File compression (not to be confused with amplitude compression) makes files smaller using a variety of algorithms. Uncompressed audio files tend to be large. CD quality audio requires ten megabytes per minute. That is not a problem with large computer hard drives available today, but it is a problem if you want to save many songs on a portable player or if you want to transfer files over the Internet. Unlike most computer data, audio data does not compress very well using typical compression methods such as those found in programs like PKZIP or WinZip. These methods preserve the data exactly so there is no loss of quality. Such compression is called lossless compression.

To make audio files smaller, complex algorithms have to used. Most of these algorithms sacrifice some quality so that when the data is decompressed, you do not get exactly the same quality you had originally. This type of compression is known as lossy compression. Ideally the quality that is lost is not perceptible, so you do not notice the difference.

The most common method of lossy audio compression is MPEG Layer-3, better known as MP3. It is capable of getting near CD quality audio in less than one tenth the size, which is about one megabyte per minute. More recent algorithms, such as AAC, Ogg Vorbis, and Windows Media Audio get even better quality in a smaller size.

Software and hardware that compress audio using complex algorithms are referred to as codecs (from coder/decoder). Compression is the same as encoding and decompression is the same as decoding.

To control the level of compression in GoldWave, use File | Save As and select a different file type and/or attributes. The lower the bitrate (the kbps number, see below), the smaller the file will be, usually at reduced quality.

When opening, editing, and saving a file repeatedly, it is best to use a lossless file format. Every time a lossy compressed file is re-opened and saved, some quality is lost. For long term editing, use the Wave type with "PCM 16 bit" attributes or one of the lossless compressed formats such as FLAC or Windows Media Audio with "lossless" attributes.

Bitrate

Many compressed audio formats measure the compressed size as a bitrate. The bitrate is the number of bits per second (bps) required to store the audio. Usually the number is given in kilobits or one thousand bits. Divide that number by 8 to determine the number of kilobytes required per second.

Internet connection speed (bandwidth) is often measured in bitrates as well. A 56k modem is capable of receiving 56 kilobits per second. If you want people to stream your MP3 audio over a modem, you'll need to compress the file using a maximum bitrate of 56kbps. Due to the connection overhead and Internet protocol, a lower rate would have to be used to ensure the audio can be downloaded fast enough. For DSL and Cable Internet connections, the standard 128kbps MP3 rate can be used.

Audio files can contain a wide range of sounds, from noisy cymbal clashes to silence. Algorithms typically get much better compression on silence or simple audio sections than on complex, noisy audio. This means that the bitrate depending on whether constant bitrate or variable bitrate compression is used.

Constant Bitrate

When using constant bitrate, each section of audio compresses to exactly the same size, regardless of the content. If the audio contains silence, then the data may be padded to fill the required bitrate. If the audio contains complex music, then quality may be decreased until it fits within the bitrate.

Constant bitrate is useful for broadcast systems where the transmission rate is fixed. It also make is easy to seek to arbitrary positions within the audio stream or file.

Variable Bitrate

Variable bitrate compression uses the smallest size possible for each section of audio. If the audio contains silence, then the bitrate will be very low. If the audio contains complex music, the bitrate will be at its maximum.

Variable bitrate gives the best compression and quality. However, it makes it difficult to seek within the stream or file since there is no direct relation between time and size.

Appendix B: Keyboard Commands

In addition to all the standard menu keystrokes, such as Alt+F O to open a file, Alt+E C to copy, Alt+E P to paste, etc., GoldWave includes a number of additional keyboard shortcuts. These are summarized in the following table. Use Options | Keyboard to reassign shortcuts.

Appendix C: Expression Evaluator