Hitting the Default button in the I/O Setup window resets all the routing for the current tab (Bus, in this screenshot). Adding Option/Alt while clicking will reset the routing for all the tabs.
Keep track of signal paths with these handy routing tips.
I always believed that an analogue mixing console was the best introduction to routing audio — a belief I held until a student approached me after a lecture. Having seen me use an analogue console side‑by‑side with a digital mixer to explain the digital console’s audio routing, he told me that he’d found the comparison really useful because it finally allowed him to understand that confusing, big old analogue thing! It seems digital natives don’t need analogue as much as I did.
But one only needs to open the Mix window in Pro Tools to see the legacy of analogue. Consoles are still very much with us in our software, and they remain a great way to represent signal flow. The basics of signal routing are simple, but it can quickly become more complex, especially when you get into surround and immersive audio, which is fast becoming a matter of ‘when’ rather than ‘if’ for budding engineers. This month’s column is about routing audio in Pro Tools, and should be relevant whether you’re working in stereo or Dolby Atmos.
I/O Setup Window
We’ve probably all had the experience of opening a new session and trying to figure out why we’re not hearing any audio, despite seeing lots of activity on the meters. In a simple session where there is no submixing to worry about, the quick and dirty way to fix this is to open the I/O Setup window and hit Default. This will reset all the routing for the tab you have open; add Option/Alt to reset all the tabs.
The problem is likely to be that the path being used to output the mix needs changing. A less nuclear option than defaulting the I/O is to change only the tracks routed to the missing output bus. One way to do this is by selecting all the tracks routed to the main output, omitting those routed to submixes, and re‑routing only the selected tracks in a single operation using the endlessly useful Option+Shift modifier, which applies changes only to the selected tracks.
If you open a session and can’t hear anything, use the Select Assignments To Mix Bus option, and then route the selected tracks to an output bus on your system.
There are other useful modifier combinations related to this which are also very useful. Option/Alt gives us ‘do to all’, and adding Command to Option will ‘cascade’ input or output assignments so that all tracks get assigned incrementally to a series of busses, inputs or outputs. Adding Shift to Option and Command applies the operation only to the selected tracks, which in practice is often much more useful than cascading all of them. This is very useful when setting up new sessions.
Manually reassigning an output is all very well, but a more elegant approach is to deal with it at source, in the I/O Setup window. One of the less glamorous but genuinely useful improvements Avid have made to Pro Tools is the introduction back in version 9 of Mapped Output Busses. This was intended to reduce the inconvenience that this sort of issue would cause when sessions were moved between systems. Output paths are no longer tied directly to physical outputs, as was originally the case; instead, they feed busses, which are then assigned to physical outputs. Because of this, Pro Tools can respond to changed hardware setups by remapping the outputs. It will use a combination of the MAC address of the host computer, the output name and the channel width to make a best guess at where to route to a valid output path automatically, but if you need to remap manually, you can do so from the Map column in the Bus tab of the I/O Setup window. This is far faster and less error‑prone than trying to manually reassign outputs track by track.
While you’re in the Bus tab you’ll notice a little loudspeaker icon next to one of the outputs. If you switch to the Output tab you’ll see the option to define this Monitor Path. This usually ensures that a session create on a different Pro Tools system will have an audible output, but if this doesn’t work, you have options for fixing it yourself. Usefully, Pro Tools automatically downmixes or upmixes the session Monitor Path to the system’s Monitor Path.
Pre Versus Post
The difference between pre‑ and post‑fader operation is one of those basics of audio which can cause novices some confusion. Once grasped, the difference seems obvious — if you don’t want to hear a ghostly reverbed version of the track you just muted, you need a post‑fader send, and if you don’t want the singer to tell you to stop messing with their headphone level while you’re adjusting levels in the control room, you want a pre‑fader send. But it can be confusing, and I’ve always thought it might be because ‘pre’ and ‘post’ both begin with the same letter. We have pre‑fader listen (PFL) and after‑fade listen (AFL), so why don’t we have after‑fader sends?
On the subject of PFL/AFL, if you’ve never tried routing your solo signal to a different output using the AFL/PLF path in the I/O Setup, I can recommend it. I have two headphone outputs on my interface and I have one set to the main output and other to an alternative output, to which I can route my PFL for tracking sessions.
The pre‑/post‑fader distinction also comes into play with inserts. The insert slots on Master Faders differ from all other track types in that they are post‑fader. This is one reason many people prefer to route their mixes to a dedicated mix bus on an aux input or to an audio ‘print track’ while mixing, because dynamics processing on your mix bus will then be unaffected by moves on your Master Fader. If you do use a dedicated mix bus, you might consider changing the relevant setting in I/O Setup to make it your default output bus. Once this is done, new tracks that you create will default to being routed there, saving you from having to reassign them.
Chain Gang
As well as routing from track inputs to outputs, busses and sends, another route by which audio can traverse a Pro Tools session is through plug‑in side‑chains. For example, it’s possible to configure a compressor so that gain reduction is triggered by a signal that is different from the audio being processed. To set this up in Pro Tools, an external key input has to be selected in the compressor plug‑in window. Regardless of the channel width of the track you’re compressing, this external key is always mono. There is no workaround for this; although you can unlink the channels in a multi‑mono dynamics plug‑in and use different settings on each channel, there is only one external key input, which is common to all the compressor’s channels. However, if you do choose to unlink a multi‑mono plug‑in, do be aware that in a plug‑in like Avid Pro Compressor, which allows you to specify the side‑chain source for the detector, this can be set on a per‑channel basis, so if you want to use an external trigger you must set it for every channel.
Here, Avid’s Pro Compressor is being set to react to an external key input signal. If Pro Compressor is inserted on a stereo or multi‑channel track and you’ve unlinked the channels, you’ll have to do this for each channel.
Naming your audio paths is really helpful and can be done much more effectively from the I/O Setup window than in the session itself.
Path Finding
If you select different colours for the output busses in the I/O Setup window (as in the first screenshot), those colours will be reflected at the bottom of the channels in the Mix window.When a session’s routing becomes a bit more complex, one of the biggest challenges is keeping track of what is going where, and this is particularly important when sharing sessions. There are a few useful tips to bear in mind here. Naming your audio paths is really helpful, and can be done much more effectively from the I/O Setup window than in the session itself. The options in the contextual menus accessed by right‑clicking on I/O selectors can also help, allowing you to select, show or hide tracks routed to or from specific paths, and an absolute gift, which came as part of Pro Tools 2023.12, was the introduction of colour to Pro Tools I/O. The Bus tab of the I/O Setup window now sports a small new column at the left, in which colours can be set for each path. This use of colour makes digesting routing at a glance so easy.
A good example of how this can be useful concerns Routing Folders. Member tracks are usually routed through the Routing Folder track, but can also be routed elsewhere at the same time. This is easy to miss, but using colour makes it very clear. You can route to multiple paths by using Control (Start on a PC) to add additional paths and this has always been indicated by the display of a ‘+’ symbol following the first path name. Multiple routing paths are now indicated using bands of their corresponding colour, which is a nice touch.
There is a lot more to say about routing, from bus and object routing in Atmos sessions to using aux I/O to route between applications, but taking control of your routing, both in the Mix and Edit windows, and in the I/O Setup window, makes for easier session management and more productive mixing.