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 Tips and Tricks: Unnatural Hall Reverbs

In one of my previous posts, I described the characteristics of “real-world” concert halls, and how to emulate them with ValhallaRoom. In general, real concert halls have a fairly fast onset of reverberation, a decay time between 1.6 and 2.1 seconds, and a somewhat longer decay time at low frequencies than at mid-frequencies. A realistic emulation would use subtle amounts of modulation, in order to create the perception of a high modal density without pitch change.

This is all fine and dandy for real concert halls, but what about unrealistic concert halls? The earliest commercial reverbs, the EMT250 and the Lexicon 224, were both created in order to emulate concert halls. With the limited amount of memory available for delay lines, both of these reverbs turned to large amounts of time variation in order to avoid metallic decays. The sonic results were big, washy, chorused decays that could stretch to 70 seconds and beyond. No “real” concert hall sounds like this – but it is a great sound. In addition, hardware units like the 224, 224XL and 480L had the ability to artificially elongate the attack portion of the reverb, such that the reverb sound would “fade in” to a much greater degree than a real hall. Not realistic, but useful for creating a bit of separation between an input signal and the reverb.

Fortunately, ValhallaRoom excels at unnatural halls, in addition to emulating the “real thing.” A few tips for dialing in an unnatural, vintage digital hall sound:

  • Set DECAY to whatever feels right. Many of the “Concert Hall” presets of classic digital reverbs have decay times of 6 seconds and up.
  • Use the LATE Bass Mult to dial in the required clarity of the decay. Setting this <1.0X will result in a reverb that gets more trebly as it decays, which can be a nice sound.
  • Turn up the modulation depth! Both Early Mod Depth and Late Mod Depth can be cranked up for that seasick decay. For less obvious detunings, use lower Mod Depth settings, but higher Mod Rates.
  • The Bright Hall reverb mode can get much deeper and random modulations than the other modes.
  • Set Early Send to 100%, Early Size to >100 msec, and DEPTH to 100%. The Depth control in early Lexicon reverbs, and the Shape/Spread controls in later Lexicons, allowed the user to dial in a slow attack on the reverb. By running the Early reverb into the Late reverb and using a large Early Size setting, you can create a reverb that fades in at the rate determined by the Early Size.
  • The Early Diffusion control can be set to lower values, to emulate the grainy sound that many of the early reverbs had during the initial attack phase.
  • Set HIGH CUT to somewhere between 5000 and 8000 Hz to simulate the dark sound of early, low-sampling rate reverbs.

The following preset implements a big digital hall sound:

<ValhallaRoom pluginVersion="1.0.5" presetName="BigDigitalHall" mix="0.425000012" predelay="0.0240000002" decay="0.0730730742" HighCut="0.44697988" earlyLateMix="1" lateSize="0.730000019" lateCross="0.709999979" lateModRate="0.155555561" lateModDepth="0.460000008" RTBassMultiply="0.166666672" RTXover="0.0787878782" RTHighMultiply="0.377777785" RTHighXover="0.474496633" earlySize="0.255655646" earlyCross="0.0299999993" earlyModRate="0.143434346" earlyModDepth="0.370000005" earlySend="1" diffusion="1" type="0.25"/>

Den (from the Gearslutz forum) posted this preset, reminiscent of the sounds of the Lexicon 224 Concert Hall algorithm:

<ValhallaRoom pluginVersion="1.0.1" presetName="BladeRunner" mix="1" predelay="0" decay="0.150750756" HighCut="0.294630885" earlyLateMix="1" lateSize="0.949999988" lateCross="0.150000006" lateModRate="0.191919193" lateModDepth="0.200000003" RTBassMultiply="0.666666687" RTXover="0.0494949482" RTHighMultiply="0.25555557" RTHighXover="0.289261758" earlySize="0.235135138" earlyCross="0.0199999996" earlyModRate="0.169696972" earlyModDepth="0.129999995" earlySend="1" diffusion="1" type="0.25"/>

A quick example of piano played through the above setting (piano played by Den):

Here’s a really big “concert hall” setting:

<ValhallaRoom pluginVersion="1.0.5" presetName="VideoIntro" mix="0.507000029" predelay="0" decay="0.300000012" HighCut="0.344966441" earlyLateMix="1" lateSize="0.579999983" lateCross="0.150000006" lateModRate="0.088888891" lateModDepth="0.280000001" RTBassMultiply="0" RTXover="0.113131315" RTHighMultiply="0" RTHighXover="0.2966443" earlySize="0.0440440439" earlyCross="0.0199999996" earlyModRate="0.169696972" earlyModDepth="0.129999995" earlySend="0" diffusion="1" type="0.25"/>

A simple electric guitar phrase (which was used on Gearslutz to test many reverb algorithms) through ValhallaRoom with the above settings:

UPDATE 1/2012: I just reread this blog post from last year (thanks to a link-back from an interesting blog post at The DIY Recordist). It is worth noting that ValhallaRoom has several new reverb modes that are well suited for emulating vintage digital halls. DarkRoom, Nostromo and Narcissus are reverb modes that are designed to have the “dark” coloration of the older reverbs, as well as a more gradual onset of echo density, noisier interpolation (to emulate the reduced bit width coefficients of the older boxes), and heavy randomized modulation.