Next generation KSCC hill communications

[walterclark]

I am working on a new design for a future KSCC headset system to replace our aging and logistically unmaintainable existing system.

What I find good and reusable from our current system is:

The long wire. While the KSCC wire itself is fast approaching the end of its life the full-duplex / party-line, multi-station, balanced twisted wire technology is still valid. I have yet to identify or seen anyone else identify an alternate technology that comes close to being as reliable and fully meeting our requirements. To this end I want to design the "new" hardware around this existing technology. The primary technical characteristics of the "wire" as we implement it is an impedance of 100-300 ohms and a nominal signal level of minus 24dbm. These represents the characteristic impedance of a twisted wire and the impedance of carbon elements used as speakers and microphones. The level is also characteristic of carbon microphone/speaker elements that dates back to around 1877. As it happens these are also the impedance and signal levels found in aviation comm systems today although you will find no carbon elements in use any longer (aviation has a habit of requiring stupid-long backward compatibility).

The ammo boxes. Tough, relatively waterproof and cheap. Heavy enough to stay put when set somewhere, but not hard to move. The ones we use offer enough space inside to store everything we need. And they are still available.

The Radio Shack speakers. Yes they are old but they get the job done. About as undemanding of battery power as I can find and they seem pretty reliable. I think I resolved the problem of feedback squealing last season by switching power off to the box when the adjacent headset mic is keyed on. I know Andy at the start of Philo had an issue but he had some bizarre configuration of extra cables and stuff where it wasnt meant to be so I am chalking that up to more evidence that nothing is idiot proof (geek-proof is more accurate in this case).

The Peltor ear protector sets converted to headsets. While the electrical part of these have aging issues - carbon elements wearing out with few replacements to be found because they are no longer manufactured, and the wiring being complicated and failing from fatigue - the headset housing and mic boom are both good. Someone did remove the sound absorbing foam from inside which compromises the outside noise attenuation but the removal of the foam probably improved the earspeaker sound levels some by not absorbing anything inside the ear cup.

And thats about it.

What I am working towards is a modern headset that uses a noise canceling electret microphone with Aviation output levels (-24dbm versus the more typical -47dbm found in electret mics for cell phones, walkie talkies, PC's and recording machines) and dynamic earspeakers with the sensitivity and impedance to match the "system" along with a volume control for the ear speakers. Noise canceling mics improve the quality of voice transmission by being immune to sounds below a certain SPL. The downside is one has to practically insert the mic in ones mouth to overcome the minimum SPL needs and this can be an adjustment problem for both the mic boom and the user. I want the headset to plug into a small box that can be attached to a the users belt or clipped on a pocket and that has a PTT switch. I want a single, relatively fine, but durable, and easy to replace wire of about 15' to connect this "belt box" to a connector in the ammo box. The ammo box will have mounted in it the RS speaker and its battery along with a power switch for the speaker battery. There will be a jack to plug the wire drop into in the ammo box as well. The ammo box will have open space for the headset and the belt box along with their respective cords.

I have in hand and am testing a low end aviation headset and a single ear, on-ear cell phone "headset". The cell phone set is being used mainly to allow me to design and test circuits using this the mic and earspeaker to make these common types compatible with our system.

I have determined that I can bump the output of the cell phone headsets mic to the levels we want fairly well with a small amp in the belt box and with a matching transformer I can up the impedance of the speaker in the ear cup so it doesnt load the wire inappropriately. These same techniques could be applied to consumer headsets including many of the gaming headsets for PC's (which are often called multi-media headsets) which can be pretty cost effective because of the high volume. It looks like all of them have similar mic elements in terms of output and many have sensitive enough earspeakers to avoid having to add an amp. For example: http://www.koss.com/koss/kossweb.nsf/p? ... pc^mm^SB45. This set costs about $23 at Amazon. It lacks a noise canceling mic however. Plantronics claims to offer several multimedia headsets with noise canceling mics but they dont publish specs so it is hard to tell if any of their headsets would work for us beyond that without buying one.

