Have a question regarding an audio recording issue in smal aircraft *SOLVED* - Page 1
Forum section: electronics / beginners

author: Synoptic
author_rank: Contributor
timestamp: May 22, 2015, 02:20:47 am
content: Hi everyone ! I record all my flights using a Sony Action Cam AS30V (actually bought it in Sydney's airport when coming back from vacation).I have this interference, high pitched hissing sound, coming from the electrical system. The pitch will follow the engine RPM.I am directly feeding the camera's MIC input from the Intercom jack. My guess is that I need to match some sort of thing between the Intercom and the MIC input.I have come across this topic here :https://www.eevblog.com/forum/projects/modding-mic-input-to-take-active-output/, but this one talks about adding a MIC input to a camera that does not have one. Although the goal is to record intercom from an aircraft, too.What I know : My headsets have 300Ohm Speakers inside and I do not hear any hissing sound with them. I know a speaker is some kind of inductor and I am not good enough in electronic to understand the difference between an inductor's Ohm value vs a resistor's Ohm value.Here's an example of audio it records (WARNING, VERY IRRITATING)https://dl.dropboxusercontent.com/u/894769/Example.mp3The sound is present whether or not the camera is powered from the aircraft's 12v supply or running on its battery.I have tried a "recording cable for aircraft" without any success (I don't even know if the cable contains anything more than just a Y connection)My question is : What can I do to eliminate this unbearable sound ? It seems like I am the only one having this issue.

author: RJFreeman
author_rank: Regular Contributor
timestamp: May 22, 2015, 03:24:51 am
content: OK listening to your recording there seems to be both buzz/hum and a squeal.Quotewhich part of the intercom are you plugging into? is it the headphone output from the Intercom or are you trying to bridge the microphone input?what model etc is the Intercom?

author: Towger
author_rank: Super Contributor
timestamp: May 22, 2015, 06:33:09 am
content: A cheap audio isolation transformer from eBay should do the trick. The are designed for going between car radios and amps etc. They tend to come with phono plugs, so you will need to convert them to jack plugs.

author: Marco
author_rank: Super Contributor
timestamp: May 22, 2015, 06:59:10 am
content: I'm going to guess that signal is way too high voltage for the mic input, probably similar to line levels ... google for line to mic attenuator.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2015, 07:21:33 am
content: Since you gave us a sample of ONLY the noise (and not the noise relative to the SIGNAL) it is impossible to judge the signal-to-noise ratio.I agree with @Marco that the intercom signal is almost certainly much too "hot" for the mic input of any camcorder. You almost certainly need to attenuate the signal. But since you didn't give us any sample of the signal, again we can only guess.I strongly expect (from previous similar experience) that you will never be able to completely eliminate that background "whine" because of the nature of electrical design and installations typical in aircraft. Don't set your hopes to suppress it completely.Many TV production people have learned the trick of simply inserting one of their miniscule "lav" mics inside the ear-cup to "hear" everything that the crew hears with none of the "intrigue" of trying to hard-wire into the craft's comms system.

author: Synoptic
author_rank: Contributor
timestamp: May 22, 2015, 01:21:21 pm
content: Quote from: Towger on May 22, 2015, 06:33:09 amSorry, I forgot to mention that I did try that and it didn't change anything.

author: Synoptic
author_rank: Contributor
timestamp: May 22, 2015, 01:28:57 pm
content: Quote from: Richard Crowley on May 22, 2015, 07:21:33 amHere's a full good example (7 Mb File) :https://dl.dropboxusercontent.com/u/894769/SignalToNoise.mp3I plug the camera directly into the Intercom Jack, I do not use a Y cable. I have tried though without success nor change.I feel like the idea of having a signal too hot for the MIC input is a good contributor of the issue. Could I simply make a voltage divider ?Also, why I do not have this issue with my headset ? There's gotta be a way to solve this...

author: LabSpokane
author_rank: Super Contributor
timestamp: May 22, 2015, 02:55:54 pm
content: Quote from: Synoptic on May 22, 2015, 01:28:57 pmMIC and Line Audio are totally different levels. You need to cut the audio in level by 20dB or so.Primer on audio levels:http://www.ovnilab.com/articles/linelevel.shtmlPi Attenuator calculator:http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

author: Synoptic
author_rank: Contributor
timestamp: May 22, 2015, 03:18:22 pm
content: Quote from: LabSpokane on May 22, 2015, 02:55:54 pmInteresting. Now I need to find the MIC in impedance my Sony expects. How do I find this ? Is it safe to say that 10kOhm is in the neighbourhood ?Using 10kOhm as output and 300Ohm as input, I get a minimum of 21.xx dB attenuation possible.Am I on the right track for experiment ?

