| When you listen to a live musical instrument, the | | | | 1. Lossless codecs. This kind of compression |
| sound you hear is an analog sound wave. Such a | | | | reduces the file size while retaining every scrap of |
| wave can take an infinity number of values. This | | | | information from the original file. This means that |
| signal has to be converted into a sequence of | | | | the original file can be reconstructed exactly as it |
| numbers (0s and 1s in native machine language) | | | | was before the compression. Lossless |
| for storage and playback on a computer. This | | | | compression does not significantly reduce the size |
| process of digitisation creates a raw digital music | | | | of an audio file. Examples of general lossless |
| file which uses 10MB or more per minute (at least | | | | compression algorithms include zip or gzip |
| 30MB for a 3 minute song). | | | | compressions. |
| On a PC, a .WAV is an example of a default audio | | | | 2. Lossy codecs. These throw away some of the |
| file, while AIFF(Audio Interchange File Format) is | | | | information from the original music file. Ideally |
| the default format on a Mac. On a Unix machine, | | | | what's thrown away is what the human ear |
| Sun Microsystems' AU(AUdio) format is the | | | | cannot hear, namely sounds below 20Hz and |
| default for audio. These file formats are an exact | | | | above 20KHz. Thus, the most successful lossy |
| replica of the original sound, except for small | | | | codecs are based on an understanding of |
| sampling errors. (They also support limited audio | | | | psychoacoustic perception. Lossy compression can |
| file compression.) | | | | reduce the size of the file to less than 10% of its |
| These files have to be compressed to the more | | | | original size. Most of the codecs you will come |
| usual levels of 3-5MB per song. Compression | | | | across are of this type. Examples include MP3 and |
| rearranges the sequence of numbers, sometimes | | | | WMA codecs. |
| throwing away less significant information, in order | | | | CONSTANT V VARIABLE BIT RATE |
| to reduce the file size. The way the compression | | | | Something else you might come across |
| is achieved leads to the many different audio file | | | | concerning compression is bit rate. This is the |
| formats available. Each compression algorithm | | | | number of bits (0s and 1s) per second in an audio |
| requires a decoder to make sense of the data | | | | stream. This kind of represents how much |
| during playback. The combination of a compressor | | | | information is being conveyed per second. The bit |
| and a decoder is called, surprisingly, a codec. | | | | rate within a single music file can be constant or |
| COMPRESSION TYPES | | | | variable. |
| There are two broad compression types: | | | | |