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Digital Sound

The ability to record sound is a necessity for journalists in the modern converged media environment. Digital sound technology is a convenient way for reporters to collect and edit audio clips from important speeches, conduct interviews with newsmakers, gather natural sounds to provide an aural context for news events, and produce news packages, newscasts, and public affairs programs. Lightweight digital recording devices are easily transportable, have removable memory cards for storage, and provide excellent sound quality.

The term “digital sound” is used to summarize digital audio techniques for capturing, manipulating, storing, and distributing sound. “Sound” is a disturbance of molecules in a compliant medium (such as air) caused by acoustical vibrations made by something moving in an environment (such as human vocal cords or the strings on a piano) and the reception of those vibrations. Sound may be described in terms of the physical properties of a pressure wave: frequency (pitch), amplitude (loud-ness), and velocity (speed). Thus, “sound” is by definition “analog,” both at its point of origination and point of reception. “Audio” is what happens in between origination and reception and refers to both analog and digital processes for converting acoustical energy to a corresponding energy form (analog) or a representational numeric form (digital). The word “digital” refers to a binary numerical encoding process used to represent sound vibrations.

Origins

An analog system uses a continuous signal or pattern that is analogous to the original waveform of a sound. Early analog devices that contributed to the development of recording include: Thomas Edison's phonograph (1878), a hand-cranked and spring-driven machine that converted sound vibrations into indentations in the groove of a tinfoil cylinder; Emile Berliner's flat disc spiral groove gramophone (1888); Lee de Forest's Phonofilm (1919), a sound-on-film technique that produced a photographic representation of sound; magnetic wire and iron oxide–coated tape processes that were developed in the early 1900s; and microphone, amplifier, and loudspeaker technologies associated with the telephone and radio. These and other analogy technologies made it possible to capture, mix, balance, edit, process (alter sonic characteristics), and record stereo or multiple audio channels, before converting various energy forms back to sound vibrations.

Digital technology converts sound vibrations into discrete numerical values that represent the physical properties of a waveform. The mathematical concept, believed to have originated in the seventeenth century, uses a binary number coding system with two values (0 and 1) to represent numeric values. But the idea to use binary digits (bits) for signal coding was developed much later—in 1929 by Harry Nyquist, a physicist working on telegraph transmission for Bell Telephone Laboratories. He discovered that the sampling rate of a binary system must be at least twice the frequency of the original continuous analog signal. Known as the “Nyquist Theorem,” its application to digital audio was made practical after the development of electronic computer technology during and after World War II.

How it Works

Digital audio converts sound to numbers (represented by electronic pulses) for recording, storage, and transmission, and then numbers back to sound for playback. This is accomplished by partitioning the waveform of a sound into discrete units called “samples.” Digital audio devices, such as the standard consumer compact disc (CD), sample sound at over 44,000 times a second. The sound pressure energy of each sample, as measured by electrical voltage, is then represented using a binary digit system of zeros and one. This “coding” process is the “digital” part of digital audio. Because each numeric place in a binary digit system can only be expressed as a zero or one, the more places in a string of binary numbers, called a “word,” the more accurate the measurement of a sample. For example, a 16-character word (16-bit system) can represent 65,536 different voltage quantities. Generally, the more samples per second and the more numeric values available to represent the voltage of each sample, the better the sound quality. During playback, the number representing the voltage of each sample is used to recreate the original waveform of a sound. The end result is a discrete stepped representation of a continuous waveform. The resulting sound, which may be played back through ordinary headphones or loudspeakers, is not the original sound, but a facsimile that resembles the original sound, to the extent that a digital system is capable of recreating the analog waveform structure.

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