It is a common audio format for consumer audio storage, as well as a de facto standard encoding for the transfer and playback of music on digital audio players.
MP3's use of a lossy compression algorithm is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners, but is not considered High Fidelity audio by most audiophiles. An MP3 file that is created using the mid-range bitrate setting of 128 kbit/s will result in a file that is typically about 1/10th the size of the CD file created from the original audio source. An MP3 file can also be constructed at higher or lower bitrates, with higher or lower resulting quality.
MP3 is an audio-specific format. It was invented by a team of international engineers at Philips, CCETT (Centre commun d'études de télévision et télécommunications), IRT, AT&T-Bell Labs and Fraunhofer Society, and it became an ISO/IEC standard in 1991. The compression works by reducing accuracy of certain parts of sound that are deemed beyond the auditory resolution ability of most people. This method is commonly referred to as Perceptual Coding.
It provides a representation of sound within a short term time/frequency analysis window, by using psychoacoustic models to discard or reduce precision of components less audible to human hearing, and recording the remaining information in an efficient manner. This is relatively similar to the principles used by JPEG, an image compression format.
From the first half of 1995 through the late 1990s, MP3 files began to spread on the Internet. MP3's popularity began to rise rapidly with the advent of Nullsoft's audio player Winamp (released in 1997), and the Unix audio player mpg123. The small size of MP3 files has enabled widespread peer-to-peer file sharing of music ripped from compact discs, which would previously have been nearly impossible. The first large peer-to-peer filesharing network, Napster, was released in 1999.
The ease of creating and sharing MP3s resulted in widespread copyright infringement. Major record companies argue that this free sharing of music reduces sales, and call it "music piracy". They reacted by pursuing lawsuits against Napster (which was eventually shut down) and eventually against individual users who engaged in file sharing.
Despite the popularity of MP3, online music retailers often use other proprietary formats that are encrypted (known as Digital rights management) to prevent users from using purchased music in ways not specifically authorized by the record companies. The record companies argue that this is necessary to prevent the files from being made available on peer-to-peer file sharing networks. However, this has other side effects such as preventing users from playing back their purchased music on different types of devices. The audio content of these files can be converted into an unencrypted format, however, because often the user permissions include "burn to audio CD". And even when that option is not available, many sound cards allow the user to record anything they play. Unauthorized MP3 filesharing continues on next-generation peer-to-peer networks, though some authorized services, such as eMusic, and Amazon.com sell unrestricted music in the MP3 format.
When performing lossy audio encoding, such as creating an MP3 file, there is a trade-off between the amount of space used and the sound quality of the result. Typically, the creator is allowed to set a bit rate, which specifies how many kilobits the file may use per second of audio, for example, when ripping a compact disc to this format. The lower the bit rate used, the lower the audio quality will be, but the smaller the file size. Likewise, the higher the bit rate used, the higher the quality, and therefore, larger the resulting file will be.
Files encoded with a lower bit rate will generally play back at a lower quality. With too low a bit rate, "compression artifacts" (i.e., sounds that were not present in the original recording) may be audible in the reproduction. Some audio is hard to compress because of its randomness and sharp attacks. When this type of audio is compressed, artifacts such as ringing or pre-echo are usually heard. A sample of applause compressed with a relatively low bitrate provides a good example of compression artifacts.
Besides the bit rate of an encoded piece of audio, the quality of MP3 files also depends on the quality of the encoder itself, and the difficulty of the signal being encoded. As the MP3 standard allows quite a bit of freedom with encoding algorithms, different encoders may feature quite different quality, even when targeting similar bit rates. As an example, in a public listening test featuring two different MP3 encoders at about 128 kbit/s,one scored 3.66 on a 1–5 scale, while the other scored only 2.22.
Quality is heavily dependent on the choice of encoder and encoding parameters. While quality around 128 kbit/s was somewhere between annoying and acceptable with older encoders, modern MP3 encoders can provide adequate quality at those bit rates (January 2006). However, in 1998, MP3 at 128 kbit/s was only providing quality equivalent to AAC-LC at 96 kbit/s and MP2 at 192 kbit/s.
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