Have a question regarding an audio recording issue in smal aircraft *SOLVED*

[dacman]

I've had exactly the same problem on my PA28, using a GOPRO and a simple cable of my own making. Never did get to the bottom of it, but it's not bad enough to make me pull my finger out to resolve it, but I would be interested in any positive resolutions.

Do you know whether or not your PA38 is using resistive spark plugs?

[John Coloccia]

Aviation plugs (most popular being Champion and Tempest if I remember correctly) generally have a built in resistor, but it's pretty small...around 1k or so.  If you start getting large, you not only get poor performance but you risk damage to the magnetos.

[Howardlong]

I've had exactly the same problem on my PA28, using a GOPRO and a simple cable of my own making. Never did get to the bottom of it, but it's not bad enough to make me pull my finger out to resolve it, but I would be interested in any positive resolutions.

Do you know whether or not your PA38 is using resistive spark plugs?

That's beyond my pay grade, but this is a replacement plug:

http://www.skygeek.com/rhm40e.html

[John Coloccia]

The "R" in RHM40E stands for "resistor". 

[Howardlong]

The "R" in RHM40E stands for "resistor". 

See, I told you it was above my pay grade!

Not a day goes by when you learn something new.

[LabSpokane]

When I first responded, I hadn't listened to the audio clip. I agree that this isn't a clipping problem.

I think what you have is common mode interference.  The audio output from your intercom is unbalanced audio, which means it will pick up the common mode interference like the spark plugs. A high quality, shielded cable that has the braid grounded at either the airframe or camera side only might do the trick.

If this interference is coming from your intercom, you might try a different one.  If you're using a  "dongle" intercom for lack of a better term, the quality on those varies wildly. If someone has a DC portable intercom, that would be a good place to start.

[Synoptic]

Good news !

I managed to get very acceptable results. I need to use both an attenuator and an isolation transformer. The attenuator will get rid of a small portion of the whining and the isolation transformer will help reduce by a lot the whining and the power supply I use to power the camera.

For the attenuator part, I have a variable attenuator that also acts as a high pass filter. Work good. I cal also use my modified ebay cable in which I added a voltage divider and it works great too.

Do anyone know of a usb cig lighter adapter that has a built in isolation transformer that has dual output and one output capable of more than 1 amp ( for iPad ).

My spark plugs are champion REM40E on a Lycoming O-360 180hp.

[BradC]

Do anyone know of a usb cig lighter adapter that has a built in isolation transformer that has dual output and one output capable of more than 1 amp ( for iPad ).

I've never seen an isolated one, and I've seen plenty that are regulated on the -ve rail (plug one end of the iPod into adapter, the other end into the Aux input of the stereo and the smoke came out). Be careful.

[Synoptic]

I've seen plenty that are regulated on the -ve rail (plug one end of the iPod into adapter, the other end into the Aux input of the stereo and the smoke came out). Be careful.

What is the layman's equivalent of -ve ?

[BradC]

I've seen plenty that are regulated on the -ve rail (plug one end of the iPod into adapter, the other end into the Aux input of the stereo and the smoke came out). Be careful.

What is the layman's equivalent of -ve ?

Gnd. So they pass the +12v right through and regulate the GND up until it sits at about 7v (7+5=12). So when I plugged the iPod in the headphone ground which was properly earthed at the radio pulled the ipod ground down to 0V, which meant the 5v input was seeing 12v (*pop fizz*).

[Howardlong]

I was sceptical about the attenuator bit, but now I've thought about it and listened a little more closely I think what's happening is that the AGC kicks in when no one's talking, amplifying the alternator whine. When there's people talking, the AGC backs off and there is no alternator whine. By adding an attenuator, the AGC dynamic range is effectively reduced.

The isolation transformer is a good idea too. Time to get the hot pointy thing out.

[Synoptic]

I was sceptical about the attenuator bit, but now I've thought about it and listened a little more closely I think what's happening is that the AGC kicks in when no one's talking, amplifying the alternator whine. When there's people talking, the AGC backs off and there is no alternator whine. By adding an attenuator, the AGC dynamic range is effectively reduced.

The isolation transformer is a good idea too. Time to get the hot pointy thing out.

Yes, I also had the strong feeling that my camcorder used some sort of AGC. In the attenuator, the isolation transformer and the ebay cable, I have added 300k resistor in serie to really try to kill the signal from the source.

Damn AGC... I think Go Pros don't have AGC.

Rainy for next two days, but will go flying as soon as weather clears up !

