System information
Glossary
A
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.
A-Law
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 non-linear
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.
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.
Chapter 34
GLOSSARY631