To elaborate, I used my notes for when I did this with the Axe-Fx III:
https://forum.fractalaudio.com/threads/keeley-halo-take-2.185495/ and the
attached Halo block diagram from Keeley.
This is a simplified block diagram for Halo core in wet-only mode using Andy's settings (the LFO phases for Delay B is not quite this, but close enough)
View attachment 55873
- Note the phase inversion in the cross-feedback from Delay A to B
- Note the phase inversion of the saturated and filtered signal at the input of Delay B.
- Note there's no cross feedback from Delay B to A.
- Note the modulation LFO phase between delay lines
- 90 degrees between Delay A L and Delay B L
- 90 degrees between Delay A R and Delay B R
- 90 degrees between Delay A L and Delay A R
- 90 degrees between Delay B L and Delay B R
Now here's a a simplified view of the Helix Stereo Crisscross delay block using the following settings:
- Pan A => Left 100
- Pan B => Right 100
- Shape => Sine
- Phase => 90%
- Mix => 100%
View attachment 55874
A couple of parallel crisscross blocks would be a good fit for emulating the Halo if we had the following additional options:
- Independently controlled cross-feedback for A => B and B => A
- With independent controls, we would set B => A feedback to 0%.
- Phase reversal option on cross-feed path
- Phase reversal option on input L and/or input R
Since we don't actually have these options currently available, we have to do the cross-feed manually, so we can apply phase reversal and only cross-feedback from Delay 1 => Delay 2.
We can rearrange the Halo block diagram in terms of two dual mono delay blocks.
To preserve the modulation LFO phase relationships between all the 4 delay lines, we swap L/R at the input and output of Delay 2.
View attachment 55875
- Essentially we have 3 parallel paths:
- Input => Delay 1 (1/8 dotted) => output
- Input => Delay 1 (1/8 dotted) => Phase inversion => 50% level reduction => Delay 2 (1/4) => output
- Input => Phase inversion => Delay 2 (1/4) => output
- Ok, so then we should be able to use a couple of Helix Crisscross delay blocks, with cross-feed set to 0% - which turns them into dual mono delay blocks with the modulation we need.
- To reverse the phase and swap the L/R channels, we can use two "Stereo Width" blocks in series which have the option to swap channels and invert the phase of the R channel (after swapping).
- For saturation I ended up using a Deluxe Comp block with fast attach and release so it kinda approximates the static non-linearity used in the Halo
- For the lowpass filtering after saturation I used an EQ Tilt block
Now, I ran into some obstacles with routing - I couldn't quite figure out how to do the 3 parallel paths described above (+ the dry) without having to replicate the whole input chain. So that's what I had to do to get this to work.
View attachment 55881
I also noticed that the feedback controls in the Crisscross delay block are not linear taper (they appear to be audio taper) so a 43-45% feedback on a linear control maps to about ~65% on an audio taper.
The preset is attached (remove .pdf extension, I couldn't upload otherwise).