magnetic sensors to detect rotational position changes. The mechanical type is commonly employed as a manually operated "digital potentiometer" control on electronic equipment. For example, modern home and car stereos typically use mechanical rotary encoders as volume controls. Encoders with mechanical sensors require switch debouncing and consequently are limited in the rotational speeds they can handle. The optical type is used when higher speeds are encountered or a higher degree of precision is required. A rotary incremental encoder has two output signals, A and B, which issue a periodic digital waveform in quadrature when the encoder shaft rotates. This is similar to sine encoders, which output sinusoidal waveforms in quadrature (i.e., sine and cosine), thus combining the characteristics of an encoder and a resolver. The waveform frequency indicates the speed of shaft rotation and the number of pulses indicates the distance moved, whereas the A-B phase relationship indicates the direction of rotation. Some rotary incremental encoders have an additional "index" output (typically labeled Z), which emits a pulse when the shaft passes through a particular angle. Once every rotation, the Z signal is asserted, typically always at the same angle, until the next AB state change. This is commonly used in radar systems and other applications that require a registration signal when the encoder shaft is located at a particular reference angle. Unlike absolute encoders, an incremental encoder does not keep track of, nor do its outputs indicate the absolute position of the mechanical system to which it is attached. Consequently, to determine the absolute position at any particular moment, it is necessary to "track" the absolute position with an incremental encoder interface which typically includes a bidirectional electronic counter. Inexpensive incremental encoders are used in mechanical computer mice. Typically, two encoders are used: one to sense left-right motion and another to sense forward-backward motion. Other pulse-output rotary encoders Rotary encoders with a single output (i.e. tachometers) cannot be used to sense direction of motion but are suitable for measuring speed and for measuring position when the direction of travel is constant. In certain applications they may be used to measure distance of motion (e.g. feet of movement). See also Analogue devices that perform a similar function include the synchro, the resolver, the rotary variable differential transformer (RVDT), and the rotary potentiometer. A linear encoder is similar to a rotary encoder, but measures position or motion in a straight line,