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I don't know where the phrase, "Fix it in the mix" originally came from. But I can imagine a recording session where a musician fluffs a note and the producer decides that the musical flow of the session is good and it would be better to continue recording rather than spend time on a small detail that can be repaired later. "That's OK, we'll fix it in the mix" the producer said.
But of course that means that sometime later the problem had to be resolved. History does not accurately record whether the problem ever was fixed to anyone's satisfaction - the musician, the band, the producer, the A&R manager, or of course the record-buying public.
Oh, and before I move on to the real meat of this article, the phrase FIITM definitely predates digital audio. Fixing problems on old-fashioned analogue tape is MUCH more difficult than fixing them digitally.
The first class of faults I'll consider are those that are musically generated.
Suppose the recording is of a band playing instruments, and not a sequenced recording. Typically the issues would be...
Of these, the first four really should have been corrected in the session.
However if the bass guitarist plays a wrong note, it isn't too much of a problem to pitch change it to the right note later.
If the rhythm guitar plays a wrong chord, then that is more of a problem. If you have Celemony Melodyne, in its Editor or Studio version, it is possible to change notes within chords. So changing a chord of C major to A minor would only require that the note G is changed to A, the other notes being the same in both chords.
This sounds good in theory, but any kind of pitch changing may work perfectly, it may work well enough, or it may not work at all. Expect your results to vary.
Out-of-time passages can be corrected through editing, or by warping, known variously as 'warp', 'elastic time', 'time stretching' depending on which DAW you use. Editing should work, but requires time and patience. Warping may work, but may sound odd. Again, results will vary.
Background vocals may need to be synchronised. This can be done through editing, or by using a correction software such as VocALign (sic).
Mistakes in the lyrics can be an insurmountable problem. If you're the mix engineer and had no responsibility for the song, you might not even know that a mistake had been made.
Sometimes mistakes might be fixable. If for example a mistake was made in one chorus, and the passage is repeated correctly in another, then a simple copy-and-paste will provide a cure.
The one problem among those listed above that probably should be left for the mix is pitch correction. Fixing a vocal that is really out of tune is one thing, but modern mixing commonly calls for auto-tuning of near-pitch-perfect vocals to a lesser or greater degree depending on genre. The mix engineer is often the person who is best placed to judge this.
Common technical faults are...
Background noise can come from a variety of sources, but one source that particularly deserves mention is the guitar amplifier.
Guitar amplifiers are often noisy, firstly because electric guitars traditionally have high-impedance outputs, and high-impedance circuits have more thermal noise and are prone to interference pickup. Secondly guitarists often seem to have an aversion to nice new, well-maintained amplifiers and prefer old, nearly worn-out models. What would they expect but more noise and interference?
Whatever its source, the key is to listen carefully for noise and edit it out from individual channels of the multitrack recording, using fades where necessary. Yes of course you can use a noise gate, but editing is a much more precise method, at the expense of taking longer.
Distortion is typically caused by setting the preamplifier gain too high so that the signal is pushed into the red in the DAW. This causes clipping in the audio file that is recorded.
This problem really, really should have been fixed in the session. But if it is printed into the recording then you'll have to deal with it. One method is to use a low-pass filter so that some of the edginess is removed. This will dull the sound however, so you might consider using a de-clip plug-in, such as that included in the Izotope RX software. This is not likely to be a perfect solution because the plug-in is trying to replace information that is lost, but often what is needed is to get as good a result as possible.
Clicks are the bane of audio engineering, in all of its forms. There are so many ways clicks can become imprinted into a recording. One is by less-than-careful editing. I have heard enough multitrack recordings where the original producer has edited some of the tracks to know that imprecise editing has the potential to be a significant problem.
The way to fix clicks is through careful editing. First you have to find the click, which is often more difficult than you might imagine. Sometimes a click is clearly audible but cannot be seen on the waveform display. Once you have found it however, then you can remove it with normal editing and fading. Alternatively you can use your DAW's pencil tool to redraw the waveform. Sometimes it is necessary to try both ways to discover the optimal method for the particular click you're dealing with.
As a takeaway from the above, my advice would be to fix any problems immediately in the session so that you know that you have a pristine multitrack recording, ready for mixing.
However, if fixing the fault in the session would disrupt the musical flow, a fault that is judged to be fixable can be left until later, but not so much later that you forget about it.
As an engineer and producer, your end-point should be a multitrack recording in which nothing needs fixing. The mix session can then be an entirely artistic process, exactly as it should be.