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Old 24th Apr 2010, 23:34   #1 (permalink)
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Recording intercom and radio transmissions

I've been trying to record the conversations and radio transmissions on my video camera (Canon HFS10) with a patch cord that looks like this:



But I'm picking up a lot of noise even when nobody is talking and I'm hoping to get rid of it. I've seen other patch cords advertise that they convert the 600 omhs headset signal to something more suited for a regular 3.5 mm recorder.
Might this be the source of the noise? I'm not sure if the cord I currently have does this, so I've included a small recording where you can hear the noise:

http://luftfartsfag.no/example.wma

The aircraft in question is a Cirrus SR20 with a Garmin GMA 340 audiopanel.

Any information and help on this subject would be most appreciated.
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Old 25th Apr 2010, 00:44   #2 (permalink)
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My knowledge of electric things varies from

(a) that bit hurts to

(b) nil

so others will, no doubt, put the technical bits in order.

However, going back a few decades, we routinely recorded flight test work to video using a home made patch cord which had nothing fancier than a variable resistor in the circuit - worked real fine. We played with a few resistance values until we ended up with something that worked OK.

I'm sure it can be done far more elegantly .. the main point is that it can be done cheaper and effectively.
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Old 25th Apr 2010, 01:02   #3 (permalink)
 
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Sounds mostly like normal cockpit background noise.
Different mics & audio ccts are more sensitve to different audio frequencies.

I doubt it's the patch lead.Noise cancelling mic's may help.
Try a recording using the same set up in a very quiet environment to see how much, if any, is generated by your equipment.Maybe some interaction with on board systems too?

The audio also sounds like it is being overdriven thus a bit distorted so impedance matching and maybe some form of attenuation maybe required.

Not at all familiar with the aircraft,sorry.
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Old 25th Apr 2010, 01:21   #4 (permalink)
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This is nowhere near how it sounds with our Bose X headset. First of all when there is no transmissions and nobody is talking in the cockpit the audiopanel's vox function keeps everything quite as long as the squelch is set correct. Atleast I belive it's the audio panel that does it and if so, the same should go for my video camera.

I've tested the camera 3.5 mm recorder by connecting it to my iPod and the sound is perfect, so I know that's not a problem.

Here you can hear a recording done while the aircraft is still on the ground with everything turned on and the engine idle. It's a lot better but theres still much noise that we don't hear with our headsets, and since the camera's only source of sound is what's coming though the cable it shouldn't matter that we have ANR in our headsets, should it?

http://luftfartsfag.no/example2.wma
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Old 25th Apr 2010, 02:43   #5 (permalink)
 
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Ampclamp is on the right track

Your recorder is using an automatic level control or ALC. When there are no transmissions, the unit appears to be jacking the gain and you hear over boosted amplifier noise from the panel itself. See if the ALC mode can be switched off and you may find better results.

Master Tullamarine's suggestion of a resistor to attenuate the output of the panel also has a good chance of helping.

<<<---out from panel-----|
...................................Z
...................................Z<-----------to recording device>>>>>
...................................Z
<<<--------earth--------|--------------earth------------>>>>>>

the device represented by the | and Z's and <-- is a variable resistor.
1K ought to do it.
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Old 25th Apr 2010, 02:57   #6 (permalink)
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I have a AGC Limit and a Microphone Attenuator option in the camera's menu and both are turned on. Might this be what you are talking about?
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Old 25th Apr 2010, 03:31   #7 (permalink)
 
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Yes, AGC (auto gain control) is another way of saying ALC (auto level control)

The mic attenuator should be on but the source may still be too hot for the input on the camcorder due to AGC limitations. Local radio shop should be able to solder in a potentiometer in an aluminium box for you. Wouldn't have to cut the wires if they also used the 1/8" stereo plugs for in/out.

you might get better results if your camera has a line level input (often RCA style-jacks)
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Old 25th Apr 2010, 03:55   #8 (permalink)
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After looking into the AGC function on my camera it seems it only controls the potential gain in the video, allowing me to film in low light conditions.

