video-cam adaptor

[Bonanza Man]

is there an adaptor that'll take the head-set
audio-out of an airplane intercom into
a jack that can be put into a standard
microphone jack in a video-cam.

[[email protected]]

I don't know of an off-the-shelf adapter, but it needs to be an
impedance adapter. Headsets are typically 150-300 Ohm, with voltages
running upwards of 10V. Portable devices like cd players and other
line-in critters use a much lower voltage, and expect a lower source
impedance. If you truly want to go into the mic port, it'd have to accept
and modulate the bias voltage coming from the video-cam as well, I'd
expect.

If it were me, I'd build one... but need to do more research
first.

-Cory

Bonanza Man > wrote:
: is there an adaptor that'll take the head-set
: audio-out of an airplane intercom into
: a jack that can be put into a standard
: microphone jack in a video-cam.




--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[Dylan Smith]

On Mon, 14 Jul 2003 21:39:49 GMT, Bonanza Man > wrote:
>is there an adaptor that'll take the head-set
>audio-out of an airplane intercom into
>a jack that can be put into a standard
>microphone jack in a video-cam.

I made one the quick and dirty way for my big trip.

All I did was stick a big resistor in-line with the audio (actually a
minature potentiometer, so I could fiddle with it). I initially underestimated
the resistance needed (I thought it might be on the order of 5k ohms,
I ended up using IIRC a 470K potentiometer maxed out). I just soldered
the pot to the pin for the intercom jack tip, then the wire to the other
end of the pot. The potentiometer fits inside the jack plug's cover,
and a bit of heat-shrink rubber makes sure the exposed contacts don't touch
the signal ground's pin. So you have


intercom jack

/-|============| 470K |---------------------left channel
/ |-----------------/\/\/\--|---------------------right channel stereo
\ | | jack
\_|============|------------ signal ground --------------------

It worked well. All the parts can be picked up at Radio Shack. I just
butchered a stereo jack extension cable for the camera side, and soldered
it into a nice screw-together large jack for the intercom. It is quick
and dirty; I think the proper method is to have some form of impedence
matching transformer, but I don't know whether such a beast exists that's
suitable for intercom -> camera.

I also made a video camera stand which could be quickly attached to and
released. It was basically a shaped polystyrene block (shaped to the
curvature of the glareshield), which was velcroed to the panel. The camera
was in turn velcroed to the top of the polystyrene block. It stayed on
even during some of the severe turbulence I encountered crossing the Sierra
Nevadas (multiple negative-g bumps which made my charts almost hit the
roof) but was quick to pull off by hand. Make sure you don't site the camera
too close to the compass - I did some testing to see where I could place the
camera without causing the compass to move.

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

[Ron Natalie]

"Dylan Smith" > wrote in message ...
>- I did some testing to see where I could place the
> camera without causing the compass to move.
>
The testing is not sufficient. One of those places where the camera sits and keeps
the compass from moving might keep it from moving even when the plane turns.

[Newps]

I have a Sony Digital 8 camcorder. All you need is the attenuating
patch cord from Radio Shack. This plugs into the camcorder audio in.
On the other end you need an adaptor so it will plug into the intercom
audio out. Whole deal costs less than $10. Sounds great. Only down
side is there is now no engine noise on the tape. To secure the camera
I bought some elastic, comes in a long string. I tripled it up and
attached it to a couple of ring terminals that attach to two screws that
are part of an avionics access panel. The camera sits on top of a
sponge. The sponge lifts it up slightly for a better view out the
window, keeps the camera from moving and also totally eliminates
vibration. Just remember to set the camera to focus on a distant
object, not the nearest thing in front of it.

Bonanza Man wrote:
> is there an adaptor that'll take the head-set
> audio-out of an airplane intercom into
> a jack that can be put into a standard
> microphone jack in a video-cam.
>
>
>

[[email protected]]

Now that I got off on this tangent on intercom audio interfaces,
I'd suggest something like this for what you want (similar to previous
post):

Headset Plug Tip|----\/\/\/-----|====== Left/Right to cam
| 680 \
| / 82
| \
Headset Ground |---------------|------ GND of cam

That'll load it so it looks something like a real headset, but cut
the voltage down by a factor of 10 or so. If you don't want to load your
intercom, make it 6.8K and 820 Ohm. Adjust the 82 (or 820) size to
plus/minus a factor of 3 or so if you need to adjust the volume.

