Computer Products Plus - Telecoupler II Acoustic Coupler Overview

Overview

The acoustic coupler was one of the must-have hacking tools back in the 1980s and early 1990s.  The acoustic coupler allowed you to connect your modem to a phone which didn't have an exposed RJ-11 jack.  Of course, what this meant, was you could now connect your modem up to a payphone.  Then, armed with a list of "borrowed" calling card numbers, a cheap laptop computer, a good external modem, and a decent terminal program - you could hack the planet!

Using payphones for your hacking expeditions yielded a certain amount of anonymity (and alot of line noise).  Hacking from a payphone using an acoustic coupler usually limited your maximum connection speed to around 1200 bps or so.  You could sometimes hit 2400 bps if there was no ketchup inside the little airholes on the receiver...  A good external modem was also a necessary tool.  External modems were used because their LEDs gave a good indication of any problems with traffic flow, and they could also be "hard" disconnected by just turning them off.  The hardest part of portable hacking was often trying to find a good terminal program which could handle all the weird emulations and connect rate settings out there.  Several sites would use weird modem speed rates, like 600 bps, as a security measure.  The terminal program (and modem) also had to allow manual dialing & connection, and still function if there was no dial tone.  You would often do all the dialing by hand using the payphone's own keypad, then when you heard the remote modem answer, try to initiate a manual connection sequence.  A portable Radio Shack telephone amplifier with an induction pick-up coil was also a handy tool to monitor the modem connection "off-hook."

The pictures shown here are of a Computer Products Plus (CP+) Telecoupler II High-Speed Modem Coupler from around 1993 or so.  As you can see, there is not much to it.  After being strapped to a payphone's handset, the coupler picks up the receive audio using an electric microphone and an op-amp with a good phase and frequency response.  The audio is then slightly amplified, RC filtered, and sent off to the modem's RJ-11 jack using a simple coupling circuit.  On the transmit side, the signal from the modem is again slightly amplified, RC filtered, and then fed into a standard 600 ohm speaker.  A portion of the transmitted signal in fed back into the receive op-amp (sidetone).  This appears to be the only "confusing" part of the reverse engineering the coupler.

At low-speeds (2400 bps and lower), the modem uses in-band audio tones to represent the ones and zeros of the digital data stream.  The modem transmits a 1070 Hz tone for a binary "0" and a 1270 Hz tone for a binary "1".  On receive, it recognizes a 2025 Hz tone for a binary "0" and a 2225 Hz tone for a binary "1".  These frequencies are for the source modem originating the transmission.  The frequency assignments are reversed for the destination modem receiving the information.

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Modem Audio Tones

                   Modem Transmitting Tone (Hz)    Modem Receiving Tone (Hz)
                         0     1                         0     1
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Modem Called Computer    1070  1270                      2025  2225
Computer Called Modem    2025  2225                      1070  1270
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Pictures

Internal view of the Telecoupler II.  The "transmit" speaker is on the left, the "receive" microphone is on the right.

The acoustic coupler is powered from a single 9 volt battery.

Alternate view.  Speaker is on the left, electric microphone is on the right.  The battery compartment is in the middle.

Closeup internal view.  The "transmit" section is on the left, the "receive" section is on the right.

Across the top, jacks "J2" are the 9 volt battery connection, "J3" is the line input (from the modem), "J4" is the transmit speaker, "J5" is the receive microphone.

The BA10358 dual ground-sense op-amp is for "transmit."

The LS204CB low-distortion, low-noise op-amp is for "receive".  The DC coupler capacitor from the microphone element is 0.1 µF.

There is a 1N5350 5 watt, 13 volt Zener diode on the line input to catch and voltage spikes on the line.

The switch on the lower left adjusts the gain of the transmit op-amp.  More speaker gain can be added for "noisy" phones.

The transistors do the normal phone line interface switching.

Coupler view without the internal PC board.  The speaker and microphone element are mounted in very soft rubber cushions.

Closeup circuit board view.  Speaker/DC/line conditioning section is on the left.

Closeup circuit board alternate view.  Speaker section is on the right.

Bottom view of the circuit board.  Speaker section is on the right.

Bottom view, now rotated.  Speaker section is on the left.







      

We are all going to die!

Telecoupler II Manual









U.S. Patent Number 4,599,491