Analog recording vs. digital recording

Analog recording versus digital recording compares the two ways in which sound is recorded and stored. Actual sound waves consist of continuous variations in air pressure. Representations of these signals can be recorded using either digital or analog techniques.

An analog recording is one where a property or characteristic of a physical recording medium is made to vary in a manner analogous to the variations in air pressure of the original sound. Generally, the air pressure variations are first converted (by a transducer such as a microphone) into an electrical analog signal in which either the instantaneous voltage or current is directly proportional to the instantaneous air pressure (or is a function of the pressure). The variations of the electrical signal in turn are converted to variations in the recording medium by a recording machine such as a tape recorder or record cutter—the variable property of the medium is modulated by the signal. Examples of properties that are modified are the magnetization of magnetic tape or the deviation (or displacement) of the groove of a gramophone disc from a smooth, flat spiral track. The key aspect which makes the recording analog is that a physical quality of the medium (e.g., the intensity of the magnetic field or the path of a record groove) is directly related, or analogous, to the physical properties of the original sound (e.g., the amplitude, phase, etc.), or of the virtual sound in the case of artificially produced analog signals (such as the output from a guitar amp, a synthesizer, or tape recorder effects playback.)

A digital recording is produced by converting the physical properties of the original sound into a sequence of numbers, which can then be stored and read back for reproduction. Usually (virtually always), the sound is transduced (as by a microphone) to an analog signal in the same way as for analog recording, and then the analog signal is digitized, or converted to a digital signal, through an Analog-to-Digital converter (an electronic device) either integrated into the digital audio recorder or separate and connected between the recorder and the analog source. An electrical digital signal has variations in voltage and/or current which represent discrete numbers instead of being continuously mathematically related as a function to the air pressure variations of sound. There are two chief distinctions between an analog and a digital signal. The first is that the analog signal is continuous in time, meaning that it varies smoothly over time no matter how short a time period you consider, whereas the digital signal, in contrast, is discrete in time, meaning it has distinct parts that follow one after another with definite, unambiguous division points (called signal transitions) between them. The second distinction is that analog signals are continuous in amplitude, whereas digital signals are quantized. That analog signals are continuous means that they have no artificially set limit on possible instantaneous levels—no signal processing is used to "round off" the number of signal levels. Fundamental laws of physics require the quantization of all analog signals (LeSurf 2007), though this fact is not commonly a limiting factor in system performance. This is because differences in quantum energy level spacing are so small as to be unimportant with typical analog signal intensities. Digitally-processed quantized signals have a precise, limited number of possible instantaneous values, called quantization levels, and it is impossible to have a value in between two adjacent quantization levels. Almost paradoxically, it is precisely this limitation that gives digital signals their main advantages.

Each numerical value measured at a single instant in time for a single signal is called a sample; samples are measured at a regular periodic rate to record a signal. The accuracy of the conversion process depends on the sampling rate (how often the sound is sampled and a related numerical value is recorded) and the sampling depth, also called the quantization depth (how much information each sample contains, which can also be described as the maximum numerical size of each sampled value). However, unlike analog recording in which the quality of playback depends critically on the "fidelity" or accuracy of the medium and of the playback device, the physical medium storing digital samples may somewhat distort the encoded information without degrading the quality of playback so long as the original sequence of numbers can be recovered.

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