ValhallaRoom: The Early Controls

ValhallaRoom can be viewed as having two separate sections for the reverberation, Early and Late. The Early controls can be accessed by pressing the Early button in the top of the plugin (under the VALHALLA ROOM text):

The Early section can be heard in isolation by setting the DEPTH slider to 0%. Higher DEPTH settings will crossfade between the Early and Late reverberation, with a setting of 100% resulting in the output coming entirely from the Late reverberation.

A quick overview of the controls:

  • Early Size: adjusts the length of the Early energy impulse, in milliseconds. A length of 10 to 50 milliseconds is useful for simulating the early reflections found in a smaller acoustic space, or for creating a wider stereo image without a strong reverberant decay (when DEPTH is set to 0%). Early Size settings between 50 and 100 milliseconds can create the impression of a “compressed” room, where the initial attack is squashed by a limiter or tape saturation. Settings above 100 milliseconds will sound like a “gated” reverb in isolation, and can create a slower attack when the Early energy is sent to the Late reverb (see Early Send for details). Larger Size settings are useful for creating long gated reverbs.
  • Early Cross: controls the stereo cross-mixing of the Early Energy. An Early Cross setting of 0% will result in no mixing of energy between left and right channels; a signal in the left channel will not generate any early reflections in the right channel. Higher settings will mix the energy between the left and right channels, and will also increase the echo density. Generally speaking, lower settings of Early Cross are useful in preserving the spatial imaging of a mix.
  • Mod Rate: Controls the base modulation frequency of the Early chorusing, in Hertz. This is an “average” rate, as there is a fair amount of randomization used for the chorusing. Values around 0.25 to 0.5 Hz are useful for warming up the sound, while around 1 to 2 Hz is useful for adding a bit of “string ensemble” effect to the sound.
  • Mod Depth: Controls the depth of the Early chorusing. In general, you will want to keep this low when creating realistic smaller spaces, and turn it up when creating large spaces or emulating older digital reverbs.
  • Early Send: Controls how much of the Early reverberation is fed into the Late reverb. The Early reverberation has been designed to interface nicely with the Late reverberation, such that sending the Early signal into the Late reverb produces a relatively colorless enhancement of echo density. A value of 0 corresponds to no send (i.e. the Early and Late reverbs are purely in parallel) while 1.0 results in the maximum send to the Late reverb. A few suggested settings:
    • Set Early Size to 10-50 msec, and Early Send to 1.0, to create a dense Late reverb.
    • Set Early Size to 150 msec or more, and Early Send to 1.0, to create a slow onset to the Late reverberation. This is useful for simulating very large halls and cathedrals, as well as adding clarity to the input signal.
    • Set Early Size to 70-100 msec, Early Send to 0.0, the Late Size to a smaller value, and Depth to 0.5, in order to simulate a smaller space that has a somewhat compressed attack.
  • Diffusion: Controls the echo density of the Early reverb. Low settings result in a less dense Early reverb, while higher settings increase the echo density. Unlike many algorithmic reverbs, high Diffusion settings will not result in a metallic decay with vocals and drums, so feel free to keep Diffusion set at 1.0 for most purposes. Low settings of Diffusion, combined with larger Early Size settings, can be useful in creating strange multitap echo sounds.

ValhallaRoom: The Reverb Modes

One of the key features of ValhallaRoom is that it is not based around a single reverberation algorithm. Instead, 4 different algorithms are provided, each with their own distinctive sonic characteristics. The algorithms are selected by the REVERB MODE combo box. A quick description of the 4 modes:

