Modern Recording Techniques
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Excerpt from Chapter 6: Digital Audio Technology of Modern Recording Techniques, 6th ed.
by David Miles Huber and Robert E. Runstein (Focal Press, 2005)
Copyright © 2005 Elsevier, Inc.. All rights reserved.
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Chapter 6: Digital Audio Technology
In recent decades, digital audio (and its related industries) has evolved from being an infant technology that was available only to a select few to its present-day position as a primary driving force in audio production, entertainment, and communication. In fact, digital audio and media production has such an impact upon our lives that it’s often an integral part of both the medium and the message within modern-day communication.
Although digital audio is a varied and complex field of study, the basic theory of how the process works isn’t really that difficult to understand. At its most elementary level, it is simply a process by which numeric representations of analog signals (in the form of voltage levels) are encoded, processed, stored, and reproduced over time through the use of a binary number system.
Just as English-speaking humans communicate by combining any of 26 letters together into groupings known as "words" and manipulate numbers using the decimal (base 10) system ...the system of choice for a digital device is the binary (base 2) system. This numeric system provides a fast and efficient means for manipulating and storing digital data. By translating the alphabet, base 10 numbers, or other form of information into a binary form, a digital device (such as a computer or processor) can perform calculations and tasks that might otherwise be cumbersome, less cost effective, and/or downright impossible to perform in the analog domain. This binary data can be encoded in such media forms as:
Logical 1 or 0
On or off
Voltage or no voltage
Magnetic flux or no flux
Optical reflection off of a surface or no reflection
After the information has been recorded, stored, and/or processed, the resulting data can be converted back into an analogous form that we humans can readily understand.
Before we delve onto the basic aspects of recording, processing, and reproducing audio in the digital domain, let’s take a look at how one form of information can be converted into another equivalent (analogous) form of information. For example, if we type the letters C, A, and T into a word processor, the computer quickly goes about the task of translating these keystrokes into a series of 8-bit digital words that would be represented as [0100 0011], [0100 0001], and [0101 0100]. These "alpha-bits" don’t have much meaning when examined individually; however, when placed together into a group, this data represents a four-legged animal that’s either seldom around or always underfoot (Figure 6.1). From this, we can deduct that whenever binary words are grouped together as a string of data that has an analogous and recognizable pattern, a meaningful message can be conveyed.
In a similar manner, a digital audio system works by sampling (measuring) the instantaneous voltage level of an analog signal at a single point in time ...and then converting these samples into an encoded word that digitally represents that voltage level. By successively measuring changes in an analog signal’s voltage level (over time) ...this stream of representative words can be stored in a form that represents the original analog signal. Once stored, the data can then be processed and reproduced ways that have changed the face of audio production forever.
The Basics of Digital Audio
In Chapter 2, we learned about the two most basic characteristics of sound:
- Frequency (the component of time)
- Amplitude (the signal-level component)
Digital audio can be likewise broken down into two analogous components:
- Sampling (which represents the component of time)
- Quantization (which represents the signal-level component)
Sampling
In the world of analog audio, signals are passed, recorded, stored, and reproduced as changes in voltage levels that continuously change over time (Figure 6.2). The digital recording process, on the other hand, doesn’t operate in a continuous manner; rather, digital recording takes periodic samples of a changing audio waveform (Figure 6.3) and transforms these sampled signal levels into a representative stream of binary words that can be manipulated or stored for later processing and/or reproduction.
Within a digital audio system, the sampling rate is defined as the number of measurements (samples) that are taken of an analog signal in one second. Its reciprocal (sampling time) is the elapsed time that occurs between each sampling period. For example, a sample rate of 48 kHz corresponds to a sample time of 1/48,000th of a second. Because sampling is tied directly to the component of time, the sampling rate of a system determines its overall bandwidth (Figure 6.4), meaning that a system with higher sample rates is capable of storing more frequencies at its upper limit.
As you might expect, the sampling process can be likened to a photographer who takes a series of shots of an action sequence. As the number of pictures taken in a second increases, the accuracy of the captured event will likewise increase... until the resolution is so great, that you can’t tell that the successive pictures have turned into a (hopefully) compelling movie.
During the sampling process (Figure 6.5), an incoming analog signal is sampled at discrete and precisely timed intervals (as determined by the sample rate). At each interval, this analog signal is momentarily "held" (frozen in time), while the converter goes about the process of determining what the voltage level actually is, (frozen in time), while the converter goes about the process of determining what the voltage level actually is, with a degree of accuracy that’s defined by the converter’s circuitry (Figures 6.6 and 6.7) and the chosen bit rate. The converter then generates a binary-encoded word that’s numerically equivalent to the analog level currently being sampled. Once done, the converter can store the representative word into a memory medium (tape, disk, disc, etc.), release its hold, and then go about the task of determining the level of the next sampled voltage. The process is then continuously repeated throughout the recording process.
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