All about MP3 format definitions

MP3 (is more accurate, Eng. Mpeg-1/2/2.5 Layer III (but not Mpeg-3) - - the third (sonic) size of coding MPEG); - - the licensed size of file for storing the audio- information.

At the given moment MP3 it is most known and most popular of the extended sizes of the digital coding of sonic information. It widely is used in the p2p networks for the transfer of musical works. Size can be lost at any operating system (for example, in the last versions Windows even there is the already built-in codec), on practically any portable audio -player, and is also supported by some models of musical it is center and DVD- players. It is necessary to note that this popularity is the "vicious circle": users code audio in MP3 in essence because of the wide support of size by the producers of program and apparatus guarantee, and producers, in turn, pack means into the size MP3most popular among the users.

History

MP3 is developed by the working group of the institute of Fraunhofer MPEG (Eng. Motion Pictures Expert Group; - - appraisal group in the region of cinematography) consisting of Johnson, Shtolla, Deyeri and Brandenburg. As the basis of development MP3 served the experimental codec ASPEC (Adaptive Spectral Perceptual Entropy Coding). The first coder into size MP3 became program L3enc, released in the summer of 1994 . After one yr appeared the first program MP3- player; - - Winplay3.

With the development of algorithm the tests were conducted on the completely concrete popular compositions. Basic experimental became Susannah Vega's song; - - Tom?s Dinner. Joke hence arose, that "MP3 was created exclusively for the comfortable hearing of the dear song of Brandenburg".

Description of the size

In this size the sounds are coded by the frequency means (without the discrete parties); there is support to stereo. MP3 is the size of compression with the losses, i.e., the part of the sonic information, which (according to psychoacoustic model) the ear of man perceive cannot or he is received not by all people, it is moved away forever from the record. Compression ratio can be varied, including in the limits of one file. The interval of the possible values of Bit rate composes 8; - - 320 kbit/.ch. For the comparison, the flow of data from the usual CD of size Audio-CD is equal to 1411,2 kbit/.ch with the frequency of discreteness 44100 Hz.

MP3 and "quality Audio-CD"

Is extended the opinion that the record Bit rate of 128 kbit/.ch is suitable for the musical works, intended for the hearing by the majority of people, ensuring the quality of sounding Audio-CD. In actuality everything is considerably more complex. First, the quality of that obtained MP3 depends not only on Bit rate, but also on the coding program (codec). In the second place, besides the prevailing regime CBR (in which, simply stated, each second of audio is coded by the identical number of bits) there are regimes ABR and VBR (in which Bit rate it varies, ensuring the more high quality of sounding). Thirdly, the boundary of 128 kbit/.ch is conditional, since it was "invented" in the epoch of the formation of the size, when on poor quality audio and columns practically it cannot be differed MP3 from the original.

It is considered at the given moment that the indistinguishable from the original sounding (with the correctly selected and special codec) usually accessibly with Bit rate from 160 kbit/.ch and higher; - - depending on initial audio file, listener and his audio system. Some audio files prefer to compress music "to the maximum of quality"; - - 320 kbit/.ch. In reality are known simply (fragments of audio records), which are not yielded to qualitative compression with the losses: on all possible Bit rates it does not comprise the special labor to distinguish the compressed audio of the original.

Regimes of coding and option

There are three versions MP3 of codec for different needs: Mpeg-1, Mpeg-2 and Mpeg-2.5. They are characterized by the possible ranges of Bit rate and frequency of the discreteness:

320 kbit/.ch with the frequencies of discreteness 32000 Hz, 44100 Hz and 48000 Hz for Mpeg-1 Layer 3; 160 kbit/.ch with the frequencies of discreteness 16000 Hz, 22050 Hz and 24000 Hz for Mpeg-2 Layer 3; 160 kbit/.ch with the frequencies of discreteness 8000 Hz and 11025 Hz for Mpeg-2.5 Layer 3.

