What does the structure of an Eventide diffchorus look like?

AnalogKid85

Member
Messages
1,318
I don't think I've ever seen a diagram of what this looks like--I've only heard "hints" as to what the structure is, so I don't have a good way to visualize this in the same way that I would with Ultratap2, or the Plex structures.

Anybody be willing to shed some light on this?
 

valhalladsp

Member
Messages
357
I don't think I've ever seen a diagram of what this looks like--I've only heard "hints" as to what the structure is, so I don't have a good way to visualize this in the same way that I would with Ultratap2, or the Plex structures.

Anybody be willing to shed some light on this?
From the Vsig description of the DIFFCHORUS module:

"Like the standard diffusor, the chorused diffusor is essentially a chain of series-connected allpass filters."

So, the DIFFCHORUS module consists of 2 to 32 allpass delays in series. Each of the allpass delays seems to have its own slow random modulation of the delay length. To my ears, it sounds like linear interpolation is used for the delay length interpolation, as the high frequency behavior of the Blackhole algorithm (which is just a few DIFFCHORUS modules) is much different at 48 kHz versus 96 kHz.
 

Brian Chan

Silver Supporting Member
Messages
314
From the Vsig description of the DIFFCHORUS module:

"Like the standard diffusor, the chorused diffusor is essentially a chain of series-connected allpass filters."

So, the DIFFCHORUS module consists of 2 to 32 allpass delays in series. Each of the allpass delays seems to have its own slow random modulation of the delay length. To my ears, it sounds like linear interpolation is used for the delay length interpolation, as the high frequency behavior of the Blackhole algorithm (which is just a few DIFFCHORUS modules) is much different at 48 kHz versus 96 kHz.
Is there any Valhalla Reverb that sounds close to the Diffchorus on Eventide?
Big fan of your plugin =D
 

hydroquebec

Member
Messages
1,933
I don't think I've ever seen a diagram of what this looks like--I've only heard "hints" as to what the structure is, so I don't have a good way to visualize this in the same way that I would with Ultratap2, or the Plex structures.

Anybody be willing to shed some light on this?
I'd be more than happy to talk about this if you catch me on Skype tomorrow. I've studied DiffChorus quite a bit and although I don't precisely understand what's going on under the hood, Sean's response above is quite accurate. I disassembled Black Hole and learned quite a bit about how to string diffusors together effectively. I can also send you a screenshot of what's available in the module, which is incredibly interesting.

Is there any Valhalla Reverb that sounds close to the Diffchorus on Eventide?
Big fan of your plugin =D
Perhaps check Valhalla Shimmer. I believe @valhalladsp mentioned that this algorithm is loosely based on DiffChorus/BlackHole. Kill the pitch, of course.

Blackhole is quite interesting. This might have been created by @italo de angelis, but he'd have to confirm. It's actually a very basic structure, but it's quite beautiful. The Space adds feedback, but I'm not quite sure what the feedback is connected to. I'd like to see that flowchart.
 

valhalladsp

Member
Messages
357
Perhaps check Valhalla Shimmer. I believe @valhalladsp mentioned that this algorithm is loosely based on DiffChorus/BlackHole. Kill the pitch, of course.
ValhallaShimmer would be the closest one. ÜberMod also uses series allpass delays for diffusion, but far less of them than Shimmer.

Blackhole is quite interesting. This might have been created by @italo de angelis, but he'd have to confirm.
I asked Andrew Schlesinger (one of the DSP4000 programmers) about the Blackhole algorithm, and this is what he had to say:

"I think I did Black Hole but I am not 100% sure. I created a lot of alga with the SIG editor at the beginning of the 4000 development working with Ken, Gill and Laura if I remember correctly.

It was either me or Bob Belcher...there were so many done at the time. I have no formal training in alg dap design and used to come at it from a 'if this sounds good then THAT should be more dense' and then tune the delays, diffusion and filters by ear always looking to reduce flutter, resonances and metallics to get the smoothest sound and width."

It's actually a very basic structure, but it's quite beautiful. The Space adds feedback, but I'm not quite sure what the feedback is connected to. I'd like to see that flowchart.
The feedback in Space could be connected in a few ways. For the sake of explanation, here's the simplified Blackhole topology:

  • Left and right inputs are summed
  • The summed inputs are processed by a diffusor (Diffusor1), consisting of 16 modulated allpass delays in series
  • The output of these 16 modulated allpasses is sent to 2 parallel diffusors (Diffusor2, Diffusor3), each consisting of [EDIT] 8 modulated allpass delays (Italo pointed out below that the parallel diffusors have 8 allpasses in series, not 16 as I wrote earlier)
  • The 2 parallel diffusors feed the left and right outputs respectively

So, you could route the feedback in the following ways:

  • Feedback around Diffusor1
  • Parallel Feedback around Diffusor2 AND Diffusor3
  • Figure 8 feedback around Diffusor2 and Diffusor3 (i.e. scaled output of Diffusor2 fed into input of Diffusor3, scaled output of Diffusor3 feeds into input of Diffusor2)
  • Feedback around the chain of Diffusor1->Diffusor3 (two allpass chains in series will still be allpass)
  • Feedback around the chain of Diffusor1->Diffusor2

No matter what way you route the feedback, I'd add an extra delay (of 10 msec or more) in the feedback path. This avoids some nasty resonance effects that occur when you route feedback around pure diffusors, which is caused by the "dry" path through the diffusors being relatively high amplitude.
 
