You used to need a separate multitrack and mixdown machine, but todays's DAWs can combine both - to good advantage
By Phil O'Keefe
Back when I first became interested in recording, most projects required two tape machines - a multitrack deck for capturing the performances and production, and a second two track recorder for capturing the stereo mix. Even well into the digital era, a second machine has remained popular with many engineers. However, most modern DAW programs can render a stereo mix from the internal tracks and mixer. In Pro Tools, this is most commonly done with the "bounce to disk" function (File / Bounce To / Disk) You can then select the format of the bounce in terms of sample rate and bit resolution, as well as whether it will save two mono files (one for left, one for right) or a generally more manageable single stereo interleaved file.
While the bounce to disk function has some advantages in terms of the file formats it outputs, it's not without its problems. First and foremost is that it is a real-time process, and the entire song has to play back, in real time, in order for Pro Tools to render and write the stereo mixdown file. If you're working on a very long song or other audio project such as a soundtrack for a short film, this can translate to a lot of waiting around every time you want to run off the latest version of the mix. Wouldn't it be nice to be able to make that "one small correction" to the mix, and punch in only that one spot to correct it? By mixing to a separate stereo mixdown track within the DAW, you can. Even better, once you've recorded the entire mix the first time, you can punch in on it at will to make any changes or adjustments, then consolidate it back to a single file and quickly export the mix file as a stereo interleaved WAV or AIF file without having to wait for the entire song to play back as you would if using the bounce to disk function.
SETTING UP - THE ANALOG PATCHING APPROACH
The object is to get all of the various tracks of your DAW to record to a stereo file. Often called a layback or internal bounce, the basic process is the same whether you're doing a submix of multiple background vocals to a stereo pair, or creating stereo stems for an outside engineer to mix, or bouncing down all the tracks in your session to a stereo mix .WAV or .AIF file. There are a couple of methods you can use to route the audio. One is to physically connect a pair of outputs on your interface to a pair of inputs, but this requires an audio interface with at least four outputs. If you have a 4x4 audio interface and want to try this, first, turn down your monitors, and then:
Route all of your DAW software mixer channels (tracks, aux returns, virtual instrument outputs, etc. - everything you want "in the mix") to outputs 3 and 4 of your audio interface. Click on the first track name to highlight it, then click on the last one while holding down the shift key - this will select all tracks. Now while holding down Shift/Option simultaneously, assign one of the tracks to output 3/4 - doing so with the shift/option keys depressed will change the assignment for all selected tracks at once, saving you considerable time.
Physically plug the hardware outputs (3 and 4) into hardware inputs 1 and 2 on your audio interface.
Create a new stereo track in your DAW. Assign inputs 1 and 2 to this stereo audio track. This will be your mixdown track. On this track, make sure the output is assigned to outputs 1 and 2 (the ones you typically have your studio speakers connected to). Make sure you DON'T assign it to output 3 and 4, or you'll create a nasty feedback loop! This is why I suggest turning down the monitors, just to be safe.
When you're ready to do your mixdown, record enable the new stereo track, and when you're ready, hit record. The mix will play back, and simultaneously record on to the new stereo track.
Whenever you want to make a correction, there's no need to re-run the entire mix, although you can if you want. In fact, I like using new Playlists in Pro Tools for saving all the different versions of the mix I create as I go along, but if you prefer, you can "punch in" on the previously recorded stereo file - just start recording on the stereo mix track a bit before the spot where you made the mix changes, and let it run a bit past the end of the changes until you punch out. You can then add crossfades and consolidate the file so it's a single continuous region. Then you can export that as a stereo .WAV file. I'll go into the specifics of how to do that in Pro Tools in a moment.
The downside to this method is that the audio is making an additional "pass" through your system's converters - both D to A on the output, then another A to D stage before it is sent to, and then recorded on the stereo track. However, if you don't have a separate mixdown deck such as an Alesis Masterlink, or analog tape deck, and you want to avoid the bounce to disk function, this can be a valid way to do it, and the degradation caused by the extra pass through the converters can be fairly slight, especially with modern equipment. Another option, and one that gets around the extra converter stages, is to use your system's digital S/PDIF input and output instead of analog I/O. The basic steps are the same as listed above, except instead of using your analog I/O, you connect your computer audio interface's S/PDIF digital output to the S/PDIF digital input, then select S/PDIF Out for all the DAW tracks used in the mix, and S/PDIF In for the stereo mixdown track's input source.
IN THE BOX - NO HARDWARE PATCHING REQUIRED
There is another way to set everything up that can be done entirely in software, without having to make any physical patch connections on your interface. I'll be using Pro Tools as an example to walk you through it, but the basic concepts are the same regardless of the DAW you're using, although you may need to check your manual for the specifics for some steps with other programs.
If the session / song file doesn't already have one, create a master fader. Assign it to your normal stereo output (typically Analog Out 1/2).
