Digital audio provides superior reproduction of sound and additional benefits useful in a radio station's operation. Except for a station's microphones (which may become the sole ana­ log source at a radio station), it is possible for a radio station's audio chain to be completely digital, from production and storage to playback and processing, before being sent to a digital exciter and on to the transmitter. In the many countries where digital audio broadcasting standards are now in place, transmitters and receivers are now also digital, completing the final links to make radio a totally digital medium. Many radio stations also distribute digital transmissions of their programming on the internet, making a digital version of radio's signal available to listeners with a computer, audio card, and internet connection.

     Digital audio has dramatically improved the quality of radio's on-air sound and has also brought many operational enhancements to the production process used to create radio programs. The clear benefits of digital audio have motivated nearly universal adoption among radio stations of some type of digital audio recording and playback equipment. A list of digital audio equipment found in radio stations today includes CD players, CD recorders, open-reel stationary-head digital recorders, rotating-head digital audio tape recorders, mini disc recorders, and hard-disk recording systems. Computers with specialized software and audio cards with inputs and outputs to interface with the other audio equipment in the station provide digital audio replacements for tape recorders, the splicing block, and other production and processing equipment. Digital versions of other equipment, such as audio consoles, telephone hybrids, effects processors, compressors, limiters, microphone processors, studio-transmitter links, on-air audio processing equipment, exciters, and transmitters, are rapidly becoming the standards as aging analog audio equipment is replaced.

Digital Audio Basics

      A sound itself is not digital. Sound is created when an object vibrates and causes the molecules of the medium surrounding it (usually air) to vibrate. These vibrations or sound waves are transferred through the air until they reach someone's ear or a microphone. At the microphone, sound is transduced (converted) into electrical energy and becomes analog audio. The characteristics of this electrical energy are analogous to the original sound energy. This electrical energy can be amplified, manipulated, stored, or transmitted as analog audio; however, it can also be digitized and then amplified, manipulated, stored, or transmitted as digital audio.

     Digital audio is created by converting analog audio into a stream of binary code, a series of ones and zeroes, representing the measurements of the characteristics of the original sound. This binary code represents measurements made of samples of the original audio representing the sound energy. The binary code can be recorded and stored on any device capable of reading and storing digital data. Magnetic tape, computer floppy disks, hard disks, and optical disks can store the digital information. These data can also be transmitted as pulses through copper wire, fiber-optic cable, or as radio frequency energy through the air. An exact, full-fidelity reproduction of the original audio can be created from the stored or transmitted digital code, copied without generation loss, and easily processed for creative and technical purposes. Unlike analog audio, the digital signal is not as subject to the limitations imposed by the storage medium or the electronics of the equipment. In the analog world, the tape itself adds noise, copying adds more noise, the amplifier adds noise, and so on. The dynamic range and frequency response of the original sound are also reduced as analog audio, because the analog system has inherent limitations in reproducing sound faithful to the original. The methods used to record and process digital audio minimize these limitations.

     The digitization of analog audio involves four stages: filter­ing, sampling, quantizing, and coding. First, the audio is sent through a low-pass filter to prevent unwanted higher frequencies from becoming audible. This process is called anti-alias­ ing. Then the analog signal is divided, which determines the sampling rate. The more often the signal is sampled and measured, the more accurate the recreation of sound will be. Sampling rates are typically 32 kHz, 44.1 kHz, or 48 kHz. This means the signal is sampled either 32,000, 44,100, or 48,000 times every second. A measurement is made during every sampling period using a multidigit binary number. This binary number is called a word. The number of bits in a word is word length. A r-bit measurement, for example, would only be able to discriminate between presence and absence of voltage. If n is the number of bits in the word length, the number of levels of measurement is 2n. An 8-bit system provides 256 levels of voltage measurement. A 16-bit system has 65,536 possible levels, and a 20-bit system provides 1,048,576 levels. Systems with more quantizing levels have more accuracy and wider signal­ to-noise ratios. The last stage of the digitizing process is the coding stage, in which the bits are placed in a precise order for recording or output to another digital device. During this coding stage, each word is identified in the bit stream. Error correction minimizes the impact of storage defects. The binary code is then distributed or recorded as pulses of magnetic energy.

     Moving audio to the digital domain for recording and reproduction purposes provides a number of advantages. Compared to analog audio, digital audio has an improved frequency response, wider dynamic range, immeasurable noise and distortion, and no degradation or generation loss in multiple digital recordings. Some audiophiles have been critical of the digital audio recording process, suggesting that when sound is digitized, it loses its warmth and can sound too sterile and even harsh. Radio has generally rejected those concerns and has continued to replace analog audio equipment with digital equivalents.

The Compact Disc

     In 1980 the Philips and Sony corporations joined forces to create an optical disc for digital audio. The two companies agreed on a CD standard, a 12-cm optical disc using 16-bit/44.1-kHz sampling. The CD player and disc were introduced in Europe in the fall of 1982 and in the United States in the spring of 1983. As record and production library companies began to release their catalogs on CD, radio began using CD players in their production and air studios. Manufacturers developed CD players with features such as a shuttle control as well as a model that played CDs inserted in a special protective case, creating a process similar to the use of a broadcast cartridge machine. The CD changer, capable of handling multiple CDs, was also found useful at many radio stations. Many broadcasters used consumer models because the audio quality was the same for both. Not only did the CDs sound better than the vinyl long-playing and 45-rpm records, but the CD format was also much more efficient to use. CDs could be cued and started faster and, with care to protect the disc from scratches, would allow endless replays without degradation of sound quality. Additional data encoded in the compact disc provided precise track timings, indexing, and continuous monitoring of playing time of tracks and programs.

