Mixing & Automation

Digidesign (Avid) Pro Tools Tips & Techniques

Published in SOS February 2002
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Technique : Pro Tools Notes

Fix it in the mix, with our useful Pro Tools mixing and automation tips.

Figure 1: The insert points at the top of mixer channels can be used either to add software plug-ins (above) or outboard hardware (right) into the signal chain.

Simon Price

This month we reach the mix stage in our discussion of traditional multitrack recording, so it's time to examine some Pro Tools-specific mixing concepts and techniques.

Mix Basics

In the jargon of traditional mixing consoles, Pro Tools' mixer is a bit like an in-line design: in fact, the same channels and faders used during the recording process are also used to manage audio coming back from the disk. If you've selected 'Linked record/playback faders' in Preferences (this feature ensures that whatever level you set for a track during recording will also apply to the track when it is played back), you'll probably already have a reasonable level balance carried over from when you were recording.

The multitrack recording mixer template I provided in December's Pro Tools Notes shows some aux channels being used as group faders (sub-mixes). One is for the drums, so you can use the track faders to set the level of the drums (if you're using them) with respect to each other, then adjust the overall level of the drum kit within the mix using the group fader. The same method can be used for setting up a balance between mic'd guitar and and DI'd guitar tracks, for example.

As Pro Tools' mixer is (obviously) a digital one, it's important to ensure that none of the individual tracks, groups, or the master fader go into the red — digital distortion is not something we want on our mix. To be sure that none of the tracks are clipping, disable Pre-fader Metering in the Operation menu. The channel meters will now show the true levels after the signal has passed through any plug-ins and the channel fader. Set the Master fader so that the loudest part of the song is as close to the red as possible. Don't use the Master fader to set your monitoring level, as this will affect your final mixdown! Also, note that the insert sections on Master faders are post-fader, so if you're using insert processing on the Master, the true final output level is reflected on the output level metering of the last insert rather than the Master metering. Look at this, rather than the Master level metering, to see if you're going into the red.

Figure 2: The Automation Enable Window, where you can globally arm or disable recording of certain parameters.
Bringing In Effects & Processing

Once you have set up some kind of working balance between your tracks, you may want to start thinking about signal processing. In Pro Tools, this can mean using internal plug-ins, linking to outboard hardware, or both. Either way, you can use an insert or a send/return routing. The send method for routing through plug-ins is discussed in the FAQs below.

Inserting a plug-in directly on an audio track couldn't be easier: just click on an insert point in the channel and choose the desired effect. To insert an outboard processor in-line with a track, you need to cable it to spare I/O connections on your interface unit. (Bear in mind that if you, for example, use output 1 you have to connect back into input 1 — and so on). Then click an insert point and choose an interface (I/O) connection instead of a plug-in (see Figure 1, above right). To use a hardware processor in a send/return configuration, connect the hardware unit, use a mixer send to split the track off to it, then bring the wet signal back into the mix via an aux input track (like a send/return structure in a normal mixer).

At some point you'll probably want to use automation in your mix, whether to achieve something as simple as tweaking the vocal level here and there, or as complex as changing the whole mix during certain sections. Automation in Pro Tools is a big subject, and I plan to cover it in depth in a future Pro Tools Notes. In the meantime, the following section covers the basics.

Mixing FAQs

What's the bare minimum I need to know to start using automation?

To record, and thus automate, channel parameter changes (fader, pan, mute, solo, send controls), first switch the channel into Auto Write mode (using its Automation Mode button), then hit Play and do your stuff. If you need to re-do something, you can drop in and out on existing automation using Auto Touch mode, which plays back earlier automation until you move the channel fader, at which point it begins to overwrite subsequent data. When you let go of the fader, the previously recorded automation takes over again. To drop in without the automated parameter gliding back to the levels set by previous passes when you let go, use Auto Latch. This, in effect, 'drops in' but doesn't 'drop out' again. However, if there is automation data later in the track which you want to preserve, don't use Auto Latch — or, at least, stop recording before this automation data is encountered. To record automation moves for plug-ins, enable the relevant plug-in controls by clicking 'Auto' on the plug-in's window. The Automation Enable Window (see Figure 2, left) lets you select or protect the different parameters to be automated.

Figure 3: Digidesign's Maxim plug-in, a mastering processor that includes both maximisation and dither.
What's 'Snapshot' automation? I don't see any menus for it.

