THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

19.2 Recording Equipment 573Tape speeds have been standardized at 30, 15, 7.5, 3.75, and 1.875 in./s foranalog recording. All of these speeds are much lower than that required for digitalrecording, which necessitates bandwidths in the megahertz range. Digital audiorecorders make use of the video recording techniques that entail rotating heads.These heads scan the tape at 228 in./s while the tape itself moves at only 0.66 in./s.The highest speed is used for master recordings, in situations where the highestquality is demanded in covering the 20 Hz–20 kHz range. Cassettes use the1.875 in./s speed.The Dolby r○ systems for noise reduction employ circuitry that pre-emphasizeshigh frequencies before they are recorded on tape in order to make them louder thanthe tape hiss with which they compete. The circuit for recording tape is amplitudesensitive with the result that only soft, high-frequency sounds are emphasized.Emphasis of loud, high-frequency sounds might drive the tape into its distortionlevels. Upon playback, a matching de-emphasis circuit is employed to restore thehigh frequencies to their proper balance with the other parts of the recorded signal.Digital RecordingThe capabilities of analog recording pale in comparison with the advantages ofdigital recording. Zero wow and flutter, more than 90 dB signal-to-noise ratio,and incredibly low-distortion levels are the most outstanding attributes of digitalrecording. To achieve digital recording, the analog signal is sampled at regularintervals. Sampling of the amplitude is done with almost absolute accuracy (onepart per 10 9 in a 96-dB signal-to-noise ratio system). The amplitude is recorded asa number, say, 59,959,498. The sampling should be done at more than twice theNyquist number (the highest frequency in the audio signal). For a signal with amaximum of 22,000 Hz, a rate of 44,000 samplings per second is quite practicable(20,000 × 2 + 10%). Because the series of digital samples occur in the megahertzrange, they are recorded and played back on videotape recorders. The audio programin digital form can be processed with digital features, error correctors, andother techniques that yield results that cannot be achieved with analog means. Thenumbers comprising the signal can be stored successively in buffers and read outin perfect crystal-controlled time intervals, even if there exist erratic mechanicalfluctuations in the tape drive. Thus, wow and flutter are eliminated. The digitalsignals are restored to analog format through digital/analog (D/A) converters.Experiments on the digitization of sound were conducted during the late 1950sand early 1960s for the purpose of computer analysis, speech synthesis, simulationof music, and simulation of reverberation, mostly at the Bell TelephoneLaboratories in Murray Hill, New Jersey (Mathews et al., 1961). In the early1970s, commercial digital recordings were commonly used by recording studiosfor master recordings, but the results were distributed by analog means.A significant breakthrough occurred in the early 1980s, when Sony Corporationof Japan and Koninklijke Philips Electronics N.V. of the Netherlands came out withthe compact disk (CD) that quickly replaced the analog long-play (LP) record andthe cassette as the most common medium for playback of recorded music. Later