Digital recording is the preservation of audio or visual signals as a series of binary numbers that can be stored on magnetic tape, optical disc, or other digital storage media. Similar techniques are used to record a wide range of scientific, monetary, and engineering data.
To make a digital recording, an analog-to-digital converter transforms the electrical audio waveform from a microphone or an analog visual image into digital information. Images may also be generated digitally by a digital camera, without any intermediate analog signal. The system that plays back or reads out the sound or image translates the binary code back into analog signals using a digital-to-analog converter. Digital tape players, compact disc players, video disc players, and CD-ROM (Compact Disc–Read-Only Memory) players in home computers utilize digital-to-analog converters to play back audio and video codes. Analog-to-digital and digital-to-analog converters are implemented on single chips.
Analog recordings were the only ones widely made until the digital revolution of the 1970s. They used a variety of methods that are now more or less outdated, like long-playing (LP) records, eight-track tapes (on either metal or magnetic tape), and audio-cassette tapes. From early in the twentieth century until the 1970s, the analog system seemed ideally suited to recording; however, with the computer revolution that occurred late in the century, the high speed and other characteristics of digital processing made digital recording not only possible but increasingly superior for many applications, as did steadily falling costs due to mass production of computers, optical discs, laser players, and other devices. Digital recording, when properly implemented, more easily achieves high fidelity (accuracy of reproduction) because it provides a wide dynamic range and low noise and distortion.
The most familiar audio system, the compact disc, uses a laser beam player to read the digital information coded on the disc. Digital audio tape (DAT) became available in the late 1980s. It uses magnetic tape and a specialized DAT recorder with a microprocessor to convert audio signals to digital data during recording and to switch the data back to analog signals for playback. DAT systems are available to the average consumer but are used extensively by professionals. In the early 2000s, they have fallen gradually out of favor because of their greater expense relative to optical media and compact, high-capacity hard drives such as those found in personal MP3 players. Digital compact cassette recorders can play both DAT tapes and the analog tape cassettes that are more common; again, however, this is a technology that is fading rather than growing.
Video systems parallel audio methods. Compact discs for video recordings were initially considered impractical because of the sheer quantity of data to be encoded, so larger-diameter laser discs were developed. Laser discs (also called videodiscs) store audio information in digital form and video as analog data. Analog videotapes in Beta and VHS formats (both analog forms) were, however, for many years easier to mass-produce at smaller cost. Another part of the video problem was that video could be recorded on compact discs, but the level of fidelity of the disc was better than any television could reproduce. The video recording industry had to wait for televisions to catch up. In the late 1990s, high-definition television (HDTV) became available, and the digital video disc (DVD) and DVD players rapidly became more popular in anticipation of better television technology. The DVD (digital video disc or digital versatile disc) can accommodate all the sound and video needed for a movie because it holds about five billion bytes of data; high-density and multi-layer versions may soon hold several times as much (prototypes already do, but as of 2006 they were not yet widespread in the consumer market). A typical CD-ROM for home computer use stores only 700 million bytes.
Digital cameras were also introduced in 1997, the same year that DVD players were first widely sold. Improvement of HDTV was given a push by government; a phase-out of nondigital TV signals is to occur over 10 years (beginning in 1998) to be replaced by the digital images from satellites, digital network broadcasts, and DVD sources. Cable systems are also converting to digital signals.
Advances in the home computer industry are closely linked with audio and video digital recording systems. First, home computers have increasingly included audio and video playback systems. Second, the mergers of audio and video giants with Internet firms have shown that all these services may soon be provided directly to our homes through one cable, phone line, or other shared system. And third, the technology for putting more and better information on a compact disc has made the disc the leading medium for sound recordings (as the compact disc), video (in the form of DVDs), and information (CDROMs and recordable and erasable CDs for data, sound, and video). Erasable and recordable compact discs are called CD-Es and CD-Rs, respectively; following their introduction in the late 1990s, the equipment for using them (with home sound systems and computers) quickly became affordable. The DVD also has a close relative for computer data storage called the DVD disc drive that replaces the CD-ROM in some personal computers (PCs). Eventually, technology may produce a single type of disc that can be encoded and played back by computer, audio recorder/players, and video recorder/players (depending, of course, on the information on the disc); or, discs themselves may be supplanted by nonvolatile random-access memory chips, doing away altogether with moving parts in the recording and playback process.