Assign all of your other mixer channel outputs to an unused bus pair. I like to use the highest bus pair available - in this case, I'm using bus 127/128, but any unused pair will work - just make sure you don't assign, or use this bus for anything else! Again, selecting all the tracks at once first, then holding down shift/option while assigning the output of one to the bus of your choice will assign all of them to that output bus and save you from having to repeat this step over and over for each track separately.
Double check to make sure that all mixer channel outputs are assigned to this same bus output. Don't forget to include all effects / aux returns, virtual instrument tracks and audio tracks - if it's part of the mix that you want to hear in the final stereo mixdown .WAV file, you need to assign it to the bus.
Create a new stereo audio track. To quickly create a new track in Pro Tools, press Command / Shift / N (Mac) or Control / Shift / N (PC). Assign the INPUT for this track to the same stereo bus you used in step 1. In the example here, we've used bus 127/128.
Assign the output of the new stereo track you just created to your main stereo output - again, typically Analog Out 1/2, or whatever outputs are physically connected to your studio monitors. (Fig.1)
Figure 1: Setting the mixer output assignments in Pro Tools. Also note the stereo mixdown track at the far right, and its I/O assignments (click on images to enlarge)
Also on the stereo track you just selected, click on the Input Monitoring icon. (Fig.2) This allows you to hear the mix from the other channels as it passes into the mixdown track.
Figure 2: Input monitoring allows you to hear the output of the DAW mixer, even when not actually recording to the mixdown track. Unselecting it allows you to listen back to the recorded mix itself after you've recorded it
Select the entire session by using the I beam cursor to drag across the entire length of the session's waveforms and MIDI tracks in the Edit window. (Fig.3) To give myself a little working room later, I like to leave a few seconds of blank space at both the beginning and end of the session, This can be trimmed away in mastering later, and is better than accidentally cutting off something at the very beginning or end of the song.
Figure 3: Select the entire length of the session by dragging across it from beginning to end with the I-beam cursor in the Edit window.
When you're ready to record the mix, hit record / play (Mac: Command / Spacebar, PC: Control / Spacebar) and let it run. (Fig.4)
Figure 4: When you're ready to record the mix, record enable the mix track, and hit record/play
PUNCHING IN FOR CORRECTIONS, AND EXPORTING THE FINAL STEREO MIX FILE
Once you've recorded the entire song's mixdown track, you can go back later and punch in on a section of the mix and correct only the small area that is problematic. Then crossfade, consolidate the waveform, then export the track. This is significantly faster than waiting for a bounce to disk. To consolidate a track that consists of several regions in Pro Tools, select all regions by double-clicking in the center of any region, then press Control / Shift / 3 (PC) or Option / Shift / 3 (Mac) on your computer keyboard. This will turn the multiple-regions into a single one, which can then be exported.
When doing an internal bounce, the files you record will be at the same sample rate and bit resolution as your session, so if you're recording at 24 bit, 96kHz, you'll need to do a sample rate conversion on the resulting stereo file as part of the mastering process. Mastering engineers invariably prefer getting the highest resolution version of the mix that they possibly can, so this is ideal from their perspective. If you're doing the mastering yourself, then you will also benefit from working with high-resolution files and saving the conversion to 16 bit and the dithering until the very end of the mastering process. Mastering within the DAW itself is becoming more popular, and some DAW programs, such as Samplitude, include some pretty impressive mastering tools, and there are plenty of mastering related plugins you can run within your DAW. Since you already have the stereo file in the DAW itself, you can disable all the other tracks, and focus on the stereo track; applying any editing, mastering EQ and compression or other processing directly to it, or to a copy of it. Once you're finished, or if you want to continue the mastering process in a different program, you can export your mix. Here's the steps to do so in Pro Tools:
In the Edit window, double click on the stereo mixdown file to select it. (Fig.5) If you've punched in on the track and there are multiple regions instead of just one continuous file, make sure you consolidate the regions first.
Figure 5: After any punch-in corrections and consolidating, double click on the stereo mixdown file (outlined here in red) in the Edit window to select it. Make sure no other regions besides the stereo mixdown are selected
If it's not already displayed, open the Clips list by clicking where indicated in Figure 6.
Figure 6: Opening the Clips list can be accomplished by clicking where indicated by the red arrow
Right click on the header where it says "Clips" (circled in red in Fig.6) to pull open the drop down menu. Select "Export Clips As Files." (Fig.7) Since the single stereo mixdown file is the only region / file selected, it will be the only file exported.
Figure 7: Exporting the stereo mixdown clip / region as a stereo file
In the Export Selected dialog box that opens up (Fig.8) you can set the various parameters for the export, including sample rate, bit depth, file type, and the destination directory (file folder) you want to save the exported mixdown file to.
Figure 8: The Export Selected dialog box
That's it! You'll find that exporting files in this way takes a tiny fraction of the time that doing a full bounce to disk takes.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.