Algorithmic Reverbs, Distortion, and Noise

Over on Gearslutz, AstralPStudios asked an interesting question:

When it comes to reverb, is there close relation to distortion in some ways? I guess when I’m listening to big verbs with high decay times it makes me wonder that because at times a big, wet verb can have some distortion-type characteristics.

The distortion observation is a good one, as this can often be heard with very long decays in algorithmic reverbs. In some cases, this is due to the high signal levels of the output. Increasing the decay time means turning up the internal feedback gains, and this often results in an output sound with a very high RMS level. However, even if the output level is reduced to a level that isn’t clipping within the DAW, sometimes various forms of distortion and noise can be heard. This can be explained as a consequence of the feedback nature of the algorithms, and how feedback causes the characteristics of the loop to be magnified exponentially.

Algorithmic reverbs tend to be made up of short delay lines, with LOTS of feedback applied. A good rule of thumb is that the average delay lines in a reverb are 0.1 seconds or shorter in length. In order to get a 30 second decay out of the reverb, the sound will be going around those loops (30.0/0.1) times…or 300 times. So any little quirk in the feedback path, like interpolation noise or fixed point distortion, will be increased on each pass through the feedback loop.

The earliest “long” electronic reverb sounds were based around tape delays, possibly running into a spring or plate reverb. Listen to “Creation du Monde” by Vangelis for an amazing “reverb” sound that comes from 3 RE-201 Space Echos:

The wow and flutter of the tape units, and the distortion added by tape saturation, results in a sound that is far more ethereal than 3 clean digital delay loops run in series or parallel.

The Lexicon 224 is an early digital reverb that was well known for its spacey, extra long decays (up to 70 seconds). Again, Vangelis was an early pioneer of the extra long decays using this unit, most famously in the Blade Runner soundtrack:

The Lexicon 224 used a fixed point processor, with a 16-bit word size, and a 20-bit saturating accumulator. Translated into English, this means that the 224 had about 24 dB of headroom internally (4X the max input volume) before things started clipping. HARD, digital clipping. The allpass delays used by the 224 also had fairly high internal gains, especially with long reverb times. This results in clipping at various points in the digital reverb network. Strangely enough, adding clipping into the nodes of a Lexicon-style reverb network doesn’t end up producing the classic tape echo runaway feedback at most settings. Instead, it tends to produce a somewhat higher noise floor with broadband audio signals (i.e. music as opposed to test signals).

The modulation in the Lexicon 224 also produced distortion, but of a different sort. The 224 had a quirky multiplier, which resulted in the linear interpolation being quantized to fairly big subsample chunks (I think it was 32 or 64 chunks per sample). This resulted in a “halo” of noise around reverbed signals, where the noise was less signal dependent than the clipping noise described above. The linear interpolation also results in attenuated high frequencies above 1/2 the sampling rate. Since the sampling rate in the 224 was pretty low in the first place, the attenuated high frequencies were quite audible with long decays. The noise and high frequency attenuation will increase every time it passes through the reverb network, and a 70 second decay time results in a few hundred passes through the allpass delays.

EDIT (1/24/2013): Since I wrote the above, I have spent a lot of time studying the Lexicon modulation. I think that the modulation source in the older Lexicons had a lot more to do with the noise performance than I had originally thought. The modulation in my PCM70 is noisy as all get out. The results of my studies can be heard in my latest plugin, ValhallaVintageVerb. In the 1970s and 1980s modes, I make use of this noisy modulation, which can really be heard on long sustained synth notes going through longer reverbs. The Concert Hall and Bright Hall algorithms are the best showcases of these “old school” artifacts.

Today, reverb plugins are usually programmed with floating point math. 32 bit floating point avoids most of the distortion issues found in the older fixed point processors, and 64-bit floating point can be used if absolute precision is desired. Generally speaking, 64-bit math is useful for very low frequency higher order filters, and for precise control of attack/decay times for dynamics processing – otherwise, 32-bit floating point has an amazingly high SNR. In addition, the last 3o+ years have seen a great deal of progress in delay interpolation techniques, so you can have modulated delay lines with lower SNR and less high frequency attenuation. If a DSP engineer wants to throw a bunch of cycles at their algorithms, they can come up with very clean reverbs, even with extremely long decays.

At Valhalla DSP, our philosophy is that a little noise isn’t necessarily a bad thing, and that the artifacts of the older reverbs often added some nice mojo to the signal. The high frequency attenuation of linear interpolation can result in a far less “glassy” high end than modern high fidelity interpolation techniques. ValhallaShimmer makes use of this in the “dark” color mode (the “bright” mode makes use of a more modern interpolation style).

