it will mute or decrease volume (your choice) your phone audio when you have incoming radio comms. I'm sure that at least the Bose would have pretty good quality for music audio as they make excellent music-centric headphones as well.
I also just found a video for a DIY Bose A-20 helmet install. Again I don't know anything about aviation helmets, just that those headsets would seem to solve your dilemma if you're able to use them.
I also just found a video for a DIY Bose A-20 helmet install. Again I don't know anything about aviation helmets, just that those headsets would seem to solve your dilemma if you're able to use them.
Dear all,
Hope to find you well.
I am new here. I am a comercial pilot and also work with program languages.
At the moment I am developing one interface, similar to that one that was discussed here, but with a different propose.
However, I am having the same problem that creates this topic, mixing amplified signal or create a true bypass line with some IC.
Below I attached two photos from the Bose A20, where is possible to see on the top 4 big components combine with some transistors?? that make exactly the same.
What I pretend is exactly the same, maintain a safe amplified line to my headset and mix the audio from the interface. Or create IC switch that changes the input from both signals.
Did you find a working solution for this?
It worked with transformers for me, but I want to avoid them, so this possibility is excluded.
Many thanks to everyone!
Hope to find you well.
I am new here. I am a comercial pilot and also work with program languages.
At the moment I am developing one interface, similar to that one that was discussed here, but with a different propose.
However, I am having the same problem that creates this topic, mixing amplified signal or create a true bypass line with some IC.
Below I attached two photos from the Bose A20, where is possible to see on the top 4 big components combine with some transistors?? that make exactly the same.
What I pretend is exactly the same, maintain a safe amplified line to my headset and mix the audio from the interface. Or create IC switch that changes the input from both signals.
Did you find a working solution for this?
It worked with transformers for me, but I want to avoid them, so this possibility is excluded.
Many thanks to everyone!
