Iz nekog iskustva mislim da je dobro staviti neke definicije masteringa na sticky, cisto iz razloga, iako siroko poznat pojam, da se razjasne neke stvari o cemu se tu radi. 
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MASTERING is - The process of preparing a body of recorded material for reproduction. THAT is the actual literal definition of mastering.
This process generally contains TWO phases. The first is a Quality  Control phase and involves correcting or enhancing the mix, removing  pops or weird noises, fading the song in or out - if anything needs to  be done. This is also where the mastering engineer checks to make sure  that the audio material fits the guidelines for the medium it will be  reproduced on. 
The second phase is called Authoring. This is where the songs are  sequenced, and encoded with data such as track information and ISR  Codes.
You do not "master a song" you master a project. The exception being a  project CAN be a single. However, one of the main things addressed in  modern mastering is the relationship between multiple songs - tonally  and volume wise.
The process of audio mastering varies depending on the specific needs of  the audio to be processed. Mastering engineers need to examine the  types of input media, the expectations of the source producer or  recipient, the limitations of the end medium and process the subject  accordingly. General rules of thumb can rarely be applied.
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Examples of possible actions taken during mastering:
 
- Editing minor flaws
 
- Applying noise reduction to eliminate clicks, dropouts, hum and hiss
 
- Adjusting stereo width
 
- Adding ambience
 
- Equalize audio across tracks
 
- Adjust volume
 
- Dynamic range expansion or compression
 
- Peak limit
 
- Dither
 
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To finish mastering a CD the track markers must be inserted along with 
International Standard Recording Code  (ISRC) and other information necessary to replicate a CD. Vinyl LP and  cassettes have their own pre-duplication requirements for a finished  master.
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Sta kazu neke poznate 'mastering face' vezano za samu definiciju:
Bob Olhsson:
"adjusting the presentation of the mix to be appropriate to its position  within the album, the genre, the contemporary marketplace and possibly  even history."
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"The presentation of a 
mix should never be confused with the  presentation of the music and the recording which is what mixing is all  about. Obviously the producer plays a key role and must approve anything  that affects the mix. The presentation of the mix must always support  the producer's, artist's and client's goals.
 
It's never a matter of homogenizing the sound to fit some genre but  rather one of understanding the context within which the song is going  to be listened to. You don't treat a Broadway musical the same way you  treat a hip hop record unless the musical is about hip hop. Even then  I'd want to discuss the goals of that kind of special situation with the  producer. Mastering to me is always a collaboration and not "playing  god."
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Brad Blackwood
"Simply put, mastering is the craft of helping the artists achieve their goals, whatever they may be."
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"Mastering is listening without prejudice - the processing that happens in mastering is a result of the objective listen."