Reverb designs

Nikko · Post by **Nikko** » Fri Jul 06, 2001 4:06 am

I've found this interesting discussion.

Just to say that 1 DSP will be filled with 4 delay lines of 1000 sample.

To create a large reverb you need at least 6 delay lines of >1800 sample (only for the comb filters) so it is impossible to use DSP data RAM to store ALL the relays without filling a scope system! (same problem with the All pass tanks)

We need to use the PC delays that strangely using the PCI to access to the RAM (I though that 32k was on board...

On 2001-06-24 23:40, Warp69 wrote:
Hi Paul,

Yeah, I know. But I just thougt that the routing was more clear, when I included the feedback modul

Another hint, when you're working with delay lines, work in 48KHz. A delay of 1000 samples in 44.1KHz is only (1000*44100)/48000 = 918 samples.

Cheers
Martin

Nikko · Post by **Nikko** » Fri Jul 06, 2001 4:24 am

I thought it could be interesting to explain here a little bit of the "Spread Engine" used in the DSPdev reverbs.

The Spread engine is included beetween the Combs and the All pass.

COmbs -> Spread Engine | AllPass | StereoSP.
|....................|
--- Taps Feedback ---|

In order to create a Leaving reberb several things are missed in the Moorer algo.
- when a sound bounce back from a wall, it does not goes back to the listener without bouncing off the other walls. That's why each Comb delay must be conserved by a feedback shame and re-injected in all the comb filters. This is what all the very knwown hardware manufacturer are doing, whatever the name of this algo is...
- BUT the sound of each Comb filter must be phased to include the domain frequency changes done by the delay (doppler).

The Spread engine splits each Comb signal in 6-8 little delays. These are not really delays they are changing in the time( like a variable delay line) and also the sound frequency changes like a pitch shifting.

The comb filter are transformed in a 2^Combfilters number of signal that are moving all the time in term of frequencies and level. That's create the density that lacks from most reverbs avalaible in plugins.

With the Verb Gold Still in development, you can tune the Spread engine to separate the Frequencies (more or less) on each CombFilter, and you can change each comb delay so you can see that how each Comb filter are transformed.

The Tap feedback is here to introduce the long feedback (like in a Canyon). In some large room, the signal will bounce like 1-2s before coming back to the listener.

This create an "echo" like effect, but it is hard to notice it because the feedback must be very low to avoid any feedback overload problems.

These were some "secrets", but like all the secrets they are very easy to program as soon as you know how it works... That's why no one like to talk about these...

[ This Message was edited by: Nicolas Choukroun on 2001-07-06 05:31 ]

onomat · Post by **onomat** » Fri Jul 06, 2001 7:35 am

What's wrong with the Timeworks 4080L?

I can't believe how good it sounds...

I have Lexicon PCM60, Alesis Midiverb II (incredibly underated and legendary reverb unit...)and various classic bits of reverb kit and think the Pulsar 4080L is awesome sounding...

far better than anything else software I've ever heard on any platform including Waves Rverb TDM...

Warp69 · Post by **Warp69** » Fri Jul 06, 2001 12:57 pm

Hi Nicolas,

Choukroun wrote:
-

when a sound bounce back from a wall, it does not goes back to the listener without bouncing off the other walls. That's why each Comb delay must be conserved by a feedback shame and re-injected in all the comb filters. This is what all the very knwown hardware manufacturer are doing, whatever the name of this algo is...

The name of that algorithm is FDN (Feedback delay network) and is not used in any of the 'high end' reverbs (afaik). They use UFDN, which is allmost the same algorithm - but do not include any comp filters. In most cases, its a allpass cross feedback with 2 additional delay lines.

For the best sounding reverbs you need time-varying delay lines and some interpolation, like 2nd-3rd order langrange -or allpass interpolation.

Cheers

[ This Message was edited by: Warp69 on 2004-03-13 09:48 ]

EarlyFirst · Post by **EarlyFirst** » Fri Jul 06, 2001 3:44 pm

Hi Guys,

Well don't forget in some cases the Allpass sections are *first* in the chain....

my secret of the day

Nickolus if you use modulated delays, they *are* on the dsp's only if you use the bigger versions will they need to piggyback the PC based delay......

Cheers
Paul

Warp69 · Post by **Warp69** » Fri Jul 06, 2001 5:18 pm

Hi Guys,

Another secret : Don't use comp filters at all

Modulated delays = Time varying delay lines??

If yes, are you using interpolation??