  • Large Room. This algorithm was designed to be the most “natural” of the 4 reverberation algorithms. Initially, the decay is a bit sparse, but it quickly builds to a high reflection density. The Large Room algorithm has a very exponential decay, with precise control over the frequency balance of the decay over time. There is a slight amount of high frequency absorption that is inherent to the Large Room algorithm, which reflects the effects of air on high frequencies. The modulation in the Large Room algorithm is designed to create a wide stereo image, but without causing random pitch shifts in the decay. Use of higher modulation depths will result in a “deeper” sound that still retains the original pitch of the input, while shallower modulation depths are useful in obtaining a natural sounding decay.
  • Medium Room. This algorithm emulates a room with walls that are somewhat wider than Large Room – more of a “square” geometry as opposed to the “shoebox” reflection patterns of Large Room. The initial echo density is the sparsest of the 4 algorithms, and audible echos can be heard at the largest settings of Late Size. This also results in a wide stereo spread, and can be useful for certain sounds. The modulation is more random than Large Room, and can cause some random pitch shifts for long decays – which is also a characteristic of many of the “classic” reverb algorithms as found in the early Lexicon boxes.
  • Bright Room: This algorithm has a slightly slower attack time than the other reverberation algorithm. This slow attack time is a characteristic of larger halls, but can also be heard in some of the “classic” digital reverbs, as well as in ValhallaShimmer (although the attack time of Bright Room is much faster than the attack times of the ValhallaShimmer algorithms). The decay of the Bright Room algorithm has more high frequency “air” and “sheen” than the other algorithms – it is fair to say that Bright Room is more “digital” than the other algorithms, in a good way. The modulation in Bright Room is random, complex, and deep, with the goal to provide lush chorusing to any input signal. At some settings, it can sound close to string ensembles, and for long decays, it turns static input sounds into evolving pads. Bright Room is the algorithm I would initially turn to for emulating those Vangelis “Blade Runner” reverbs.
  • Large Chamber: This algorithm shares many characteristics with Large Room, but has a much more “even” early echo density. Instead of emulating the initial echo sparseness of most rooms, it starts off with a very high echo density, similar to the high echo densities found in echo chambers and similar spaces (I’ve spent a lot of time clapping my hands in multi-story concrete stairwells and listening to the decay). The goal with Large Chamber was to get a sound that was “larger than life,” with the echo density of a small space, but the modal density of a large hall. The result is a fairly “colorless” reverb that suits a wide variety of input sounds, and can be used to emulate chambers, rooms, halls, what have you, without imparting a specific “space” on the sounds. You hear the decay, not the walls. The modulation and decay time are similar to Large Room, with the difference that the initial “detuning” of the decay is far more diffuse than the Large Room algorithm.

ValhallaRoom: The High Level Sliders

The ValhallaRoom GUI is divided into several sections. The sliders in the left part of the GUI represent the high level parameters, in that they have the most dramatic and immediate effects on the sound.

From left to right, the parameters are:

  • MIX: The mix slider controls the balance of the dry input signal to the “wet” reverberated signal, expressed as a percentage. 0% represents a fully dry signal, 100% is fully wet. The Mix slider uses a sine/cosine crossfade, such that the signal is balanced in volume at all settings of Mix.
  • PREDELAY: The standard predelay control, which delays the onset of the Early and Late reverberation. The delay time is expressed in milliseconds. Predelay is useful in establishing the “size” of the room, in that the first reflections won’t be heard until after the predelay time. It can be viewed as moving the “walls” of the space in and out. From a non-physical perspective, the Predelay control is used to add some “space” between the input signal and the onset of the reverb decay, which can add clarity to the source signal. In the olden days, tape echos were used to add 15 to 30 milliseconds of predelay to the input of a reverberation chamber or plate.
  • DECAY: The high level decay control for the Late reverberation. The Decay control could also be labeled “RT60″ which stands for the time (T) needed for the reverb (R) to decay to a level 1/1oooth of the initial level (-60 dB). The decay time is based on the mid frequencies – the Bass Mult and High Mult controls in the Late section are used to dial in the decay times at low and high frequencies, which will be explained in more detail later.
  • HIGH CUT: Controls the cutoff frequency of a -12dB/oct lowpass filter, with the units measured in Hertz. The relatively steep slope of this filter creates a more “natural” sound in the high frequencies, which reflects the air and wall absorption found in real acoustic spaces. Setting this to a range between 3000 and 7500 Hz is optimal for most larger rooms, while higher settings of High Cut are useful for emulating chambers and plates, as well as the brighter sounds of many digital reverberators.
  • DEPTH: Controls the balance between the Early and Late reverb sections, measured as a percentage. 0% represents a signal that is only from the Early Reverb, while 100% represents a signal that is all Late reverb. The Depth control uses a sine/cosine crossfade. In addition, a great deal of effort went into “normalizing” the levels of the Early and Late reverb sections, such that the output levels are balanced over virtually the whole Decay range. This control was called “Depth” as a tribute to some of the classic digital reverberators such as the Lexicon 224 and Eventide SP2016, which featured Depth controls that adjusted the balance between early and late reverberation. The effect simulates moving a microphone further away from the source, or (more accurately) controlling the mix between close mikes and room mikes. The Depth control could have been called the “Early/Late Mix” control, but I didn’t like how that looked in the UI.

