D-1
Rev 2.2, 10/31/94
D. Glossary and Bibliography
Some terms used in this manual may not be familiar to you. Their definitions are presented in
the following glossary. At the end of this chapter, you will find a short bibliography which is a
good starting point for further research.
Many of the glossary items and their definitions are taken (with permission) from
The Audio
Dictionary
, by Glenn White. In the interests of brevity, some of Glenn's definitions have been
abridged and Glenn's extensive cross-referencing has been removed. Definitions taken from this
book have the notation "(TAD)" appended to the definition. A more complete bibliographic entry
for this book may be found in the bibliography.
D.1 Glossary
In this glossary, words typeset as follows: digital are cross-references to other words in this
glossary. Glossary entries followed by "(TAD)" may be found in The Audio Dictionary.
Analog
An audio signal is an electrical replica, or analog, of the waveform of the sound it represents.
The voltage of the signal varies up and down (negatively and positively, in electrical terminology) the
same way as the sound pressure varies up and down at the microphone.
As long as the signal is in this form, i.e., is a voltage that varies directly with the sound pressure, it is an
analog, and audio devices which use such signals are analog devices. The majority of audio devices are
analog in nature, though
digital
devices are increasing in popularity.
An analog audio device need not be electrical; the Edison mechanical phonograph was an analog device,
the groove depth being an analog of the sound pressure at the recording diaphragm. (TAD)
AES/EBU
A
digital
audio transmission system standardized by the Audio Engineering Society and
the European Broadcast Union. An AES/EBU signal carries two audio channels as well as status
information. The AES/EBU interface is balanced and uses XLR connectors. There are subtle differences
in the actual signal format from the
S/PDIF
system.
AGC
An automatic gain control (AGC) circuit adjusts the gain of an audio device in inverse proportion
to the signal level entering the device. An example is a portable tape recorder which is designed for
speech recording. When the talker is close to the microphone, the gain is reduced so as not to overload
the tape. As the level from the talker decreases, for instance because of a greater distance, the
gain
increases to keep the recorded level the same.
This type of machine is often used for radio interviews, and usually the gain changes can be plainly
heard as the background noise rises each time the speaker pauses for a few seconds, only to suddenly
fall the moment the next syllable is uttered. (TAD)
A more recent meaning for AGC is the combination of the device described previously and a signal-
sensing circuit that prevents the gain from changing when there is no valid signal present. This prevents
the rising and falling background noise heard when a simple compressor is used as an AGC. The
601
uses this technique.
Analog to Digital Converter (ADC)
In
digital
audio systems, the audio signal (
analog
) must first be
converted to digital form before it can be further processed. This entails
sampling
the signal at very short
successive time intervals, and converting the height of each sample to a digital word, which is simply a
binary number indicating the amplitude of the waveform at that instant.
See also:
quantization
)
The output of the A/D converter is a series of digital "words," expressed in binary form. Before the signal
can be fed to an amplifier so it can be heard, it must undergo
digital-to-analog conversion
. This recovers
a replica of the original audio signal from the digital words. (TAD)
Anti-Aliasing Filter
Before a signal is subjected to the process of A/D conversion, it must be passed
thorough a low-pass filter to remove any components that are higher in frequency than one-half the
sampling frequency. This is because it requires at least two samples per cycle to determine the existence
and strength of a frequency component, that is, it would require at least one hundred samples per
second to encode a tone of 50 Hz. The A/D process will create spurious signal, called aliased
components, if this rule is not followed.
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