The aviation headsets do these things already. I have this set now: http://www.warrengregoire.com/aviation-headsets.htm It does what I need it to do electrically and will connect to our system with no added amplifiers. It feels nice on the head and the sound quality of the speakers is good. The mic boom has a flexible memory which in my short experience makes it a pain to get just right and the noise canceling mic or the circuit used to amplify it within the headset may be an issue - it is a very narrow range between inaudible and overloaded when speaking into it though it performs great with constant amplitude sounds like test tones and pink noise. I will be spending more time on this. The big problem with the aviation sets is price. This model is really cheap (about $85 delivered for the cosmetic-reject version) compared to most others where $400-$800 seems perfectly normal. I am sure part of the reason it is low relates to some of the shortcuts. For instance, I removed one of the ear pads to see what it used for an amplifier, speakers and level control. What I saw was there is an add-in mic preamp consisting of 2 transistors, 2 capacitors, and 4 resistors soldered together in a little ball of parts and leads, dangling in space on the thin stranded copper mic wires and more substantial shields. A small transformer is soldered to the speaker leads in that earcup using the leads of the transformer to support and suspend it (at least they seem to be solid strand tinned steel wire and not stranded copper). My guess is this wont hold up long under any shock and vibration testing - or use. At least the speaker and level control were mounted well.

Based on what I have learned from the aviation headset and the cell phone headset I think I can update the Peltor sets we have now with electret noise canceling (or non-nc) mics and dynamic speakers in the earcups. I will plan to not use the ball-of-parts-suspended-in-space approach. Along these lines, it seems very doable for me to make up our own noise canceling electret element that would fit on the end of the existing flex in a "cigar tube" that would be about 1/4" ID and maybe an inch long. I would just need to find the "cigar tube". The electret element is available at Digi-Key for 3 bucks. And mylar speakers that should work are about $4 each. I would not like to try to retain any of the existing wiring and hookup we have with the Peltors we have now because I would have to hack them anyway and that stuff is already getting. to the end of its useful life - wires are breaking and switches going bad at almost every event.

I found a great looking boomed noise canceling electret with aviation outputs from Gentex http://www.gentexcorp.com/default.aspx? ... edit=false that would work great with future Peltor ear-protector sets. Gentex is in NH. Anyone have an in there? I would guess retail prices are very high but we might be able to afford them at cost.

[Mopar 151]

Walter - FWIW, I did a machine installation in a Gentex (main plant in Carbondale, PA)plant a couple years ago. You would be hard pressed to find a more screwed up company! I'd avoid 'em 'cuz of 2 things - if they lose a couple contracts with Uncle Sugar, they'll likely tank, and their quality guy - it would be an insult to idiots to call him an idiot!

On a better note, the "mike booms" on the "KSCC style" headsets are machine tool coolant line, and they have lots of differrent ends. Link to MSC catalog page http://www1.mscdirect.com/CGI/NNPDFF?PM ... T4TP=*LTIP IIRC, we used a 3/8 (may have been 1/4) kit with 1/4 npt end, and added a 1/4 npt coupler on the end.
http://www1.mscdirect.com/CGI/NNPDFF?PM ... T4TP=*LTIP
We sourced the headsets from them as well - catalog page http://www1.mscdirect.com/CGI/NNPDFF?PM ... T4TP=*LTIP
but I've used these folding ones in their normal application, and they are much easier to store, withour compromising comfort.
http://www1.mscdirect.com/CGI/NNPDFF?PM ... T4TP=*LTIP Hopin' this all helps!

