MP3 File Compression - What is MP3 Compression
Audio files with no compression, as found on a normal audio cd, can be very large - around 10 MB per minute of
stereo sound. Without a way to compress this data, portable audio players such as MP3 would probably not exist.
Fortunately, with today's technology, there are many ways to compress digital data. computer data files, for
instance, can be compressed with the popular ZIP format. The resultant zip file is normally much smaller than the
original file and can be unzipped so that the original file is intact.
Compression formats which retain all of the original data are called 'lossless'. Certain types of files like
audio, video and graphics can also be compressed by discarding some of the original data. This form of compression
is called lossy compression.
MP3 compression is a lossy format as there is no way to reconstruct an MP3 file to its original sonic quality.
However, the major advantage in this is a much smaller file size.
One of the main advantages of MP3 files is that they still retain acceptable audio quality, even after
compression, while greatly reducing the size of the file. This is accomplished by discarding certain parts of the
audio spectrum which are considered to be less important. This includes higher frequencies from the audio spectrum
and sounds which may be hidden behind louder sounds.
Audio compression depends a lot on something called psychoacoustics which is the study of how sound is perceived
by the ear and by the brain. The MP3 audio compression codec uses a model that determines how a particular audio
file is heard by the average person. This model can decide how much of the audio data can be removed while still
retaining an acceptable sound.
There are a number of compression schemes, and each of them has their own model for figuring which parts of the
audio spectrum are removed. This results in different sound quality between MP3, WMA, Ogg Vorbis and AAC formats.
The actual sound quality is subjective as some people may favour one format over another.
MP3 Compression - Bitrate
Besides the actual algorithm used for audio compression, the bitrate has the biggest influence on sound quality.
Bitrate represents the amount of streaming audio data and is expressed as kilobits per second. The standard bitrate
for MP3 files is 128 kb per second, and this represents a good compromise between sound quality and file size.
Higher bitrates remove less data and consequently have a higher quality sound. WMA and AAC can realize
comparable sound quality to MP3 at lower bitrates, so these formats generate smaller file sizes.
In the early years of MP3, all files were encoded at a constant bitrate or CBR. This means that a bitrate of 128
kb per second (for example) is used for every section of the music no matter whether it was silent or loud. Musical
passages which are relatively dense, however, have more audio data than quiet passages, so variable bitrates or VBR
can be used to achieve a higher quality sound. As the music becomes more complex, the bitrate rises, and conversely
when the music thins out, a lower bitrate is utilised.
When you are encoding your own music files it is a good practice to try out a variety of bitrates and file
formats to see what works best to you.
You should also bear in mind, that certain kinds of music such as classical, pop or jazz, may sound better with
a particular codec. For instance, it is reported that the Ogg Vorbis codec is ideal for classical music.