There are several schemes for modulating a signal for data transmission. Phase-shift keying (PSK) is a method of digital communication in which the phase of a signal is varied in order to transmit information. There are several methods that can be used to accomplish PSK. The simplest is called binary phase-shift keying (BPSK), which uses two opposite signal phases (0i – 180i) to encode bits. For example, the phase shift could be 0i for encoding a “0,” and 180i for “1,” or the phase shift could be -90 for 0 and +90i for a 1. In either case, the representation of 0 and 1 are a total of 180i apart. If the phase of the wave doesn’t change, then the signal state stays the same. If the phase of the wave changes by 180 degrees (reverses), then the signal state changes. Because there are two possible wave phases, BPSK is sometimes called “bi-phase modulation.”
In digital clocking, jitter is reduced with bi-phase modulation. Other modulation schemes, such as pulse position modulation (PPM), require control of time positions on a pulse-to-pulse basis. Therefore a series of wide-bandwidth circuits are required. This is where jitter accumulates. A bi-phase system only needs a stable, low-phase-noise clock, since the spacing of the pulses is constant. As such, narrowband synchronization circuits that won’t add significant jitter can be used.
Bi-phase is also an older synchronization technology used in the film industry. To synchronize audio systems with video equipment, devices such as the Digidesign Universal Slave Driver offered bi-phase sync, which was a clock reference signal that used two square waves 90 degrees out-of-phase to generate pulses. While bi-phase signals don’t contain positional information (forward or reverse), the Digi USD could determine the direction based upon which wave is read “high” relative to the other. For example, with some film equipment, when the device is running forward, it will generate a bi-phase signal where the A wave peaks before the B wave peaks. When the device is in reverse, the B wave will lead the A wave.