My guess it that'll do it.
-Cory

Dylan Smith > wrote:
: On Mon, 14 Jul 2003 21:39:49 GMT, Bonanza Man > wrote:
:>is there an adaptor that'll take the head-set
:>audio-out of an airplane intercom into
:>a jack that can be put into a standard
:>microphone jack in a video-cam.

: I made one the quick and dirty way for my big trip.

: All I did was stick a big resistor in-line with the audio (actually a
: minature potentiometer, so I could fiddle with it). I initially underestimated
: the resistance needed (I thought it might be on the order of 5k ohms,
: I ended up using IIRC a 470K potentiometer maxed out). I just soldered
: the pot to the pin for the intercom jack tip, then the wire to the other
: end of the pot. The potentiometer fits inside the jack plug's cover,
: and a bit of heat-shrink rubber makes sure the exposed contacts don't touch
: the signal ground's pin. So you have


: intercom jack

: /-|============| 470K |---------------------left channel
: / |-----------------/\/\/\--|---------------------right channel stereo
: \ | | jack
: \_|============|------------ signal ground --------------------

: It worked well. All the parts can be picked up at Radio Shack. I just
: butchered a stereo jack extension cable for the camera side, and soldered
: it into a nice screw-together large jack for the intercom. It is quick
: and dirty; I think the proper method is to have some form of impedence
: matching transformer, but I don't know whether such a beast exists that's
: suitable for intercom -> camera.

: I also made a video camera stand which could be quickly attached to and
: released. It was basically a shaped polystyrene block (shaped to the
: curvature of the glareshield), which was velcroed to the panel. The camera
: was in turn velcroed to the top of the polystyrene block. It stayed on
: even during some of the severe turbulence I encountered crossing the Sierra
: Nevadas (multiple negative-g bumps which made my charts almost hit the
: roof) but was quick to pull off by hand. Make sure you don't site the camera
: too close to the compass - I did some testing to see where I could place the
: camera without causing the compass to move.

: --
: Dylan Smith, Castletown, Isle of Man
: Flying: http://www.dylansmith.net
: Frontier Elite Universe: http://www.alioth.net
: "Maintain thine airspeed, lest the ground come up and smite thee"


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[Newps]

wrote:

> I don't know of an off-the-shelf adapter, but it needs to be an
> impedance adapter. Headsets are typically 150-300 Ohm, with voltages
> running upwards of 10V. Portable devices like cd players and other
> line-in critters use a much lower voltage, and expect a lower source
> impedance. If you truly want to go into the mic port, it'd have to accept
> and modulate the bias voltage coming from the video-cam as well, I'd
> expect.
>
> If it were me, I'd build one... but need to do more research
> first.

If you have a Sony with a mic in a simple cord from Radio Shack will
suffice.

[Ross Oliver]

Bonanza Man > wrote:
>is there an adaptor that'll take the head-set
>audio-out of an airplane intercom into
>a jack that can be put into a standard
>microphone jack in a video-cam.


Aircraft Spruce P/N 13-01456,
Patch Cord Cockpit Voice Recorder
$24.95

See their catalog under "Headset Adapters and Extensions"

I have one, works great.


See also:

P/N 11-00694, CD Player Adapter
$29.30
Allows you to connect your CD player or other audio out
source to your headset, while remaining connected to the
aircraft radio.



Ross Oliver

[Newps]

Dylan Smith wrote:
> On Tue, 15 Jul 2003 11:02:19 -0400, Ron Natalie > wrote:
>
>>"Dylan Smith" > wrote in message ...
>>
>>>- I did some testing to see where I could place the
>>>camera without causing the compass to move.
>>>
>>
>>The testing is not sufficient. One of those places where the camera sits
>>and keeps
>>the compass from moving might keep it from moving even when the plane turns.
>
>
> The testing did involve a flight (I don't need much of an excuse to go flying,
> and testing the camera stand etc. was ample excuse).

Who gives a rats ass if the compass is affected? We're talking VFR flight.

[Craig Prouse]

Newps wrote:

> wrote:
>
>> I don't know of an off-the-shelf adapter, but it needs to be an
>> impedance adapter. Headsets are typically 150-300 Ohm, with voltages
>> running upwards of 10V. Portable devices like cd players and other
>> line-in critters use a much lower voltage, and expect a lower source
>> impedance. If you truly want to go into the mic port, it'd have to accept
>> and modulate the bias voltage coming from the video-cam as well, I'd
>> expect.
>>
>> If it were me, I'd build one... but need to do more research
>> first.
>
> If you have a Sony with a mic in a simple cord from Radio Shack will
> suffice.

