A recent post asking about file zipping made me wonder…does audio compression do the same thing? Is it finding pieces of the sound that are identical and then saving them only once in the MP3 file? It’s one thing to identify patterns in a text file and only save one version of the repeating parts, but somehow that doesn’t seem feasible with audio since things like music have so much complexity.
In: Technology
The encoder takes a short window of time and transforms it from time into frequency domain. This gives the intensity over a range of sound pitches, as opposed to instantaneous amplitude. These intensity values are then rounded to reduce the number of digits required to store them.
A listener usually cannot distinguish sounds of a similar pitch that differ significantly in volume. So some noise is acceptable in the presence of a louder signal, within a limited band of frequencies. Rounding directly in time domain would affect all frequency ranges equally, and noise would become audible in those that are quiet.
A sound signal is usually quite chaotic in nature and it is impossible to find exact repeating patterns in it. Compression usually takes advantage of neighboring samples having similar amplitude, and stores the difference between them with fewer digits. This is how multimedia or delta compression works in a general purpose archiver (Rar), and is also used in frequency domain samples.
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