[walterclark]

Cool. It looks like maybe the elbow fitting on 2622 might work on the end to hold the mic element and turn the opening 90 degrees. It will be good if the opening for the mic points directly at the talkers mouth - critical if we can use noise canceling mics, otherwise helpful with omni pattern units . I will have to study this more. One of the mic elements I am looking at is 1/4" D (OK 6mm) and the other is 3/8 (well 9mm) so either 1/4 or 3/8 should work. I will pull on of our Peltor sets out and check. Prices on the Peltor head protectors seem reasonable from them too. Since we dont really have extra headsets right now and assuming we dont hang onto the aviation unit I picked up (I plan to return it) it would make sense to start with another Peltor and build it up. Once we have used it and decided the rest should be that way, then swap it into the 13 active sets to free up one to rebuild or replace with the new design, and so on.

Thanks!

Looked at the headsets and the catalog a little more. the headsets use the1/4" snap stuff and I think the 1/4" elbow 48485171 will do nicely as a mic holder on the end of the "boom".

[walterclark]

So John. What were you saying about the ear protection exactly? The second and third links both go to the same page. Was there a model besides the H7 we use now that you were suggesting? Thanks.

[Mopar 151]

Try this - Peltor and MSC house brand
http://www1.mscdirect.com/CGI/NNSRCM2?PARTPG=GSDRVSM or http://www1.mscdirect.com/CGI/NNSRCM2?PMS0NO

Also, MSC and McMaster-Carr are cutthroat competitors so you can use the other's part # in each co's online search.

[walterclark]

OK, I see now. Wouldn't hurt to give a pair a try. I will get set of these and build the prototype "new" headset from them and we can see how they do in the hands (and on the ears) of checkpoint workers. Thanks. Walter

[walterclark]

The MSC stuff (one set of earmuffs plus enough of the snap-lock bits for 2) and a couple variations of mics and small speakers from Digikey should be here by the end of the week.

[walterclark]

The MSC order arrived. The earmuffs look good, but... I think the headband is going to be a problem. It puts a lot of pressure on the earmuffs when I put them on. I could never see wearing them for a few hours at a time the way they are.

John - is the plastic headband metal cored and can it be over-bent to relieve the pressure?

If it is just plastic I cant see using them. Too much chance of either someone over bending the band to loosen them up and breaking them or the band getting softer on its own and not holding the headset in place.

Walter

[walterclark]

I went ahead and ordered a set of Peltors - I found some H10A sets for $19 delivered. I used this model as the practice/transit headsets in the GTI for rally and loved them. Easy to modify, VERY quiet, very comfortable. They might be overkill for many checkpoints but I would bet for the Start/Control along with a noise canceling mic, they would be great. They might not be here in time to build a set for the KSCC banquet though...

[walterclark]

The earmuffs arrived in plenty of time and I have them set up with the electret mic, "receivers" (that is what the phone company called the earspeakers) from what had been a cannibalized pair of the gray headsets, and an amp/PTT switch in a used PTT box from the KSCC headset spares bin. I will bring them and another of our sets with me tomorrow to the banquet for anyone wanting to try. The mic/amp is powered by a 9V NiMh rechargeable battery in the PTT box and my tests indicate it is good for more than 50 hours talk time.

On the subject of earspeakers... I picked a couple sets from the Digikey catalog that represent what I thought stood the best chance of substituting for the old ITT phone "receivers" we use today. I was wrong. Those ITT units (and probably the Western Electric equivalent) are not only functionally better suited to our use but they make the stuff I bought look like junk. I dismantled a receiver that had a defective coil to see what made them tick. The "cone" of these appears to be made from an alloy including beryllium, a moisture barrier, cage that is the heaviest part of the whole piece and rare earth magnets and it is wrapped in a protective aluminum shell. I cant imagine what these things would cost to make today. Anyway I want to locate some on the surplus or old phone restoration circuit as spares. I plan to reuse the ones in the gray headsets as I one by one replace them with Peltor based earmuffs with electret mics. I would ask anyone having an old ITT/Western Electric/Bell desk or Princess phone that is not of use to them...we could sure use the receiver (the element behind the screw-off earpiece on the handset).