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Double tracking seems to have fallen out of favor as a production technique in recent years. This is probably because in the early days of the modern era of recording, the 1960s, double tracking was one of the few specialized studio techniques available. Double tracking a vocal was, and still is, intrinsically a studio technique because it can't be done live (unless the singer has a twin).
These days however there are innumerable ways of processing vocals and instruments with plug-ins, all of which can be done instantly with very little extra time and effort involved. So double tracking, which does take time to do right, seems more like a chore than an exciting production technique.
But of course, when everyone else is ignoring double tracking, anyone who incorporates it into their working methods will sound different to the masses. So that it itself is a good reason to try it. And it actually can sound really great and beat almost any plug-in for richness of texture.
To double track a vocal, the singer should go through the normal process of recording. If this involves comping and punch-ins, that's fine. A complete take at as high a standard as possible is what's required, just like normal recording. But then...
...The singer records the vocal again. The slight differences between the new take and the original create a wonderful richness of sound. Having the same voice doubling itself is a completely different sound to two singers singing in unison. Oddly enough, double tracking makes precise intonation less of an issue, so even if the original vocal was comped, the double-tracked version doesn't necessarily need to be, although punch-ins should be expected.
Inevitably however, there will be discrepancies in timing. Where the timing is accurate, the listener will perceive the sound as a single, very rich-sounding voice. But where the timing slips, it will be clear that double tracking was done. Often this doesn't matter and timing discrepancies can be accepted as part of the overall texture. But sometimes you would like to get things as perfect as possible, which is where punching in will be useful.
One tip however is to under-pronounce 's' and 't' sounds in the double track. Timing differences in these sounds are obvious and unpleasant, so eliminating them as an issue makes a lot of sense.
Way back in the 1960s, performers including the Beatles eventually got bored with having to sing the vocal twice, and so the technique of automated double tracking, or ADT, was invented. One way of doing this is to delay the vocal a little, then modulate the delay time up and down. The delayed version then has a cyclic change in pitch which, when mixed with the original, creates a reasonable illusion of double tracking. It has to be said though that it isn't nearly as rich. Doing it the hard way, or at least let's call it the more long-winded way, always produces a better result.
One instrument that is commonly double tracked in modern production technique is the guitar. Let's consider the acoustic guitar because it is rather more demanding of care and attention than the electric, where any problems are more easily disguised.
Suppose you have a song that benefits from a strummed acoustic guitar accompaniment. So you record the guitar and it sounds fine. If it is important in the arrangement, then it will need to be high up in the mix, and to avoid an unbalanced stereo image you might pan it center. The problem is that to an extent it will obscure the vocal.
So in response to this, you double track the guitar playing exactly the same chords in exactly the same rhythm, and pan the two versions hard left and hard right. By doing this, the vocal is separated spatially from the guitars and the mix can be much more clear.
What you will commonly find however is that the guitars don't always match precisely. Sometimes the guitarist will play a slightly different rhythm. Sometimes the precision of the timing will slip.
Now if you listen carefully to the recording as a whole, you will notice that at moments when the guitars are precisely together, the sound is much better than at other points in the recording where the timing slips, even slightly.
If you pay attention to this, then you will realize that having two guitars playing exactly in time is far better than two guitars that are roughly together. The solution of course is to retake the sections that are out, or do punch-ins. You might try editing, but it will probably take longer. Any extra time spent, whatever your method, will be well worthwhile.
Well you can double track anything. A lead guitar can sound fantastic when double tracked. In the 'olden days' of multitrack tape, the recorder would be slightly sped up or down for the second take, so that there was a slight pitch difference between the two takes. This makes for a very rich sound. Varispeed, as it was called, is an unfortunately rare feature in modern DAWs, and using a pitch-changing plug-in doesn't sound quite the same.
Another instrument that can effectively be double tracked is drums. Done well, this can create a huge sound to drive the rest of the band. It is difficult to achieve as good an effect with a virtual drum instrument, but doubling the drum track with two different kits, and experimenting with the timing, is worth a try.
In summary, double tracking is a powerful technique. It requires care and attention to detail, but it can create a sound that is difficult to achieve any other way.