For modulation a monolithic phase-locked loop (PLL) IC xr2211 used.
The demodulation XR-2206 a monolithic function generator was used .
Calibrate the Mark tone 1200Hz at low TXDATA level , and
the space @ 2200Hz at high TXDATA .
Why do I need to build such an old style modem ?
1st because it a piece of the digital communication history .
2nd its a very good demo tool for the digital modulation topics.