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