Abstract A technique is described which uses a digital circuit based on a phase-locked loop to permit viewing, photographing, or measuring phase-dependent phenomena in cyclic systems. This eliminates the need for expensive and tedious high-speed cinematographic methods that have been used thus far for visual observations of cyclic systems. The technique provides accurate quantitative data on visual cyclic phase-dependent phenomena that occur within the system with the same frequency as the forcing vibration. The technique has been used in studies of a vibrated bed. It may also be used with vibrating gas-fluidized beds. With little modifications, the technique may be applied to study phase-dependent phenomena in other cyclic systems. In direct observations at a series of phase angles, using a strobe light activated by the digital circuit, particle-free air gaps appear above and below a horizontal heat-transfer tube placed within a two-dimensional vibrated bed. The time-integrated percentage of heat-transfer surface blanketed by air, estimated from back-lit photographs, explicates trends in the heat-transfer data. A rarefied zone of reduced particle density forms at the top surface of a vibrated bed. In a bed in which solid circulates, down at walls and upward at the center, the rarefied zone is the major path for return flow of solids from the center to wall. Phase-shift photographs show that the rarefied zone develops during lift-off of the bed from the vibrating plate, indicating that rarefaction occurs because a downward flow of gas, necessary to supply gas to the gap forming between the bed and plate, exerts a lesser drag on the top layer of particles in the bed that it does on the remainder of the bed. The phase-delayed trigger system ha also been used to facilitate measurements of non-visual phase-dependent properties such as gas pressures below the bed throughout a cycle of vibration.