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APPENDIX B GLOSSARY
APPENDIX
377
Glossary
A-Law
A companded compression algorithm for voice signals defined by the Geneva Recommendations (G.711).
The G.711 recommendation defines A-Law as a method of encoding 16-bit PCM signals into a nonlinear
8-bit format. The algorithm is commonly used in United States telecommunications. A-Law is very similar
to µ-Law, however, each uses a slightly different coder and decoder.
Adaptive Delta Pulse Code Modulation (ADPCM)
A method of compressing audio data. Although the theory for compression using ADPCM is standard, there
are many different algorithms employed. For example, the ADPCM algorithm from Microsoft® is not
compatible with the International Multimedia Association’s (IMA) approved ADPCM.
Aliasing
A type of distortion that occurs when digitally recording high frequencies with a low sample rate. For
example, in a motion picture, when a car’s wheels appear to slowly spin backward while the car is quickly
moving forward, you are seeing the effects of aliasing. Similarly, when you try to record a frequency greater
than one-half of the sampling rate (the Nyquist Frequency), instead of hearing a high pitch, you may hear
alias frequencies in the low end of the spectrum.
To prevent aliasing, an anti-aliasing filter is used to remove high-frequencies before recording. Once the
sound has been recorded, aliasing distortion is impossible to remove without also removing other frequencies
from the sound. This same anti-aliasing filter must be applied when resampling to a lower sample rate.
Amplitude Modulation (AM)
A process whereby the amplitude (loudness) of a sound is varied over time. When varied slowly, a tremolo
effect occurs. If the frequency of modulation is high, many side frequencies are created which can strongly
alter the timbre of a sound.
Analog
When discussing audio, this term refers to a method of reproducing a sound wave with voltage fluctuations
that are analogous to the pressure fluctuations of the sound wave. This is different from digital recording in
that these fluctuations are infinitely varying rather than discrete changes at sample time (see Quantization).
ASIO
ASIO (Audio Stream In/Out)™ is a low-latency driver model developed by Steinberg Media
Technologies AG.
ASIO audio drivers are only supported in the full version of Vegas® software.
Attack
The attack of a sound is the initial portion of the sound. Percussive sounds (drums, piano, guitar plucks) are
said to have a fast attack. This means that the sound reaches its maximum amplitude in a very short time.
Sounds that slowly swell up in volume (soft strings and wind sounds) are said to have a slow attack.
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