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First things first, whenever you're taking musical advice from someone, ALWAYS listen to their music to see if it's any good. If you like the mixes in my songs (try http://www.newgrounds.com/audio/listen/754531) then use my guide, otherwise don't! I freely admit I have a long way to go, though I feel my mixes have the basics in place.
HOW 2 MIX, sorted by importance.
If you're using FL Studio, for gods sake, remove any limiter on your master mixer channel. When learning, never put a limiter on master. This makes it impossible to mix when stuff starts hitting the limiter's threshold.
Are your songs clipping?
Clipping is the sound of distortion that happens when your songs go too high into the red on the volume meter. If you don't recognize clipping immediately, I'd advise you to always keep your master volume meter in the green until you do.
"My tracks sound muddy!"
This is because of TOO MUCH REVERB. There's a lot of misinformation about this that if you just do random EQ cuts or something all the mud will go away. No no no. If your song sounds muddy, step 1: TURN OFF EVERY REVERB AND DELAY. "Wow, it sounds crisp now!" (If it doesn't, you missed some reverbs. Check in your VSTs, they often have built-in reverb and delay modules.) Yeah, I know, it's amazing.
"But now it's a little dry." Put a SMALL amount of reverb on ONE instrument. Put some SMALL delay on your leads. Add it back gradually, and constantly check your mix for mud as you do so, turning it back when it's too muddy. You should now have a better, non muddy mix.
I think what happens is that to an amateur, each of their 6 instruments with reverb sounds great soloed, so they join them all together, not realizing that the reverbs will also combine. That's how you get mud.
Put a high pass filter on every non-bass instrument.
The quantity of frequencies in your song should look like an inverted pyramid. Tons of reverb tails and delay on top, a few mid range instruments, and ONE SINGLE BASS INSTRUMENT. If you have multiple things in your bass, they will make the low end sound muddy.
"But I don't have tons of stuff in my bass" This is where I'm going to surprise you. Almost every instrument has bass frequencies. Put a spectrum analyzer on your different instruments and test them out. Yep. Better safe than sorry. Always put a high pass to trim off those redundant frequencies that muddy up your low end!
Side chain your bass to your kick so that they don't overlap.
Learn how to sidechain in your DAW. The idea is you want the bass to get quiet when the kick hits, then be loud when the kick is not hitting. If you don't do this, your kick won't be audible in your song.
Take care with your volumes.
80% of mixing is getting volume levels to be right. For some reason, everyone is obsessed with EQ. NO NO NO. EQ is helpful, but, aside from the bass high pass I just mentioned, VOLUME should always be your first go to.
How to make good volume decisions:
Keep practicing, you'll eventually be able to do this in your sleep. Until that point, there's a trick: make all mixing decisions with the volume turned down low. The brain can make better mixing decisions when a track is very quiet.
Make your drums louder.
If you're reading this guide, you got the levels of your drums wrong. Sorry, but it happens to every amateur musician for years.
90% of people who do drums wrong get them too quiet. Compress them, MAKE THEM LOUDER. (The remaining 10% are weirdos who somehow made them too loud.)
Don't have overlapping frequencies.
Imagine that your song has a couple of slots:
• midrange (melody/singing)
• midrange pads
• high range (high arps, or sometimes nothing).
If you put one instrument in each slot, the song will sound full. (Assuming you didn't make any of the above errors)
If you put more than one instrument in the same slot, they will fight with each other and make each other inaudible.
My bass doesn't sound... basey enough
USE SUB BASS.
High pass your bass instrument. Now, put a sine wave under it, playing the note that your bass instrument played. A sine wave is like the ultimate bass note, it's tough for anything to sound as big.
When done correctly, it will sound like your bass instrument is huge and super basey. (If it doesn't, you probably need to high pass some other instruments that are interfering with the bass, like I said above.)
"But I wanna have two things in the same slot! I WANNA!"
There's really only two things you can do. First, you can pan - put one thing left and one thing right. This can work, but it can also make people go insane if you have two melodies going at the same or something. (Not that I would ever do that...)
It's generally a good idea to be aware of panning, though. Panning e.g. hihats alternatively left and right can help fill out a mix.
The other trick is stereo width. When you have a melody and a pad, and they're both at the same frequency, you're going to have some clashes. One cool trick is to adjust the stereo width of the pad to spread it out. (You can do this with the width parameter of Utility in Ableton - not sure about other DAWs). This will have the weird feeling of panning it both left and right simultaneously. It makes it sound wider, but with space in the center for something else.
It's hard to explain, but it's a great way to mix something like a pad with something like a melody. I did it in this song http://www.newgrounds.com/audio/listen/622648 - see around 1:20 where the stabs and the melody are going at the same time, but not clashing somehow.
How to improve
Always always always ALWAYS be listening to other people's mixes. Compare your mix to the mix of other songs. Is it as good? Is it lacking in some way? What are they doing better?