The analogue-to-digital conversion (ADC) process is used to convert analogue signals to digital data. The diagram shows the stages of this process. (a) Desc...

(a) The process of analogue-to-digital conversion for a sound recording:

1. A microphone converts sound waves in the air into a continuous analogue electrical signal whose voltage varies with the amplitude and frequency of the sound. [1]
2. The sampler measures the amplitude of this analogue signal at regular intervals determined by the sample rate (for example, 44,100 times per second at CD quality). [1]
3. Each measured amplitude value is rounded (quantised) to the nearest available level determined by the bit depth. [1]
4. The rounded values are encoded as binary numbers and stored as a sequence of digital data in a sound file. [1]

(b) Quantisation is the process of rounding each measured amplitude to the nearest available level, because there are only a finite number of levels determined by the bit depth (for example, 256 levels with 8-bit depth). [1] The difference between the actual amplitude and the nearest available level is called quantisation error. With a low bit depth, fewer levels are available, so the rounding error is larger, producing audible noise or distortion in the recording. [1]

(c) The reverse process (digital-to-analogue conversion):

1. A digital-to-analogue converter (DAC) reads the binary values from the digital file and converts each sample back into an analogue voltage level. [1]
2. The resulting analogue electrical signal is sent to a speaker, which vibrates a diaphragm to recreate sound waves that travel through the air and can be heard. [1]