types of models. The transmitter radio broadcasts using AM or FM using PPM or PCM. Each aircraft needs a way to determine which transmitter to receive communications from, so a specific channel within the frequency band is used for each aircraft (except for 2.4 GHz band, and amateur radio-only 70 cm systems; which use spread spectrum modulation, described below). Most radio control systems traditionally on low-VHF-band frequencies before the 21st century have traditionally used crystals to set the operating channel in the receiver and transmitter. It is important that each aircraft uses a different channel, otherwise interference could result. For example, if a person is flying an aircraft on channel 35 (used for 72.490 MHz in North America), and someone else turns their radio on the same channel, the aircraft's control will be compromised and the result is almost always a crash. Since multiple RC frequency use began in the RC hobbies in the mid-20th century, so-called "frequency pins" have been used to ensure that only one modeler was using a particular frequency at any one time, for "traditional" style, crystal-controlled RC system use . The common, spring-loaded two-piece wood clothespin marked in some manner with text and/or color-coding for the designated frequency it references, usually with an added piece of thin plywood or plastic on the clothespin to place the text or color-code upon for greater visibility is the usual basis for these. Usually, the model club itself will possess some sort of "transmitter impound" facility at their modeling site for secure storage of the modelers' transmitters when not in active use while visiting the facility, and usually provides some sort of fixed "frequency control board" nearby to the impound area. The "frequency control board" at a modeling club's facility is used in one of two ways: either the club provides sets of frequency pins itself, already clipped onto the control board for the modeler to take the appropriate pin for their modeling activity (clipped onto their transmitter's antenna, in a so-called "subtractive" method) while their transmitter is in use away from the impound area, or with the modeler required to provide them for their own transmitter(s), and places them on a club facility's existing frequency board (the "additive" method) whenever they are using their RC transmitter. A modern computer radio transmitter and receiver can be equipped with synthesizer technology, using a phase-locked loop (PLL), with the advantage of