author: dacman
author_rank: Frequent Contributor
timestamp: May 22, 2015, 03:38:10 pm
content: Are you using resistive sparkplugs in the aircraft engine?

author: Synoptic
author_rank: Contributor
timestamp: May 22, 2015, 03:39:58 pm
content: Quote from: dacman on May 22, 2015, 03:38:10 pmI don't know, but I do not have the choice as I am using the onces that are certified for it.Again, no issue with my headsets so I don't see the need of changing anything on the aircraft.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2015, 03:45:54 pm
content: The mic input impedance is not really a major factor. The primary issue is the amplitude (voltage) of the audio signal. Yes, a simple attenuator is what you need.The new signal + noise sample demonstrates that you are massively overloading the mic input with probably a linevel signal (something on the order of 1 volt) where the mic input is expecting something between 100 and 1000 times lower than that!That is exactly why they make attenuators. I have made many such gadgets with nothing more than an input and output jack and a simple potentiometer (volume control). You could even take your existing cable, chop it in half, and insert a pot to knock down the level.Another problem demonstrated by the signal + noise recording is that low frequencies are causing a lot of interference as well. I would certainly use a high-pass filter to eliminate those low frequencies which are nothing but noise and do not contribute to any intelligible speech.You claim that you don't hear the "engine whine" in your earphones, but I would bet that the reason is because the sound is masked by the acoustic sounds from the engine as you hear it. Of course the recorder is not hearing the acoustic sounds, only the electrical signal from the intercom system.You could try using a $5 "computer microphone" slipped inside your ear-cup. Professional production crews often use one of their little clip-on "lav" microphones slipped inside the ear-cup to capture the intercom sound without any direct connection. It is a rare example in the Real World where you get all three: better + faster + cheaper.

author: LabSpokane
author_rank: Super Contributor
timestamp: May 22, 2015, 04:04:17 pm
content: Quote from: Synoptic on May 22, 2015, 03:18:22 pmThat is the right direction. Don't worry too much about impedance matching. That was a big deal in the days of tubes, tail fins, and beehive hairdos but modern electronics are looking for essentially "voltage transfer". And before people dogpile on me I know that isn't a technically perfect explanation but it's how I used to teach it to techs and they understood it well enough.

author: Synoptic
author_rank: Contributor
timestamp: May 24, 2015, 02:30:30 am
content: Thanks for all your input. Here's a followup update on my project.* I did not test it in the plane yet *I have modified one of my cable to divide the input voltage by a lot, getting about 0.002v from a 1v input source.To test, I have hardwired my camera MIC input to an audio Line-Out (headphone) from my old trusty Roland MT-32. I have recorded the same thing with my modified cable and with a non modified straight cable.The left channel on the MP3 file is with the regular straight cable and the right channel is with the modified cable.Here's also what it looks like in Audiacity.https://dl.dropboxusercontent.com/u/894769/followup.mp3

author: LabSpokane
author_rank: Super Contributor
timestamp: May 24, 2015, 02:42:55 am
content: That's about 54 dB is attenuation which may be too much of a good thing. I'd start with 20-30 dB and see what happens.dB = 20*Log(V/Vref)

author: Synoptic
author_rank: Contributor
timestamp: June 04, 2015, 04:20:20 am
content: I went flying and tested. It's worst than before.I am really desperate now. I don't want to go and buy a small microphone to put in the earcup. I can't accept that there is now way of recording the audio when I see so many people on youtube doing it without difficulties and without anything more than what I have.Here's the video with audio to give you an idea. The signal to noise ratio is worst than before.You also can see the difference when I turn the main power off after stopping the engine.http://youtu.be/J2ohayfeCuo

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 07, 2015, 02:48:27 am
content: So you still have severe clipping from excessive audio amplitude.A proper attenuator would solve that problem. However.....You also have severe power supply noise on the audio.If you are not hearing that in your headphones, then it is caused by the way you are connecting to the aircraft comms circuits.It is possible that an isolation transformer could reduce (or possibly even eliminate) the power "whine".You can hear how the whine frequency drops after you power down. That is likely the APU winding down.Or alternator or rotary generator, or whatever they use in those kind of aircraft.Dunno what is your objection to a small microphone? The kind I am talking about cost US$ 1 on Ebay.You have already burned through an order of magnitude more $$ than that without success.http://www.ebay.com/itm/New-3-5mm-Mini-Studio-Speech-Mic-Microphone-w-Clip-for-PC-Desktop-Notebook-SU-/201191176283

