To create a digital signal from an analog source -- light, sound or pressure -- you turn a continuously variable input into a result made up of binary data. Because digital signals consist entirely of on and off states, represented by ones and zeros, binary signals typically contain steps in multiples of two. The orderly nature of a digital representation simplifies the task of separating relevant signal information from disorganized noise. Analog-to-digital converters process data through circuits designed and tuned for specific types of signals.
Processing the Signal
The resolution at which you translate an analog input into a digital signal determines how accurately the conversion represents the original information. Increasing the resolution reduces the effects of quantization error, the variance between the analog signal coming in and the digital reproduction coming out. Bit depth determines dynamic range, which represents the difference between the maximum output of an analog-to-digital converter and its noise floor, the total of all unwanted signals. Sampling rate determines how often the converter grabs data from the input signal, specified in samples per second, with higher values capable of yielding greater fidelity to the original. For example, CD-quality sound uses a sampling rate of 44.1 kHz, or more than 44,000 samples each second.
Musical instruments, such as guitars, pianos, violins and saxophones, create analog audio, as do singing, speech and sounds from the natural world. Likewise, microphones capture sound as an analog signal. To bring audio information into a computer so you can edit and manipulate it, you must translate sound waves into digital values. An analog-to-digital converter bridges the gap between the two realms. Some "digital" microphones include built-in converters that translate audio to bits and bytes in real time. If you translate a digital audio file from a lossless format, such as WAV or AIFF, into a lossy format, such as MP3, a digital conversion process alters the data to suit the destination.
Whether you use a digital camera or camcorder or rely on a film camera or analog camcorder to capture still images and video footage, the original signal consists of analog information. Digital cameras and video gear incorporate the translation from light -- the analog signal -- to digital directly into the gear itself. To translate a film image into a digital signal, you use a scanner that illuminates the image and captures it frame by frame, translating the light and dark areas of the original into values along a digital continuum from light to dark.
Before the era of smartphones and digital communications over VoIP lines, the telecommunications system consisted largely of analog equipment. Your voice creates an analog signal, which the microphone inside a traditional telephone passes on in analog form as an electric current through an analog signal chain. Digital phone gear requires an analog-to-digital conversion to send your voice over the Internet datastream that carries VoIP calls or the laser pulses transmitted through a fiber optic cable. Converting analog to digital produces a datastream made up of series of ones and zeroes that a digital communication system can transmit at high speed, conveying thousands of conversations over a single fiber optic cable strand.
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