What is MP3
MP3, designed by the Moving Picture Experts Group (MPEG) as part of its MPEG-1 standard and later extended in the MPEG-2 standard, is a rather popular audio coding format. It was developed by a group of engineers of Fraunhofer-Gesellschaft, a research organization in German. Since the release of MP3, it uses inexact approximations and partial data discarding to reduce size significantly. For most common listeners, MP3 still sounds like the original uncompressed audio. And the popularization of MP3 has a great impact on music industry.
There are many technologies has been used in MP3. Psychoacoustics is one of them. It determines which parts of audio data can be discard. MP3 can compress audio according to different bit rate and provide basis that can recognize data volume and audio quality. MP3 used a compound translation that can change time domain signal into frequency domain signal.
According to MPEG standard, the Advanced Audio Coding(AAC) is going to be the next generation of MP3 despite the struggling of other competitive formats. However, judging from its important position, other formats could not be a threat to MP3. MP3 owns not only widely supports of users software, but also by other hardwares like portable audio player(normally refers to MP3 player), mobile phone, DVD and CD player.
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In order to create a compatible MPEG Audio file (Layer 1, Layer 2, Layer 3), the members of ISO MPEG Audio commission has developed ISO 11172-5, a simulation software developed by C-language. In some non real-time operation system, MP3 can displays us the first compressed audio codec based on DSP real-time hardware.
On 7 July 1994, Fraunhofer-Gesellschaft launched the first MP3 player called l3enc.
On 14 July 1995, the develop group of Fraunhofer used ".mp3" as filename extension to replace its old one.
On 9 September 1995, the first real-time MP3 player makes encoding and replaying possible on personal computer. For the storing room of hard disc at that time is rather small, this technology has been a vital importance on storing music on PC and greatly facilitated individual's personal life.
Bit rate is changeable to MP3 files. The higher the bit rate, the more original data it would be in the file. The more original audio it contains, the higher quality of replaying. In the early encoding years of MP3, it used a stable bit rate, which is latter called Constants Bit Rate(CBR).
The bit rate that allows to use in MPEG-1 Layer 3 is 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kbit/s, with sampling rate 32, 44.1 and 48kHz. Among these rates, 44.1kHz is the most common used rate and 128kbit/s represents a good quality even though 320kbit/s gains growing popularity in P2P file sharing online. Other bit rates of MPEG-2 and MPEG-2.5 are included: 6, 12, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160kbit/s.
Variable bit rate is also existed. The MP3 audio is divided into frames with different bit rate, which makes it changeable when encoding. VBR had been widely used in audio/video encoding field although it is not exited in the early time. Lots of encoders are now relying on this new technology.
A high bit rate of 640kbit/s can be achievable in LAME codec and open format. But it cannot be used in low version MP3 player since it was not the standard bit rate.
Since MP3 is a lossy container format, it supports various bit rate. Apart from the bit rate, the audio quality is also related to codec and coding signal. Most of the time, some users advocate that this lossy quality is less acceptable while taking it willingly when they moved to another place such as sit in the noisy car or takes part in a loud party. Apparently, the defect of MP3 is almost implicit when playing in a low version player but undesirable when it connected into Hi-Fi or high quality headphone.
On the one hand, there is no definite standard of MP3 encoder in MPEG-1 standard. On the other hand, it has more detailed definition on decoding algorithm and format. During the encoding process, 576 samples of time domain are converted into 576 frequency domain sample. If it is a transient signal then uses 192 sample points rather than 576.
As a result, it leads to a variety of MP3 encoder, which makes a different audio quality. With a already existed comparable results, potential users may easier to choose the best suitable encoder. Nonetheless, we also need to know that the higher bit rate may not always performs better than the lower one when encoding.
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