Control modes of coding the sonic channels

Since size MP3 supports the two-channel coding (stereo), 3 regimes there are:

Stereo; - - the two-channel coding, with which the channels are coded independently of each other. Thus, assigned Bit rate is divided into two channels. For example, if assigned Bit rate of 192 kbit/.ch, then for each channel it will be equal to only 96 kbit/.ch. Mono; - - single-channel coding. If we code two-channel material in this manner, the differences between the channels will be completely effaced, since two channels are mixed up into one, it is coded and the very same is reproduced in both channels of stereo-system. Only plus of this regime can be only output quality in comparison with the regime of stereo with the identical Bit rate, since to one channel is fallen doubly a larger quantity of bits, than in the regime of stereo. But you will not hear the differences between the channels, since channel here only one. United stereo (Joint Stereo); - - the most optimum method of two-channel coding, with which left and right channels are converted into their sum and difference. For the majority of sonic is file the channel with the difference it is obtained considerably quieter than the channel with the sum; therefore for the sum is removed the big part of Bit rate. Thus, the quality of output file striking is differed to the best side from the regime of stereo with the identical Bit rate, especially with the low. Existing the opinion that this regime does not be suitable for the sonic stereo-material, in which in two channels absolutely different material is reproduced subjectively, since it erases the differences between the channels. This erroneous opinion, since in actuality MP3- codec operates with frequencies, and the specific frequencies in the majority of the cases intersect in both channels, i.e., identical information nevertheless is present, and different is coded separately. Is especially effective this method of two-channel coding with the use of variable Bit rate, the discussion with which will deal below.

CBR, VBR, ABR

CBR is deciphered as Constant Bit Rate, i.e., constant Bit rate, which is assigned by user and does not change with coding work, thus to each second of work is an identical quantity of coded data bits. In reality this regime of coding is not optimum, since it does not befit for the majority of dynamic musical works with Bit rate below 256 kbit/.ch.

VBR is deciphered as Variable Bit Rate, i.e., being varied Bit rate or variable Bit rate, which dynamically changes with program- coder with the coding, depending on the saturation of coded audio material and established by user quality of coding. This method MP3- coding is most progressive and, until now, is developed and is improved, since audio material of different saturation it can be coded with the specific quality, which is usually higher than with the installation of average value in method CBR. plus to that, the size of file decreases for the calculation of the fragments, which do not require high Bit rate. The complete impossibility to predict the size of output file is only minus of this method of coding. But this deficiency VBR- coding grows dim before its merits.

ABR is deciphered as Average Bit Rate, i.e., Middle Bit rate, which appears the hybridism?s VBR and CBR: Bit rate into kbit/.ch is assigned by user, and program varies it, constantly driving on under assigned Bit rate. Thus, coder cannot assign maximally and the smallest possible values of Bit rate, since it risks not to be entered in Bit rate assigned by user. This is explicit minus of this method, since it affects the quality of output file, which will be somewhat better than with use CBR, but it is considerably worse than with use VBR. from other side, this method makes it possible to most flexibly assign Bit rate (it can be any number between 8 and 320, against the exceptionally multiple 16 numbers of method CBR) and to dimension of output file.

Merits and the deficiencies MP3 is leader on the prevalence, but in this case it is not best according to the technical parameters. For example, there are sizes, which make it possible to obtain the comparable quality (subjectively) with the less density. Also in size MP3 there is no regime of coding without the losses Eng. lossless, that is suitable for the professionals. For the domestic musical collection (when there is no need for losing compositions in the musical center or extending them through the Internet) it is possible to use the competing sizes.

MP3 is unfit for the professional use by musicians already because data are compressed with the losses, and quality deteriorates with each editing of file. In this case the size completely approaches (from a professional point of view) for the propagation of the demonstration compositions or other methods of the "distribution" of its music because of the general prevalence of record players.

LAME - popular free mp3 codec LAME, which is been one of the good, even if not best EncSpot - utility for determining the quality mp3 it is file Audiophile'.s Webpage - the collection of utilities for the coding The period of the action of patents for the technology elapses into 2010 (it is Russian.) CompuLenta.ru (on 11 September 2006)

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