Intercom

Perhaps you might to try starting here:

?-)

On a sunny day (Tue, 09 Aug 2011 08:31:43 -0700) it happened John Larkin wrote in :

OK, this is how you can do it with just Linux software and existing hardware.

Per car: headset1 -> USB soundcard - headset2 -> USB soundcard - headset3 -> USB soundcard - USB hub -- netbook -- wireless access point -- antenna on roof of car. headset4 -> USB soundcard - | headset5 -> USB soundcard - control

The idea is to send the streams mp3 encoded as UDP on different ports for each speaker to the other WAP. You can then select to talk to only one or any number of persons in the other car, so a one-to-one is possible too.

If you had laptops with a decent 5.1 soundcard you could use front-left middle front-right rear-left and rear-right in one AC3 stream.

I you MUST design hardware you could replace laptop + USB sound card + hub with

headset - opamp - PIC(digitiser) -- ethernet chip ---- wireless access point

---antenna on roof of car. LM386(audio power) All of the above configurations will run on 12V.

Keywords: patent prior art seen before inter car communication

BTW the little PMT walky talkies are now on sale here for about 6 Euro (10$) a set... Not sure if that includes the 6 AAA batteries. Those PMTs should work too, but from *inside* a car? 8 channels with beep and scan mode.

The WiFi setup described above should work, not sure about speed changes, but cars would have low relative speed to each other in traffic.

BTW many people have a GSM, some have push-to-talk, handsfree sets exist.

Aw, c'mon Rich, you know better than that ... I think. I've designed (and my company sold) seven different aviation headsets. I learned how to use shooter's noise deadening headsets and mate them to boom microphones.

It may interest you (or maybe not) that an aviation headset has to mimic a carbon microphone and telephone earphone impedances. Why you ask? Because aircraft were among the first to try this newfangled two-way radio stuff and what could be more convenient than to rip a

1920 era telephone apart to use the carbon button microphone and magnetic earpieces.

Um, no it can't. I can squeeze the eagle as well as anybody but by the time you are done, you've got about $35 in piece parts. Go by the traditional x5 for the list price or by the less common x3 for a kit price and you wind up, as you noted, at the $100-150 price.

Thanks,

Jim

Because I know the aviation side stone cold and am totally ignorant of the bluetooth/zigbee/whatever modern low-power wireless systems. I'm here to learn about that, not be told to go to a pilot shop to learn about headsets. I supply the pilot shops. I am dumb about modern wireless.

OK?

Jim

Thanks. That's the sort of pointer that I was looking for, although they are VERY protective of their information. You dig through three layers of "approval" before you get the CHANCE to dowload any data. I'm on level two with them now.

Thanks again,

Jim

OK, fair enough. I still can't figure out how bluetooth can separate its user's voice from ambient noise. Check and see if your passengers' phones will do intercom mode - I'm almost sure it can be done.

Other than that, I'll bow out now and defer to the experts.

Cheers! Rich

It is really quite simple. The microphones used on aircraft headsets are noise cancelling. Both sides of the electret element are exposed to ambient noise (which cancels out the noise), but only one side of the element to the user's voice. If it were not for this, we'd have a hell of a time making a squelch circuit using voice amplitude as the trigger.

In nearly every case, you can expect the user's voice signal to be about 20 - 30 dB above the noise.

Check and see if your passengers'

The passengers' phones are nearly identical in operation, if not in quality, to the pilot/copilot headsets. I did my first aircraft

2-person intercom in 1975 and my twentieth design about three years ago. I haven't gotten into DSP yet, but that's on my list to learn about once I get the wireless gig down pat.

Jim

So what you seem to want is a Bluetooth aviation headset and a intercom "server"?

Try comp.arch.embedded. The regulars there have more experience in that application area.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Plus active noise cancellations via sampling and phase shifting. Digital comm along can't help with acoustic noises.

The regulars usually goes on both CAE and SED, but overwhelmed by Signal to Noise ratio of SED.

No master unit necessary...

Each box could consist of a mic pre, a squelch circuit to mute when signal below a threshold (unless the headsets have their own PTT switches), resistor feed to a pair of parallel jacks for chaining units, summing headphone amp so all chained units mix together. Each unit powered by its own

9V battery. Cheap.. probably about $20 parts or so for each unit not counting case/connectors/etc. No fixed limit to how many units can be chained together, interconnect using common 1/4" to 1/4" shielded cables (guitar jack cords).

Terry

When you get there let me know if it was actually helpful.

?-))

The idea was to wirelessly connect them all together.

Jim

I presume that since it's for a car, they won't be wearing helmets, but only naked headsets?

Why not buy two matched sets of RS (or whatever) walkie-talkies (i.e., all 4 on the same freq.) and hack them to use the plug and jack?

You'd still have the battery life thing, unless the back seats have lighter outlets or something.

Have Fun! Rich

...

Yea, probably would need a master unit for that... and much more expensive than wired units. Perhaps off-the-shelf transceivers could be dropped into an otherwise simple analog circuit...

mic pre stuff >----*---R---*---> summed headphone amp | | | `--R---< RX transceiver `--------------> TX module

... then in the master unit each transceiver input would sum the outputs of the other units, but not from itself to avoid echo/phasing from the transmission delay.

Terry

That be true. Naked headsets. No helmets.

What can you tell me about heterodyne squeal if two people try to talk simultaneously?

You all still don't have the idea. I want a wireless system that is low power, light enough for the passenger to either put in their shirt pocket or clip to their shirt, and of my design. I have no interest in hacking somebody else's design.

Jim

It will probably annoy everybody and the two who were trying to talk over each other will be shunned. >:->

Good Luck! Rich

Ah, so it's an ego thing after all.

Thanks! Rich

Like a DECT phone as someone mentioned earler?

--Winston

Those look nice... not quite the "$100" price point but close, probably can connect straight to the headsets.

When it comes to wireless it takes something existing and mass-produced to be affordable, once I looked into using bluetooth for a new product - forget it unless selling tens of thousands. Just the licensing and testing would have been a huge investment, even using a precanned solution. Jacks and wires are much cheaper :-)

Terry