Ideal Binary Pulse Transmission by AM and FM

1358 1360 1363 1365 1367 1370 1372 1376 1379 1382 1386 1389 1391 1394 1398 1400 1403 1406 1411 1413 1413 1416 1424 1426 Transmission of digital or analog information by binary rather than by multilevel pulses offers significant advantages in systems design. For one thing, it simplifies the implementation of regenerative repeaters and various kinds of terminal equipment, such as carrier modulators and demodulators, and devices for timing-wave provision, coding and storing of messages and automatic error-checking or correction. For another thing, binary pulse transmission imposes less severe requirements on the transmission medium with respect to signal-to-noise ratio, amplitude and phase deviations over the channel band, and tolerable transmissionlevel variations. Because of these advantages, binary rather than multilevel pulse transmission is ordinarily the more practical and economical method, even in existing channel facilities designed primarily for voice or other analog transmission, where consideration of the rather high signal-to-noise ratio alone would permit a much greater number of pulse amplitudes and, thus, a substantially greater channel capacity than could be economically realized. The three principal methods of binary pulse transmission by carrier modulation now in use are double- and vestigial-sideband AM, in the form of "on-off" keying with envelope detection, and FM in the form of "frequency-shift" keying. With synchronous or homodyne detection in AM, other methods are feasible that afford a bandwidth saving or improved signal-to-noise ratio or, like FM, have the advantage over "on-