Unless you have an intercom that can't drive an 8 ohm load, in which case
you'll get nothing on the tape and nothing in your headset. Or a fire in
your intercom. Your intercom sees that Radio Shack cord as almost a short
circuit.

[Newps]

I have the PS Engineering PM 1000 II, the specs say the headphone
impedance range is 150-1000 ohms.

Craig Prouse wrote:
> Newps wrote:
>
>
wrote:
>>
>>
>>>I don't know of an off-the-shelf adapter, but it needs to be an
>>>impedance adapter. Headsets are typically 150-300 Ohm, with voltages
>>>running upwards of 10V. Portable devices like cd players and other
>>>line-in critters use a much lower voltage, and expect a lower source
>>>impedance. If you truly want to go into the mic port, it'd have to accept
>>>and modulate the bias voltage coming from the video-cam as well, I'd
>>>expect.
>>>
>>>If it were me, I'd build one... but need to do more research
>>>first.
>>
>>If you have a Sony with a mic in a simple cord from Radio Shack will
>>suffice.
>
>
> Unless you have an intercom that can't drive an 8 ohm load, in which case
> you'll get nothing on the tape and nothing in your headset. Or a fire in
> your intercom. Your intercom sees that Radio Shack cord as almost a short
> circuit.
>

[Newps]

Craig Prouse wrote:
> Newps wrote:
>
>
>>I have the PS Engineering PM 1000 II, the specs say the headphone
>>impedance range is 150-1000 ohms.
>
>
> That's right. When you use that Radio Shack adapter cable, you're plugging
> an eight-ohm load into your headphone jack. Eight is not between 150 and
> 1000. Consequently you're asking your amp to drive nearly 20 times its
> rated output on that jack.

I don't know what the load is. The cable isn't relavant, it's the mic
jack off the camera that matters. And looking thru the camera manual it
does not list the load for that jack. In any event I don't care. The
audio laid on the tape, both intercom and radio transmissions, are
perfect and at the proper volume. If I turn my walkman type AM/FM radio
that I have velcroed to the window and fed into the intercom that really
sounds good too.

[Craig Prouse]

"Newps" wrote:

>>> I have the PS Engineering PM 1000 II, the specs say the headphone
>>> impedance range is 150-1000 ohms.
>>
>> That's right. When you use that Radio Shack adapter cable, you're plugging
>> an eight-ohm load into your headphone jack. Eight is not between 150 and
>> 1000. Consequently you're asking your amp to drive nearly 20 times its
>> rated output on that jack.
>
> I don't know what the load is. The cable isn't relavant, it's the mic
> jack off the camera that matters. And looking thru the camera manual it
> does not list the load for that jack.

The cable is totally relevant. It determines your load.
Radio Shack provides the spec for the cable:

http://support.radioshack.com/support_audio/doc33/33094.htm

It places a 10 ohm resistor directly across the plug contacts, in parallel
with the voltage divider (attenuator). This is to provide the proper load
to consumer audio devices which are designed to drive eight ohm speakers and
headsets. When you put a low impedance in parallel with a much larger
impedance, the lower impedance prevails and you can essentially ignore the
large impedance; the effective impedance of the circuit will be just
slightly less than the smaller parallel impedance. The impedance of the
camera's mic input basically doesn't matter at all. All the amp sees is
that tiny little 10 ohm resistor.

Another concern would be the amount of power dissipated in that 10 ohm
resistor. The intercom can put 70 mW into a 150 ohm load. If you
extrapolate, then that 10 ohm resistor ought to be rated for about a watt or
so, and would therefore be somewhat physically large. The actual resistor
in that patch cord is probably rated no more than a quarter of a watt.


> In any event I don't care.

Your personal experience notwithstanding, it's poor advice to recommend this
product to others for this application. EE101 says that something is likely
to get hot.

[Dylan Smith]

On Wed, 16 Jul 2003 18:59:38 GMT, Newps > wrote:
>> The testing did involve a flight (I don't need much of an excuse to go flying,
>> and testing the camera stand etc. was ample excuse).
>
>Who gives a rats ass if the compass is affected? We're talking VFR flight.

Erm, it occasionally comes in handy when the goal is to do a coast-to
coast flight across the USA using a map, clock and compass!

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

[Ron Natalie]

"Craig Prouse" > wrote in message ...

> That's right. When you use that Radio Shack adapter cable, you're plugging
> an eight-ohm load into your headphone jack. Eight is not between 150 and
> 1000. Consequently you're asking your amp to drive nearly 20 times its
> rated output on that jack.
>

No, no, no. Power transfer is NOT directly proportional to impedence.