The new Dark Room mode for ValhallaRoom deliberately introduces noise into the linear interpolation, and downsamples the entire reverb algorithm, to produce more audible noisy artifacts. As the noise is very signal dependent, and the algorithm is true stereo, this will result in different noise signals in the left and right channels, which increases the decorrelation of the algorithm, producing a very spacious feel. With short decays, the noise won’t be that audible, as it will only pass through the delays a few times before decaying away. With long decays, the noise floor builds up, resulting in a big, washy block of “spectral plasma,” to borrow a term from Christopher Moore. The other ValhallaRoom algorithms are fairly clean, but I felt it was important that users had the choice between modern clean and vintage dirty.

ValhallaRoom V1.0.6: Introducing Dark Room

I have just released version 1.0.6 of ValhallaRoom. You can download the demos from the usual place, and all current ValhallaRoom customers should have received a link to the updates (send me an email if you haven’t received your links yet).

In addition to a few bug fixes, version 1.0.6 introduces a new reverb mode to ValhallaRoom: Dark Room. This new reverb departs from the high fidelity path taken by the other 4 reverb modes in ValhallaRoom. It is deliberately low-fi, with noisy interpolation, no high frequencies above 11 kHz, and a late reverb that can have a low initial echo density. It also has a wide stereo image, a clear decay with lush randomized chorusing, and sits in a mix quite nicely. An added bonus is that the CPU is significantly lower than the other ValhallaRoom reverb modes.

Why add a lo-fi reverb mode to ValhallaRoom? I’m not really sure. After doing this for about 12 years, I’ve learned to follow my instincts on this stuff, even if it takes me in strange directions. Plus, I wanted to add something new to ValhallaRoom, as a way of saying thanks to all of the customers who have supported my work.

I was also inspired by some recent studies of the Lexicon 224, the EMT250, and other vintage reverbs. These early digital machines often had very noisy interpolation, sparse initial echo density (at least by modern standards), and sampling rates that seem primitive today. They also were useful for creating a “larger-than-life” sound, that is described to this day as warm and spacious. I noticed that a lot of these classic reverbs had a very limited frequency response, so I figured it would be worth adapting some of these old-school limitations to the more modern algorithm architectures found in ValhallaRoom.

Dark Room has identical controls to the other reverb modes, but produces a noticeably different initial sound. With Early Send set to 0, the Late Decay can have a marked amount of initial “flutter” or “grain,” similar to the 224 Concert Hall with the Diffusion control set low. A few usage tips:

  • The Late Size control can be used to adjust the speed of the “flutter,” with larger sizes corresponding to more obvious and slower echos.
  • By setting Early Send to 1.0, and adjusting the Early Size to 40 msec or later (depending on the Late Size), the flutter in the Late reverb can be totally eliminated. This is similar to how the Diffusion control works in older Lexicons, but with the advantage that the Early reverb has far less coloration than the series allpasses used for the diffusors in many “classic” reverbs. The Late Size can then be adjusted to get the desired stereo width – this can get really big.
  • Setting DEPTH to 1.0 results in the most “vintage” sound, while values less than 1.0 allow the user to dial in some early reflections.
  • With Early Send set to 1.0, and using larger Early Size values (>100 msec), the Late Reverb will have a slower initial attack. This is similar to how the Depth control worked on the 224 and 224X/L, as well as the Shape and Spread controls on the 480L and later reverbs.
  • The Late High Mult and Late High Xover have an effect on the initial tone of the late decay, similar to the Concert Hall algorithm on the 224XL and the Small/Large Concert Hall B on the 224. By setting High Mult to 0.1X, the user can simulate the -6dB/octave filters used on these older boxes.
  • Turn up the Early and Late Mod Depth when using Dark Room. The older algorithms used a LOT of pitch modulation to avoid metallic decays. The Dark Room algorithm uses a different architecture that is less prone to sounding metallic, but if you want that big, lush, spacey decay, modulation is a must.

Here’s a preset that can be used as a good starting point for the Dark Room reverb mode:

<ValhallaRoom pluginVersion="1.0.6" presetName="DarkStartingPoint" mix="0.289999992" predelay="0" decay="0.0265265256" HighCut="0.58523488" earlyLateMix="1" lateSize="0.720000029" lateCross="0.25999999" lateModRate="0.202020198" lateModDepth="0.430000007" RTBassMultiply="0.413333327" RTXover="0.0666666701" RTHighMultiply="0" RTHighXover="0.410067111" earlySize="0.0581581593" earlyCross="0" earlyModRate="0.0909090936" earlyModDepth="0.800000012" earlySend="1" diffusion="1" type="0.416666657"/>