Snapshot automation refers to the process of writing a static mixer setup across a section of your track. You might want to do this, for example, when your song has sections that vary dramatically in sound or instrumentation. Think of the standard Nirvana song structure: "this is the gentle bit; this is the buildy bit; this is the loud shouty bit!" Using snapshot automation in this case, you could set up all the levels, mutes, voice and guitar processing, and so on, for each section, and write them as the foundation of your mix. Simply use the Selector tool to highlight across the area where you want to write your snapshot — making sure the selection extends across all the tracks you wish to include. Enable playback looping, and click 'Auto Suspend' on the Automation Enable Window. You will now be able to loop around the section, setting up the mix as you want. When you're happy with it, stop, and choose 'Edit > Write Automation to all Enabled Parameters', then take off 'Auto Suspend'.

What's the deal with aux inputs and sharing plug-ins?

Aux inputs are channels that are not used for playing audio recorded to disk. They are versatile and can be used as sub-mix groups, 'live' input channels, or effect returns. Their most common use is for sharing plug-ins between several tracks. Most of us don't have a powerful enough computer to apply plug-ins willy-nilly, but we can still give all the tracks access to one or two really good reverbs. Create a stereo aux input track and bring up your plug-in on this track (with its 'dry/wet mix' set to 100 percent wet). Set the input of the track to one of your internal stereo busses. Now create sends on each track that you wish to add reverb to, choosing the same stereo buss going to the aux input. The send level will now adjust the amount of each track going through the reverb.

What about mastering plug-ins and dither?

Plug-ins billed as 'mastering processors' are mainly maximiser or dither effects, or both. A maximiser reduces peaks in the mix, and uses the headroom this frees up to boost the whole level of the song (see Figure 3, left). Dither is a perceptual trick used when creating a digital-format master, creating the impression that the digital representation of the waveform is more accurate than it theoretically should be. Anyone who's used dither in a graphics package to smooth the boundary between two colours, or text against a background, will have seen the same effect in the visual sphere.

  Phase Correction: Using The Time Adjuster Plug-in  

Figure 4: The Time Adjuster plug-in, which can be used for correcting phase discrepancies.
Craig Anderton's 'Direct Dilemma' piece in SOS July 2001 goes into detail about phase correction, which is something you may need to attend to in certain mix situations, As usual, I'm presenting a 'Pro Tools Appendix', as opposed to a general discussion of the topic.

If your multi-mic recording technique is as shoddy as mine, you'll need to correct the small timing differences between different (related) tracks to reduce the 'comb filtering' effect heard on the combined sound (the classic example is when spill between drum kit mics causes phase problems). There will certainly be a discrepancy — which Craig's article focuses on — when you've recorded both a DI and mic feed from a guitar or bass. Finally, on TDM systems you occasionally have to correct for the tiny delays caused by going through a plug-in.

Figure 5: Waveform Alignment. Here I've identified a matching peak in both the mic'd and DI'd recordings, and can use the Grabber tool to shift them into sync.
Pro Tools ships with the Time Adjuster plug-in (see Figure 4), that provides a small delay (0.1 to 45ms), along with a phase-invert button. By putting this plug-in across the 'earlier' signal (which would be the DI'd one in the case of the mic-d/DI'd bass example), you can follow the tried-and-tested method of phase alignment. Play both tracks, panned centrally, and click the phase-flip button on Time Adjuster. Then play with the delay slider until you find a point where the combined level drops significantly, un-flip the phase and you're done. LE systems don't have the Time Adjuster, but the Short Delay plug-in is equally effective. With Short Delay, you should hold down the command key when dragging the delay slider, to achieve finer resolution.

Alternatively, you can take an editing approach by moving the recorded audio into alignment manually. Start by zooming in to an extreme level where you can see the individual peaks and troughs of the waveforms. Locate a point where you are can see where the waveforms should line up on the different tracks — a transient such as a drum hit or guitar pluck is best. Now, triple-click with the Selector in the track you want to move. This selects everything on the track, ensuring all regions are moved together. Now switch to the Grabber tool, and shift the audio until the waveforms are aligned (see Figure 5).


  Quick PT Plug-In Tips  
  Since PT 5.1, TDM systems are able to use both TDM and RTAS plug-ins, which could get TDM users out of a tight spot if they've maxed out their DSPs. However, to use plug-ins of both formats on the same track, one must place the RTAS inserts before the TDM ones (higher up on the screen). Luckily, version 5.1 also introduced the ability to rearrange plug-in order by dragging and dropping.

Some stereo plug-ins have separate parameter controls for left and right channels. To link both sides when making changes, hold down the Shift key while you make adjustments.

Although there are only five insert points on each track, if you're going really plug-in crazy and need more, simply route the track to an aux input to get another five.

All Pro Tools systems include the Trim plug-in, which provides an extra 6dB of gain. However, on really quiet recordings, try using a Time Adjuster plug-in (TDM systems only) instead, with the delay set to zero, because this plug-in includes a gain control with a whopping 24dB boost.


DAW Techniques


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