[[email protected]]

OK... I can't take this crap anymore. He was right saying that
power is NOT proportional to impedance, NOR is it *inversely*
proportional. If the voltage source has a zero output impedance, then it
would go with 1/Z. HOWEVER, the output stages on these doodads are
typically impedance protected so a short circuit won't blow them out. In
other words, the perfect source that drives them has a series matching
load (probably of 100 Ohms or so). That means you'll get the most power
out of it at 100 Ohms total impedance. If you hook up an 8 Ohm load, you
will get much less power output than your factor of 20. Might get a
factor or 2 or 3, but at the expense of killing everything else on the
same bus expecting high-Z. Also, a typical walkman-style headphone tends
to have a higher than 8 Ohm impedance. Often it's 16 or 32. In any
event, you don't need much power into it, so a voltage divider to cut
down the voltage while not swamping the output should work fine. Look at
my schematic from a few days ago... should work.

-Cory


Craig Prouse > wrote:
: "Ron Natalie" wrote:

:>> That's right. When you use that Radio Shack adapter cable, you're plugging
:>> an eight-ohm load into your headphone jack. Eight is not between 150 and
:>> 1000. Consequently you're asking your amp to drive nearly 20 times its
:>> rated output on that jack.

:> No, no, no. Power transfer is NOT directly proportional to impedence.

: Of course it's not. I said it's inversely proportional.

: Now do you want to argue the wisdom of sticking a hairpin in a light socket
: or do you want to argue my precise derivation of the number 20?


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[Ron Natalie]

"Craig Prouse" > wrote in message ...
> "Ron Natalie" wrote:
>
> >> That's right. When you use that Radio Shack adapter cable, you're plugging
> >> an eight-ohm load into your headphone jack. Eight is not between 150 and
> >> 1000. Consequently you're asking your amp to drive nearly 20 times its
> >> rated output on that jack.
>
> > No, no, no. Power transfer is NOT directly proportional to impedence.
>
> Of course it's not. I said it's inversely proportional.
>
> Now do you want to argue the wisdom of sticking a hairpin in a light socket
> or do you want to argue my precise derivation of the number 20?

No, I'm saying that unless you know the characturistics of the last amplifier
stage you don't have a clue how it's going to perform when you feed it into a
different impedence than what it is spec'd for.

[[email protected]]

That's what I was driving at.

-Cory

: No, I'm saying that unless you know the characturistics of the last amplifier
: stage you don't have a clue how it's going to perform when you feed it into a
: different impedence than what it is spec'd for.



--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[MikeM]

Ron Natalie wrote:

> No, I'm saying that unless you know the characturistics of the last amplifier
> stage you don't have a clue how it's going to perform when you feed it into a
> different impedence than what it is spec'd for.

But you can make a pretty good guess... A lot of headphone amps used in
intercoms
and audio panels use an off-the-self IC audio amp to drive the headphone(s).
This this application, the amp is running "closed loop" (i.e. its gain is
determined by putting some resistors around it). This makes it have low output
impedance (i.e. it looks like a voltage source). Practically, the loudness you
hear in your headset isn't effected by having additional headsets plugged in.
Put another way; the voltage out of the amp is independant of how many headsets
are
plugged in (up to 4 or 6).

This would make you think that it could drive an unlimited number of 150 Ohm
aviation
headsets or even an 8 Ohm speaker! The fly in the ointment is that the poor
little
audio amp is limited by how much current it can deliver without "clipping" or
"distorting". The amp is selected to be able to drive up to 6 150 Ohm headsets.
If you try to plug in a "low-impedance" load like a 4 to 8 Ohm home HiFi headset
in parallel with your aviation headsets, then two things happen.

First, the amp distorts like mad due to its inability to deliver the peak
currents required by the low impedance load. Second, the power delivered to
the low impedance phones would blow the eardrums of the wearer, while the
aircraft headset wearers would complaining about low levels
and distortion.

Now, back to the original poster's problem: Hooking the headphone bus to
the line input of a CamCorder. Since the audio line input impedance of a
CamCorder is likely to be in excess of 1K Ohm, and since the headphone amp is
already capable of driving 37 Ohms (4ea 150 Ohms headsets in parallel), then
adding the CamCorder will have negligable impact. Any patch cord that
has the appropriate plugs will work.

MikeM
Skylane '1mm
Pacer '00z