In addition to the high level sliders, the left part of the GUI contains the Reverb Mode selector, which will be the subject of the next blog post.

Reverbs: Diffusion, allpass delays, and metallic artifacts

One of the most common controls found in reverberation algorithms is the Diffusion control. This is usually described as increasing the echo density, either the initial echo density (for Lexicon algorithms) or the rate at which echo density builds over time. The manual for the Lexicon LXP-15 has a somewhat typical description of the Diffusion parameter:

Diffusion: Controls the degree to which initial echo density increases over time. High settings of Diffusion result in high initial buildup of echo density; low settings cause low initial buildup. Echo density is affected by Size — smaller spaces will sound denser. To enhance percussion, use high settings of Diffusion. For clearer and more natural vocals, mixes and piano music, use low or moderate Diffusion settings.

If you read a lot of manuals for reverb products, you will often see similar descriptions of the Diffusion control, as well as the recommendation to use lower settings of Diffusion for clearer vocals. But why is this? A real room or hall tends to start with very high levels of diffusion, due to the objects typically found in the space – chairs, furniture, intricate wall patterns, etc. It would seem that a given echo density should be a characteristic of the space, not of the signal being sent into that space.

The answer lies in the signal processing tricks used to generate the initial high echo density. Manfred Schroeder, in his seminal 1962 AES paper “Natural Sounding Artificial Reverberation,” discusses using very short feedback delay lines in series to increase the echo density. Schroeder developed a very clever feedback/feedforward trick, such that the resulting delay line has a “flat” frequency response. The resulting delay unit is referred to as an allpass delay:

In the late 1970′s, James Moorer published an optimized version of the allpass delay, which used less multiplies, and is more commonly used today:

The earliest commercial digital reverbs, such as the EMT-250 and Lexicon 224, made use of several series allpasses at the inputs of the reverberation algorithms to increase the echo density. Lexicon was the first company to allow the user to directly control the coefficients of the input allpasses, and labeled this the “Diffusion” control.  The practice quickly spread through the audio industry.

EDIT: Chuck Zwicky, in a comment to this post, points out that the Diffusion parameter wasn’t originally present in the Lexicon 224, but was introduced with the Version 4.0 software. He also points out that most of the successful early reverberators up to 1984 did not have adjustable diffusion. The Eventide SP2016 had adjustable diffusion for some of their reverb algorithms, but this would have been around the 1984 to 1985 time frame.

The problem with generating echo density through series allpass delays stems from the definition of “allpass.” An allpass system will pass all frequencies with equal amplitude, over time. There is no guarantee when a given frequency will make its way out of the allpass delay. In practice, allpass delays don’t sound flat. Much like comb filters, a short impulsive sound sent through an allpass delay will result in a “ringing” sound, where only certain frequencies are resonating. Run an impulsive signal through several short allpass delays in series, and the result is a metallic decaying sound.

For percussive instruments, the metallic coloration might be an acceptable tradeoff, versus the “chattering” sound that occurs when the initial echo density is too low. Plus, snare drums have a metallic coloration in their own right, so a bit more coloration is OK. For vocals, the coloration produced by short allpass delays can be very unpleasant. Even though vocals are usually perceived as a “smooth” or continuous signal, the actual waveform produced by the glottis is very pulse-like, and can cause short series allpasses to ring out. This is especially audible on male vocals.

Some of the possible solutions to the issues with series allpasses:

  • Embrace the metallic coloration, use a bunch of series allpasses, and call the resulting algorithm a plate reverb. This is a fairly common approach, with most of the “plate” algorithms having very little to do with a physical plate, so much as having a lot of initial echo density and a somewhat metallic sound.
  • Use fewer series allpasses at the input. This works in eliminating coloration, but can result in a lower initial echo density. Many “hall” algorithms use this trick.
  • Use a larger number of series allpasses, with the idea being that the larger number of resonances will end up smearing out the metallic sound. This works, but a side effect of cascading a larger number of series allpasses is that the attack time can be extended to the point where the sound seems to “fade in.” This is a great sound if you like it, but doesn’t work for small room simulation.
  • Modulate the delay lengths within the allpasses. For longer allpasses, this helps reduce coloration. For the short allpasses used in the input diffusion section, this ends up producing too audible of a chorusing sound, or a sound similar to water sloshing around in a metal pan.
  • Reduce the coefficients of the allpass delays. This will reduce coloration, but will also reduce the echo density.