     The CD player uses a laser to read the data encoded in the microscopic circular pits on the disc. The binary code is stored in a series of pits and lands in the disc. A pit is an indentation in the groove; a land is a flat area with no indentation. A photoelectric cell reads the amount of light reflected from the pits and lands and emits voltage to recreate the digital code representing the audio waves. As needed, the digital output of the CD can be converted back to analog audio or sent as digital output to a digital recorder or console. The only problem with the CD was that its content was limited to prepackaged material offered by the manufacturer. However, the recordable CD was soon on the way.

     The recordable CD (CD-R) was launched in 1988 but initially was not widely adopted as a production tool by radio. The first CD recorders were relatively expensive, and the disc recording was permanent: the disc could not be erased and recorded on again. Recently, with the introduction of the rewritable CD (CD-RW), lower-cost CD-R recorders, and CP­R drives installed in computers, the recordable CD has attracted more attention from broadcasters, for use as an archival and production tool and as a component in digital automation systems.

     The digital versatile disc (DVD) is not yet a factor in the radio environment, but it most likely will be. The DVD is the same diameter and thickness as a CD, but a difference in design and manufacturing provides eight times the capacity of a CD by creating a dual layer. There are currently five recordable DVD formats.DVD-Rand DVD+R discs can be recorded only once. DVD-RW and DVD+RW can be rewritten. The DVD-RAM is used for recording computer data only.

Digital Audio Tape and Mini Disc Recording

     Although the CD was quickly adopted by most radio stations shortly after its introduction, digital audio recording had a more difficult time gaining a foothold in radio. Commercial digital audio recorders have been available since the 1970s and early 1980s. Sony and Denon introduced adapters that made it possible to record digital audio on videotape recorders. Open­ reel two-track and multitrack digital recorders (Digital Audio Stationary Head) were employed in recording studios but were not widely used in radio. By the early 1990s, however, the rotary-head digital audio tape recorder (R-DAT) was finding a place in radio production.

     R-DAT machines use essentially the same digitizing scheme as the CD, but they use a rotating helical-scan tape head to record and read the large quantity of information representing the audio signal on the small cassette tape. The result is audio recordings with characteristics similar to the CD with the additional flexibility of being able to record and rerecord. R-DAT recorders were adopted as a cost-effective, high-quality production and on-air playback tool, especially in automation systems. The use of R-DAT has been supplanted somewhat in recent years by other digital formats, including the mini disc.

     Although originally intended as a consumer product, the mini disc recorder is finding a niche in broadcasting. The mini disc offers a more portable, less expensive alternative to hard disk recording, and the portable minidisc units provide a digital alternative for field recording. The mini disc offers nonlinear access, track identification, and a recording time of 74 minutes. It has low noise, low distortion, and a wide dynamic range, but its use of data compression limits its use in critical recording.

Computer-Based Recording, Editing, and Digital Distribution

     Open-reel analog recordings have at least one advantage over open-reel digital tapes: analog recordings are easier to edit than a digital tape. Because a digital tape has to be running at speed in order to decode the data to recreate the digital audio, digital open-reel recorders record and play on an analog head for editing and cueing purposes. The digital tape can then be marked, cut, and spliced like an analog tape. Physical editing is not possible with the DAT cassette tape, and electronic editing on a DAT recorder requires some finesse. Moving the recorded digital information to a computer hard disk opened the way for the rapid deployment of computer-based digital audio recording and editing systems, which have revolutionized audio production for radio.

     By the early 1990s, audio could be recorded to media other than tape. Increased hard disk capacity, faster computer processing speeds, and new compression methods combined to make recording directly to a computer's hard disk a viable alternative to recording on analog tape. Software programs and audio input/output cards were developed to be used on inexpensive personal computers to create digital audio recordings that sounded better than the recordings created on professional analog equipment. Even consumer products could create professional-sounding results in radio production studios. Editing software was introduced that would allow nondestructive editing of the audio material. These programs typically provide a visual representation of the audio waveform, which can be marked, highlighted, cut, copied, pasted, and moved within and between sound files. Precise, noise-free edits are performed that can be readjusted and fine-tuned as needed without destruction of the original sound file.

     There are numerous multitrack recording and editing programs used by radio stations, which, when combined with compatible high-quality computer audio cards, allow desktop computers to perform the same functions as multi channel recorders, production consoles, and effects processors-which cost thousands of dollars more-all in one computer.

     Once the digital audio exists as a file in a networked computer, local area networks, wide area networks, and the internet allow these sound files to be distributed and shared internally or externally. An increasingly common distribution approach is the use of MP-3 files. The MPEG-1 layer 3 recording technology (commonly known as MP-3) is a digital audio file compression method increasingly used by radio stations to send and receive programs and programming elements through the internet. This form of distribution becomes cost-effective and important as advertising agencies and production companies start to distribute commercials, programming, and other information digitally. As radio groups consolidate and combine station operations and look for economies of scale, digital distribution of content will become even more important. After a commercial is created in the production studio of one of the stations in the group, it can be distributed instantly to all the other stations on the computer network.

Digital Audio Processing

     After audio has been converted to digital form, it can be manipulated or processed for creative and technical reasons. Modern radio production studios often have at least one digital effects processor, which efficiently creates various combinations of digital effects, such as echo, reverb, pitch changing, phasing, flanging, and many others. Computer software-based recording and editing programs also have digital audio processing and effects as part of the package. Digital audio processing is also used in the station's air chain, running microphones through processors that convert the signal to digital before processing to strengthen and improve the sound quality of the announcer's voice. Digital processing of the audio signal before it is sent to the transmitter provides one last measure of limiting, compression, and other subtle adjustments to give the station's audio a distinctive, full sound.