User Manual

GLOSSARY | 463
Appendix B
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 a low-frequency rumble.
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.
Alpha Channel
Alpha is a fourth channel that determines how transparency is handled in an image file. The RGB channels are blended to
determine each pixel’s color, and the corresponding alpha channel determines each pixel’s transparency. The alpha channel can
have up to 256 shades of gray: 0 represents a transparent pixel, 255 represents an opaque pixel, and intermediate values are
semitransparent.
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 (Audio) on page 471.)