And as you say the Microphone Attenuator should be turned on as it is, so it seems theres nothing I can change on my camera to fix the problem.

I fear that there aren't any local radio shops where I live but this potentiometer isn't something I can find on the Internet?

My knowledge in electronics is fairly limited but doesn't this cable provide the resistance that the potentiometer would create?

Quote:
  • Integrated electronic circuitry provides impedance balancing between aircraft intercom system and microphone interface of recording device, applying voltage step-down to prevent distortion of recorded sound.
AIRCRAFT INTERCOM RECORDING CABLE from Aircraft Spruce
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Old 25th Apr 2010, 05:04   #9 (permalink)
 
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An Answer to the Noise but Not the Problem

Good day. I am not a pilot but I am a broadcasting engineer. Excuse my interloping. Your problem was simple to diagnose and is somewhat easy and inexpensive to resolve and is something we deal with every day.

The attenuator and ALC in your camera or any other consumer or prosumer camera may not be sufficient to counteract the sound power levels required to deal with the noisy environment of a cockpit. For example, the power needed to listen comfortably in your home is less than needed in your car. So the earphone amplifier in the comm system has been designed to provide all the gain needed for that environment.

I will make the assumption that to avoid missing an ATC transmission the volume level of aircraft comm radios cannot go to zero. Even at minimum that appears be too much for the ALC and attenuator in your camera.

I heard two things in your recording. Distortion: the comm was over driving the input to the camera. And background hiss. This hiss is thermal noise in the comm system's headphone amplifier and is only objectionable when over amplified above background levels as demonstrated in the recording.

The suggestion vapilot2004 made is on point. If you have access to a technician in an avionics shoppe they can cobble together a 15 or 20 dB pad from spare parts in their tool box. Except for the special aircraft connectors, the rest are readily available at an electronics hobby shoppe. I Googled this commercially available device for about 14 Pounds from http://www.canford.co.uk Place it in line between the camera and the wiring harness.

A final comment about ANR headsets. The magic you hear is all in your head! There are microphones inside each ear cup. The circuitry analyzes what it hears inside the cup and creates an audio signal 180* out of phase. When mixed with what is coming up the wire, leaves only the radio or intercom. The squelch action you describe is just the ANR readjusting to blessed silence.

Good luck,
Bob Sudock/Fox Television Engineering (retired)
Los Angeles
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Old 25th Apr 2010, 05:33   #10 (permalink)
 
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From what I can tell, the cable has a built in attenuator so the issue most likely is within the camera itself in its automatic level control.

In your posted recording, one can hear the gain surging upwards when there is no transmitted audio. That is when the hiss comes in which is amplifier noise - most likely coming from the audio panel - but being amplified many times which is why this is not heard on the AC speaker or headsets.

Is there not a line in available on the camera?
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Old 25th Apr 2010, 05:41   #11 (permalink)
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The cable I linked to in the last post is not the cable I used in the recordings, its a cable I believe might fix the problem because of the resistor. The cable I did use looks similar to the one in the link but I'm not sure if it has any resistors.

The camera only has a 3.5 mm microphone input, the rest are outputs.
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Old 25th Apr 2010, 05:52   #12 (permalink)
 
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Ah. Incorrect assumption on my part. Cable sounds like a good investment.
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Old 25th Apr 2010, 06:56   #13 (permalink)
 
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Icenor, before you try anything else, try this. I use a small magnetic telephone pick-up, available from a high-street electronics shop (Maplin in the UK) for a few $ or , and attach it with its sucker pad (and a bit of tape) to the outside of one headset earphone. It is plugged straight into the camera socket, which of course disables the camera mic.

The result is near broadcast quality of any intercom or radio transmission in the headset, and of course no ambient engine or aerodynamic noise at all. In the background I hear a faint clatter from the magnetos, but that's all (and in fact is quite reassuring!).

