In the past, for simple, relative synchronization, a "sync" or "tach" pulse was sometimes used. For example, a camera might send out a square wave pulse when it exposes each frame or a motorized system may send out a pulse train that varies in frequency as it runs. With either of these systems, other devices which receive the information, can synchronize to the pulses. Another, analog approach, found in older film production equipment, is use of the "pilot tone." With such a system, a tone is recorded on a media's track. As the device moves, a tone of a specific frequency is generated- as the device speeds up, the tone's frequency will increase.