This is where the Diffusion control comes in. Instead of being a compromise solution that works OK for all signals and not great for any signal, it allows the user to adjust the algorithm to suit the input signal. It places the burden of balancing echo density and coloration on the end user, instead of on the algorithm designer. By knowing how the Diffusion control works, the end user can make their reverbs work better for them.

Is this an ideal solution? Probably not. But in the limited hardware processors of the late 1970′s, or the low-CPU plugins of today, it can be a reasonably effective solution.

EDIT #2: ValhallaRoom uses some clever signal processing tricks to avoid the issues associated with series allpass delays described above. A high level explanation of the Early Reverb section of ValhallaRoom can be found here. Even though ValhallaRoom has a Diffusion control, it is not being used to control allpass coefficients – the Early Reverb has no allpasses in it.

EDIT#3: ValhallaShimmer is built around a large number of cascaded, modulated allpass delays, and the artifacts that are generated by such a structure (see this blog post for more details). In addition, many of the “classic” digital reverbs relied heavily on series allpasses, so it isn’t to say that they produce a sound that is unusable – just that this sound isn’t necessarily reflective of what is found in a “real” acoustic space.

ValhallaShimmer Tips and Tricks: Chorus

ValhallaShimmer was primarily designed as a reverberator. However, it can also get some cool chorus sounds. The Chorus preset is a good place to start. Some general tips:

  • Set Size as low as possible. This will keep the reverberant quality to a minimum
  • Use one of the smaller reverbMode settings. The Chorus preset uses smallStereo, but mediumStereo can be used for a more diffuse, washy chorus.
  • Set Diffusion up around halfway to start with, and go from there. Too high of a Diffusion setting will result in more of a small room sound, but this might be what you are looking for.
  • modDepth should be set to about 0.5 for starters, and modRate should be adjusted to taste.
  • Note that higher settings of Diffusion result in more pitch change for given settings of modRate/modDepth, so you may want to turn down Diffusion and/or modDepth if things get out of hand.
  • The bright colorMode will result in a full-bandwidth chorus signal, while the dark colorMode will be closer to the older BBD based choruses.
  • Use highCut to control the overall brightness.
  • lowCut can be used to shave away the low frequencies, which can add clarity to a chorused signal.
  • Feedback should be left at 0 for standard chorusing. Turning it up with the above settings will result in a very metallic sound, which, again, might be exactly what you are looking for.

ValhallaShimmer Tips and Tricks: Using Diffusion instead of predelay

A number of users have asked why there is no pre-delay parameter in ValhallaShimmer. I decided to exclude a predelay parameter, partly due to the desire to keep the UI as simple as possible, but mainly because I feel that the Diffusion parameter can be used to serve a similar function: to create a sense of separation between the source signal and the reverbed signal.

Here’s a quick tutorial in adjusting Diffusion to create the proper separation/blending between dry and wet signals:

  • Create the desired reverb “size” and decay, through the use of ReverbMode, Size, Diffusion, and Feedback, as described in an earlier tips and tricks entry.
  • Gradually back down on the value of Diffusion. Remember that a value of 0.9 and above will result in a fast attack for the reverb envelope, while a Diffusion value of 0.5-0.618 will result in the reverb fading in very slowly. Setting the value somewhere in between will create an attack that isn’t instantaneous, and that will sit behind the dry signal in a way that is similar to how predelay is often used in reverbs.
  • Listen to the decay after adjusting Diffusion. If it is shorter than desired, turn up Feedback to get the desired decay rate. You can also adjust Size, but this will also affect the fade-in rate.

ValhallaShimmer Tips and Tricks: Shimmering

ValhallaShimmer was designed to get a variety of big reverb sounds, with the option of adding pitch shifted feedback to the decay. The “Shimmer” in the title refers to the classic shimmer effect, as used by U2, Brian Eno, Daniel Lanois, Coldplay, etc. There are a few presets that ship with ValhallaShimmer which reproduce this effect, but if you want to dial in your own version, here’s some tips:

