Back in the 1980s, a viable modem cost hundreds of dollars. Even in the 1990s, you were looking at spending a a Benjamin or two to get computer squawking down the phone lines. According to [Cameron Kaiser], though, it’s possible to whip up a softmodem using a Commodore 64 for much cheaper than that. How much? Just 35 cents, we’re told!
The inspiration was simple—Rockwell apparently used to build modems using the 6502. The Commodore 64 has a 6502 inside, pretty much, so surely it could be a softmodem, right? Indeed, one [John Iannetta] had done this in a one-way form in the 1980s, using the Commodore 64’s SID audio chip to output data in sound form. In 1998, he espoused the 35-cent modem—basically, the price of buying an RCA jack to hook up a phone line to your Commodore 64.
As [Cameron] found out, the concept still works today, as does [John’s] code, but it’s more like 68 cents in 2025 dollars. With the right bits and pieces, and a little code, you can have your C64 modulating data into sound at rates of 300 baud.
It’s hacky, slow, and there’s no real way to receive—the C64 just doesn’t have the chops to demodulate these kinds of signals on its own. You also shouldn’t use it on a real phone line if you don’t want to damage your C64. Still, it’s a wonderful bit of hackery, and it’s fun to see how well it works. We’ve seen some other great Commodore 64 modem projects before, like the ever-useful RetroModem. Meanwhile, if you’ve got your own communication hacks for the computers of yesteryear, don’t hesitate to let us know!
Q: What does the “-dem” part in “modem” stand for?
Demodulator. Turn in your geek card.
Demodulator. Something this does not have, making it ‘a Commodore 64 softmo’. Hence the original question.
Well since the SID chip is basically a hardware modulator, i guess it’s actually a ‘mo’ .
I think the point is that technically is a mod-, there is no -dem to it.
besides that it’s also illegal to hook up to a POTS, but would be fine for an internal PBX
Why would it be illegal?
It’s not type-approved.
If I understood it correctly, the proposed modem can only send but not receive.
The main challenge is to receive an audio signal with the processing power of the 1980s.
Even that isn’t technically difficult – the only question is what speed you can achieve.
The vast majority of 8-bit computers in the 80’s could save and load data to tape, and generally this was implemented entirely in software. Realise that that is also a fully-functional modem, it’s just simplex and connected to a tape recorder instead of a phone.
Beauty of Bell 103/V.21 is there is little to process, you just need two analog filters (four if you want both originate/answer) and a comparator. Maybe another $5.
Same goes for Bell 202/V.23half-duplex 1200 baud, commodore should be able to handle it no problem.
V.22 full-duplex 1200 baud is another story and requires more electronics to handle phase shift keying.
Anything faster a we start talking about QAM/Trellis and echo cancellation meaning requires real DSP.
Transmit only and would break if connected to a phone line?
I feel like this is maybe 33% of a modem at most.
Rockwell might have used 6502 to co-ordinate operations, but the data pumps did the hard work of modulating and demodulating. The hybrid took care of removing the transmitted signal from the received signal to make it easier to demodulate. Modems, even a V22bis one (2400bps), are surprisingly complex!
I miss my LM741 “modem” on the ol 8086.
Was it Hamcomm and JVFAX. Was amazing what could be done
with RS-232 port and a few cheap parts.
Also ran “Digicom” on the C64 with a 2206/2211 dumb modem.
AX25 at its best…lol