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.

About the author:

Sean Costello is the "algorithmic reverb plugin wizard" [citation needed] at Valhalla DSP.

Comments (8)

  1. Actually I’ve been curious about to pinning down exactly what Vangelis was using prior to the 224. I’m 100% certain “Creation du Monde” and “La Mere Recomence” are half speed playback of mixed tracks done at normal speed and one can hear the actual performance and reverb at double speed. Beautiful effect but the extreme length is from the tape playback slowdown.

    I’m pretty sure the multiple Space Echo I.D. comes from photos his 1974 live show in Paris where it’s established at least some of his gear was borrowed. Also I think the interesting Nemo studios someone puts together has a later shot of 3 RE-201s. I’m not saying that’s impossible but chances are he wouldn’t be able to get a hold of 201s in Paris, 1973 for this recording though he could have gotten some earlier Space Echos. One thing that is well established from photos is he used multiple Echorec models too. For what it’s worth sometime in the 70s, besides the spring reverbs in Space Echos, he had a BX-25 and an unknown model Master Room spring with a 7 second decay

  2. This is another great article on reverb development! I would be interested to hear your take on various method of modulation in the reverb process.

    Also, have you ever studied the Roland RSS processes? The use of phase and Haas effect in conjunction with good reverb design makes for some very interesting and often disorienting spatial effects.

    Small correction: 4 bits is 24dB of headroom not 12. I’m not sure the accumulator clipping is very common in the 224, unless there was a bug in the code, as that is a lot of headroom for processing. Maybe the original had such a bug and it was removed in the XL, which some claim is not as ‘rich’ as the first 224?

    1. You’re right about the dB of the headroom – I’ve corrected it in the original post. 4 bits of headroom isn’t that much when you are dealing with nested allpasses using the DFII topology. Check out Tim Stilson’s Ph.D thesis for a description of the internal gain of DFII allpass delays, & Dattorro’s website for a 224-esque hall algorithm.

      As far as the difference between the 224 and 224XL, I think that the two processors share similar heardroom characteristics. The 224XL has a much higher sampling rate than the 224. The 224XL also appears to have introduced some new algorithm topologies (the “Rich” algorithms), but kept the 224 algorithms in somewhat modified variants.

      I’ve never studied the Roland RSS stuff. Any patents that cover the processes?

  3. I knew you’d pinpoint it for me Sean lol. So basically what I’m hearing is the repeated pass through of the SNR/feedback building up then?

    Then that leaves the question of: “Well wouldn’t changing the feedback setting (if available) reduce apparent distortion?” Similar to how a delay unit typically has a feedback setting.

    :).

    1. Well, the decay time is directly linked to feedback. So if you reduce the feedback, with all delay lengths being the same, you will reduce the decay time.

      If you want a “cleaner” sound, increase the Size control of the reverb you are using. This will increase the delay lengths (in most algorithmic reverbs), so that the feedback gains can be reduced for a given decay time.

      As far as what you are hearing, there are a lot of different things going on in a reverb. The SNR can result in strange effects, but there are other artifacts that get more obvious as you turn up the decay time. For example, all algorithmic reverbs will have an audible artifact that is linked to the sum of the delay line lengths. So if you use 1 second of total delay length, the artifact will be heard at a 1 Hz rate, unless modulation is used.

  4. Always fascinating to hear the technical behind the magical. The 224 and PCM-70 to me are what fantasy sounds like. I’m glad I bought Valhalla Room and Vintage to support your work Sean. Please keep the refinements coming as your knowledge grows.

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