Using your cell phone to record cockpit audio

Gents,

I decide to take the plunge and do some GoPro video and record cockpit audio separately and planned to use an old Samsung smart phone as my digital voice recorder.

I have a Garmin GTR200 radio that has audio output that typically goes to an audio panel but the manual says it can be used for external recording. The manual says the audio out (called receiver out in the manual) has an impedance of 600 ohms.

Yesterday I installed an audio out jack (isolated from the airframe ground) in my panel wired to the GTR200. I made an adapter cable to interface with the 4 ring connector the cell phones used. I did some test recording and wasn't happy with the results.

I tried recording on two of my phones, my S5 (currently the phone I use) and the Fascinate (5+ years old). I used the Samsung recording app on both phones. I had nothing else powered up on in the plane except the radio.

The recording on the Fascinate had a lot of what I call static or background noise. When I recorded the ATIS it was even worse. There was no background noise present in my headsets while recording. My voice (testing 1,2,3 etc) sounded very tinny.

The recording on the Samsung S5 had static and a small amount background noise. It too sounded tinny.

Doing some research on the Samsung phone mike interface I found that the phones expect to see an mike impedance of 1200-1600 ohms. Since the output impedance from the Garmin GTR200 is only 600 ohms, would the impedance mismatch cause the noise and tinny recording?

Since there are only two pins for the mike input to the phone is there a polarity to be concerned about?

Any insight would be appreciated.

Not sure about the impedance question, but I got this adapter from nflightcam http://nflightcam.com/products/smartphone-audio-cable and it works great with my droid turbo.

If you are recording video with it, the stock camera app won't recognize the external mic, but I use an app called Cinema FV-5 that does let me use it.

Chris

Yes, impedance mismatch can distort the audio and make it sound as you describe. Difficult to tell if that's the sole problem, however. Could also be a mismatch between levels even if the impedance match is good i.e. too much volume coming out of the radio.

I'm curious as to why you wouldn't record audio into the GoPro directly?

I too am confused as to why you would not just record the audio on the GoPro. I do this all the time and the audio is nice and clear. I use the following cable:
http://www.sportys.com/pilotshop/vi...-and-accessories/gopro-hero4-audio-cable.html

Many reasons why

To answer a few questions posted:

1. I want to be able to record audio of a flight for review later. To improve my radio work. So an independent recorder separate from the GoPro is needed. Also if the GoPro is mounted outside the cockpit I can capture the audio recording.

2. I believe the headset adapter to smartphone either taps into one side of the headset (right or left). If it combines the two then it defeates the stereo function of the headset. The GTR200 has some really nice 3D audio effects and tapping into only one side of the headset would not record volumes at the correct levels (those flying with the Garmin with 3D audio should know what I am taking about).

3. Less mess in the cockpit with a dedicated audio out jack in the panel.

I'd probably disagree with Brad about the impedance 'mismatch'. If it went the other way, with a 1200 ohm source feeding a 600 ohm load, it could cause low signal level & high distortion, but at audio frequencies in solid state circuits, the source impedance is almost always significantly lower than the load. (The output impedance of a radio headphone output or an intercom is only a few ohms. There's typically a 500-600 ohm resister in series with the output, but that's more to protect the source from shorted output wiring.

If your source has an audio transformer in its output (extremely unlikely, these days), it may cause more of an issue.

Lots of unknowns.
1st, it's difficult to quantify 'static'; one person's static is another's 'distortion'. Hard to know what you're hearing, without actually hearing it. Distortion is relatively easy to diagnose; static can have *lots* of causes (usually external, but not always). Have you tried recording a short segment with the adapter plugged in the phone but *not* plugged into the radio jack? Is the static still there? What's it sound like using an external mic that's intended to be used with the phone? What's it sound like feeding it audio from your home stereo?

2nd is signal *level*. If the output is 'line level' (it almost certainly is) and the input to the phone is a mic input, you'll be severely overdriving the mic preamp in the phone. Check the docs on the phone, & see if they supply both mic & line level inputs in that jack. Note that even if they do, the GTR200's output may still be a bit 'hot', because consumer electronics 'line level' is typically at least 6 db lower that 'real' line level as defined in pro grade audio gear and avionics.

Charlie

edit, unrelated to troubleshooting: Others have achieved what you're after by simply placing a small mic inside the earcup of their headphones. It will get both the sidetone of your transmissions and reception audio from your radio.

The first drawing in this thread http://www.vansairforce.com/community/showthread.php?t=111560&highlight=Bluetooth+audio shows how to interface high level audio into a cell phone.

On my first flight I used the earbuds/mic accessory that came with my Samsung Galaxy 4. I stuck the mic part inside the ear-seals of my headphones then just used the sound recording app that came with the phone. It came out ok although I had a difficult time matching the audio with the edited video....You should be able to get an adapter from Radio Shack to go from your passenger headphone out to your phone for a more permanent solution. I was so busy with other things, I just took the easy way out.