That mythical parameter that “everyone” wants, but no one really knows what is. Should you even care? Below are a few quick snippets from a couple of threads to get you going. (The most useful of all though would be to grab a PCM70 and try for yourself.)

But first, let me just throw in my 2c.. I think the topic of diffusion (at least as discussed on HRI) is a classic example of how something of less importance grew into this fabled topic, and while being explained several times by different folks, the majority still didn’t get it, mostly because no one was able to actually explain it by just saying how it sounds without adding ten thousand words on the technicality behind it, and when people start using the terminology in different ways and had their own version of how it affected the sound.. well, let’s just say that a better name for the whole thing would be confusion.

The short and immediate real life explanation of diffusion on delays in the PCM70 is that the repeats appear to have a short ambience/room reverb on them instead of being totally dry. Just try it for yourself. And, should you care? No, not really. Make music instead.

Posted by Sorbz62

Sun Mar 01, 2015 11:42 pm

What is this mythical ‘Diffusion’?
………. That I keep reading about??

Posted by threeofaperfectpair

Mon Jul 27, 2009 1:55 am

From the pcm70 v2 manual:


Diffusion controls the density of echoes. High levels of diffusion thicken or smear the echoes. This is especially noticeable on material with sharp transients.

Diffusion controls the degree to which echo density increases over time. High diffusion settings result in high initial buildup of echo density, and low settings cause low initial buildup. After the initial period (in which echo buildup is controlled by diffusion) density continues to rise at a preset rate determined by the program.

Posted by cliffc8488

Mon Jul 27, 2009 3:11 pm

Diffusion is technically known as an Allpass filter. It has unity gain at all frequencies but the group delay varies with frequency. Another way to say that is that the phase response is not linear.

Nonlinear phase response causes dispersion (unequal delay vs. frequency). Dispersion is mostly noticeable on transients since transients contain all frequencies (the Fourier Transform of an impulse is equal power at all frequencies). Humans hear dispersion as a smearing of transients.

Using dispersion on delays makes the sound more diffuse. If the Allpass filter is in the feedback loop the echoes become more and more diffuse each time.

Posted by Sorbz62

Tue Mar 03, 2015 12:12 am

So to summarise, it’s just a series of very short delays with filters to simulate space, as opposed to reverbs??

Posted by italoop

Thu Jun 02, 2011 4:31 pm

DIFFUSERS are groups of 2 or 4 or even more short delays. Depending on who wrote the code, there are different diffusers structures. Sometimes their delays can be totally independent in their paths, or they have some kind of connection between them to augment their effect. Usually these delays are set between 4 and 20 milliseconds, so VERY short, and they have, like most delay have, feedback paths. By raising their feedback a thick, dense ambience is obtained. Eventide is the only company that offers modulated diffusers, basically short delays with feedback and modulation rate & depth.

Diffusers and early reflections are typically used in reverb algorithms! They are needed to simulate the environment in which reverberation takes place. By using these clusters of short delays the hearing has a perception of density caused by the “walls” of a reverberant space and early reflections typically produce a perception of a wall on which sound reflects, providing the listener position within the reverberant space.
Diffusers control the degree to which echo density increases over time within the reverb delay clusters.

Diffusers are placed in different places of the reverberant algorithms. If you take a look at the PCM80/81/90/91 users manuals, the diagrams show you that diffusers are placed before the reverb in most smaller reverb algorithms; Hall instead doesn’t have pre_diffusers but diffusion is internal to the reverb itself. This is because a huge reverberant space such as a big hall, doesn’t produce distinct perception of walls and echo attacks. All is smeared within an undefined big space.
When it comes to delays diffusion…it controls the density of echoes in the delay voices, independently of Reverb diffusion. High levels of diffusion thicken, or smear, the echoes. This is most noticeable on material with sharp transients. Your delays will sound with some ambience on them, particularly more audible on each feedback cycle.

In some PCM algorithms diffusers are shared between Chorus and Reverb algorithms so the Size parameter is important here. Size sets one room dimension that defines its volumetric size. Even though you may kill the reverb level and have no verb at all, diffusion AND size will still be active for your chorus or chorused delays.
So, it’s not a fuzz/gain effect as previously described and it’s not reproduced by hicutting delay tone at all. You need a machine that has diffusers in the feedback path ot allows the user to place that in a modular algorithm.
Some processors have only a diffusion parameter that takes care of setting delays AND their feedback.
Others have fixed feedback and diffusion only changes their feedback.
Others offer Diffusion Size and Diffusion Amount, setting respectively delays and feedback (Eventide). Or they may offer Diffusion and Size, like some Lexicon algorithms.

IMPEDANCE 101…ohms are not your enemies
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