These days, most radios have a digital tuner; one that changes the station by pressing a button. Pressing the button alters the frequency that the radio is receiving by a very small, discrete amount, allowing one to tune in a station more clearly. Formerly, most radio tuners were analog. Tuning in a station was accomplished by turning a knob, which in turn, moved a piece of wire or string inside the radio. This string was attached to an electronic component called a capacitor. The movement of the string moved metal plates inside the capacitor, and this physical change of the position of the plates caused the radio to turn up or down the freqency spectrum. The mechanical linkage allowed a person to adjust the signal very precisely (especially important for the weaker signals in the shortwave spectrum).
Changing the frequency by turning a knob produces a very subtlely shifting auditory experience, one that has an infinitely fine gradient. A person can use this to find exactly the right place, "the sweet spot" where the reception is best. On modern radios, this is usually not necessary unless the signal is very weak. If so, it can be very difficult or impossible to tune such a radio, with a digital tuner, to the very best spot that is available. At times like these, an analog control can be a nice thing to have around. Recognizing this need, manufacturers of digital music synthesizers and samplers have responded by adding to newer models special knobs that simulate the analog controls that former synths used to have - with the benefit of being able to store and recall the position, and thus the sound. Pretty neat. Systems that use digital interfaces that replace analog, mechanical linkages are called "fly-by-wire" systems in the airline industry, and "guide-" or "drive-by-wire" in other industries. Paradoxically, the phrase "-by-wire" refers to the electrical wire connecting the circuity, not the mechanical wire that connects parts of analog control systems.
In terms of user interface design, the difference is important to consider. Such decisions change how people perceive and interact with devices. The digital guide-by-wire devices increment in steps - hopefully ones that are small enough to be useful, but not so small that they are tedious. They also can usually store their state for later recall. Analog devices can be better for some tasks and provide extremely fine control, but are often more expensive to manufacture, and cannot store their state. The mertis of each approach must be weighed carefully in the design of any physical interface.