author: Synoptic
author_rank: Contributor
timestamp: June 07, 2015, 12:36:31 pm
content: I bought the ebay microphone, but :Using a microphone has other issues. It will pickup ambiant noise (GA aircrafts lime mine are not really sound proofed. The headset serve two purposes, ear protection and audio). one could say that I could put the mic in an unused headset. Well then I need to buy a headset. one can say to put mic in the headset I'd wear, but this isn't practicable and could be distracting. When you fly, you do not want distraction.I think what I really need to do is to analyze the issue. Right now I don't have the full picture. All I know is :1. Use of isolation transformer on the audio feed does nothing on the sig/noise ratio and whining.2. Attenuator does nothing on sig/noise ratio and whining.3. Modified cable with voltage divider accentuate sig/noise ratio bringing more noise.4. Having the camera operate on battery has no effect on the signal/noise ratio and whining.5. The whining is always present. The hum we hear in the last vid could be caused indeed by my night equipment being turned on - I will put this aside for now. I want to put all my energy on the whining.Here are my next steps in no fixed order :A. Use a headset that also has a female 1/8 jack and see if it can act as a filter of some sort since I do not hear the whining in headsets.B. Use a dedicated recording device with a Line-in.C. Check if by anychance, the wiring of the audio in the aircraft is reversed. How can I verify if audio GND and audio SIG are correclty wired up ?.Quote from: Richard Crowley on June 07, 2015, 02:48:27 am

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 07, 2015, 01:28:43 pm
content: It is easy enough to use a cheap little DMM to check the polarity/grounding of the comms connection and wiring. It seems likely that the outer "sleeve" is connected to the airframe ground. But it is possible that the sleeve of the plug may NOT be connected to the airframe.I fail to see how sticking a little mic capsule under your ear-pad is "distracting". If you are that easily distracted, maybe you shouldn't be flying airplanes.Sticking the mic INSIDE the earpad gives it the same "sound" as what is reaching your ear. Including reducing the acoustic noise which is the major point of the heavy ear-cups.At this point, you may be better off simply buying a commercial product. There are adapters of many varieties available athttp://www.mypilotstore.com/

author: John Coloccia
author_rank: Super Contributor
timestamp: June 07, 2015, 01:40:16 pm
content: I don't hear any clipping at all. All of the voice audio sounds clear to me. What I do hear is classic alternator whine, and somehow it's making it to the connector you're using. You don't hear any of this except when you're recording? Next time you start up, do an experiment and plug your headset into whatever plug you're using for recording, and see if you can now hear it. That particular jack might just be grounded to the airframe, whereas the others might be floating.edit: I just listened to the last video you posted too. It's interesting that there is still a lot of noise with the engine off, and it sounds like you can hear the gyros spinning down too? What do you have for electric gyros in there other than a turn coordinator?Anyhow, one thing you can try is that you probably have an unused output somewhere on the intercom. Maybe talk to your A&P (or whatever they're called where you live), tell him what you're trying to do and the problems you're having, and maybe he can stick a proper line-out on your panel.

author: Synoptic
author_rank: Contributor
timestamp: June 07, 2015, 04:46:36 pm
content: Quote from: Richard Crowley on June 07, 2015, 01:28:43 pmAlready bought one and didn't help at all.

author: Synoptic
author_rank: Contributor
timestamp: June 07, 2015, 04:48:45 pm
content: Quote from: John Coloccia on June 07, 2015, 01:40:16 pmAh ! Someone that knows Aircraft !The only electrical Gyro I have is the turn coordinator.At this point I am going note how the wiring is made on the jacks I use and take a decision from there.I will report back here with my findings. Today is a good day to start the engine and make some tests.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 07, 2015, 05:27:46 pm
content: Quote from: Synoptic on June 07, 2015, 04:46:36 pmToo many unknowns to know whether it was even the right one, used in the right way, or whether there is actually something wrong with the wiring in that plane.Have you done any of these experiments in more than one aircraft?

author: Synoptic
author_rank: Contributor
timestamp: June 07, 2015, 06:00:42 pm
content: Quote from: Richard Crowley on June 07, 2015, 05:27:46 pmI haven't experimented on more than one aircraft, but other youtubers use the same wire as I do and they don'T have any issue. I'm guessing the trouble is either my camera or my plane audio system. I will go and analyse the intercom connections very soon and write down some notes.