As a solution it is cheap, cheerful and effective, and when I suggested it in another forum some time ago at least one person came back with a huge thumbs up. Do let me know if it works for you.
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Old 25th Apr 2010, 21:30   #14 (permalink)
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I've ordered the pick-up and it will hopefully be here within the week. Thanks for the tip
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Old 26th Apr 2010, 01:55   #15 (permalink)
 
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good luck with the device.Could be the fix.

It does sound like 2 issues to be dealt with
1/ over driven audio
2/ background noise

The overdriven audio should be dealt with with either a matching device or attenuation.

The apparent background noise could be a few different things.Overly sensitive mic, poor matching, circuitry derived noise for example

Remembering that an aircraft headset mic has a very close range (and noise cancelling in many types) so it wont pick up all the hash / noise generated in an aircraft cockpit.
Devices like your camera will have a mic more sensitive to more distance noises and also be sensitive to a greater range of audio frequency over a headset mic.
You generally need to close talk a headset mic to make them work nicely whereas a camera mic is an area mic similar to a cvr mic.

PS how were you affected by the Icelandic volcano ?

all the best from the other side of the world.
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Old 26th Apr 2010, 02:14   #16 (permalink)
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I'll keep that in mind

Initially the ash only closed the airports in northern Sweden, but after a couple of days it reached us in the south. So the airspace was closed for IFR, but we worked out a deal with ATC to allow us to fly local IFR flights because we only fly piston. So overall the ash hasn't really affected us, but only limited our destinations.

Hopefully it will all be over soon
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Old 26th Apr 2010, 21:17   #17 (permalink)
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Wow this is strange, even though robert.sudock's post is dated the 25. it didn't show up in the forum before 3 o'clock today...

Anyway thank you for the information, but I'm not really clear on how I can convert the RCA to the 3.5 mm (minijack) that the camera uses.

Is it possible to use some kind of "minijack female to RCA male" cable, connect it to the RCA Attenuator and then use a "RCA female to minijack male" adapter and connect it to the camera?
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Old 27th Apr 2010, 03:09   #18 (permalink)
 
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Icenore pleased the volcano has not hurt too much.I was wondering if piston engined craft would be allowed to fly.Different risks altogether.

Robert sudock is probationary so perhaps his post was delayed? Not sure but the only thing I can think of.

I do think he is right (basically agrees with me ) although I do bow to his superior knowledge.Been out of this kind of thing for a while. You tend to become a box changer / systems expert in the airline line maint game and not dabble so much in the home brew side of electrics/electronics.

RCA to 3.5 mm jacks can be readily home made , purchased retail or on the net.Just search for what you want am I am sure it will be obtainable.

Good luck,
ac.
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Old 29th Apr 2010, 04:33   #19 (permalink)
 
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... with your permission

Excuse my verbosity -- often necessary to explain techie things to my former, non-engineering superiors.

Correct about a probationary member.

There is nothing wrong with the cable harness. You need to absorb more earphone power before going into the camera.

I am sure you can easily obtain the parts. For example, In the States there is Radio Shack who sells the parts to build the pads (that is what they are called). Then there are purveryors to musicians like Guitar Center and Sam Ash Music, who sell pre-built adapters to help musicians plug various types of audio gear together.

Again let me suggest you make friends with a technician in an avionics shoppe. He, most likely has the parts to build the pad in his tool box. Protocol in the States is a 6-pack of beer. With parts in hand, it should take maybe 15 minutes to build. I suggest a 15 or 20 dB pad. Anyone with a modicum of knowledge will know what that means! You can also find a local Amateur Radio operator who is usually suceptible to the same payment protocol or even a tour of the flight deck. Maybe that is the best idea because he can test and make adjustments on-site.

E-mail directly if you have questions.

robert AT sudock.com

Cheers!
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Old 29th Apr 2010, 12:24   #20 (permalink)
 
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I see the 'beer economy' operates in the states as well as it does here in Oz.

cheers!
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