I hacked together a tty demodulator using some old 88 mH toroids and surplus caps. A boat anchor HP signal generator, old frequency counter and crappy old Heathkit scope provided the means to tune the coil/cap combos. I made a "ratio" detector from those parts that could key my old 160 mA loop using nothing but the audio coming from the speaker output of my old Yaesu FT221. The model 15 teletype I had refurbished was happy to capture the single strike art that Don Royer (WA6PIR) would put out on a Thursday night.
I'm still quite capable of designing that kind of gear from scratch. As long as I have a modern computer, I can do most of that modulation/demodulation of the sound card alone. There are great open source software defined radio packages available on the net. In a pinch, psk31 works a bit better than the old 170 Hz shift AFSK for simple "chat" over the air.
It appears Don Royer is a silent key now. His RTTY art work is still available via archives on the net.
Yep, you are correct, sound cards work well for a/d and d/a conversion on the limits the FCC sticks us with. I understand there are some physics limits but Nyquist theory suggests we could do better than 110 baud on the low frequency bands.
I do know about AFSK on 2 meters and above.
It just gripes me that hams invented most of this stuff and we get very limited by rules and regulations.
Even the origins of packet are obscured today in modern documentation.
CCIT takes credit, but it was hams that built it.
/johnny