artificial reverberation: from mono to true stereo

“True stereo” is a term used in audio processing to describe a stereo recording, processing or playback technique that accurately represents the spatial location of sound sources in the stereo field. In true stereo, the left and right channels of a stereo recording contain distinct and separate audio information that accurately reflects the spatial location of sound sources in the recording environment.

This is in contrast to fake/pseudo stereo, where the stereo image is created through artificial means, such as by applying phase shifting techniques to create the impression of stereo. True stereo is generally considered to be superior to fake stereo, as it provides a more natural and immersive listening experience, allowing the listener to better locate and identify sound sources within the stereo field. In the domain of acoustic reverberation, this is essential for the perception of envelopment.

Artificial reverberation has come a long way since its early beginnings. The first mechanical devices for generating artificial reverberation, such as spring or plate reverberation, were initially only available as mono devices. Even when two-channel variants emerged, they usually did summing to mono internally or did processing in separate signal paths, known as dual mono processing. Typically, in a plate reverb, a two-channel output signal was achieved simply by mounting two transducers on the very same reverb plate.

The first digital implementations of artificial reverberation did not differ much from the mechanical ones regarding this principle. Quite common was summing the inputs to mono and the independent tap of two signals from a single reverb tank to obtain a two-channel output. Then, explicit early reflection models were added, which were typically processed for left and right separately and merged into the outputs later to preserve a basic representation of spatial information. Sometimes, also the first reflections were just taken from a (summed) mono signal. The Ursa Major 8×32 from 1981 is a good example for this design pattern. Later, the designs became more sophisticated, and even today it is common to distinguish between early and late reverberation in order to create a convincing impression of immersion.

However, ensuring proper sound localisation through early reflection models is a delicate matter. First and foremost, a real room does not have a single reflection pattern, but a vast variety of ones that depend on the actual location of the sound source and the listening position in that room. A true-to-life representation of this would, therefore, have to be represented by a whole set of individual reflection patterns per sound source and listening position in the virtual room. As far as I know, the VSL MIR solution is the only one that currently takes advantage of this, and with an enormous technical effort.

Another problem is that first reflections can also be detrimental to the sound experience. Depending on their frequency and delay in relation to the direct signal, the direct signal can be masked and affected in terms of phase coherence so that the overall sound becomes muddy and lacks clarity. This is one of the reasons why a real plate reverb is loved so much for its clarity and immediacy: it simply has no initial reflections in this range. As a side note, in the epicPLATE implementation, this behaviour is accurately modeled by utilizing a reverberation technique that completely avoids reflections (delays).

Last but not least, in a real room there is no clear separation between the first reflections and the late reverberation. It is all part of the same reverberation that gradually develops over time, starting with just an auditory event. This also means that there is no clear distinction between events that can be located in space and those that can no longer be identified – this also continuously evolves over time.

A good example of how to realise digital reverb without this kind of separation between early and late reverberation and at the same time in “true stereo” was impressively demonstrated by the Quantec QRS back in the early 80s already. Its ability to accurately reproduce stereo was one of the reasons why it became an all-time favourite not only in the music production scene, but also in post-production and broadcasting.

Artificial reverberation is full of subtleties and details and one might wonder why we can perceive them at all. In the end, it comes down to the fact that in the course of evolution there was a need for such fine-tuning of our sensory system. It was a matter of survival and important for all animal species to immediately recognise at all times: What is it and where is it? The entire sensory system is designed for this and even combines the different sensory channels to always answer these two questions. Fun Fact: This is exactly why some visual cues can have a significant impact on what is heard and why blind tests (in both meanings) are so important for assessing certain audio qualities. See also the “McGurk Effect” if you are interested.

Have fun listening!

dear acoustics researchers!

Thank you for all the latest research papers in acoustics and especially the basics of acoustic design for concert halls. I have learned so much about the ambivalance of early reflections, auditory proximity, the critical timing of sound distribution but also amazing things about natural frequency dependent compression in real rooms. Thanks also for making many contributions available as an easy introduction via YT.

However, many of these YT contributions shine through the poorest audio quality imaginable. Recorded in bad acoustic environment (!), poorly miked, badly placed headsets, hissing, booming, humming, dropouts – the whole lot. Resulting in recordings of the lowest audio quality and speech intelligibility. Sometimes so bad that you can hardly follow the content. Seriously, guys, the kids over at Tiktok can do better. So next time please do your homework and walk what you talk, okay?

Sincerely,
Herbert

something epic is coming

Stay tuned!

epicPLATE released

epicPLATE delivers an authentic recreation of classic plate reverberation. It covers the fast and consistent reverb build up as well as that distinct tonality the plate reverb is known for and still so much beloved today. Its unique reverb diffusion makes it a perfect companion for all kinds of delay effects and a perfect fit not only for vocals and drums.

delivering that unique plate reverb sound

• Authentic recreation of classic plate reverberation.
• True stereo reverb processing.
• Dedicated amplifier stage to glue dry/wet blends together.
• Lightweight state-of-the-art digital signal processing.
  

Available for Windows VST in 32 and 64bit as freeware. Download your copy here.

The former epicVerb audio plugin is discontinued.

that unique plate reverb sound

Unlike digital reverberation, the plate reverb is one of the true analog attempts in recreating convincing reverberation build right into a studio device. It is basically an electro-mechanical device containing a plate of steel, transducers and a contact microphone to pickup the induced vibrations from that plate.

The sound is basically determined by the physical properties of the plate and its mechanical damping. Its not about reflecting waves from the plates surface but about the propagation of waves within the plate. While the plate itself has a fixed, regular shaped size and can be seen as a flat (two dimensional) room itself it actually does not produce early reflection patterns as we are used to from real rooms with solid walls. In fact there are no such reflections distinguishable by human hearing. On the other hand there appears to be a rather instant onset and the reverb build-up has a very high modal density already.

