A Modem Case

A project log for An Old Fashion Acoustic Modem for the iPhone

My son recently returned from a holiday in China. His biggest complaint was the blocking of FaceBook!

agp.cooperagp.cooper 01/29/2017 at 12:550 Comments

A Modem Case

Spent the day designing a case for the modem. Basically a piece of 18 mm thick MDF.

I chose MDF as it has a soft edge when cut and should suit this application. The design consists of a top layer for the iPhone and mounts for the microphone and speaker. The second layer (to do) will hold the electronics etc. The iPhone will be fully below the top of the MDF so that the modem case can have a lid. Here is the top layer:

This morning I backed out of the "carved" modem case and redesigned a plywood version:

Th disks are temporary feet.

Here are the cut pieces:

And assembled with the microphone and speaker in place (to be glued later):

And the iPhone happily sitting on top:

Now to design the electronics compartment. The simplest power supply and serial interface is to use the Arduino/Nano USB plug and a USB cable. I will also need a switch (on the base) for send or receive mode.

Half duplex means you need two modems and iPhones on each end (unless I can get the back channel to work).

Testing Speakers

Even though I have made the top of the case for the small speaker I had not actually tested the speaker. I assumed that the smaller speaker with less inertial would be better - wrong!

Here is the small (26.7 mm diameter) speaker signal as recorded by the Nano:

Looks good except for the AM modulation. Here is the signal after the bandpass:

Looks even better except for the AM modulations. Here is the signal after the lowpass:

See how the AM modulation upsets the decode and in fact the decode fails (repeatedly at this point). Here the medium (39.5 mm) speaker signal:

It looks worse but look at the signal after the bandpass:

It is pretty good. Here is the signal after the lowpass:

No problems. All of the above is repeatable. No difference with a Zobel network and no hint of issues when measuring the speaker terminals.

So I will have to rework the case for the larger speaker.


I have some 6.4 mm thick plywood in the shed so I will cut the electronics enclosure and the base tomorrow. I will cut the strip-board the same size as the other pieces and sandwich it near the base.

I don't have bolts long enough for the complete sandwich pile but that can be fixed later.

Designed the rest of the case. Decided to glue the top sandwiches and conceal the bolts for the bottom sandwiches.


Cheap Chinese CNC Machine

I have a cheap Chinese CNC machine, so I can make neat little wooden objects (such as the modem case) but I can tell you these machines are hard to use and are very slow:

On the user side:

So tonight I had one success out of three attempts. Not happy! I will have another go tomorrow, five pieces remain. Here is the stack so far:

I have re-cut the speaker hole to fit the 40 mm diameter speaker and added a 6.4 mm thick cover to hide the assembly bolts. The iPhone will now sit just below the top of the case.

Speaker Tests

I tested the modem at 1200/2200 Hz and not too bad.

Tested the modem at 400/500 Hz and evidence of strong AM modulation upsetting the demodulator:

I suppose it is too much to ask for a speaker not to resonated at any frequency of interest. Not keen on a two speaker design.

I wonder if paper cone speakers are better? I will have to buy a couple and try them.

I suspect stopping the speaker from resonating may be a bit of an ask so I need to look at suppressing AM modulation in software or going directly to a FM discriminator. Looking at he above, it looks like close to 100% AM modulation, that may be difficult to remove.

Thoughts on Speakers

Unsoldered a small paper speaker from my 2 transistor MW radio:

But it was worse, AM modulation (~120 Hz) and frequency was way off (slow to respond):

This is what the LP is producing:

Three More Speakers Tested

Bought three more small speakers, 30 mm mylar and 50 mm mylar and a 30 mm metal ("modem") speaker. The modem and 50 mm speakers failed but the 30 mm just worked (or could with improved filters). So a quality speaker (and these speakers are not quality) would likely work. However I have move on to the Bell 103A specification so the issue is not important anymore.