Most DAWs default to routing audio directly to a physical output. This is a common, trouble-free signal flow that generally works very well. But there are several advantages to summing your signal at a Bus, acting as a master subgroup, before it reaches the final stereo output.
Easy Summing to Mono to Check Mono Compatibility
Regardless of whether we are mastering our own music or not, it is common practice to incorporate some stereo bus processing on the full mix. This is easily done in most DAWs by placing effects processing on the stereo output. What isn’t always easily done, however, is temporarily switching between stereo and mono while preserving the final effects processing.
This is an important step when mixing, in order to ensure mono compatibility. We hear this term “mono compatibility” often, for good reason. Put simply, it means that there is no significant imbalance to the mix, or cancelling of frequencies when the two channels of a stereo mix are summed so that the same signal appears in both the left and right channels. We need to make certain that spacial effects we have carefully crafted are not lost due to phase cancellation when the two sides are summed. This can include anything from chorus or flanging effects to the doubling, delaying, EQing, and panning of important tracks.
The best way to check this is to toggle back and forth between mono and stereo frequently as you are mixing, to make sure your mix dynamics translate well. The ability to do this easily ensures you don’t get too deep into your mix before realizing there may be problems you need to fix.
By routing to a stereo master bus you can safely set up your stereo processing in place there, and then simply toggle your final stereo output between stereo and mono. This way, all your stereo processing is included in the “testing” when toggling between mono and stereo.
Change the stereo output format button state to toggle between mono and stereo while preserving the master bus processing.
Use the master bus fader to control the level reaching the first plug-in on your stereo output.
Easier Gain Staging into the First Plug-In Slot on Your Stereo Output
If your DAW does allow for easy mono/stereo toggling on the final stereo output and you work with your stereo bus processing in place there, it is very useful to have single-fader control over the level arriving at this final output. There has been a lot of virtual ink spilled in recent years about gain staging within DAWs. The bottom line is, it simply cannot clip or overload at the final stage before hitting the physical outputs of your audio interface.
This in itself is not a problem that requires master bus routing to control. We can simply turn down the stereo output channel strip, or the output of the last plug-in in the effects chain.
Some plug-ins, however, particularly the variety that emulates analog circuitry (like tape modeling plug-ins you might typically place on a final stereo bus) are designed to operate optimally with specific target input levels. In this scenario, it is important to have control over the level of the signal that reaches the first plug-in in your final effects chain.
Routing to a master bus before this stage allows you to easily see and control the level being sent to the first plug-in on the final stereo output. Because no processing will be in place on the master bus in this scenario and you are monitoring the unprocessed stereo signal, it is less likely that you will paint yourself into a corner with a raging hot mix feeding into the top of your potentially delicate stereo output processing chain. Sure, you could simply place a gain-type plug-in in the first slot of your stereo output channel strip to attenuate (or boost) the gain there. But the master bus approach allows you to continually visually monitor your levels as you are mixing.
Easy Parallel Processing of the Entire Mix
Parallel processing is a great technique that works not only on instrumental or vocal subgroups but in many cases on the full mix as well. The basic idea is that you create a duplicate audio signal running alongside the original, which you enhance in some way, and then blend in with the original unenhanced signal. Parallel processing is often set up with a send, which routes a duplicate signal to another track. Routing the signal to the input of two separate tracks or channel strips also works.
When parallel processing the full mix using a master bus, both approaches work. You can simply set up a send on the master bus, route it to another bus/ aux, and then output both of them to the final stereo output. Or, create two separate channel strips, each set up to receive the master bus signal at their input, and route the output of each to the stereo output.
Either way, the result is a duplicate audio stream that you can process separately and blend with the original. For example, you might apply some fast aggressive FET type compression to the duplicate signal and then blend only a small amount in with the unprocessed master bus signal to enhance the attack and transients of the percussive elements in the mix. Or you might compress and enhance only the high end of the frequency range on the duplicate signal, and blend that in to add some weight and presence to the top end of the mix.
Use two channel strips to blend the parallel processed and unprocessed master bus signal.
“Often times though it is useful to omit one or more elements from two bus processing.”
Processing the Entire Mix Except For Specific Tracks
When routing everything to a master bus before it reaches the stereo output, effectively, both the master bus and the stereo output are receiving the same signal. Processing on either the master bus or the stereo output affects everything. Often times though it is useful to omit one or more elements from two bus processing. Routing one or more elements directly to the stereo output effectively bypasses any processing on the master bus. Why would you want to do this? Here are a few examples:
It is often helpful to omit the drums from two-bus compression. The sharp kick and snare transients will not trigger the compression. The compressor(s) will instead react to the other elements in the mix, without over compressing the kick and snare peaks.
Another scenario where this is useful is when mixing a music bed track with a voice over. By routing the voiceover directly to the stereo output, only the music will be processed on the master bus. This set up also lends itself well to automating/ducking the music bed from a single fader without it affecting the voice over.
Rerouting the Mix to Alternate Outputs
It is good practice to check your mixes on alternate sets of monitors as you are working. A common scenario is to have a multi-output audio interface with each pair of outputs feeding different sets of monitors. By routing the entire mix to a master bus in your DAW, it is possible to create several channel strips receiving this same master bus input. Set the outputs of each channel strip receiving the master bus signal to send to alternate outputs on your audio interface, and ultimately to alternate monitors. Mute/unmute them as needed to route the mix through the different outputs.
This is particularly useful if you are using monitor calibration software like Sonarworks Reference 4. You can set up a unique calibration profile for each set of monitors and place them on the corresponding master bus channel strip.
Use multiple channel strips to route your mix to alternate physical outputs.
External Processing of the Entire Mix
If you process your full mixes with outboard gear, routing to a master bus is invaluable. Route the output of the master bus to the physical output on your audio interface that is patched to the input of your external processing. Bring the signal back from the outputs of the external processing unit back into your DAW on a channel strip routed directly to the stereo output.
Monitoring a Reference Track
It is common practice to monitor a commercial track as a reference as you are mixing. The objective is to compare your mix with the reference track as you are working. If the reference track is playing back from within your DAW, and all the audio in your session arrives at a processed output, the reference track will be processed as well; which defeats the intended purpose. However, if you route the entire mix minus the reference track to a master bus, you can process your mix there. Route the reference track directly to the stereo output. This way your mix gets the intended to bus processing, while the reference track does not.
These are just some of the many useful routing benefits available when summing your entire mix to a subgroup before the final output.