Also reverb diffusion appears to be quite unique within the plate. The wave propagation through metal performs different compared to air (e.g. speed/frequency wise) and also the plate itself – being a rather regular shape with a uniform surface and material – defines the sound. This typically results in a very uniform reverb tail although the higher frequencies tend to resonate a little bit more. Also due to the physics and the damping of the plate, we usually do not see hear very long decay times.

All in all, the fast and consistent reverb build up combined with its distinct tonality defines that specific plate reverb sound and explains why it is still so much beloved even after decades. The lack of early reflections can be easily compensated for just by adding some upfront delay lines to improve stereo localization if a mix demands it. The other way around, the plate reverb makes a perfect companion for all kinds of delay effects.

everything just fades into noise at the end

When I faced artificial reverberation algorithms to the very first time I just thought why not just dissolve the audio into noise over time to generate the reverb tail but it turned out to be not that easy, at least when just having the DSP knowledge and tools of that time. Today, digital reverb generation has come a long way and the research and toolsets available are quite impressive and diverse.

While the classic feedback delay network approaches got way more refined by improved diffusion generation, todays computational power increase can smooth things out further just by brute force as well. Still some HW vendors are going this route. Sampling impulse responses from real spaces also evolved over time and some DSP convolution drawbacks like latency management has been successfully addressed and can be handled more easily given todays CPUs.

Also, convolution is still the ticket whenever modeling a specific analog device (e.g. a plate or spring reverb) appears to be difficult, as long as the modeled part of the system is linear time invariant. To achieve even more accurate results there is still no way around physical modeling but this usually requires a very sophisticated modeling effort. As in practise everything appears to be a tradeoff its not that much unusual to just combine different approaches, e.g. a reverb onset gets sampled/convoluted but the reverb tail gets computed conventionally or – the other way around – early reflections are modeled but the tail just resolves into convoluted noise.

So, as we’ve learned now that everything just fades into noise at the end it comes to no surprise that the almost 15 years old epicVerb plugin becomes legacy now. However, it remains available to download for some (additional reverb) time. Go grab your copy as long as its not competely decayed, you’ll find it in the downloads legacy section here. There won’t be a MkII version but something new is already in the making and probably see the light of day in the not so far future. Stay tuned.

the twisted world of guitar pedals II

Meanwhile I had the opportunity to put my hands on some Fairfield Circuitry effect pedal stuff mentioned earlier here and the “Meet Maude” analog BBD delay was right here on my desk for a deeper inspection. My actual experience was a rather mixed one.

Focusing on a rather dark and LoFi sound quality on the one hand plus a rather simplistic feature set concept wise on the other, they do not appear to be very flexible in practise and this at a rather steep price point. They appear to be very noisy featuring all kinds of artifacts even when integrated to the mixing desk via reamping. One may call this the feature itself but at the end it makes it a one-trick pony. If you need exactly that, here you have it but you get nothing beyond that. To me this trade off was too big and so I send it back.

However, I found their nifty low pass gate implementation (very prominently featured within their “Shallow Water”) that much unique and interesting that I replicated it as a low pass filter alternative in software and to have it available e.g. for filtering delay lines in my productions. The “Shallow Water” box made me almost pull the trigger but all in all I think this stuff seems to be a little bit over-hyped thanks to the interwebs. This pretty much sums it up for now, end of this affair.

Timeline & BigSky – The new dust collectors?

Going into the exact opposite direction might be a funny idea and so I grabbed some Strymon stuff which aims to be the jack of all trades at least regarding digital delay and reverb in a tiny stomp box aka desktop package. To be continued …

Further readings about BBD delays:

• the Ibanez AD202 analog delay
• the Dynacord VRS-23 analog delay

utilizing early reflections in a production

A quite often underestimated or even forgotten production technique is to take advantage of artificial early reflections which could be added somewhere during the mixing process. Without inserting any fully fledged reverberation at all, applying such techniques allows to dramatically increase stereo width and depth perception as well as a way better instrument localization even in a busy mix. Creating density is not the goal here but the opposite is the name of the game: achieving a clear and intelligible mix.

In a simple case, one can place a short and plain delay (a slap-back echo) on a track and properly place it in the stereo field – maybe on the opposite side of the source but that’s just an example. More sophisticated tap delays could be used to create a sort of room experience and some reverberators are allowing to disable the late reverb diffusion and just to use their early reflection generation. There are no restrictions in general – allowed is what gets the job done in that specific mixing situation.

the Lexicon 224 reverb sound

As one of the first digital reverbs ever, the Lexicon 224 indeed is a classic device and even today, the Lexicon 224 reverb has its place in quite a lot of studios and productions. Whenever it comes down to that larger-than-life sound or that certain graininess, which cuts through a busy mix that easily, the 224 delivers. Of course, it can’t compete with todays smooth and silky reverb algorithms at all but instead and with its typical movement and animation, the 224 reverb tail offers tons of charm and character.

the many shapes of delay

About the different applications of audio delay effects.

There are quite a number of different types and applications for the audio delay effect in the audio production ranging from plain technical delay application up to all the musical and creative ranges of application. The rather technical or correctional delay typically is a plain digital delay which serves as a sample accurate alignment tool. Such alignments might be necessary for example for plug-in delay compensation (when a plug-in introduces latency) or during the mixing process to align a group of recorded tracks or samples. The so-called pre-delay can also be seen as an alignment sort of thing where the direct source signal has to be aligned in a positive or negative manner in relation to a processed signal, e.g. in a reverb effect.