MP3 is actually MPEG-1 Audio Layer III or MPEG-2 Audio Layer III. It is a method of lossy compression of digitally stored audio data. MP3 makes use of psychoacoustics with the aim of storing only perceptible parts of the signal for humans. As a result, with barely reduced perceived audio quality, a strong reduction in the amount of data is possible.
With an example data rate of 192 kbps, which already allows for high quality, the compression rate of an MP3 audio file is about 85% compared to an uncompressed audio CD. MP3 is the dominant method of storing and transmitting music on computers, smartphones, the Internet, and portable music players (MP3 players), although there are now a number of technologically advanced options. In May 2017, the developers stopped licensing the format, after the last patents expired in the U.S. Since then, it has been a freely available standard.
Read this article to learn about various types of audio file formats.
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The MP3 format was developed in 1982 for Integrated Circuits. Later, it was codified as part of the MPEG-1 standard. The file name extension .mp3 was established on 1995 following an internal survey. Previously, .bit filename extension was used internally. Brandenburg has received several awards for the development of the format.
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Procedure That MP3 Audio Format Uses
Like most lossy compression formats for music, the MP3 process exploits psychoacoustic effects of human perception of tones and noises. For example, humans can only distinguish two sounds from each other from a certain minimum difference in pitch, before and after very loud noises they can perceive quieter sounds worse or not at all for a short time. So you don’t have to store the original signal exactly, but the signal components that the human ear can also perceive are sufficient. The task of the encoder is to prepare the original sound signal according to fixed rules based on psychoacoustics in such a way that it requires less storage space, but still sounds exactly like the original to the human ear. In the case of subjective complete correspondence between the original and MP3 variants, this is referred to as transparency. In principle, however, due to the lossy compression, it is not possible to reconstruct the original signal from the MP3 signal exactly. There are also lossless methods of audio data compression such as FLAC, but these achieve much lower compression rates and are even less common, especially in the field of playback hardware.
When playing the MP3 signal generated in this way, the decoder generates an analogue sound signal from the reduced data, which sounds original to the majority of listeners, but which is not identical to the original signal, since information was removed during the conversion to the MP3 format. If one were to compare the temporal signal curve of the MP3 sound signal with the original, for example on the screen of an oscilloscope, clear differences would be recognizable. Because of the above-mentioned psychoacoustics of human perception, the MP3 signal still sounds exactly like the original to a listener – provided that a mature encoder is used and a sufficiently high data rate (bit rate) during encoding.
While decoding always follows a fixed algorithm, coding can be done according to different algorithms (e.g. Fraunhofer encoders, LAME encoders) and accordingly delivers different acoustic results. The question of whether a loss of quality is perceptible by some or many listeners depends, among other things, on the quality of the encoder, on the complexity of the signal, on the data rate, on the audio technology used (amplifier, loudspeakers) and, finally, on the hearing of the listener. In addition to fixed data rates of 3 kbit/s up to 8 kbit/s, the MP320 format also allows arbitrary free data rates of up to 640 kbit/s (Freeform MP3) in free format mode. However, only a few MP3 player decoders are designed for higher bit rates than those from the ISO standard (currently up to 320 kbit/s).
The quality impressions are quite subjective and vary from person to person as well as from hearing to hearing. At a higher bitrate and using a sophisticated encoder, most people can no longer distinguish the encoded material from the source material, even with concentrated listening. Nevertheless, it may be possible to distinguish certain pieces of music from CD quality, even at 256 kBit/s if older MP3 encoders has been used. However, in people with “abnormal” hearing (e.g. hearing damage caused by bang trauma), the mechanisms used sometimes do not work as intended, so that they are more likely to notice differences between coded and source material. The test person who was best able to detect differences in the above-mentioned test, even at high data rates, has impaired hearing.
In addition to encoding with a constant data rate (= fluctuating quality, accompanied by the changing complexity of the sound signal over time), encoding with constant quality (and thus fluctuating data rate) is also possible. This avoids (largely) drops in quality at music passages that are difficult to encode, but on the other hand saves on the data rate and thus on the final file size in quiet or even completely silent passages of the audio stream. The quality level is predetermined, and you get the minimum file size required for it.
