(5 min read.)
Parallel processing is a technique used in audio production to create a fuller and more dynamic sound. In the context of bass guitar, it involves running the bass signal through two or more processing chains in parallel before combining into a single outputted signal.
This technique used to require a DI-box and sending different copies of the signal to various amps and hardware effects. In the modern recording studio this is no longer necessary, as basically every DAW has a way to split and recombine signal paths. In this article I'll be using Ableton Live, as that's my DAW of choice. I'll also be referencing my own Preset Bass signal chain, which I use all the time. It includes both Parallel and traditional processing, but I'm only going to dive into the former in this post.
Here's a quick before and after, the bass in this demo is from PDQBass:
Note: The only difference between these two demos is the bypassing of the Audio Effect Rack involved with Parallel Processing, all of the other Track effects remained active.
Your milage may vary.
First, we need to split the signal. Ableton has a handy feature called Grouping, which we can use inside an Audio Effect Rack to create any number of signal chains.
To create an Audio Effect Rack & Group, add any effect to the track (I'm using Utility) and hit Control + G. Now select the chain and hit Control + D to duplicate it. In this case I'm using 3 chains: Low, Clank (Mid) and Grind (High).
The low chain is usually heavily compressed (or limited) to keep the sub-bass under control. Having too much low-end bouncing around can really muddy up a mix and wreak havoc with your Master Chain. I usually keep this chain pretty clean and dead-center.
Clank / Mid (150hz ~ 3k)
I'm looking for a twangy, bitey mid-range that helps the guitars create a tonally consistent image across the stereo field. First the signal is limited, albeit less aggressively than the low-end. It then goes into an 1176-style compressor achieving about 6dB of gain-reduction with a slow-attack time to preserve the pick transient.
This is followed by an EQ with a slight boost at around 2.4k to add some of that upper-mid clank. Next it goes into an amp, in this case I'm using TSEX50 on the Red channel with default settings and cab. If I think this is too overdriven, I'll switch to the green channel and play around with different cabs.
Finally, I'm using the stock EQ to remove some lower-mid muddiness at around 350hZ. I'll also reach for a tool like Trackspacer if I find this chain is clashing too much with the lead vocal, since they both tend to occupy similar spaces.
Grit / High (3k ~ 10k)
Okay, this is the chain that really nestles into the heavy guitars. I'll typically push this through the same amp and settings as the guitars, to create a tonally consistent sound across the stereo image. I'll also treat it like a guitar with the EQ that I use, like taking out some 2.5k and 4k to reduce the overall harshness. Again if this chain starts clashing with other important elements, I'll use Trackspacer.
The final piece to the puzzle is the balancing act of levelling these three signals. You want a nice, powerful low-end alongside a clanky but not overbearing mid with some high-end sizzle. Too much of the latter will make your bass harsh and thin, too much of the former will make it boomy and cover everything else with mud. I typically level the three channels in order of loudness as follows: Low first (loudest), high second, mid last (quietest). I won't give specific dB values here, as the ratio is something you'll have to play with and will change depending on the song.
The Rest of the Chain
After combining the signals, the rest of the Bass Processing Chain includes the usual suspects like surgical EQ, maybe a highpass filter to remove any sub garbage, and sometimes Chorus to add a bit of stereo width and flavor to the sound. Tape Saturation (or any subtle saturation) is also often a good choice for taming any harshness or rounding out the lower-mids. Trackspacer, or any variant of a dynamic EQ to sidechain with is very powerful to clear up space when you find certain elements are clashing too much.
One final little tip is to add Utilities to each Chain and automate the amplitude of them over the session. This is handy when you need a cleaner Bass for the verse, and a loud gritty one for a chorus or breakdown. Just automate the levels as necessary without the need for adding an entirely new Bass Guitar track(s).
In summary, parallel processing is a powerful technique that can not only tonally sculpt the bass, but can clean up a mix and ensure elements aren't clashing. I hope this has been helpful to you.