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.

Feedback, anti-feedback, and complexity in time-varying systems

“For my birthday I got a humidifier and a dehumidifier. I put them in the same room and let them fight it out.” – Stephen Wright

When I was researching the Eventide H910 Harmonizer, I found it curious that the box had controls for both feedback and something called “anti-feedback.” The service manual explains the anti-feedback control as follows:

Increasing clockwise rotation of the ANTI-FEEDBACK control progressively adds a small up and down frequency shift to the output signal, which serves to decrease the effect of room resonance peaks on the signal which ultimately re-arrives at the microphone.

In modern terms, I would call this a chorus effect, with a triangle wave modulator. Pretty simple. However, it is interesting to see how such a simple process can have a significant effect in a PA system – by turning on the Anti-Feedback control, you can increase the gain of a microphone being fed into the H910.

The idea of using a time-varying system, such as pitch shifting, delay modulation, or frequency shifting, to increase the maximum gain of a system before oscillation occurs, dates back many decades. In 1962, Manfred Schroeder (of digital reverb fame) published an article in the AES Journal about using frequency shifting as a method of increasing the gain of a PA system by up to 6 dB. A picture tells a thousand words, especially if it has a bunch of words attached to it:

Schroeder also discusses what happens if the gain is turned up beyond the feedback suppression limits of the frequency shifter:

For example, when using the frequency shifter, an excessive gain announces itself by a faint but easily recognizable “growl” or “chirp.” When this sound is heard, the operator decreases the gain by one or two decibels and the system continues to operate without the audience having heard any adverse effect.

This works well for gain increases up to a certain limit, but what happens when the gain is increased well beyond that point? The answer can be found in ValhallaFreqEcho. As the feedback gain is pushed beyond a certain level, the plugin will enter into a self-oscillating region, but one that has a huge amount of complexity. By controlling the shift frequency, delay, tone controls, and feedback gain, a variety of constantly evolving patterns can be produced. The overt goal of ValhallaFreqEcho is to get those chirps and growls that Schroeder described.

The Eno/Lanois “shimmer” sound works along similar principles. Pitch shifting, in and of itself, is a useful way of avoiding oscillation, as it pushes the feedback energy into regions that are above or below the original energy in frequency. However, if you turn the feedback gain up high enough, the system will start to self-oscillate, but in a highly chaotic manner. Keeping the gain just below self-oscillation will result in a sound that slowly evolves into a huge orchestral wash, that fades away into tinkling high octaves.

From a DSP developer’s perspective, delay modulation, frequency shifting, and pitch shifting all fall under the category of time-varying systems. Conventional digital signal processing theory concerns itself with linear, time-invariant (LTI) systems. Once time-variation is introduced, conventional LTI theory falls apart. There has been some research performed on what time-variation will do in otherwise linear systems, but there is no simple answer.

In some systems, time variation will make a simple system become unstable, such that its output amplitude grows out of bounds. Reverb developers call that “blowing up,” as that is the best way to describe the sound that comes out of the speakers. However, in the systems described above, time-variation serves to make a system more stable, in that it allows for the feedback gain to be increased. The onset of oscillation in such systems is something that is usually avoided in academic DSP, but in musical audio it is an area rich for exploration.

In my next post, I will look at an early digital reverb in which the entire theory of operation was based upon the increased gain obtainable through time-variation.