acoustic modems and cellular phones

No, but you realize that you'll be lucky to get even a 2400bps modem to work with such a method, right? The CoDecs in cell phones assume they're taking human voice input and being heard by a human ear, and there's lots of "psychoacoustic" compression that throws out lots of audio information that humans can (pretty much) do without but that a typical modem absolutely needs to maintain its data transfer model.

Mmm... LM386?

---Joel Kolstad

I am aware that my connection speed would be less than spectacular. I work in the oil fields of a third world country and I am sometimes in the field for several weeks at a time. We can get a mobile phone signal most everywhere but internet is extreemly scarce sometimes. 300 baud would be a luxury.

Thanks for the amplifier suggestion... however I was kind of hoping for a ready-made solution. I'll be back out in the field soon and won't have time to build anything. Maybe like an amplifier out of some small powered computer speakers or something. I thought maybe someone would have an idea about a perfect amp for the job. But I'm probably on my own here from the looks of my responses.

Dave

You'd probably be better off to get a modem-ready cell phone driven from a modem card in a laptop.

...Jim Thompson

I'll second that... but I admire your initiative in just getting something, _anything_ to work.

There are still acoustic couplers out there that would probably work for low data speeds (newer ones had velcro straps that hold the speaker and microphone to the phone rather than the old 'cradle' style that only worked with the old generic Bell phones); this is probably the easiest solution. For a 'ready made' amplifier solution out in the middle of almost nowhere, I'd be tempted to tear apart some small amplified computer speakers, a boombox, etc.

---Joel

I have a design (proven in production ~1977) for a "muff-cup" acoustically-coupled 300-baud modem. But I found I got better distortion by driving the EARPHONE magnetically (for outbound signals), rather than acoustically thru the carbon mike.

But times have changed.

I'll see if I can find it in my archives. It used active-filter "S-curves" for demodulation.

...Jim Thompson

Cheers! Rich

Jim, you really need to take your show on the road one of these days and give all of us wanna-be analog designers some pointers. :-)

National lets Bob Pease out of his cage from time to time, although -- and this is nothing against the man, he's been working hard for decades after all -- he seems more interested in talking about how poorly his grandkids drive than how to design circuits anymore!

Back in the late '80s, GemStar came out with one of their VCR "programmers" (you entered a magic number printed in the TV guide that encoded a show's start time/date, it spit out the IR codes at the appropriate time to record the show) that could be programmed over the phone for your particular model of VCR and TV... you held the thing up to the phone after called a 1-800 number and touch-toned your make and model. The guy who designed that -- I think he was hanging out here for awhile -- said it was the cheapest speaker he could find, an op-amp and software for the microcontroller he already had. I.e., the feature cost something under a buck, and apparently was a big selling point for GemStar.

---Joel

I'd have to let MYSELF out of the cage ;-)

I'm heading that way myself... oldest granddaughter now has her learner's permit.

...Jim Thompson

If it is a digital phone, like a GSM, it will not work.

OTOH I've seen an NMT phone (narrowband FM) working on a 300 baud acoustic coupler once.

As I suppose all phone systems are digital now, look for a different solution.

Thomas

I have read bits and pieces out there on the internet. Some say it won't work and some say it just won't work at high speeds. As far as I'm concerned 300 baud would be ok and 1200 baud would be smokin'. Are you certain it won't work?

Unfortunately I cannot use a cell phone with built-in modem. GSM is the only available service and GSM phones do not use standard modem protocols. I would have to subscribe to WAP service in order to connect to the internet with my phone. This would be fine and good but the third-world-country-factor needs to be taken into account. I have to jump through all kinds of crazy bureaucratic hoops to get WAP access from the phone company. Even then, I've been told that the service is expensive and rarely functioning.

Dave

Other issues aside, what about the other end? Unless you are connecting to something you have set up, there is less and less chance that there'll be anything that connects at 300baud. I gather the modems at ISPs tend to be set so they can't connect at such a low speed. Not really a surprise, given that 300baud modems haven't been sold in twenty years or so.

Michael