author: Howardlong
author_rank: Super Contributor
timestamp: June 07, 2015, 06:42:42 pm
content: 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.http://youtu.be/7xjCqncO1QQ

author: dacman
author_rank: Frequent Contributor
timestamp: June 07, 2015, 07:38:09 pm
content: Quote from: Howardlong on June 07, 2015, 06:42:42 pmDo you know whether or not your PA38 is using resistive spark plugs?

author: John Coloccia
author_rank: Super Contributor
timestamp: June 07, 2015, 07:50:00 pm
content: 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.

author: Howardlong
author_rank: Super Contributor
timestamp: June 07, 2015, 07:54:56 pm
content: Quote from: dacman on June 07, 2015, 07:38:09 pmThat's beyond my pay grade, but this is a replacement plug:http://www.skygeek.com/rhm40e.html

author: John Coloccia
author_rank: Super Contributor
timestamp: June 07, 2015, 08:01:47 pm
content: The "R" in RHM40E stands for "resistor".

author: Howardlong
author_rank: Super Contributor
timestamp: June 07, 2015, 08:17:43 pm
content: Quote from: John Coloccia on June 07, 2015, 08:01:47 pmSee, I told you it was above my pay grade!Not a day goes by when you learn something new.

author: LabSpokane
author_rank: Super Contributor
timestamp: June 07, 2015, 11:07:24 pm
content: 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.

author: Synoptic
author_rank: Contributor
timestamp: June 08, 2015, 01:17:12 am
content: 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.

author: BradC
author_rank: Super Contributor
timestamp: June 08, 2015, 04:58:25 am
content: Quote from: Synoptic on June 08, 2015, 01:17:12 amI'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.

author: Synoptic
author_rank: Contributor
timestamp: June 08, 2015, 05:05:21 am
content: Quote from: BradC on June 08, 2015, 04:58:25 amWhat is the layman's equivalent of -ve ?

author: BradC
author_rank: Super Contributor
timestamp: June 08, 2015, 05:27:53 am
content: Quote from: Synoptic on June 08, 2015, 05:05:21 amGnd. 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*).

author: Howardlong
author_rank: Super Contributor
timestamp: June 08, 2015, 05:48:23 am
content: 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.

author: Synoptic
author_rank: Contributor
timestamp: June 08, 2015, 12:07:47 pm
content: Quote from: Howardlong on June 08, 2015, 05:48:23 amYes, 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 !

author: Howardlong
author_rank: Super Contributor
timestamp: June 08, 2015, 05:59:12 pm
content: 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.

author: Synoptic
author_rank: Contributor
timestamp: June 08, 2015, 06:01:14 pm
content: Quote from: Howardlong on June 08, 2015, 05:59:12 pmWhining change pitch with RPM. Maybe there are two whining, gyro + Engine.

author: Synoptic
author_rank: Contributor
timestamp: July 09, 2015, 04:49:07 pm
content: Hi, here's the end result. MUCH MUCH better :https://www.youtube.com/watch?v=XLfvxE0US1A&feature=youtu.be&spfreload=10

author: dentaku
author_rank: Frequent Contributor
timestamp: July 09, 2015, 11:29:02 pm
content: 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

author: John Coloccia
author_rank: Super Contributor
timestamp: July 10, 2015, 01:14:12 am
content: That definitely sounds normal. What was the ultimate solution? The isolation transformer?

author: Synoptic
author_rank: Contributor
timestamp: July 10, 2015, 02:16:17 am
content: 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 ?

author: jc101
author_rank: Frequent Contributor
timestamp: July 10, 2015, 09:04:34 am
content: Quote from: Synoptic on July 10, 2015, 02:16:17 amDo 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.

author: John Coloccia
author_rank: Super Contributor
timestamp: July 10, 2015, 11:36:30 am
content: Quote from: Synoptic on July 10, 2015, 02:16:17 amI 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.

author: Synoptic
author_rank: Contributor
timestamp: July 10, 2015, 12:37:15 pm
content: Quote from: John Coloccia on July 10, 2015, 11:36:30 amMaybe, 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.

author: Synoptic
author_rank: Contributor
timestamp: July 10, 2015, 12:38:59 pm
content: Quote from: jc101 on July 10, 2015, 09:04:34 amYes 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 ?

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2015, 01:00:38 pm
content: 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.

author: John Coloccia
author_rank: Super Contributor
timestamp: July 10, 2015, 02:29:06 pm
content: Quote from: Synoptic on July 10, 2015, 12:37:15 pmOh, 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.

author: Synoptic
author_rank: Contributor
timestamp: July 10, 2015, 02:32:16 pm
content: Quote from: John Coloccia on July 10, 2015, 02:29:06 pmYes, but at 80$/hour, I'll pass on this non-necessary fix