Analogue Signals
Analogue signals are transmitted continuously. VCRs, walkie-talkies and turntables use analogue signals. They are usually sent as electrical impulses, but in the case of walkie-talkies, they are transmitted via radio waves.
Digital Signals
A digital signal is composed of a stream of binary data. Binary is a computer language that consists of a series of 0s and 1s that are interpreted by digital devices. Digital cable, HD radio and MP3 players utilize digital signals to create output.
Conversion
During conversion, the analogue signal is passed to a device for interpretation. A computer's audio input, for example, accepts an analogue signal such as one from a microphone or tape player. The electronic impulses of this signal are interpreted by the analogue to digital converter (ADC). The ADC converts the interpreted analogue signal to a stream of binary data before passing it to the appropriate program.
Sampling
When used for audio, the ADC determines the quality of the resulting sound by sampling the audio. The process of sampling is similar to the frame rate of a video camera. The higher the sampling rate, the more snap shots of the audio the ADC captures. Just as a film recorded with a higher frame rate will appear smoother and more realistic, an audio stream with a higher sampling rate will sound more like the original analogue audio.
Video Applications
ADCs are also used in devices that capture the output of a video stream. Digital video recorders (DVRs) and up converters (i.e., boxes that convert an analogue television signal for a digital input such as HDMI) utilize ADCs. The process of conversion is not strictly real time, so the ADC must buffer the input signal as it is converted. This can result in choppy video, especially with lowered quality ADCs. This is more noticeable on high-definition televisions (HDTVs) than on older TV sets because HDTVs rely on clean digital signals.
Reserve Conversion
Digital audio must be converted back to an analogue signal for proper playback. This is done by the inverse of an ADC, a digital to analogue converted (DAC). These devices, which are found in digital audio players such as iPods, convert the digital signal to an electrical impulse that can be output over speakers or headphones. DACs are also used for older TV sets that cannot interpret digital signals, such as those that require a digital converter box for the June 2009 digital TV switch.
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