When mixing you need to create space. It's so easy to keep adding different instruments when recording. However, when it comes to mixing you'll probably end up with 2 or more instruments fighting for the same frequency. So what do you do? Well, Firstly it depends on the instruments. Sometimes you may have to be harsh & think: "Do i really need these 2 parts or can i mute one (or even delete)?" This is what i normally do. Now maybe you're thinking: "What about using EQ?" Or "Why not use panning?" Yes i could use these tools. However is this additional sound going to add anything to the mix? Is this part necessary? If not, then im afraid the part either gets muted or deleted.
A few days ago i was recording some vocals, for a middle 8 (on a new song). Let's just say i got a bit carried away & ended up recording a few harmony parts too many. I can tell you now that some of those harmony parts will probably be muted. (Why were those parts recorded in the first place? Well, it seemed a good idea at the time & you sometimes have to try these things)
I was once told "You cannot be too precious about certain instruments, in a song." There will be some instruments (when mixing) that are the main feature of the song, these obviously stay.
You don't really want all the instruments playing at the same time, all the time.
So sometimes 'Less is More'
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