31. These receivers include a photo-diode, an automatic gain control ( AGC ) circuit, and a demodulator . 32. Hybrid Analog / digital demodulators have permitted continued access to free-to-air national channels during the transition. 33. As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. 34. In this case, the circuit is sometimes called a " ring demodulator ", one of many possible chopper circuits. 35. To the receiver, the signals of other users will appear to the demodulator only as a low-level noise. 36. A demodulator can only be used to observe a conversation between two fax modems, usually to record the image transmitted. 37. The demodulator consists of a delay line interferometer which delays one bit, so two bits can be compared at one time. 38. During this period the demodulator was usually a carborundum point contact diode, but sometimes a vacuum tube grid-leak detector. 39. A typical optical system for such a purpose is Mach-Zehnder interferometer or Michelson interferometer, forming an optical DPSK Demodulator . 40. The signal is then processed by a burst demodulator , ( using the MF-TDMA protocol via the data scheduler ).
