A time-varying parameter which carries information. In the musical context, the signal is typically a fluctuating electrical voltage which, after appropriate amplification, can be applied to a loudspeaker to generate an audible sound. A musical signal may be generated in a variety of ways. A microphone senses the fluctuating pressure in a sound wave and converts it into an electrical signal. The pickup on an electric guitar generates a signal which depends on the string motion. In a synthesizer the signal is generated purely by electrical circuits. Once the signal has been generated, it can be modified by a range of techniques known collectively as signal processing. The most common of these are amplification, in which the signal is multiplied by a constant factor, and filtering, in which selected parts of the frequency spectrum of the signal are amplified or attenuated. The voltage output from a microphone is a continuously varying representation of the sound pressure. Neglecting any distortion introduced by the microphone, the voltage waveform is a strict analogue of the pressure waveform, and this type of signal is known as an analogue signal. Most modern signal processors and recording systems require that an analogue signal is first passed through an analogue-to-digital converter (ADC), which periodically samples the signal. The result is a sequence of numbers, representing the signal values at the sampling times, known as a digital signal.
Digital signals have two major advantages over analogue signals. Firstly, they can be copied and transmitted effectively without degradation. An analogue signal is always in practice accompanied by unwanted noise. Each stage of processing, transmission or recording introduces additional noise, and the signal-to-noise ratio, by which the signal's quality is measured, decreases. In contrast, digital signals are represented by sequences of logic pulses of fixed height; as long as the noise level is kept well below the pulse height it does not affect the information content of the signal. Secondly, digital signals can be stored, edited and processed on a computer. A digital signal is a sequence of numbers, and cannot be fed directly to a loudspeaker. A digital-to-analogue converter (DAC) is required, which recreates an analogue signal in the form of a continuously varying voltage matching the rise and fall of the numbers in the digital signal. The fidelity of the reproduction improves as the interval between the samples diminishes; the digital signal stored on a CD is obtained by sampling the original analogue signal 44,100 times per second.
See also Recorded sound, II.
MURRAY CAMPBELL, CLIVE GREATED