THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

582 19. Sound Reproductionuse of MP3 compression, this bitstream is greatly reduced by a factor of 8–12. Atypical MP3 file will require 0.828 MB/s, and hence 1 min of music is reducedfrom 10 MB to about only 1 MB. Even greater compression is possible for use onthe Internet, for example, but at the expense of a decrease in sound quality.MP3 uses two compression techniques to achieve size reduction from uncompressedaudio: one lossy and the other lossless.1. The process begins by breaking the signal into smaller components calledframes, each frame typically lasting a fraction of a second. The signal is analyzedto determine its spectral energy distribution. Because different portionsof the frequency spectrum are most efficiently encoded via slight variant ofthe same algorithm, the signal can be broken into sub-bands, which can beprocessed independently for optimal results.2. The encoding bitrate comes into play, and the maximum number of bits that canbe allocated to each frame is calculated. For example, if the encoding is set at128 kbps, there is an upper limit on how much data can be stored in each frame.This step establishes how much available audio data will be stored and howmuch will be discarded. This constitutes a lossy phase of the MP3 procedure.3. The frequency spread for each frame is compared to mathematical modelsof human psychoacoustics, which are stored in the codec 5 as a reference table.From these models, it can be determined which frequencies need to be renderedaccurately, because they are most perceptible to humans, and which ones canbe dropped or allocated fewer bits, owing to the fact that they are less likely tobe perceived by human hearing. This stage is also a lossy procedure.4. The bitstream is run through the process of Huffman coding that compressesredundant information throughout the sample. The Huffman coding does notwork on the basis of a psychoacoustic model but achieves additional compressionthrough more traditional means so that even less space is required forstorage. The action is quite lossless, similar to the “zipping” and “unzipping”of files on a computer.5. The collection of frames is now assembled into a serial bitstream, with headerinformation preceding each data frame. (The header contains extra informationabout the file to come, such as the name of the artist, the track tile, the nameof the album from which the recording came from, the recording year, genre,and personal comments that may have been added—this is referred to as anID3 tag).iPodThe iPod r○ is Apple Computer’s multiuse record/playback ultraportable unit thatmakes use of a mini disk drive to store data. It plays music and other audio that5 The word “codec,” a shortening of the words compress and decompress, refers to any of a class ofprocesses that allow for the systematic compression and decompression of data. Various codecs arefundamental to many formats and transmission methods, for example, image and video compressionformats. Here we are concerned with the audio MP3 codec.