Hi,
If I have a group of 4 tracks : snare, kick, toms, hit hat; and a bus “drums” and I want to apply some compression to every tracks of the group.
What’s the difference between using the “send” menu (aux or ext) and connecting the output of each track to input of the bus?
Thanks.
If you send each track to the bus and mute them, thus only having sound coming out the bus, it would be the same as connecting them to the input. But as you probably won’t be doing that the difference is that you would be having two signals coming out at the same time, one “dry” out of each track on its own, and the other wet coming out from the bus to where you made the sends.
Don’t know if I made myself clear but you surely would want to look up “parallel compression” : )
Output to bus is directly affected by the fader level on a 1:1 basis and is after all processing.
Aux send has it’s own level as well on top of this to modify even farther and can be placed before the fader (Or anywhere in the chain). Aux send is more flexible, but outputting to the bus is simpler.
It becomes more a workflow terminology and workflow. If I want to process things together, outputting to a bus becomes the better way so I don’t need to manage the level except through the channel fader, and don’t have to worry about where it goes in the processing chain, or if I add a new processor remembering to move that processor before the send. I use aux sends to handle additional sends other than this, such as sends to reverbs, sends for external mixes such as for headphones, etc.
Now one thing to note, is that there is a difference to applying compression to a bus with multiple channels feeding the input of the bus like you describe, and applying compression individually to each channel. Make sure you think t his through and decide what it is you actually want, it is rare I have the same compression on snare, kick, toms, etc. and often times I might have individual compressors and a bus compressor, depending on the style of music I am mixing.
There is a clear use case for using busses that might make it more clear why busses exist.
You are mixing a song where each instrument was recorded directly (dry) to the DAW. Now you want to add some reverb to drums, guitars, etc and the reverb type and preset needs to be the same for all so that it sound like they are performing in the same room. Now you create one bus and put a single reverb effect on it, set it to output only “wet sound” then you route all instruments via aux sends to the same bus the reverb plugin sits on.
If you now decided to change the settings of the reverb you can do it once and it applies to all tracks. The audio tracks outputs now only “dry” sound of the instruments and the reverb bus only the wet reverberated sound. You can adjust the dry / wet balance for a single instrument by adjusting the amount it sends to the aux. And you can control the total reverberation with a single fader (the reverb bus fader).
This is s very common way of mixing music as it is easier to control one reverb plugin than many and this also saves computer resources and lets you run more plugins on your hardware. Also if you want to automate reverb parameters (reverb fades out for example) then you just need to automate the reverb bus volume fader.
That is more a clear cut case for busses in general yes, and aux sends to those busses, but the OP actually asked about the differences in routing from th e output of a track to the bus or via using an Aux Send, both of these still use a bus:)
I suppose one of these days I need to actually start recording nice explanatory videos on the topics I see here, it is a good question, and one that is easier to demonstrate than to explain.
Using a bus to group all the outputs from a set of tracks is often known as “subgrouping”. It is a typical workflow when you know for certain that you are not interested in continuing to access the “dry” version of each track individually. Subgrouping disconnects the track outputs from the master out, and wires them into a bus used (only) by the group.
Ardour makes this very easy to do: select N tracks, right click in the track tabs area, select “Create Group from Selection”. Then right click on the group tab, and select “Add New Subgroup Bus”.
If you think you may still want access to the “dry” individual track outs, then use “Add New Aux Bus”, and Ardour will add the bus and set up the sends for you. With this arrangement, the track outputs are still sending output to the master bus (or wherever else you might connect them to).
Thanks everybody that’s very clear.
Aux send is more flexible, but outputting to the bus is simpler.
That’s more or less what I thought, but wasn’t sure. It seems that one solution may be more pratical than the other, but it doesn’t much as for the sound.
you surely would want to look up “parallel compression” : )
It’s after watching stuff about “parallel compression” on utube that this question came to me. 
The video (don’t remember which one, I saw many) made it sounds that it was a way to blend the dry sound coming from one track with the compressed sound from a parallel track; and I wondered what was the point since every compressor have a wet/dry knob…
Maybe the sound is a little bit thicker with parallel comp however, since it comes from two tracks instead of one…
From what I understand that mix knob fulfills exactly the same goal as that of setting the aux bus in this scenario. It would be a matter of workflow or habit, if you set a compressor in the drums group bus with heavy settings and lets say 10% mix, that would be paralell compression as you would have a portion of processed signal along with the dry one.
Not every compressor has a wet/dry knob. Actually I would say most do not, it is more common to see such a control on reverb or other time based processors for instance.
Seablade