Cheers

[ This Message was edited by: Warp69 on 2004-03-13 09:48 ]

spacef · Post by **spacef** » Mon Jul 09, 2001 2:03 pm

Isn't there another parameter to add and that causes reverb modulation ? like a fine phaser. It doesn't sound like a phaser on verb, but may create nice subtele evolution.
I only saw this in EPS 16+ 44.1 reverbs (i guess it's on DP 4 too) and is something you could hear in a natural reverb (i've experienced large hall reverb everytime i went out home,for more than 20 year, the sound of this hall is a really cool reverb

However, i don't know if it happens in all reverb spaces (but it doesn't in most software verbs).
Also, how to modelize a cavern or underground lakes with stalamites

[ This Message was edited by: spacef on 2001-07-09 15:10 ]

spacef · Post by **spacef** » Mon Jul 09, 2001 2:07 pm

More seriously,how gated reverbs are made : i compared 2 of them that I own, namely the very old Boss RRV10, which has a really cool gated reverb, with guts and hard gate (the rest is crap compared to what you can have now).
On the other hand, the STW 4080L, which is great except for gate (my opinion): i guess it is because it seems to cut reverb when decay reaches a certain treshold, and that's a very different effect than the Boss one, which seems based on TIME rather than Treshold, so the Boss is very steady and not input-sound-volume dependant.
i don't know if that make sense though,
cheers

[ This Message was edited by: spacef on 2001-07-11 13:35 ]

EarlyFirst · Post by **EarlyFirst** » Wed Jul 18, 2001 1:59 pm

Hi,

Can you send me a wav with some examples?
I understand what you say with the *Time* not threshold thing but I need to hear it...

paul

spacef · Post by **spacef** » Thu Jul 19, 2001 10:14 pm

Yes Paul, but please be patient i'm totally submerged with work during day and night.

the "non time" option is better explained as "gate" ("reverb cuts when its decay goes under a certain treshold", whereas the other option, as you understood, could be "reverb cuts after x seconds").

spacef
http://www.spacef.com

[ This Message was edited by: spacef on 2001-07-19 23:16 ]

madmod · Post by **madmod** » Tue Sep 04, 2001 8:08 am

Is there a place when those pictures are
stored i would love to see more

plz

[ This Message was edited by: madmod on 2001-09-04 09:11 ]

Nikko · Post by **Nikko** » Tue Sep 04, 2001 1:39 pm

Dear Folk, I've released the Verb GOLD.

I think that some explanation on how it works could be interesting for all, first there is a very bad picture of its design here http://www.dspdev.com/products_verbgold.asp

I think that COMB filters and ALL pass Filters tanks are both worth. They do not give the same result.

The GoldVerb do not uses any CombFilters. But AllPass needs to be encapsuled in a Feedback delay network otherwise you loose too much of you original signal dynamics.

The goal of the FDN is to keep the signal and re-inject it as much as possible in order to have the best density.

In the Verb GOld I've used 3 FND in parallel.
- one with 2 delays (use the Del1 & del2 button to play with them)
- one with the Vtube + a Waveshaper (the preset "Distorsion" boots the waveshaper for fun.
- one with several filters (Filter button) and a panning effect.

When I was studying the Lexicon (I've sampled almost all their presets), I've came to thses conclusions :
- they use a panning effect at the end of the chain.
- they use a flanger that they sync with the delay length to obtain this kind of doppler effect.
- they boost the bass and the treble.
- they use a waveshaper on the bass and on the treble to give the bass a "fire burning" like sound and to the treble the smooth that is not naturaly included in the original sound.

Even if I've not the same sound in the Verb Gold (it is not a copy and it was not the idea), the results can be very similar.

The difficulty with a FND is the feedback loop.
To have a very warm sound, you must maximize the feedback and stay at the limit of larsen. That's why each preset must be tunned internaly carefully.

Several other effects must be included in order to create more harmonics and more effects, flanger, and chorus are in play.

Finaly a Reverb is very close to a multi effect included in a feedback loop

rpd · Post by **rpd** » Thu Nov 01, 2001 6:57 am

There will be a Reverb available soon, that will make you forget all the other stuff...

STAY TUNED !!!

EarlyFirst · Post by **EarlyFirst** » Fri Dec 07, 2001 2:37 pm

Yup on Monday the PT-FREE-96K will be released

jupiter8 · Post by **jupiter8** » Tue Apr 09, 2002 4:52 am

I want to know what happened to the Lexicon PCM 91 killer. Was it ever finished?