I’ve been working long hours on getting the next Valhalla DSP plugin out to beta testers. Today, I decided to convert some of the parameters to use percentages instead of just going from 0.0 to 1.0. Apparently I goofed up the math. Here’s what got barfed out of the plugin:
This is NOT what the plugin is supposed to sound like. This is a nasty, brutal bug. So I’m going to include it as a mode, alongside all the non-busted modes.
To quote Brian Eno:
“Honour thy error as a hidden intention.”
Another Oblique Strategies quote, that is equally appropriate:
“Work at a different speed.”
EDIT, 9 YEARS LATER: This ended up being the OverMod control in ValhallaÜberMod. I had been working with the Mod Depth control, which works in a range from 0-100%. I forgot to scale that down to the 0.0-1.0 parameter range the plugin was expecting. The results were weird enough that we moved it to a dedicated control!
A couple of great sounding examples of the new reverb modes in ValhallaRoom, Dark Chamber and Dark Space, have been posted on SoundCloud. The first is from Elan Hickler, and goes from a distorted explosion noise storm to beautiful cinematic ambient, all processed with the new Dark Space mode in ValhallaRoom:
Next up, Simon Stockhausen has posted an example of his beautiful soprano sax playing (from one of his Kontakt sample libraries) through the Dark Space mode:
Finally, Simon Stockhausen processes an electronic drum beat (twisted by Turnado) through the Dark Chamber mode:
Note that Stockhausen is automating the reverb DECAY control, which creates a cool sounding “fade-in” of reverb energy when it goes from long decays to short ones.
ValhallaRoom has been updated to version 1.0.7. Changes since 1.0.6:
Fixed a GUI bug in older versions of Pro Tools (7.3/7.4).
Improved interpolation quality for PPC versions of ValhallaRoom
Improved “short names” in Pro Tools insert/send windows. The shortened name now shows up as “VRoom” instead of “Valh.”
Two new reverb modes: Dark Chamber and Dark Space.
A bit about the new reverb modes:
As the names suggest, they are dark. All frequencies above 1/4 the sampling rate are completely attenuated.
The modulated delays are cleaner than the deliberately dirty delay modulation used in Dark Room.
The modulation in both modes is more balanced than the modulation used in Dark Room, resulting in less random pitch shifts and more of a “detuned” decay.
Dark Chamber has a fairly even initial echo density. In this way, it is similar to the Large Chamber reverb mode, but with deeper modulation and a much bigger size.
Dark Space has a lower initial echo density, and can have a bit of a delay of the onset of reverberation with high settings of Late Size. This also results in a wide stereo image.
The links to the new ValhallaRoom version have been sent out to all VRoom customers, and new demo links have been posted to the ValhallaRoom page.
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.
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:
The 1.0.4 version of ValhallaRoom added a set of Cathedral presets, that were dialed in from published acoustic measurements of various Italian cathedrals. A few hints in dialing in a cathedral sound:
Turn DECAY up to correspond to the midrange decay rate. Gothic cathedrals can have decays up to 12 seconds long, while cathedrals from other eras tend to be smaller and have shorter decay times.
Late Size should be set to a high setting, to reflect the high modal density of these spaces.
The Early Size control should be used to generate a bit of a “fade-in” for the decay. Due to the large size of cathedrals, it takes a fair amount of time for the energy to build up in the space, which translates to a slow attack for the decay time. A good rule of thumb is to use an Early Size setting of 1/20th of the decay time (up to 500 msec or so for truly large cathedrals).
In order to get the “fade-in” effect from Early Size, set Early Send to 1.0, and DEPTH to 100%.
The Late High Mult should be set to a low value, such as 0.1x. The sheer volume of air contained in a cathedral adds a huge amount of high energy loss.
Set the HIGH CUT and Late High Xover to get the required amount of brightness in the sound. Lower frequencies would be more realistic, but higher frequencies might be better for that “heavenly” sound.
For a realistic sound, leave the Early Mod Depth low, but don’t be afraid to turn up the Late Mod Depth. In order to properly capture the modal density of a cathedral, a digital reverberator would have to use several minutes of delay memory, which would put the CPU and memory consumption well outside of the real-time range for any modern computer. Modulated delays are a good way of creating the impression of higher modal densities. Plus, they sound purty.
The first reverb I ever programmed was a minimal feedback delay network based around a design Puckette and John Stautner published in the Computer Music Journal in 1982. Michael Gerzon had originally described the theory behind feedback delay networks in 1971/1972 in a British journal that saw almost no distribution within the United States, so Puckette’s work was a case of convergent evolution. The Puckette reverb had great spatial characteristics and used random delay line modulation for a fantastic long decay.
