Shielded Audio Question

PaulR · #1 12-29-2009, 07:42 AM

I'm planning on putting the headset/mic jacks behind the cabin frame (in a tipper) on the bottom of the brace. I'm short of 3 conducter shielded wire and, being the cheapskate that I am, have some wire that I think will suffice.
PSE says shielded wire is a must so I'm following their direction.

Anyway, I have a pretty good amount of 6 conducter shielded and was wondering the effect this might have on the stereo headset jacks, if all the headset wires are shielded together.

I believe it will simplify the routing also.

As always, thanks in advance.

Walt · #2 12-29-2009, 12:37 PM

Should be no problem, I'd use it.

SteinAir · #3 12-29-2009, 03:25 PM

We thought some years ago that a single cable with 6 conductors in it would be much easier than 2 separate 3 conductors. But....turns out that's not the case and I was quickly scolded for attempting sucha thing by a nubmer of people who know more than me about it.

So, I would say that when it comes to Audio stuff, I'd follow the mfgr recommendations . Perhaps Mark S. can chime in and give is the bits/bytes reasons for such things, but I just do as I've been told.

Cheers,
Stein

mscheuer · #4 12-29-2009, 03:40 PM

Because of the capacitance associated with wires that are closely coupled together, such as wires that are in the same bundle, there is a high likely hood that you will have cross talk.

And if you were to include a mic line in there too, you will also create an oscillator! Because the capacitance from the mic audio wire and the headphone audio wire will create a feedback path. Once you turn your intercom volume up so you have a gain greater than 1, you will hear sqealing in your headset.

I know, I'm a bit anal when it comes to wire harnesses. But I have good reason. From experience I know that there is a high probability you WILL get the audio performance you had paid for when investing in one of our product if it is wired per our print. Conversely, there is a likely hood that you will NOT have that same satsifying outcome if you stray from our wiring diagram.

OK, back to vacationing.....but thanks Stein for asking, I do like this kind of stuff.

Mark Scheuer
PS Engineering, Inc.

Quote:

Originally Posted by SteinAir

We thought some years ago that a single cable with 6 conductors in it would be much easier than 2 separate 3 conductors. But....turns out that's not the case and I was quickly scolded for attempting sucha thing by a nubmer of people who know more than me about it.

So, I would say that when it comes to Audio stuff, I'd follow the mfgr recommendations . Perhaps Mark S. can chime in and give is the bits/bytes reasons for such things, but I just do as I've been told.

Cheers,
Stein

jdeas · #5 12-29-2009, 03:53 PM

If you have individually shielded pairs of wire with each pair in their own shield then this should not be an issue but is not as safe as separate cables. If you have a single shield over all six wires then not a good idea.

PaulR · #6 12-29-2009, 06:23 PM

I'll go with Stein and Mark's suggestions. I needed to call Stein and order some other "Stuff" tomorrow anyway. I"ll have to find something else to keep me busy till I go back to work on the 4th.

Happy New Year!!!

N941WR · #7 12-29-2009, 07:56 PM

Mark,

I don't ge this. I have a Sigtronics 200S in my RV and none of the wires they supplied are shielded. With over 250 hours on the plane I have had no problems of any kind.

What is different about this intercom that it doesn't require shielded wires?

breister · #8 12-29-2009, 08:56 PM

Quote:

Originally Posted by N941WR

Mark,

I don't ge this. I have a Sigtronics 200S in my RV and none of the wires they supplied are shielded. With over 250 hours on the plane I have had no problems of any kind.

What is different about this intercom that it doesn't require shielded wires?

Bill,

I have the same intercom. BUT - all of the wires from the jacks are only inches from the unit in my setup, and the separation of the jacks from each other assures some separation of the pairs for all but a couple of inches of exposed wire.

SOoooooo, I'll take a stab that since the phenomenon has to do with an induced current, the length of the run and proximity of the pairs determines how much "crosstalk" actually occurs. Since RF declines as the inverse cube of the distance, 1/4" has only 1/9th as much inductive RF as 1/8" wire separation; 1/2" would only be 1/27th as much; etc.

Did I get it right, engineer-folks?

SteinAir · #9 12-29-2009, 11:24 PM

Quote:

Originally Posted by N941WR

Mark,

I don't ge this. I have a Sigtronics 200S in my RV and none of the wires they supplied are shielded. With over 250 hours on the plane I have had no problems of any kind.

What is different about this intercom that it doesn't require shielded wires?

True that Sigtronics harnesses are unshielded, but comparing a Sigtronics to a PSE is like comparing a chimp to a human....both primates, but....you know what I'm getting at!

Lost of differences both inside and out. For example, some companie's EFIS systems require every wire to be shielded, some don't. Some mfgr's want shields sinked at both ends, some don't. Just one of those things I guess!

Cheers,
Stein

mscheuer · #10 12-30-2009, 05:26 AM

Great questions!

As far as cross talk, I think I can answer that one. It is my belief that our intercoms provided more "Volume" to the headsets due to our increased gain. If signals are small, while the same amount of cross coupling is present, the one with more gain will exhibit cross talk more pronouncedly because of the higher gain.

Capacitance in a capacitor is determined by the size of the plates, the type of insulator, and the distanced between the plates.

In this case, the plates are wires, the insulator is the insulation on the wires, and the distance the wires are away from each other.

Place two different signals and a common ground in a shielded bundle and the signals will couple together because of the inherent capacitance between the wires.

When we introduced our aerocom in 1985, we introduced two new things in the field of aircraft intercoms, individually gated microphone and more gain than any other intercom, at that time.

As far as why you don't have to use shielded wires, I don't know the answer to than one. I never had a need to disassemble one and look at how they did things. Maybe a Sigtronics representative could answer that for you, I would very much like to know that answer as well.

But please let me assure you, we have gone to great lengths to minimize RFI. And while I know it bares little importance to homebuilders that products are TSO?d or not, our intercoms are (except the PM1200 but is built exactly the same way as our others) have gone through a rigorous certification process. It includes susceptibility, emissions, and voltage spikes that are "coupled" with the entire bundle.

It's not easy to pass these tests, but we do and had lead us to a better products.

Long answer, but you had a great question.

Hope this helps.

Mark

Quote:

Originally Posted by N941WR

Mark,

I don't ge this. I have a Sigtronics 200S in my RV and none of the wires they supplied are shielded. With over 250 hours on the plane I have had no problems of any kind.

What is different about this intercom that it doesn't require shielded wires?