  • Use the mediumStereo or bigStereo reverb modes for the smoothest shimmer sounds. The mono reverb mode will have a stronger sense of pitch shifting in the feedback signal, while the other modes have a gentler onset of the pitch shifting.
  • Set the Feedback control for the desired amount of pitch shift in the output signal, and then use the Size control to dial in the decay.
  • The Pitch control should be at +12 semitones.
  • Diffusion works best at around 0.9 for reverberant sounds. If you set Diffusion < 0.5, it will sound closer to a pitch shifted echo, which is another cool sound.
  • The different pitch shift modes have different levels of “smoothness”:
    • The single and dual pitch shift modes have more noisiness in their decay. This is better for emulating the orchestral sounds as heard in “Deep Blue Day.”
    • The singleReverse and dualReverse pitch shift modes are much smoother, and are better for organ-esque sounds.
  • colorMode should be set to dark. This produces a natural roll-off of high frequencies, which eliminates almost all of the aliasing noise in the feedback path of the pitch shifter.
  • Set the modDepth control to a fairly low value at first, as the pitch shifting provides its own random modulation to the signal.

ValhallaShimmer Tips and Tricks: Bloom

The Alesis Midiverb II featured a unique reverb algorithm, which Keith Barr labeled “Bloom.” From the Midiverb II manual:

Programs 45 and 49 are extraordinary variations on the reverse reverb theme. They are exclusive Alesis programs that are unobtainable in any affordable signal processors other than MV II. They are named Bloom and have an envelope that rises (blooms) to a rich and highly diffuse reverb with a smooth decay. These are the ultimate for ethereal effects and long, slow introspective musical passages.

The architecture of ValhallaShimmer lends itself nicely towards emulating the Bloom algorithms of the Midiverb II. A screenshot of the settings:

The key to the emulation is to combine Diffusion of 0.5 with a fairly high feedback setting, and use the reverb mode and size control to tune the attack time of the reverb.

  • In the above example, the mediumStereo mode is being used, as it has a fairly lengthy attack with this diffusion setting. This will sound the closest to the original Midiverb II Bloom algorithms.
  • The bigStereo mode can give you super long attacks, as will the mono mode.
  • The smallStereo mode doesn’t have enough density to get this effect – it will sound like a diffuse echo with a Diffusion setting of 0.5.

The Midiverb II Bloom algorithms didn’t use modulated delays, probably due to cost restrictions of the hardware. Adding modulation to the above settings results in a beautiful, washy soundscape, that works well for ambient synthesizer and guitar.

Keith Barr talked about using allpass coefficients of 0.5 in the original Midiverb (due to this being an easy value to get with bit shifts), and that people complained about the reverb taking too long to build. My guess is that Barr created the Bloom algorithms for the Midiverb II in order to take advantage of this “flaw.” Apparently My Bloody Valentine used the Bloom patches a lot – recent photos of Kevin Shields’ rig show two Midiverb IIs in there.

ValhallaShimmer Tips and Tricks: Adjusting the reverb envelope

In order to dial in the desired reverb characteristics while using ValhallaShimmer, it helps to understand how the Feedback, Diffusion, Size, and Reverb Mode parameters work together:

  • The Feedback parameter controls how much of the output signal is fed back into the inputs. If ValhallaShimmer is viewed as a delay line (and it is far more complicated than that, but with Diffusion at zero this is a fair approximation), the Feedback parameter controls the number of repeating echos before the signal decays to inaudibility.
  • The Feedback parameter also directly affects the perceived intensity of the pitch shifted signal when the Pitch Shift mode is not set to bypass. A higher Feedback setting will result in a more intense pitch shifted sound.
  • The Size control changes the overall delay length(s) in ValhallaShimmer. A larger setting of Size will result in longer delay lines, which results in a longer time for the echos generated by the Feedback parameter to decay away.
  • The Diffusion parameter adds echos to the “delay line” at the heart of ValhallaShimmer. The echos increase with each feedback pass through the network, so combining Feedback with Diffusion results in echos building exponentially in density, until the signal is no longer perceived as discrete echos, but as a reverberant decay. Higher settings of Diffusion result in the echo density building up more quickly with a given Feedback setting.
  • The Diffusion parameter can also add its own reverb decay, even without any Feedback being used. If Diffusion is set around 0.9, the result will be a reverb sound that is considerably longer than the delay length would be without any Diffusion. Applying Feedback to this network will result in a much longer reverb than the same Feedback setting with a lower Diffusion setting.
  • The Reverb Mode parameter has a global effect on the lengths of the delay lines, as well as the density of the echos produced by the Diffusion parameter. The larger the Reverb Mode, the longer the delay lines, and the higher the density for a given setting of Diffusion.