Puckette invented Max & Pure Data
Outside of my admittedly nerdy world, Puckette is best known for inventing Max during his time at IRCAM and for developing Pure Data as an open-source alternative to the Max/FTS hardware/software platform. The DSP code in Pure Data was later adapted to become the basis of MSP in Max/MSP. Pure Data is now making its appearance in all sorts of environments, from mobile audio to video games.
And also wrote some excellent tutorials on reverbs
Miller Puckette’s website at UC San Diego is the home of his excellent tutorial, “Theory and Techniques of Electronic Music.” This provides an overview of basic DSP processes with an informative introduction to unitary matrices in reverberators. Highly recommended to DSP programmers and all electronic musicians.
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:
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.
Don Gunn just sent me a link to one of his latest projects – mixing a live performance by Death Cab for Cutie that will be shown on VH1’s Storytellers this Friday at 9pm. A few links (which I can’t embed here for some reason, so just click ’em):
Three instances of ValhallaRoom were used for mixing the live musical performances – two vocal reverbs (one long, one short) and a drum room reverb. Don was kind enough to share the .vpreset settings that he used:
ValhallaRoom was named for its ability to get realistic room sounds. From this perspective, a concert hall is nothing more than a really big room. The physics and reverberant characteristics of concert halls have been studied extensively in the last century, starting with the work of Wallace Sabine (the acoustic consultant for the design of Boston Symphony Hall) and continuing with the work of Leo Beranek, Yoichi Ando, Manfred Schroeder, and many other acoustic researchers.
Concert halls that are highly ranked by classical music listeners tend to have the following characteristics:
A mid-frequency decay time of 1.6 to 1.8 seconds for music from the “Classical” period (1750 to 1820), and around 1.9 to 2.1 seconds for music from the “Romantic” period.
An initial time-delay gap at or below 25 msec. This is the time between the direct sound and the first reflection, and produces a sense of “intimacy” for shorter settings. This is why the “shoebox” shaped halls tend to be preferred by conductors and audiences – the initial lateral reflections start rather quickly, due to the relatively short distance that sound has to travel from the orchestra to the side walls to the listeners.
“Warmth,” in that the bass tones are strong. This translates to a low-frequency decay time that is somewhat longer than the mid-frequency decay time.
Spaciousness, in that the sound seems to come from a space wider than the instrument making the sound. This tends to be tied into phase differences in the signals hitting the left and right ears (which is often referred to as the IACC, interaural cross-correlation).
Envelopment, in that the reverberation appears to come from all directions, rather than from limited directions. In practice, this means that an ideal hall will mix the reflections together rather quickly, and won’t have any strong discrete echos from any one location, or a part of the hall where the reverb hangs around too long (as can happen in cathedrals with high naves).
Starting with the above criteria, we can dial in a realistic concert hall preset in ValhallaRoom. A few general principles:
Set DECAY to 1.6 to 2.1, depending on the type of music that will be played in the space.
Set HIGH CUT to a fairly low frequency, between 4500 Hz and 7000 Hz, to simulate the air absorption in the space.
PREDELAY should be set to match the initial time delay gap of the hall being modeled, with 25 to 35 milliseconds being a more “realistic” setting, and shorter settings useful in generating intimacy.
The Early Size parameter should be between 20 and 50 msec, with Early Send turned up full and Early Diffusion at max, in order to product a diffuse onset of reverberation.
The Late Size should be set greater than 0.5. The largest settings of Late Size may produce audible reflections, depending on the mode used, so tune this by ear.
The Late High Xover should be set low enough to cause a bit of rolloff in the decay (around 2 to 4 KHz), and the Late High Mult should be set to values significantly lower than 1.0X.
Late Cross should be set higher than 0.0, in order to simulate the envelopment of real halls. Late Cross values less than 1.0 will help retain the spatialization of stereo inputs, so this should be tweaked according to taste – start at 0.5 and work your way up or down.
The Early Mod Depth should be set to 0 for realism – higher values result in an unnatural panning of the early decay. The Late Mod Depth can be set higher than 0, and a little bit of modulation helps enhance the realism, but keep the Late Mod Rate in the 0.25-1.0 Hz range to avoid obvious pitch changes.
As a quick example, here’s a preset based on the measurements of Boston’s Symphony Hall, as described in Leo Beranek’s “Concert Halls and Opera Houses”:
A quick tweak of the above, with a change of the Reverb Mode from Large Room to Large Chamber (just to shake things up), and we have a model of Vienna’s Grosser Musikvereinssaal:
It is worth noting that I haven’t been to either of the above halls in person, so take the above presets as a rough starting point for creating your own concert hall presets.
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