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Master Bus Processing
Should you mix with compression across the master bus? If you choose to, go easy, especially on the attack. I believe keeping the master bus free from heavy processing while you mix is crucial in getting the best from your mastering service. Some may choose to mix with a limiter on the master bus to help fatten up the mix. Whereas compression can be acceptable when not over done, limiting the masters should not be a part of the mixing process as there is little or no room for any more processing following a limiter. If you choose to use a limiter on the masters, be sure to remove it when you’re ready to use an audio mastering service. One of the most common problems I come across when audio mastering is receiving tracks that have already experienced heavy processing on the master bus in the form of compression, limiting or reverb (often all three). The amount of stages involved in processing the master bus should be kept to a minimum. Applying further audio mastering techniques after a compressor/limiter will not produce quality results. It will surely increase distortion and important dynamic range will be lost. It’s fair to say that the dedicated bus processors found in the audio mastering studio are probably the best for the job so there’s no real need for audio mastering techniques during the production/mix down. Achieving a good balance of levels with no, or little master bus processing is a key part of achieving a good master.
As mentioned in the previous paragraph, getting a good balance of levels with no/minimal compression and no limiting on the master bus is a key part of getting the best audio mastering. In most situations, the vocals should sit in front (that doesn’t necessarily mean louder). A common problem I find is the lead vocal being buried under the guitars or the synths. When performing live, a lead vocalist will usually be at the front of the stage, this arrangement should be reflected in the mixing. In fact, having an imaginary image of the position of each instrument doesn’t just help you arrive at balanced levels but can also help with your stereo placement. Read more about stereo in the Stereo Imaging article. Ensure no levels are hitting the red on the master bus and all individual mixer channels. It sounds simple but this is all too often missed. A quiet mix with head room for mastering will produce more pleasing, and louder results than a mix that has been pushed up too high. If you are finding yourself pushing up level after level trying to hear one instrument over another, it maybe that you may need to lend some attention to separating your tracks using your Pan and EQ. The next paragraph covers this in more detail.
Separation with EQ
A mixing engineers channel EQ is a very powerful tool. It can be used to separate instruments that occupy similar frequency ranges. Lets say there were two very similar sounding acoustic guitar parts playing simultaneously (not advisable but just as an example). To help the listener distinguish between the the two, channel EQ can be used to make one guitar sound a little more “twangy”. The other guitar could me made to sound a little “warmer”. Also, a little panning can help with the separation too, which will make for a wider stereo image (see the next section for separation using stereo placement). When this technique is considered through out the whole mixing process it contributes to the quality of the final audio master. A low cut on your electric guitar parts can help clarify the low mid by separating from the bass regions. In isolation, an electric guitar may sound a little thin if it’s missing 150Hz and below but when this technique is used in a whole mix it can do wonders for tightening up the bass and giving definition to the lower mid and bass. This is a link to a great article covering this very important subject in more detail… Separation with EQ tutorial
Stereo Imaging and Depth
Audio mastering can only enhance what is already there. Good stereo width and depth in a mix will be emphasised during the audio mastering process, producing that desired “big” sound. Audio mastering engineers are equipped with tools for focussing on the stereo elements in a mix to help bring a shallow or narrow mix to life but they should not be relied upon. They are no substitute for good stereo imaging and depth in the mix. Having an imaginary view of the soundstage and where all the instruments are placed or sounds are coming from can help achieve good stereo imaging and depth. With the careful use of panning, EQ, reverb, chorus and delay you can literally place each instrument where you want it in the soundstage, left right, front or back. Here are links to a couple of great articles providing easy steps to achieving good stereo imaging and depth in your music… Stereo Imaging Depth
Digital Audio File Format
When you are finished mixing you need to bounce to at least 24bit – 44.1kHz wav or aif. 16bit is acceptable but not preferred. Audio mastering studios are equipped to deal with almost any file format so higher word length/sample frequencies are encouraged. Wma files are also acceptable. Do not bounce to, or convert to mp3 before your audio is mastered. This is best carried after the audio mastering process in the correct way by the audio mastering engineer (yes there is a right and not so right way to do this!). If you are reducing word length (bit rate if you prefer) when you bounce down your music, 32bit to 24bit for example, remember to add “dither” to avoid truncation. In fact, it is good practice to dither every time you bounce anyway. Below is a link to a fantastic explanation about what dither and truncation are… Dither and truncation explanation