"Digital" means information stored in a binary format; "Audio" refers to sound. In general, the term refers to using computers to record, play, and manage audio stored in a digital format. Digital audio systems can be used to transfer audio from a tape, CD or live performance into a format playable on computers (like AIFF or WAV files); make editing changes (like cuts, fades, and special effects) to those audio files; and output audio files back to tape.
What kind of equipment do you need to edit audio on a computer?
Capturing audio, or taking audio from an analog source such as an audio tape or microphone, requires a sound card. A sound card plugs into an internal slot and has audio-in, audio-out, microphone and speaker jacks. The card contains chips that compress incoming audio data and allow your computer to save the audio data to disk. All Macintosh computers and most Windows computers have built-in sound cards. With the appropriate sound editing software, most audio can be captured and digitized using a USB connection or using a FireWire connection and a conversion box such as a Sony Media Converter or an iMic. sound stored on a CD is already in digital format, and it can be copied to an internal hard drive, external hard drive or back to a CD. Audio stored on a hard drive or removable disk can be played back, recorded to tape, added to a computer presentation, or even pressed to a CD. With such programs as iMovie and itunes installed on all Macintosh computers and MusicMatch Jukebox on PCs, one can play all types of digital audio files.
How does digital audio work, and how good is the quality?
When a sound card is digitizing audio, it is measuring the voltage coming from the audio source many times per second. These measurements are called "samples", and they are saved to the hard drive as a list of numbers. When it is time to play back the sound, the computer can read the numbers from the hard drive and reproduce the original voltage levels.
The quality of digital audio depends on two factors--the "sample rate" and the "sample size". Sample rates usually lie between 8kHz and 44.1 kHz; a sample rate of 44.1 kHz means that the sound is sampled, or measured, 44,000 times per second. Eleven kHz is considered "phone quality", 22.5 kHz is "radio quality", and 44.1 kHz is "CD quality". Digitizing with a high sample rate results in a closer approximation to the original sound with better capture of high-frequency sounds, but it requires more disk space. The sample rate of a sound is analogous to the resolution of an image.
Sample size, usually 8 or 16 bits, tells how accurately the voltage is measured. For an 8-bit sample, numbers between 0 and 255 are used to describe 0%-100% voltages. for a 16-bit sample, numbers between 0 and 65,536 are used to describe 0%-100% voltages. Again, using a higher sample size gives a closer approximation to the original sound with smoother transitions between voltages. The sample size of a sound is analogous to the color depth of an image.
CDs use a 44.1-kHz sample rate and a 16-bit sample size; that is 44,000 samples per second, with each sample taking up 16-bit sample size; that is 44,000 samples per second, with each sample taking up 16 bits(two bytes). Therefore one second of CD quality sound would take up 88,000 bytes, or 88k of disk space. this would be doubled if the sound were in stereo.
What is MIDI, and can you convert audio files to/from MIDI?
MIDI, which stands for "musical instrument digital interface", is an electronic music format which contains a list of notes to play and descriptions of the instruments to play them on. Since MIDI files contain lists of musical notes, and not actual sounds, they are much smaller than WAV or AIFF files of the same duration.
However, MIDI files can only create music, not sound effects or speech. It is not possible to convert a WAV or AIF file into a MIDI file, just as it is not possible to write sheet music that describes the sound of a lion's roar or of a person's voice.
What is an MP3 and how do to get audio files in MP3 format?
Perhaps the most common digital audio format today is MP3. An MP3 stands for "MPEG-layer 3" which is a video file that can be compressed small enough to be easily streamed over the internet and for many files to be saved and played on a disc CD players with MP3 reading capabilities can read up to 5 hours of information. All computers made now have MP3 playing capabilities such as iTunes, RealPlayer, Windows MediaPlayer.