[Howardlong]

By the way I'm not certain it's all alternator whine. When you switched off your engine, that sounded a more like a gyro, probably the turn coordinator as the other steam gauges are usually on the engine vac system assuming your a/c is of a certain age.

[Synoptic]

By the way I'm not certain it's all alternator whine. When you switched off your engine, that sounded a more like a gyro, probably the turn coordinator as the other steam gauges are usually on the engine vac system assuming your a/c is of a certain age.

Whining change pitch with RPM. Maybe there are two whining, gyro + Engine.

[Synoptic]

Hi, here's the end result. MUCH MUCH better :

https://www.youtube.com/watch?v=XLfvxE0US1A&feature=youtu.be&spfreload=10

[dentaku]

That sounds good. It has the typical high pitched squeal but nothing annoying at all.

Does it ever get confusing over already difficult to understand radio transmissions to figure out right away if the person is speaking French or English?
I have a hard time understanding certain Québécois accents anyway

[John Coloccia]

That definitely sounds normal.  What was the ultimate solution?  The isolation transformer?

[Synoptic]

Solution was to combine both the isolation transformer, in series 300ohms resistor and a voltage divider.  The side tone is still not perfect (when I talk to ATC) but its better than nothing.

Question remains : why is my intercom so damn noisy. On other airplane with same camera it does not do that and the headset have good sound. Could a balun help even more ?

[jc101]

Solution was to combine both the isolation transformer, in series 300ohms resistor and a voltage divider.  The side tone is still not perfect (when I talk to ATC) but its better than nothing.

Question remains : why is my intercom so damn noisy. On other airplane with same camera it does not do that and the headset have good sound. Could a balun help even more ?

Do you use a cheap plug in USB charger in this plane?  Some of them generate horrendous RF and conducted noise which can screw things up quite nicely in random areas.  If so, try just unplugging that and see if things improve.

[John Coloccia]

Solution was to combine both the isolation transformer, in series 300ohms resistor and a voltage divider.  The side tone is still not perfect (when I talk to ATC) but its better than nothing.

Question remains : why is my intercom so damn noisy. On other airplane with same camera it does not do that and the headset have good sound. Could a balun help even more ?

I still think the ultimate problem might be tracked to how the jacks are mounted, or something like that.  You can get a lot of ground noise through the airframe.

[Synoptic]

I still think the ultimate problem might be tracked to how the jacks are mounted, or something like that.  You can get a lot of ground noise through the airframe.

Maybe, but with certified airplanes, you are not allowed to do much more than what is specified in the installation manuals of every equipment. I wonder what I could do.

[Synoptic]

Do you use a cheap plug in USB charger in this plane?  Some of them generate horrendous RF and conducted noise which can screw things up quite nicely in random areas.  If so, try just unplugging that and see if things improve.

Yes I do, unplugging the USB removes some of the noise, but it is not the prominent one. It is really the Alternator Whine + all Avionics + strobe lights that we hear.

Still asking, whould a balun or simply coiling the audio wire through a ferrite would help ?

[Zero999]

Your isolation transformer is already acting as a balun.

Ferrite beads may help but only if it's RF which is being demodulated by the audio amplifier.

[John Coloccia]

I still think the ultimate problem might be tracked to how the jacks are mounted, or something like that.  You can get a lot of ground noise through the airframe.

Maybe, but with certified airplanes, you are not allowed to do much more than what is specified in the installation manuals of every equipment. I wonder what I could do.

Oh, I know this well.  That's why I was into building experimental aircraft...

Your A&P (or whatever they call airplane mechanics where you live) has latitude to make repairs, upgrades, etc. He probably can't go into the radio/intercom itself, you can't here without an FCC license, but I don't see any reason he shouldn't be able to have a look at the installation, and make modifications as long as they still fall within the installation guidelines.

[Synoptic]

I still think the ultimate problem might be tracked to how the jacks are mounted, or something like that.  You can get a lot of ground noise through the airframe.

Maybe, but with certified airplanes, you are not allowed to do much more than what is specified in the installation manuals of every equipment. I wonder what I could do.

Oh, I know this well.  That's why I was into building experimental aircraft...

Your A&P (or whatever they call airplane mechanics where you live) has latitude to make repairs, upgrades, etc. He probably can't go into the radio/intercom itself, you can't here without an FCC license, but I don't see any reason he shouldn't be able to have a look at the installation, and make modifications as long as they still fall within the installation guidelines.

Yes, but at 80$/hour, I'll pass on this non-necessary fix