Given that the controls have a fair amount of interaction with each other, there is no one method to get a reverb decay of a given length. The user can decide if it is best to use a larger Size in conjunction with a smaller Feedback setting, or to rely on high Diffusion settings and less Feedback, and so on. ValhallaShimmer is meant to encourage exploration on the part of the user!

An example approach:

  • Start with the Reverb Mode parameter. mediumStereo is best for halls and other “kinda” large spaces. bigStereo and mono are huge, and are a good starting point for very long ambiences. smallStereo is best for short ambiences, spring reverbs, and other sounds that are more metallic.
  • Next, set the Diffusion parameter for the desired attack. Low values will start off as echos that slowly build to reverbs, values around 0.5 to 0.618 will cause the reverb to slowly fade in, and values between 0.8 and 0.91 will have a relatively quick attack.
  • The Size parameter can be used to adjust the precise attack time of the reverb, as well as the amount of “color.”
  • Once the attack time is dialed in with Diffusion and Size, use Feedback to get the desired decay time.
  • After this, adjust the tone controls and modulation controls to taste.

ValhallaShimmer Tips and Tricks: Diffusion

In the next week, I will be posting some Tips and Tricks for ValhallaShimmer, to help users dial in the sounds they want. The Diffusion parameter is a great place to begin, as it is one of the most powerful parameters in ValhallaShimmer.

I’ll start from more of a “meta” perspective, and quote Wikipedia: Diffusion, in acoustics and architectural engineering, is the efficacy by which sound energy is spread evenly in a given environment. In the case of ValhallaShimmer, the environment is the plugin. The Diffusion control allows the user to shape how energy from an input signal leaves the plugin. At low settings of diffusion, all of the energy that enters the plugin leaves it at about the same time (i.e. a single delay line). At higher settings of diffusion, the energy is more spread out over time and frequency, with randomization of phase. This is perceived by the listener as sounding reverberant.

From a less physics oriented perspective, the Diffusion control is used to adjust the degree of echo density in ValhallaShimmer. At its lowest setting, Shimmer acts like a delay line (actually 2 delay lines, one per channel, with slightly different lengths). Turn up the feedback for the stereo modes, and the plugin acts like a ping-pong delay.

Once you turn up Diffusion beyond 0.0, the echo density starts to increase. For settings around 0.2 or lower, the increased echo density is fairly subtle for a single repeat of the delay. If you increase the Feedback setting, the first repeat will sound close to a single echo, but later repeats will sound more and more reverberant.

At Diffusion settings around 0.5, the signal will have a fairly high amount of echo density. However, the energy will still be distributed in such a way that short repeats will still have an audible repeating echo. Setting Diffusion at 0.5, and Feedback at 0.5 or greater, is a great way to get the “Bloom” sound from the Midiverb II.

A Diffusion setting of 0.618 has some very unique characteristics. 0.618 is known as Phi (Φ), or 1/goldenRatio. With diffusion set at 0.618, the attack and decay of the reverb signal have the same lengths. The reverb response looks like a Gaussian, or bell curve. This produces a neat “backwards” reverb sound.

If Diffusion is set to values greater than 0.618, the decay characteristics start becoming closer to an exponential decay. The reverb will still “fade in” to a greater or lesser extent, but the decay will be far longer than the attack. In addition, the decay time itself increases. With a Diffusion setting of 0.8 to 0.91, you can get some very long reverb decays, even with Feedback gains of 0.0. Turn up the Feedback, and the reverb decay time gets closer and closer to infinity.

Diffusion settings above 0.91 start to sound weird. The decay lasts longer and longer, but the reverb itself gets quieter and quieter. The explanation for this is that the Diffusion control doesn’t add any energy to the signal – it just redistributes it. So the super long decays come at the expense of quieter signals, as the reverb is taking the same energy and spreading it too thin. This is why the Feedback control is useful for long signals, as it adds gain to the system.

A few other quirks about the Diffusion control in ValhallaShimmer:

  • For higher levels of Diffusion, the sound can become somewhat more metallic, especially for smaller settings of Size. This is fairly common with reverbs that use diffusor sections as their main building blocks. Modulation is the quickest way of reducing metallic artifacts. Another way of getting a similar reverb time with less metallic coloration is to turn Diffusion down, and Feedback up.
  • Higher settings of Diffusion also increase the perceived chorusing in the algorithm (as will smaller settings of Size with the same modulation speed/width).
  • The Diffusion control is smoothed, in order to avoid clicks when changing the Diffusion amount.