1. Not trained hearing skill
This point above is the principal headache of vast number of the newbyes as well as the professional mixing engineers. Really, the lack of skill is the only prevailing serious barrier to produce a high-fidelity mix. The listening skill is one the basic ones every engineer of the studio of professional mixing should have. It should be practiced on and on by regular mixing. Doing that your ears will start percieve incomparably greater than ordinary human being can hear. To identify basic equalization, to see the numerous musical effects and not to ball up the instrumental legibility in mix with its rate, - now you have the first acts of being a studio sound master. To be skilled sound engineer, a real pro in the industry one have to hear and identify details of such subtle mix components as audio compression, sound character, soundstage, structure of the overall sound etc. The more regularly you train your listening competency, the better is your nuances hearing becomes.
2. Details neglegence
Concentration and listening are rather essential characteristics to be a sound editor, but here let us speak about thoroughness and a hard work. Engineer's work contains huge number of tiny aspects. Ignoring these details, paying not enough attention to study them and self education as well you are getting at risk to ruin next mix completely. Sounds fancy, yet in reality just couple or even single wrong note, some unpleasant vibrations or the dearth of controls automation at the right place - any of these criterias are able to transmute your song had power to become a fantastic mix into shocking one.
3. Ignorance of toolset
I met people, who alternated several mixing application types (DAWs) one by another in a short time period, who hunted out supernatural plugins while having exceptional ones, people who ignored hotkeys… In a word, I met numerous precedents of resource & time wasting. Infrequently, though a few cases were actually justified. After all, there is no any "magical" digital audio workstations or plug ins, specially designed to convert poorly mixed song into award winning mix.
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