Mention the term “digital audio” to anyone who is not a digital native (basically a person who grew up using computers and other digital devices) and chances are that person will assume you are talking about high quality audio. The mere fact that audio is digital says nothing about its quality. Digitizing audio solves some problems and introduces others.
For example, unlike analog recorders, recording audio digitally introduces no new noise. The most serious problem with digital audio is the loss of data. As discussed earlier, in the article on how digital audio works, an analog-to-digital converter uses a process called sampling to create discrete (numerical) values to represent the signal, but removes some of the information from the original analog signal. The two key elements that determine the quality of digital sound are the sampling rate (how often the converter samples the sound) and bit depth (the precision with which changes in amplitude are recorded. For example, CD-quality audio is sampled 44,100 times a second, or 44.1 KHz. The drawback to higher sampling rates is that they require more bandwidth for transmission and consume more storage space.
There is a good and easy to understand explanation of digital audio quality at the University of Medicine and Dentistry, New Jersey’s website. You may access it at: http://www.umdnj.edu/idsweb/idst3400/audio.htm .
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How Digital Audio Works
Why do we need digital audio, when analog audio so accurately represents the original sound? We need digital audio because analog audio has several limitations. First, since analog audio uses a continuously fluctuating voltage level to represent sound, any variation in that level such as a bit of electrical interference, will be heard when the signal is passed through another transducer such as a loudspeaker and becomes sound again. Second, you can’t process analog audio in your computer without first converting those signals into numbers.
A digital audio system uses numbers to store, process, and transmit data. Digital equipment uses an analog-to-digital converter (A/D converter) to convert analog signals into digital form. The converter uses a process called sampling to create discrete numerical values. The A/D converter measures the voltage level at regular time levels. In the second illustration in the screen shot below the numbers on the x-axis represent a certain number of snapshots per second. The y-axis represents signal strength, or the numerical values of those snapshots. The third illustration shows how the digital converter rounds off the continuous levels of the analog signal to the numerical values, resulting in the digital signal shown in the fourth illustration.
After the A/D converter converts the sound into digital form you can process it or store it on your computer. If you want to play the sound back through your speakers or headphones, it must be converted back into analog form. We use a digital-to-audio converter, or DAC, to perform this task.
Some common digital devices include:
- Computer audio interfaces
- Digital effects, such as guitar-amp simulators
- Portable digital recorders
- Digital instruments, such as MIDI synthesizers
- Digital connections
We will begin this discussion by talking about how the term analog is used in its broadest sense. According to Wikipedia the term means “an object, concept or situation which in some way resembles a different situation.” (Wikipedia, 2012). When used in literature the term “analogue” refers to “a work which resembles another in terms of one or more motifs, characters, scenes, phrases or events,” or “an individual motif, character, scene, event or phrase which resembles one found in another work.” An example would be the story of Noah and the Flood from the Bible and the epic of Gilgamesh. In language an analog is a comparison between concepts. Another literary analogy is: “Life is a road.”
According to PC Magazine, analog audio, “Refers to recording audio in a format of continuous vibrations that are analogous to the original sound waves.” (PC Magazine, 2012). You may be surprised to know how many devices we use today employ analog signals. These devices include:
- Microphones and speakers
- Standard audio connections, such as headphone jacks and RCA phono plugs on a stereo
- Analog musical instruments, such as analog synthesizers and electric guitars
- Analog audio processors