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I'd be more than happy to talk about this if you catch me on Skype tomorrow. I've studied DiffChorus quite a bit and although I don't precisely understand what's going on under the hood, Sean's response above is quite accurate. I disassembled Black Hole and learned quite a bit about how to string diffusors together effectively. I can also send you a screenshot of what's available in the module, which is incredibly interesting.



Perhaps check Valhalla Shimmer. I believe @valhalladsp mentioned that this algorithm is loosely based on DiffChorus/BlackHole. Kill the pitch, of course.

Blackhole is quite interesting. This might have been created by @italo de angelis, but he'd have to confirm. It's actually a very basic structure, but it's quite beautiful. The Space adds feedback, but I'm not quite sure what the feedback is connected to. I'd like to see that flowchart.

Black Hole was written by Andrew, yep.
The summed inputs go thru a tone module.
The two parallel diffusors run on 8 voices each, so its 16v > 2x8v 3 diffusors.
The Eclipse version is 2x8v diffusors > hicut filters feeding the same structure again, in series, 2 more 8v diffusors > hicut filters.
Being different from the original... it was rightfully called "BlackHole2" to point out a non 100% exact version of the original, which can't be done on Eclipse, unless a 16v diffusor algorithm is added to the unit.
Space.... well... this can't simply do "Black Hole" so its version is something else, possibly beyond "3". Its parameters show it ain't THE thing anymore. Bummer.
It's marketed with a definition of a "an H8000FW classic".... when it's been made on the DSP4000 and remained thru all the later units.
I hate when people "put their hands" in others work and mess with it...
The decency!
 

hydroquebec

Member
Messages
1,933
Excellent info, guys! So with the Space algorithm, I wasn't a huge fan of feedback - there should be a better way to extend the verb. While the feedback extended the duration, there was also a buildup of density that made the Blackhole lose it's character. I think only people with rack Eventides will notice this.

Just out of curiosity, I'm going to try the methods you mention above @valhalladsp. And yeah, Sean, I found my playground!
 

valhalladsp

Member
Messages
357
Excellent info, guys! So with the Space algorithm, I wasn't a huge fan of feedback - there should be a better way to extend the verb. While the feedback extended the duration, there was also a buildup of density that made the Blackhole lose it's character. I think only people with rack Eventides will notice this.
Feedback is a nice way to turn the loooong reverb of Blackhole into an INFINITE reverb. However, it does change some of the characteristics of the sound. One of the things that is nice about Blackhole is the non-exponential attack and decay of the reverb. Add feedback to this, and it turns into a more standard exponential decay.

You can always increase the decay of Blackhole by:

- increasing the lengths of the individual delays. Just make sure that they have the same delay length ratios.
- add more diffchorus in front of the unit.

Feedback is a cheap way of extending the decay, as it costs almost nothing CPU-wise. Plus, you can put things in the feedback loops. Cool things.:D
 

hydroquebec

Member
Messages
1,933
Feedback is a nice way to turn the loooong reverb of Blackhole into an INFINITE reverb. However, it does change some of the characteristics of the sound. One of the things that is nice about Blackhole is the non-exponential attack and decay of the reverb. Add feedback to this, and it turns into a more standard exponential decay.

You can always increase the decay of Blackhole by:

- increasing the lengths of the individual delays. Just make sure that they have the same delay length ratios.
- add more diffchorus in front of the unit.

Feedback is a cheap way of extending the decay, as it costs almost nothing CPU-wise. Plus, you can put things in the feedback loops. Cool things.:D
Good stuff, Sean.
Have you any insights on calculating delay times? I normally start with a low value 30-50ms and apply an exponential curve (based on Fibonacci). My arrays usually look something like this:

45.04
52.11
60.28
69.74
80.67
93.33
107.97
124.91

Several Eventide presets follow this convention:

67.10
70.74
74.57
78.62
82.88
87.37
92.11
97.11

Am I overthinking these types of things?
 

hydroquebec

Member
Messages
1,933
So, you could route the feedback in the following ways:

  • Feedback around Diffusor1
  • Parallel Feedback around Diffusor2 AND Diffusor3
  • Figure 8 feedback around Diffusor2 and Diffusor3 (i.e. scaled output of Diffusor2 fed into input of Diffusor3, scaled output of Diffusor3 feeds into input of Diffusor2)
  • Feedback around the chain of Diffusor1->Diffusor3 (two allpass chains in series will still be allpass)
  • Feedback around the chain of Diffusor1->Diffusor2

No matter what way you route the feedback, I'd add an extra delay (of 10 msec or more) in the feedback path. This avoids some nasty resonance effects that occur when you route feedback around pure diffusors, which is caused by the "dry" path through the diffusors being relatively high amplitude.
This method worked quite well! I played around with the delay time in the feedback path and it did indeed smooth things out a bit. I've also thrown a touch of distortion and filtering in there. That's one helluva a Blackhole now... Thanks!
 

valhalladsp

Member
Messages
357
Good stuff, Sean.
Have you any insights on calculating delay times? I normally start with a low value 30-50ms and apply an exponential curve (based on Fibonacci). My arrays usually look something like this:

45.04
52.11
60.28
69.74
80.67
93.33
107.97
124.91

Several Eventide presets follow this convention:

67.10
70.74
74.57
78.62
82.88
87.37
92.11
97.11

Am I overthinking these types of things?
Fibonacci and exponential curves would work well. My advice is to plot out the Black Hole delays, and see what pattern you can find. Also remember that things are modulated, so slight deviations from a pattern will get smoothed out.
 




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