What is MP3 file format?

For the first time, MP3 files, allowed people to copy tracks from their CDs to their computer in a practical way but what is MP3 and where does it come from? This page provides you information on the two following topics:

Technically Mp3 is not a compression technique like .zip as it does not retain all the information of the original file, it actually wipes the part of sound that is not audible.

MP3 files are generated by removing the sound that human ear cannot hear. This would allow the digital audio file size to be reduced to one twelfth of the original CD but maintaining CD quality. The human ear cannot hear about 90% of the audio in a CD audio disks track. These are the extremely high frequencies which only animals, such as dogs, can hear.

The standard MP3 rate is 128 kbps (killo bits per second), which uses approximately 1 Mb of hard disk space per minute of MP3 files. This allows for 1:12 compression; maximum sound quality with minimal disk space. Higher bit rates generally give higher sound quality as well as larger files, but human ears cannot much distinguish the difference. A four minutes track, encoded at 128 kbps will occupy 3.6 Mb. The same file, encoded at 192 kbps, occupies 5.5 Mb which uses twice as much disk space without any recognisable difference in the sound quality.

Information about the author, artist, song names, etc is also stored. This information is stored as the ID TAG, which helps the player to identify the track name and to display the picture of the artist.

History of MP3

In January 1988 in the Centro Studi e Laboratori Telecomunicazioni (CSELT labs) in Turin, Italy the 'Moving Picture Experts Group' (MPEG) developed the most popular compression technique. Their research provided a worldwide standard for moving images, audio, and images combined with audio.

MPEG-1, the standard for storage and retrieval of moving pictures and audio on storage media was standardised in November 1992. This compression encodes the combined audio-visual signal at a bit rate of about 1.5 Mb/s and with a slightly below TV quality. File space is around 1.2 Mb/s for video and 0.3 Mb/s for stereo sound.

A couple of years later the 'Moving Picture Experts Group' developed MPEG2 that is the heart of Digital Video Disks (DVDs).

Around the same time as MPEG, another European group was tackling the audio compression. The perceptual audio coding was started in one of the European collaborative EUREKA project EU147. The main collaborators were mainly the University of Erlangen and the Fraunhofer IIS-A institute.

At the time of this research the MPEG was already well established, however what the Fraunhofer IIS final discovery was a very powerful algorithm that was standardised as ISO-MPEG Audio Layer-3 and MP3 was born.

What the Fraunhofer people did was use what they learned from MPEG, applied only to audio, added additional functions and created an audio compression algorithm with some very impressive characteristics.

Digital Sampling and different sampling rates

When converting an analogue signal to a digital file, the sampling frequency with which samples are taken and converted into digital form must be at least twice that of the maximum analogue frequency being captured.

The CD quality playback is sampled at 44,100 times per second (44.1KHz), This is slightly more than double the maximum 20KHz frequency a person can hear which needs more than 1400000 bit to represent just one second of stereo music in CD quality.

The digitising voice for a telephone-quality is sampled at 8,000 times per second (8KHz), i.e. twice the 4KHz required for the full spectrum of the human voice.

As the sampling rate is increased, so too is the quality of the digital recording. The faster the sampling rate, the larger the sample sizes, the more accurately the sound can be reconstructed and replayed. Good quality sound is very large and with the average 56K modem it would take more than 3 hours to download a single track. This is the reason for the big demand on compression techniques enable the compact of a digitised file to a fraction of its original size since the introduction of the Internet .