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Interfacing a iPOD to an Aircraft Intercom/Headset

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Anonymous
May 10, 2004 12:18:58 AM

Archived from groups: rec.audio.tech (More info?)

Howdy I need some help with 2 scenarios:

1. First one is I would like to connect my Apple iPOD to my
airplane's intercom. The intercom does not have an AUX input like
most others. So the solution seems to be to make a cord to plug the
audio output of the iPOD into the MIC jack of the intercom. We tried
this with a friends iPOD and it started acting pretty weird, it would
reboot, show that the battery was low even though it was normal and it
would act OK as soon as we disconnected it.

I'm guessing that there is voltage on the mic jack for the electret
mic. I asked someone and they said that there is 12 volts on the mic
jack and only a couple of mv of current. So what is the best way to
put something on the cable that will block any DC voltage from the
intercom from entering the iPOD? I've read about 1:1 audio
transformers or using a Capacitor. I did swing by the local
electronics shop and he suggested a CAP but I forgot what value he
specified. Also when I tried that it seemed to drop the audio level
and made it a little hard to hear.

I'll admit, I made a mistake and ended up just connecting the R(+) and
the L(+) together where I see now that that should be done with a
couple of 10K resistors.
I did a little more research while typing this and maybe the problem
with the iPOD was the left and right (+)'s being tied together and not
the 12v from the mic jack. Still probably a good idea to block that.

Second scenario is what if I just want to add the audio to the
earphones on MY headset while also listening to audio coming from the
intercom.

Intercom--------------+---------------Headset
|
|
|
iPOD------------------+

I don't need any kind of squelching of the iPOD signal over the
intercom signal, just simply combine the two. I may have to do a
MONO-TO-STEREO before I include the iPOD audio because my headset is
STEREO(Has it's own MONO/STEREO switch) and the intercom in the
airplane is MONO. Right now I have my headset in MONO so I get audio
in both L/R ears. So I'd split the intercom MONO to L/R and then
insert the iPOD L/R and send it on to the headset.
Again, I would want to make sure that I don't get any DC or anything I
shouldn't into the iPOD and do I also need to be concerned about
anything going from the iPOD and backing up into the intercom?


Thanks for all of your help. I'm fairly good at electronics and
putting things together and building stuff but just have zero in the
design and technical aspects of it.
Anonymous
May 10, 2004 7:39:06 AM

Archived from groups: rec.audio.tech (More info?)

Forgot to add that when I'm plugging the iPOD into a mic jack, it is
an unused mic jack in one of the back seats. My headset is plugged
into its own jack in the front seat. Scenario two would be combining
the two audio sources (intercom and iPOD) into my headphones.

Also, most aviation headphones/microphones are 600 Ohms and the only
specs I could find on the iPOD is that it is at 32 Ohms.

Thanks in advance!!!
Mark
Anonymous
May 10, 2004 12:40:26 PM

Archived from groups: rec.audio.tech (More info?)

Mark Antry wrote:
> Howdy I need some help with 2 scenarios:
>
> 1. First one is I would like to connect my Apple iPOD to my
> airplane's intercom. The intercom does not have an AUX input like
> most others. So the solution seems to be to make a cord to plug the
> audio output of the iPOD into the MIC jack of the intercom. We tried
> this with a friends iPOD and it started acting pretty weird, it would
> reboot, show that the battery was low even though it was normal and it
> would act OK as soon as we disconnected it.

You're lucky that there wasn't one of those really expensive puffs of smoke
from the iPod. However there's got to be some current limiting built into
the mic jack, and that probably saved you.

> I'm guessing that there is voltage on the mic jack for the electret
> mic. I asked someone and they said that there is 12 volts on the mic
> jack and only a couple of mv of current. So what is the best way to
> put something on the cable that will block any DC voltage from the
> intercom from entering the iPOD? I've read about 1:1 audio
> transformers or using a Capacitor. I did swing by the local
> electronics shop and he suggested a CAP but I forgot what value he
> specified. Also when I tried that it seemed to drop the audio level
> and made it a little hard to hear.
>
> I'll admit, I made a mistake and ended up just connecting the R(+) and
> the L(+) together where I see now that that should be done with a
> couple of 10K resistors.
> I did a little more research while typing this and maybe the problem
> with the iPOD was the left and right (+)'s being tied together and not
> the 12v from the mic jack. Still probably a good idea to block that.

A 50 volt 10 uF electrolytic capacitor with the positive side attached to
the mic input would make a good DC blocking capacitor. It should cut way
back on odd behaviour from the iPod.

