Sometimes when creating sounds in Fruity Loops, you want to add effects only to certain frequencies. For example you have made a great bassline synth, which you could make slightly warmer with adding distortion and reverb, however that completely destroys the original sound. Unless... you add distortion only to high frequencies.
Step 1 - Create simple saw wave sound
1) Right click on any sounds in Step Sequencer (press F6 if it isn't visible) and take Insert > 3x Osc
This is a very basic synth where we will be creating our saw sound
2) A new window should open. Set Osc 1 to saw wave, turn down the volume of Osc 2 and Osc 3, in the bottom of 3x Osc window, right click on C8, like in image below
Press W on your keyboard, you should be hearing a saw wave.
Step 2 - Splitting frequencies
1) In the same 3x Osc window set the little 'fx' box to channel 1 (hold your mouse on the box and drag upwards)
2) Now go to the Mixer (key F9), take Insert 1 and disable "Send to Insert 1 to Master" by clicking the little orange button (like in image below)
3) Click the same button (it will be gray this time) from Insert 1 to Insert 2 and from Insert 1 to Insert 3, so sound signal from Insert 1 is being sent to 2 and 3.
4) Right click on Insert 1 and select Rename. Name it to "SOUND"
5) Rename Insert 2 to "SOUND LOW" and Insert 3 to "SOUND HIGH"
6) Now take a Mixer channel 'SOUND LOW' and on the right side add Fruity Parametric EQ 2
7) Yet another window pops up, click the small button in the top-left corner and select Presets > 20Hz + 18kHz cut
8) You have a line in the middle of your window, take it's right point and drag it to the area where bass fequencies end
You may close the Fruity Parametric EQ 2 window
9) In the Mixer click on channel 'SOUND HIGH' and on the right side add Fruity Parametric EQ 2
10) Like before, select Presets > 20Hz + 18kHz cut
11) Now take the left point of line and bring it where bass frequencies end, the same frequency where you put the right point for SOUND LOW channel
You can close the Fruity Parametric EQ 2 window now
Step 3 - Add effects to individual frequencies
Now when the frequency splitting is done, you can do whatever you like, for example add compression to low frequencies, distortion to high frequencies and reverb to everything.
Use SOUND channel when you want to add effects to every frequency
Use SOUND LOW channel when you want to add effects only to low (bass) frequencies
Use SOUND HIGH channel when you want to add effects only to mid and high frequencies
This is an advanced method which you should use when adding effects to sound as whole is not enough or in case you want to do something like sidechain kick hits to bass frequencies so they don't conflict and make messed up sound. You also may split it in even more channels, like Insert 2 for low frequencies, Insert 3 for mid frequencies and Insert 4 for high frequencies, I believe that after this tutorial you will know how to do that, and it's not that hard to do in Fruity Loops.
If there is anything unclear, please leave a comment and I'll try to impove the tutorial and answer your question.