acoustic modems and cellular phones

[Dave]

Hey guys, just wondering if anyone has any experience in using a
cellular phone with an acoustic modem. I have an acoustic modem but my
cell phone simply will not fit nicely into the cradle. I wired the
hands free cord directly into the modem but it seems there is a
problem. I don't think the signal from the hands free output is large
enough for the modem to detect. I know that the volume of the sounds
emitted from the modem's mouthpiece are very low when I plug in my
phone and dial a BBS. Also, the sounds I hear from the remote modem
tell me that my modem isn't detecting anything.

I thought about wiring in some type of amplifier but I wanted to ask
here first. Has anyone got experience with cell phones and acoustic
modems? What would be a good amplifier to use to bring the cell phone
hands free output up to adequate volumes?


Thanks

Dave

 

[Joel Kolstad]

I thought about wiring in some type of amplifier but I wanted to ask
here first. Has anyone got experience with cell phones and acoustic
modems?

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.

What would be a good amplifier to use to bring the cell phone
hands free output up to adequate volumes?

Mmm... LM386?

---Joel Kolstad

 

[Dave]

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

 

[Jim Thompson]

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

 

[Joel Kolstad]

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

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

 

[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

 

[Rich Grise]

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

http://www.google.com/search?q=cell+phone+modem
first hit:
http://mobileoffice.about.com/od/usingyourphone/a/cellmodem.htm

Cheers!
Rich

 

[Zak]

Hey guys, just wondering if anyone has any experience in using a
cellular phone with an acoustic modem.

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.

I thought about wiring in some type of amplifier but I wanted to ask
here first. Has anyone got experience with cell phones and acoustic
modems? What would be a good amplifier to use to bring the cell phone
hands free output up to adequate volumes?

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


Thomas

 

[Joel Kolstad]

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

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

 

[Jim Thompson]

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.

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

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!

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

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

...Jim Thompson

 

[Dave]

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

 

[Michael Black]

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?

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