hey
@PeteS Welcome to the Rabbit hole
First of all...
'From an engineering perspective, consider routing the backing track to a bus and compressing it carefully with a compressor that is keyed by the vocal track. That would make the instruments duck under the vocals when they are there.'
I haven't seen this comment in context but I would personally
NOT recommend this approach for general music mixing as it could lead to a very unnatural and distracting listening experience
IMHO it would be MUCH better to automate the vocal to ensure it can be heard above a consistent backing track - and automate individual elements so that they shine in a mix
The only exception to this would be for a "voice over" track - eg for a nature documentary or advert - where you would want the music to get out of the way of "spoken word" - although even in this case I would approach it with automation
However -let's get back on the BUS
A "bus" is a single track (usually stereo) where you can route other tracks to
You can then route a "bus" to another "bus" (and then to another and another etc.) until you finally route everything to your MASTER BUS
To understand this I will follow through a typical routing for a drum kit....
DRUMSIf you mic a drum kit up you might multiple mics that you want to group on to 1 fader/channel
Eg if you have a mic on the top and bottom of the snare you will have tracks called "Snare Top" and "Snare Bottom"
You may want to feed these tracks to a bus called "Snare Bus"
You may then want to route this "Snare Bus" to a "Drum Bus"
The "Snare Bus" is then referred to as a "sub bus" to the "Drum Bus"
The "Drum Bus" may have lots of other sub busses being routed to it eg a "Kick Bus" (made up of an "inside kick mic" and "outside kick mic") a "Overheads Bus" (made up of "overhead right" and "overhead left" tracks) etc. etc.
The first advantage of using busses is that you can really BUILD the elements of your mix in detail (eg balance the levels and shape the tone of the snare top and bottom mics) and then move on to the "bigger picture" of mixing the "Snare Bus" with the rest of the drums
When you have processed and balanced all of the busses feeding the "Drum Bus" you will have drum sound you want going to one fader channel (is the "Drum Bus") so when you need to balance the level of the drums with the rest of the track you only need to change 1 fader
As you have already mentioned the use of busses also makes it possible process a whole group of instruments with a single effect - eg adding reverb to the bus would help give the impression all instruments are in the same "space"
Adding very light "compression" to a bus can also help to "glue together" the various tracks and busses feeding a bus as they will all take on the subtle tonal characteristics and "movement" from the single compressor
You might also have various percussion elements feeding a "Percussion Bus"
In this case you might want a "Master Drum Bus" that is fed by the "Drum Bus" AND "Percussion Bus" - this means that ALL of your drums and percussion can now be controlled on a single fader. This makes it easier to make broad changes to the mix - if you want all of the drum elements, including percussion, louder you just turn up the "Master Drum Bus" fader
Remember - you can still go back to any of the busses or tracks at any time to tweak - eg if you want just the Drums louder you can turn up the "Drum Bus" - if you want the snare louder you can turn up the "Snare Bus" and if you want more of the top of the snare you can turn up the "Snare Top" track
This helps with mixing a song that has loads of different tracks and elements because as the mix progresses you start mixing using the busses (groups of instruments) rather than trying to juggle a gazillion tracks (safe in the knowledge that you can dive in to the detail at any time to tweak anything)
The key to busses is BE LOGICAL AND ORGANISED - I can't stress this enough
I have consciously avoided using the word "SEND"
I find it useful to refer to "routing to" of "feeding" bus - ie you send ALL of the signal from the track or bus to another bus
I then think of "SEND" as tapping off a bit of the signal (controlled by the "send level") to another bus (usually an FX bus)
It can be confusing but stay organised and logical and busses are an absolute godsend for lots of different reasons (many of which I haven't covered in this example)
Most DAWs allow you to create "templates" - so that when you open a new file to start recording on mixing all of your "routing" (tracks to busses and busses to other busses) are already set up and ready to use
Here are some examples of the routing I have set up:
Lead vocal -> Lead Vox Bus -> Master Vox -> MASTER BUS
Lead Double -> Lead Vox Bus -> Master Vox -> MASTER BUS
Backing Vocal 1 -> BV Bus -> Master Vox -> MASTER BUS
Backing Vocal 2 -> BV Bus -> Master Vox -> MASTER BUS
Backing Vocal 3 -> BV Bus -> Master Vox -> MASTER BUS
Snare -> Drum Bus -> Master Drum Bus -> MASTER BUS
Kick -> Drum Bus -> Master Drum Bus -> MASTER BUS
Tom1 -> Toms Bus -> Drum Bus -> Master Drum Bus -> MASTER BUS
Tom2 -> Toms Bus -> Drum Bus -> Master Drum Bus -> MASTER BUS
Floor Tom -> Toms Bus -> Drum Bus -> Master Drum Bus -> MASTER BUS
Overhead Left -> Overhead Bus -> Drum Bus -> Master Drum Bus -> MASTER BUS
Overhead Right -> Overhead Bus -> Drum Bus -> Master Drum Bus -> MASTER BUS
Room mic -> Drum Bus -> Master Drum Bus -> MASTER BUS
Acoustic Guitar -> Rhythm Guitar Bus -> Master Guitar Bus -> MASTER BUS
Rhythm Guitar Clean verse -> Clean Guitar Bus -> Rhythm Guitar Bus -> Master Guitar Bus -> MASTER BUS
Rhythm Guitar Clean with delay -> Clean Guitar Bus -> Rhythm Guitar Bus -> Master Guitar Bus -> MASTER BUS
Rhythm Dirt 1 -> Dirty Guitar Bus -> Rhythm Guitar Bus -> Master Guitar Bus -> MASTER BUS
Rhythm Heavy Chorus -> Dirty Guitar Bus -> Rhythm Guitar Bus -> Master Guitar Bus -> MASTER BUS
Lead guitar intro ->Lead Guitar Bus -> Master Guitar Bus -> MASTER BUS
Lead guitar solo ->Lead Guitar Bus -> Master Guitar Bus -> MASTER BUS
Lead guitar harmony left solo -> Harmony solo bus ->Lead Guitar Bus -> Master Guitar Bus -> MASTER BUS
Lead guitar harmony right solo -> Harmony solo bus ->Lead Guitar Bus -> Master Guitar Bus -> MASTER BUS
Lead guitar outro ->Lead Guitar Bus -> Master Guitar Bus -> MASTER BUS
Etc. etc.
At first glance this may seem complicated - but with templates you only need to do it once and you can always build it up over time
But as you can see from the example above I could take ALL of the rhythm guitars down with one fader (Rhythm Guitar Bus) OR take all of the guitars down with a single fader (Master Guitar Bus)
This is particularly useful for creating different versions of mixes to compare - eg to create a "guitar heavy mix" you only need to nudge up one fader
Did I mention it was important to be LOGICAL AND ORGANISED?
It can help to actually draw the routing and groupings you want on paper before touching your DAW
I am not sure if GaragBand has busses but if you are already looking at this type of thing I think you will quickly outgrow GarageBand
I can recommend STUDIO ONE as a solid fully fledge DAW - along with Logic, Cubase, Sonar
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