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Glossary
Appendix E
A-Law
A-Law is a compounded 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.
Acoustic Signature
The acoustic signature of a system is data containing all of the sound characteristics of a system. This includes such things as reverb
time, frequency response and other timbral qualities. Impulse files used by Acoustic Mirror can be thought of as acoustic signatures.
Activation Number
This number is based on the Computer ID number of the computer on which Sound Forge software is installed. Each computer has a
unique number, similar to a license plate. An activation number is created based on that number. When you register the software, Sony
will generate an activation number for you. Once the activation number is entered, the software will not time out. Since the activation
number is based on the Computer ID, it is important that you have Sound Forge software installed on the computer where you will be
using it.
See also Computer ID on page 346.
ActiveX
A Microsoft technology that enables different programs to share information. ActiveX extends Microsoft Windows-based architecture to
include Internet and corporate intranet features and capabilities. Developers use it to build user interactivity into programs and World
Wide Web pages.
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, Microsoft’s ADPCM algorithm is not compatible with the International Multimedia Association’s
(IMA) approved ADPCM.
Advanced Streaming Format (ASF)
See Windows Media Format on page 358.
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 a low-frequency rumble. See also Nyquist Frequency on page 352.
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
Amplitude Modulation (AM) is 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 that can strongly alter the timbre of a sound.
APPENDIX E | 343