How to add Bluetooth to a Dynon intercom system

[RaymondJ]

Hi all,

I wanted to share how I added Bluetooth capability to my plane, despite having only the Dynon intercom installed (the SV-INTERCOM-2S).

Purpose and goals

The things I wanted out of this setup were:

• Ability to playback music from my phone (easy!)
• Ability to call via my phone -- and have the other side hear me without pushing the PTT or any strange shenanigans (hard!)

For context, here's a picture of my panel, in a Rutan VariEze. The panel is fairly small, and I use it for light IFR operations fairly often (enough to punch through the marine layer and shoot an approach now and then).

Over the past 3 years of having the SV-INTERCOM-2S, I've loved the ability to have stereo music input, but having an additional wire in an already cramped space proves incredibly annoying. Additionally, it got in the way of picture taking: if I saw something good out the window, I often had to unplug my phone, lift it up, take the picture, then plug it back in.

Lately, I've been flying IFR more out of uncontrolled fields / at night when the tower's closed. Calling approach on speakerphone, one side of my headset off, was doable but got kind of old quick.

My panel's size & setup makes it hard to shoehorn in a more "complete" audio panel; the typical Garmin / PS Engineering / Trig units just won't fit! (Yes, there's some stuff I could tweak to get more space -- like replacing my Trig TT22 with the SV-XPNDR-261, but I'm not ready for that yet.)

Approach

There are many, many Bluetooth-to-audio out converters available for sale (usually for folks who have Aux In jacks on their cars), but very few that match the requirements needed here:

• No integrated battery (want to power off of ship power)
• Mic / line-in input (most have integrated microphones)

I ended up using a Tinysine TSA6015 Bluetooth module:

The TSA6015 is very much a developer's sort-of-module: no box is provided, and you need customize the firmware settings a bit in order to get it to work well in this scenario, so buyer beware. This setup lies squarely in the "experimental" part of "experimental avionics"!

For mounting, I used some 3M Dual-Lock fasteners (like Velcro, except both sides are symmetric and don't wear out) to place it on the underside of the SV-INTERCOM-2S.

For power, the TSA6015 requires +5V DC. I grabbed the USB +5V output (and ground) from the 7" HDX display, on the main 37-pin connector.

Overall, the wiring diagram looks like this:

The microphone input to the TSA6015 is picked off of the copilot's right channel to minimize interference in case anything fails: if the module dies/shorts out, it should only result in the passenger's right ear losing signal, at which point they can turn their headset to mono to keep on trucking in both ears.

 

[RaymondJ]

continued

Audio playback worked out of the box, but I needed to change some firmware settings in the TSA6015 in order to get everything to really work. Follow the guide on TinySine's site to do so; you'll need to also buy the Qualcomm programmer as well as the TinySine Tag-Connect line kit: https://www.tinyosshop.com/index.php?route=information/news&news_id=96.

• Change to single mic mode: In order for the line-in functionality to work, the "HFP Plug-in" (hands-free profile plug-in) needs to be set to "CVC HandsFree 1-Mic". This is done via the ADK Configuration tool, under the "Voice" configuration menu. If this isn't done, folks on the phone won't be able to hear you at all.
• Reduce input gain (optional): The output signal to the TSA6015 might be too high, depending on where you like to have the SV-INTERCOM-2S volume knob set. This shows up as clipped / compressed-sounding audio on the phone. To turn down the TSA6015 input gain, use PSTool and search for input gain settings; they can be turned way down.
• Disable LEDs (optional): Since I mounted the TSA6015 upside-down on the bottom of my intercom, it lights up the cabin with a bright blue LED -- not great for night flying. I turned off the LEDs using the ADK Configuration tool; go to "User Interfaces -> LED" and change the "LED A Mapping" and "LED B Mapping" to "N/A" for the Default Pattern. I also deleted most events since I didn't feel they were relevant.
• Rename the Bluetooth device (optional): Out of the box, the TSA6015 shows up as a Bluetooth headset with the name "TSA6015". Lame. I changed the device name to my tail number.

For the audio engineers out there, here's how this system avoids feedback loops:

• Local-side loop: The TSA6015, by default, does not produce local sidetone when in call mode, so local squeal is not an issue.
• Remote-side loop: The TSA6015 has an acoustic echo canceller, which gets rid of the the remote side contribution to the audio bus. (This is a particularly aggravating case, since the TSA6016 mic input is hooked right up to the mixing bus output of the SV-INTERCOM-2S. It's akin to the worst case of placing the speaker right next to the microphone in a speakerphone setup, but the echo cancellation algorithm handles it nicely.)

