This is my professional view as an audio engineer:
First of all, I have been a DJ for 20 years and a producer for the past 7 years. I have been producing and engineering remixes since 1991. Through out the years of remixing I have developed and perfected my own technique to for mastering songs. I use a unique combination of EQ's, Filters, Dynamics, and Enhancers to improve sound quality and level a song. I will get more in depth in this process later on.
The main reason to get your song mastered by a professional is to get a perfect balance between instruments and vocals in a song. Another reason is to maintain an equal sounding balance between all of your songs on a particular album. Professional opinions state it is always good to have a fresh set of ears to master your song. Most professional recording artist use one studio to record and another studio to master song and album, just check their credits and you will see what I mean.
What mastering does
Mastering accomplishes four principal tasks:
1. Format conversion. For CD release, an audio file must be converted to 16 bit resolution and a 44.1kHz sampling rate. Many mixes come to the mastering engineer at a higher resolution, such as 24 bits at 96kHz, or even on analog tape.
2. Assembly. This process takes a collection of individual tracks and turns them into a single, cohesive "album" or listening experience. This means no awkward volume or tonal changes between tracks, as well as a "flow" that provides the optimal artistic impact.
3. EQ. This has two main functions in mastering. One is to ensure consistent tonal qualities from track to track, while the other is to fix specific tonal problems within individual tracks (like reducing resonances—buildups in certain frequency ranges).
4. Dynamics. Vinyl had a limited dynamic range, so the wide dynamic range of music had to be reduced in order to fit. This meant bringing down the peaks so that the needle wouldn't jump out of the grooves, while bringing up softer passages to raise them above the surface noise. Even though CDs don't have these limitations, dynamic control is still used (unfortunately, often to excess) because it can create a CD that sounds "louder" and "jumps out" at the listener. Although a CD that's cut really loud has a dramatic initial impact, it eventually becomes fatiguing. A well-mastered piece of audio should retain as much dynamics as possible, as dynamics are a huge part of the emotional impact of music. Paradoxically, a lot of older vinyl records have more dynamics that today's CDs; many feel those old records provided a more satisfying listening experience.
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