Recording, particularly of music and video images, consumes large amounts of digital memory. Consumer-style tape recording, though convenient and easy, could not easily store large quantities of digital data and play it back at high speed until the development of the DAT tape. (Ordinary audiocassettes could be, and were, used to store digital data on occasion, but with low efficiency.) On CDs and other disc formats read by laser, the physical structure uses “lands” or slightly raised areas and the low stretches or “pits” between them to encode the zeros and ones.
The sampling frequency used in converting from analog to digital is critical to sound recording. The analog signal is measured or sampled many times per second—tens of thousands of times—and these samples constitute a picture or recording of the changing analog wave over time. Increasing the sampling frequency improves the sound quality by providing a more perfect picture of the analog waveform, but requires greater data storage. The system of using a binary word (set of bits) to stand for each regularly spaced sample of an analog waveform is called pulse code modulation. Pulse code modulation is actually an old development in the history of recordings. It was developed in 1939 by A. H. Reeves, but it took electronic technology many years to find practical uses for Reeves’invention.
Resolution is another important specification and describes the number of bits used to represent the amplitude of an instant on the recording. Each bit doubles the possibility for representing instantaneous amplitude levels. Typically, 14-bit resolution is used to give a range of 16, 384 possibilities for representing instantaneous amplitude values.
Recording media are all imperfect, thanks to specks of dust or other contamination that prevents equipment from imprinting the data on the medium. In analog recording, imperfections take the form of audible noise; in digital recording, they cause errors in the bit stream, which may translate to noise or to failed playback. To deal with this problem, error correction-codes are built into the data stream. Some of these error correction codes can be very complex, and they also cause the data to consume more storage space. The result, however, is highly reliable playback of discs at reasonable levels of dust and scratching.
- Bit and byte
- A bit is the smallest element representing data in a computer’s memory. A byte consists of eight bits that the computer processes as a unit.
- High density
- In recording systems, the ability to store large audio, video, or information files in a small space.
Analog systems also have the disadvantage that, when a recording is played back and rerecorded, distortion is increased. Each subsequent copy will be audibly worse. In a digital recording system, this distortion does not occur. The master recording may have minimal quantization errors, but these do not compound when copies are made: copying is simply a matter of copying a string of numbers. In this case, the zero-orone character of the digital world works to an advantage because the copy is equally absolute unless the digital recording is reconverted to an analog signal. Thousands of copies can be made from a digital master without distortion; similarly, digital media on CDs can be played back thousands of times without distortion.
Recordable and erasable CDs are giving the compact disc greater versatility. Compact disc recorders allow the user to record audio from various sources on CDs. The recorders require attentive use because the recording procedure depends on the type, quality, and input device of the source material. If the source is a CD that can be played on a machine with digital optical output, it can be connected directly to the CD recorder as input and be dubbed like an audiotape. The recorder evaluates the sonic range of the original and digitally synchronizes it; if tracks are recorded from several CDs, the recorder must resynchronize with each track.
Erasable CDs followed recordable CDs quickly. Erasable CDs (CD-RWs and DVD-RWs) can be overwritten at will, with manufacturers claiming cycle lifetimes of 1, 000 erasures. High-density CDs are also being developed and are anticipated by the music industry because they can store music detail more completely. Enhanced audio CDs include music videos, lyrics, scores that the home musician can play, and interviews with the musicians. Enhanced audio CDs can be played on a CD-ROM drive and viewed on a monitor or connected television set.
Since 2001, when the iPod was introduced, digital music (with, more recently, integrated video) has become the definitive listening experience for scores of millions of people. Relatively few people, and fewer each day, are listening to music stored in any analog form. Digital sound was once advertised as music’s future; by 2006, it was more accurate to recall analog recording as music’s past.
Fries, Bruce, and Marty Fries. Digital Audio Essentials. Sebastopol, CA: O’Reilly Media, 2005.
Gibson, Bill A. Producing Music with Digital Audio Software and Plug-Ins. Boston: ArtistPro, 2005.
Huber, David Miles, and Robert E. Runstein. Modern Recording Techniques. 6th ed. Boston: Focal Press, 2005.
Pohlmann, Ken C. Principles of Digital Audio. New York: McGraw-Hill/TAB Electronics, 2005.
Gillian S. Holmes