Radio Shack TRS-80 PT-210 Service Manual page 17

Demodulator
COMPARETER
RxD
0
Â
+5V-+12V
-
Figure 14.
The analog data signal input through MIC IN is amplified by the amplifier, its noise is cut by the band
pass filter, and it is then input to the demodulator. The demodulator contains a frequency comparison
circuit, and it outputs the difference in frequency by a digital signal.