There are two types of body waves: compressional (P) waves and shear (S) waves. Primary waves (P-waves) travel the fastest, so they arrive at the recording station (seismograph) first. Secondary or shear waves (S-waves) travel slower than P-waves so they arrive second.
Modified frequency modulation (MFM) is an encoding scheme used by PC floppy disk drives and older hard drives.
From Wikipedia, the free encyclopedia. Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of possible required time-shifts. This is repeated every T seconds, such that the transmitted bit rate is bits per second. complete PPM frame is about 22.5 ms (can vary between manufacturer). Signal low state is always 0.3 ms. It begins with a start frame (state high for more than 2 ms). Each channel (up to 8) is encoded by the time of the high state (PPM high state + 0.3 x (PPM low state) = servo PWM pulse width).