PULSE POSITION MODULATION ENCODING
Pulse position modulation (PPM) encoding is the system where encoding is carried out by positioning the pulses. In the PPM method of signal encoding, the transmitted signal takes the form of a binary coded word formed from a succession of short bursts of ultrasound. Ultrasound frequencies are not included in the code. The code is determined by the time interval or space between successive ultrasound bursts. This principle is illustrated in Fig. 35.4. Here the frequency of ultrasound is 40 kHz.
In Fig. 35.4 (a), a regular train of six ultrasonic pulses, each of duration 3 ms, is repeated at 27 ms intervals. The separation rate is therfore 1000/27 or 37 Hz and the separaton time from trailing edge to leading edge ...
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