when activated. By placing the phosphors directly next to each other, and activating them with different intensities, color monitors can create an unlimited number of colors. In practice, however, the real number of colors that any monitor can display is controlled by the video adapter. Auditory A speaker is an output device that produces sound through an oscillating transducer called a driver. The equivalent input device is a microphone. Speakers are plugged into a computer's sound card via a myriad of interfaces, such as a phone connector for analog audio, or SPDIF for digital audio. While speakers can be connected through cables, wireless speakers are connected to the host device through radio technology such as Bluetooth. Speakers are most often used in pairs, which allows the speaker system to produce positional audio. When more than one pair is used, it is referred to as surround sound. Certain models of computers includes a built-in speaker, which may sacrifice audio quality in favor of size. For example, the built-in speaker of a smartphone allows the users to listen to media without attaching an external speaker. Interface The interface between an auditory output device and a computer is the sound card. Sound cards may be included on a computer's motherboard, installed as an expansion card, or as a desktop unit. The sound card may offer either an analog or digital output. In the latter case, output is often transmitted using SPDIF as either an electrical signal or an optical interface known as TOSLINK. Digital outputs are then decoded by an AV receiver. In the case of wireless audio, the computer merely transmits a radio signal, and responsibility of decoding and output is shifted to the speaker. Form factors Computer speakers While speakers can be used for any purpose, there are computer speakers which are built for computer use. These speakers are designed to sit on a desk, and as such, cannot be as large as conventional speakers. Computer speakers may be powered via USB, and are most often connected through a 3.5mm phone connector. PC speaker The PC speaker is a simple loudspeaker built into IBM PC compatible computers. Unlike a speaker used with a sound card, the PC speaker is only meant to produce square waves to produce sounds such as beeping. Modern computers utilize a piezoelectric buzzer or a small speaker as the PC speaker. PC speakers are used during Power-on