Acoustic Couplers

Jun 10 04:39 UTC 1996

This got started somewhere else, but I'd like to seek more extensive
assistance, so am starting a new item about Acoustic Couplers. 

response 2 of 23:

Jun 10 05:18 UTC 1996

I bought a "surplus" Acoustic Coupler from Alltronics ($4.95), and have
tried to get it to work on a 2400 baud modem. The Coupler consists of two
rubber cups connected by a wire, with a wire to a modular phone plug. A
9-volt battery is used internally. I tried using it with an INCOMM Midget
24 from a Mac running Versaterm. I could not connect. Here are
observations on what occurred, and some information on internal circuit
details: 

First - the instructions that came with it do not identify a make, nor is
there anything on the coupler except a sticker saying "Made in Hong Kong".
However the instructions say that it is to be used with the "Datalink 1000
telephone receptacle". 

When attempting to connect, the modem does not appear to hear the dial
tone.  It is very faint from the modem speaker, although when left on
longer and the telephone warning is sent, I can hear and understand that.
The coupler earpiece seems pretty sensitive, as touching it makes noises
on the modem speaker. 

When I take the modem off hook without a phone in the coupler, and send a
atdt###, the modem speaker responds but not the coupler speaker (I checked
this also after atm0, to make sure it wasn't just faint.)

All 4 wires of the modular connector enter a circuit board in the earpiece
cup (where the 9-volt battery is also). Only two wires go on to the
speaker-cup. On the circuit board is the microphone, plus two resistors
(1,500 and 10,000 ohms), and a solid state device with three heavy leads.
It looks like a triac or rectifier, but without the heat-conduction tab.
The numbers on it are 226 and (E or F or 5)301, along with the Motorola
"M".  The board has been buried in a moderately clear, hard, casting resin
so its underside cannot be inspected to see the "wiring". 

There is 8.2 V across the middle red (+) and green (-) pair on the modular
connector. There are no voltages on the outer black and yellow pair, nor
are these connected to the inner pair even with high resistance. However
across the outer pair one finds 1,000 ohms (perhaps the speaker?). 

That's all I know. I speculate that the "Datalink 1000" speaks across the
outer pair and listens on the inner pair. Of course, a standard modem does
everything on the inner pair. I don't know how a modem does this so I
don't know how to adapt this device to work with an ordinary modem. 

Any suggestions?

response 4 of 23:

Jun 10 07:36 UTC 1996

Another place you could probably find a coupler is at UM property disposition.

response 6 of 23:

Jun 10 15:08 UTC 1996

An older phone won't provide much more volume. The rubber cups fit OK
on the squarish ear/mouth pieces of the phone I used. I think that the
"Datalink 1000" might have some amplification for the microphone, since
an acoustic link is much weaker than a direct link. Unless, of course,
this unit is supposed to have more amplification but is defective. There
may also be an impedance mismatch into a modern modem compared to the
Datalink 1000. Another possiblity is to gut this unit and just use the
rubber cups (sounds like arthurup's plan) and install thingies that will
work into a modern modem. Does anyone know what those might be? How does
a modern modem switch between send and receive - just very fast? 

Alltronics sells just the carrying case that comes with this unit, by
itself, for $2. I wonder if they just trash the coupler if someone wants
to buy just the case (!?).

response 8 of 23:

Jun 11 06:26 UTC 1996

My phone is as loud as I would want any phone to be. That's all that
matters as the phone earpiece is just "talking" into the coupler
microphone (from 1/4" away). Apparently, though, it isn't loud enough
when it couples to an ordinary modem to trigger the dial tone detector.
Hmmm...if I just put the microphone and speakers pairs in parallel, I
might get some feedback (i.e., sidetone) gain. Worth trying........

response 10 of 23:

Jun 12 05:33 UTC 1996

Yes, it works by sound. It is called an *ACOUSTIC* Coupler in the slip
enclosed (and catalog), and the microphone and speaker cups have the
patterns of holes usually associated with each kind of transducer.
,

response 12 of 23:

Jun 13 18:59 UTC 1996

You can buy acoustic couplers that work up to 14,400 baud or so...if you 
want to spend money.  They're not really cheap.

response 14 of 23:

Jun 13 22:51 UTC 1996

The problem is the processing to separate the incoming from the outgoing...
the classic two wire to four wire problem.  If you don't solve that, you get
feedback.  Solving it isn't hard, just expensive.  Have you ever seen a 2 line
phone with a conference feature that actually worked?  Same problem, too much
$$ to fix it right.

response 16 of 23:

Jun 14 06:55 UTC 1996

I have a 2-line phone with a conference button - and it works. What
do you mean? Ordinary phone do solve the coupling problem to separate
the incoming and outcoming audio between the ear and mouth pieces - with
some ingenious balancing transformers. If I could get at the *separate*
input and output signals of the modem, then it would be possible to
install amplifiers - and decouple. However, as you say, $$$$$$. That
must be what the Datalink-1000 does. Too bad they aren't available surplus
like the coupler is.

response 18 of 23:

Jun 14 18:22 UTC 1996

Ordinary phones were designed almost from the beginning to let you hear
the other person *and not yourself* in the earpiece. However the phone
designers circumvented this by adding an imbalance to create the
"sidetone", which would not normally exist. It was found in early phone
design that without sidetone, people tended to shout. As I said, how this
is done with transformers is very ingenious. I'm setting up a similar
circuit using RS transformers to accomplish the same thing (for a field
phone system). 

On my two-line phone I can call someone, put them on hold, call someone
else on the other line, and then press "conference" - and voila, it is a
conference call. I have not tried it with two people calling in (no one
calls on our data line as it is unlisted - well, no one except salepeople.
We ignore it when it rings.) Interesting question though - it should work
the same. I'll have to arrange to try it. The phone is a GE Model 2-9420 -
but I thought this was a common two line phone feature. 

response 20 of 23:

Jun 15 15:33 UTC 1996

Rane, I think you're misunderstanding what Scott is saying. When you are
"the man in the middle" most 2-line phones with a conferencing feature
work fine for *you*, but have you ever tried being out on one of the ends
of such conversations? On almost every phone I've tried this on, it is
ussually very hard to hear the *other* person out on the other end. For the
guy in the middle, such setups ussually work just fine, it's the 2 othe
people that have a devil of a time understanding each other.

response 22 of 23:

Jun 17 06:27 UTC 1996

Got it! (finally) We'll try that out next time. The few times we have used
it the people out on the limbs didn't complain...but we'll have to ask.