(And I know I really shouldn't be mixing power/signal grounds like this. The system doesn't add any additional noise once Bluetooth is connected, though there's a slight hum while it's trying to pair. I'm currently investigating the best way to isolate +5V for this setup, and it's possible, though likely overkill, to isolate the microphone input using a wideband isolation transformer.)

Results

I can play music from my phone, no problem! It's really nice not to have to worry about that additional wire in my already-snug cockpit. The phone links back up whenever I turn on the master and the HDX display has booted, so it's been very seamless.

Phone calls via my headset also work great. While I haven't needed to call approach for an IFR clearance yet, the test calls I've done seem to work well. As a benefit, if you have cell signal while flying, this will also filter ATC/audio alerts into the call -- which allows the other side to figure out why you've suddenly stopped talking. (Folks on the phone cannot talk to ATC, however, as PTT takes only from the associated station microphone.)

Bugs & future work

Like all experimental efforts, there are still some bugs that I'm working on. I get strong interference when I transmit on certain frequencies (like a loud feedback squeal); I'm fairly certain this is due to my plane's construction (fiberglass, no self-shielding) and proximity of the TX antenna. I'm working through adding ferrites at key locations and also adding a ground strap between COM radio chassis and the intercom chassis (as recommended in this post: https://forum.flydynon.com/threads/squeal-when-push-to-talk.5237/). I'll report back when I get that eliminated. To mitigate this, I turn off the intercom (via pressing the volume knob) when I need to transmit on affected frequencies, and this eliminates any squeal.

I'm also based in Socal, where the radio can get quite busy, totally blocking out any music. I'm working on reverse engineering the SV-INTERCOM-2S to add a switch to prevent the muting action; it will be the responsibility of the pilot to music volume so that this doesn't interfere with ATC. (The design decision Dynon made here is correct to maximize safety, but I would like to listen to some trance and instrumental tracks without totally missing it all thanks to ATC...)

Let me know if you all have any questions, I'm happy to answer as best I can. The majority of the mod was in finding the right Bluetooth module & settings; the wiring & mounting itself I thought was annoying but not too hard.

 

[RaymondJ]

Fixed the interference issue -- in my zealous attempt to eliminate any sorts of radio interference, I'd been placing ferrites over cables and cable bundles, including the audio wiring. (Something like these, though I forget the exact ferrite blend that I purchased: https://palomar-engineers.com/tech-support/tech-topics/ferrite-tutorials/-p82737002).

Ferrites are supposed to present a higher impedance on wires in the RF spectrum, so that any RF trying to get in has a harder time doing so. However, when you wrap them over a bundle, they also increase the coupling between the lines themselves (it's effectively a half-turn or n-turn transformer) -- and for sensitive analog signals, this can be a problem.

I removed the ferrite I placed over my COM radio cable bundle (which included COM headphone output and COM mic together) and the problem went away! It didn't really need to be there after all, I'd placed it as a "preventative measure" and it definitely hurt me more than I thought. I suspect, though can't confirm, that having the headphone output line (which possibly has some RF leaking out of it), in a ferrite next to the microphone input line (very sensitive), resulted in a transmit-to-input feedback path.

Why did the feedback only happen when the SV-INTERCOM was on? I suspect it's something to do with the input impedance of the intercom unit -- probably higher impedance (normally a good thing!) than the mic input of my radio.

This setup now works like a charm (and should work for most folks without ferrites at all, that's just something specific to my setup). And for anyone playing with ferrites, be careful with their application!

 

[Rhino]

Curious where you got the idea to use pins 16 and 17 on the HDX for power. The installation guide says not to connect those pins to anything. Could it be they have an amperage limit? In any case, for anyone not wanting to jumper off the DB37 connector, you can get a 12 volt to 5 volt converter pretty cheap ($10) on Amazon, with overvoltage, overcurrent, overtemp and short circuit protection.

https://www.amazon.com/BINZET-Converter-Regulator-Regulated-Transformer/dp/B00J3MHRNO

 

[vlittle]

For reference, here's how I did BT. From 2014

Vern