Split-band filtering is a great technique whereby you separate an audio signal into different frequency bands so that you can process each band separately. You then combine the respective bands together to form a more coherent and dynamic sound. Split-band filtering comes in really handy with bass sounds where you wish to 'beef-up' the bass and spread it across the audio spectrum by adding different processing effects to different ranges of the bass sound. In electronic music which is mostly drums and bass driven, you want to get the most out of your bass source. This is where split-band filtering comes into play. I am going to be using Logic Pro, but this technique can be set up and applied in any DAW.
Step 1 - Create Your Initial Bass Sound
First let's create a bass sound as a starting point. I am using Logic's ES2 and have dialed in the following settings represented in the diagram below. I've used 3 different oscillators with different waveshapes to build up the bass sound. I have increased the amount of voices to 8, and slightly detuned the oscillators. The sound is a bit 'buzzy' but this will be resolved by our split-band filtering.
Step 2 - Buss It Out
To get the separate frequency bands we need to send the bass out to multiple auxiliary channels. I have chosen 3 busses. This will give us buss channels for the Low, Mid and High frequencies, respectively.
On your Bass instrument choose 'no output' (in the I/O section, below 'ES2') and increase the busses' volumes to 0 dB so that you are getting equal amounts of the signal sent to the busses. This can easily be achieved by Option-clicking on each bus send.
Step 3 - Create the Filter Bands
Now insert a Channel EQ on each buss. For the Low buss, I have dialed in a Low pass filter of 300Hz, and a slope of 48dB/Oct.
For the High Buss I created a High pass filter at 2000Hz with a slope of 48dB/Oct.
The Mid bus will include the remaining frequencies as reflected in the diagram below:
Step 4 - Now Process Those Bands
This is where the excitement starts. You can really go crazy here. For my Low Bus I have compressed it heavily and then added some Distortion to drive the sounds. I have cut away any frequencies below about 50Hz to get rid of any low rumble that make take up valuable headroom in our mix.
For the Mid Bus I have added some more compression, some Distortion, some Chorus, and an EVOC Filterbank to change up the sound and give it a different color.
For the High Buss I have added an Exciter, a Microphaser, some SilverVerb, a Bitcrusher to gritty-up the high frequencies, and a Tremolo to add some movement to the sound.
Here are the audio examples of the Low:
and High Bass channels:
As you can see you can go quite wild. But the real test is to see how it all sounds together and if it all gels. It can take a few attempts, but keep at it; you will be amazed at how your bass can become "more alive".
Step 5 - Mix Them Together
For the final step, listen to all the bus channels together and not in isolation. Adjust the volumes to taste. To get an idea of how far you have come, compare the split-band filtered bass sound to the original and you will notice a drastic change. You can achieve things with split-band filtering that are not possible with just the single channel strip and software instrument.
Here is the sound of the Bus channels mixed together:
And here is the Bass sound mixed together with a beat, to give you an idea of how it would in the context of a drums and bass mix:
Step 6 - Conclusion
As you can see you can take an average bass sound and turn it into something quite extraordinary with split-band filtering. The bass source doesn't have to just sit in the bass frequencies. You can use the harmonic content in the higher registers and process this for extra effect in your productions. I hope you guys have fun with this like I do. Split band Filtering can also be applied to other instruments such as beats, and synth lead lines. So split and get filtering!
For another handy bass tutorial on The MPV Hub check out G.W Childs' article on Bass Randomization in Reason.
And for more information on Bus processing you can read the following tutorial by Mo Volans on Creating dynamic effects with busses.
For more great Logic tutorials check out the tutorial-videos available here.