Your two 10 K resistors running to L & R might form part of a good
attenuator. Mic inputs tend to be pretty sensitive and some attenuation
might be in order. Start out by hooking the point where they are soldered
together to the negative side of the blocking cap. If the volume is too low,
go to smaller resistors. Resistors as small as 22 ohms might not be a
problem.

If the volume is too high, try either larger resistors or a parallel
resistor from the point where the resistors join to ground.
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Anonymous
May 11, 2004 11:41:01 AM

Archived from groups: rec.audio.tech (More info?)

Arny, thanks for your message.

What about scenario 2 where I want to combine the output audio from
the intercom and the ouput audio from my ipod and feed it only to my
headset?

Any issues there? I will be feeding stereo from ipod to the headset
so I do not need to add the 2 resistors to tie the L/R together.
Should I put a CAP on both the R and the L to keep anything from
backing up into the iPOD?

Should I do anything to keep the iPOD signal from feeding back into
the intercom via the HEADPHONE jack?

Thanks in advance!!!!

Mark
Anonymous
May 11, 2004 4:19:47 PM

Archived from groups: rec.audio.tech (More info?)

Mark Antry wrote:
> Arny, thanks for your message.
>
> What about scenario 2 where I want to combine the output audio from
> the intercom and the ouput audio from my ipod and feed it only to my
> headset?

Looks like you need to interpose some kind of mixer between the various
components.

> Any issues there? I will be feeding stereo from ipod to the headset
> so I do not need to add the 2 resistors to tie the L/R together.
> Should I put a CAP on both the R and the L to keep anything from
> backing up into the iPOD?

Depends on how you drive the headsets. I was thinking of a headphone amp
that acted as the mixer.

> Should I do anything to keep the iPOD signal from feeding back into
> the intercom via the HEADPHONE jack?

It's possible that your headphones have a high impedance

http://www.davidclark.com/PDFfiles/19515p39.pdf

suggests that pilot's headsets have a ca. 300 ohm impedance. This is high.

The schematic also shows that the headset is wired for mono. The same
earpieces can't be wired for mono and stereo. Some kind of buffering or
isolation device is needed.

iPods are designed to work with ca. 16 ohm headsets. There's probably a big
difference in the voltage required to drive the pilot's headsets and what
the iPod can put out all by itself.

I'm still thinking about a separate headphone amp that works as a buffer and
mixer.
Anonymous
May 12, 2004 2:35:46 AM

Archived from groups: rec.audio.tech (More info?)

On 11 May 2004 07:41:01 -0700, mark.antry@xmission.com (Mark Antry) wrote:

>Arny, thanks for your message.
>
>What about scenario 2 where I want to combine the output audio from
>the intercom and the ouput audio from my ipod and feed it only to my
>headset?
>
>Any issues there? I will be feeding stereo from ipod to the headset
>so I do not need to add the 2 resistors to tie the L/R together.
>Should I put a CAP on both the R and the L to keep anything from
>backing up into the iPOD?
>
>Should I do anything to keep the iPOD signal from feeding back into
>the intercom via the HEADPHONE jack?


As an Avionic technician and an ex owner of an audio store, I'd suggest running
the iPOD into the aux input on the intercom. That's what it is designed for!

Exactly what kind on intercom do you have??
Anonymous
May 12, 2004 8:13:22 AM

Archived from groups: rec.audio.tech (More info?)

Arny, yes the headset is 600 Ohms and is stereo ready. It has a
mono/stereo switch on it that right now is in MONO because the
intercom output is MONO. So if I had the headset in STEREO I would
only get sound in one ear. So I figure I will have to do a
stereo-to-mono combine BEFORE I insert the audio from the iPOD.

I have connected the iPOD directly to the headset jack just with some
bare wires and it sounds just great.

Alan, yes I understand it would be best to use the AUX input of the
intercom but it is not available.
Anonymous
May 12, 2004 11:06:15 AM

Archived from groups: rec.audio.tech (More info?)

I'm guessing I just need to build a 2 input mixer:

10K
Intercom +----\/\/\/-----------+------- Headset L
Mono | |
----------| |
| 10K |
+----\/\/\/--------------+---- Headset R
Grnd | |
--------------------------------------+----- Headset Grnd
| | |
| | |
IPOD 10K | | |
L----------\/\/\/---------------+ | |
10K | |
R----------\/\/\/------------------+ |
|
Grnd----------------------------------+

Will the 10K resistors provide too much attenuation? How low can I
go? I seen somewhere that mention 22 to 10K would work. Maybe I
should use lower ones.

Should I put a blocking DC Cap on both the L and R of the iPOD line to
make sure nothing bad gets back to the iPOD? Should they go before or
after the resistors?

Thanks again for all your help!
Mark
!