sidechain linking techniques

How an audio compressor responds to stereo content depends largely on how the channel linking is implemented in the sidechain. This has a major influence on how the spatial representation of a stereo signal is preserved or even enhanced. The task of the compressor designer is to decide which technical design is most suitable for a given overall concept and to what extent the user can control the linkage when using the device.

In analog compressor designs, in addition to unlinked “dual mono” operation, one usually finds simple techniques such as summing both stereo channels (corresponding to the center of the stereo signal) or the extraction of the maximum levels of both channels using a comparator circuit implementing the mathematical term max(L,R).

More sophisticated designs improve this by making the linking itself frequency dependent, e.g. by linking the channels only within a certain frequency range. It is also common to adjust the amount of coupling from 0 to 100%, and the API 2500 hardware compressor serves as a good example of such frequency dependent implementation. For the low and mid frequency range, simple summing often works slightly better in terms of good stereo imaging, while for the mid to high frequency range, decoupling to some degree often proves to be a better choice.

The channel coupling can also be considered as a summation of vectors, which can be easily realized by sqrt(L^2+R^2). As an added sugar, this also elegantly solves the rectification problem and results in very consistent gain reduction across the actual level distributions that occur between two channels.

If, on the other hand, one wants to focus attention on correlated and uncorrelated signal components individually (both of which together make up a true stereo signal), then a mid/side decomposition in the sidechain is the ticket: A straight forward max(mid(L,R), side(L,R)) on the already rectified channels L and R is able to respond to any kind of correlated signal not only in a very balanced way but also to enhance its spatial representation.

More advanced techniques usually combine the methods already described.

TesslaPRO mkIII released

the magic is where the transient happens

The Tessla audio plugin series once started as a reminiscence to classic transformer based circuit designs of the 50s and 60s but without just being a clone stuck in the past. The PRO version has been made for mixing and mastering engineers working in the digital domain but always missing that extra vibe delivered by some highend analog devices.

TesslaPRO brings back the subtle artifacts from the analog right into the digital domain. It sligthly colors the sound, polishes transients and creates depth and dimension in the stereo field to get that cohesive sound we’re after. All the analog goodness in subtle doses: It’s a mixing effect intended to be used here and there, wherever the mix demands it.

The mkIII version is a technical redesign, further refined to capture all those sonic details while reducing audible distortions at the same time. It further blurs the line between compression and saturation and also takes aural perception based effects into account.

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

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.

BootEQ mkIII released

BootEQ mkIII – a musical sounding Preamp/EQ

BootEQ mkIII is a musical sounding mixing EQ and pre-amplifier simulation. With its
four parametric and independent EQ bands it offers special selected and musical
sounding asymmetric and proportional EQ curves capable of reproducing several
‘classic’ EQ curves and tones accordingly.

It provides further audio coloration capabilities utilizing pre-amplifier harmonic distortion as well as tube and transformer-style signal saturation. Within its mkIII incarnation, the Preamp itself contains an opto-style compression circuit providing a very distinct and consistent harmonic distortion profile over a wide range of input levels, all based now on a true stateful saturation model.

Also the EQ curve slopes has been revised, plugin calibration takes place for better gain-staging and metering and the plugin offers zero latency processing now.

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

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:

interview series (11) – Andreas Eschenwecker

Andy, your Vertigo VSC compressor has already become a modern classic. What has been driven you to create such a device?

I really like VCA compressors. VCA technology gives you a lot of freedom in design and development and the user gets a very flexible tool at the end. I was very unhappy with all VCA compressors on the market around 2000. Those were not very flexible for different applications. These units were working good in one certain setting only. Changing threshold or other parameters was fiddley and so on. But the main point starting the VSC project was the new IC VCA based compressors sounded one dimensional and boxy.

Does this mean your design goal was to have a more transparent sounding device or does the VSC also adds a certain sound but just in a different/better way?

Transparency without sounding clean and artificial. The discrete Vertigo VCAs deliver up to 0,6% THD. Distortion can deliver depth without sounding muddy.

Does this design favour certain harmonics or – the other way around – supresses some unwanted distortions?

The VSC adds a different distortion spectrum depending when increasing input level or adding make-up. The most interesting fact is that most of the distortion and artifacts are created in the release phase of the compressor. The distortion is not created on signal peaks. It’s becoming obvious when the compressor sets back from gainreduction to zero gainreduction. Similar to a reverb swoosh… after the peak that was leveled.

Where does your inspiration comes from for such technical designs?

With my former company I repaired and did measurements on many common classic and sometimes ultra-rare compressors. Some sounded pretty good but were unreliable – some were very intuitive in a studio situation, some not…
At this time I slowly developed an idea what kind of compressor I would like to use in daily use.

From your point of view: To which extend did the compressor design principles changed over the years?

The designs changed a lot. Less discrete parts, less opto compressors (because a lot of essential parts are no longer produced), tube compressors suffer from poor new tube manufacturing and some designers nowadays go more for RMS detection and feed forward topology. With modern components there was no need for a feedback SC arrangement anymore. I think RMS is very common now because of its easy use at the first glance. For most applications I prefer Peak detection.

Having also a VSC software version available: Was it difficult to transfer all that analog experience into the digital domain? What was the challenge?

In my opinion the challenge is to sort out where to focus on. What influence has the input transformer or the output stage? Yes some of course. Indeed most of the work was going into emulating the detection circuit.

Which advantages did you experienced with the digital implementation or do you consider analog to be superior in general?

I am more an analog guy. So I still prefer the hardware. What I like about the digital emulations is that some functions are easy to implement in digital and would cost a fortune in production of the analog unit.

Any plans for the future you might want to share?

At the moment I struggle with component delays. 2021/22 is not the right time for new analog developments. I guess some new digital products come first.

Related Links

the twisted world of guitar pedals I

Quite recently I had a closer look into the vast amount of (guitar) effect pedals out there. Most are already DSP based which surprised me a little bit since I still ecpected more discrete analog designs after all. While looking for some neat real analog BBD delay I finally stumbled across Fairfield Circuitry’s “Meet Maude” which got me intrigued, having a rather rough look&feel at first sight but some very delicate implementation details under the hood.

Their delay modulation circuit has some randomness build in and also there is a compression circuit in the feedback loop – both designs I’ve also choosen for NastyDLA and which makes a big impact on the overall sound for granted. But the real highlight is the VCF in the delay feedback path which actually appears to be a low-pass gate – a quite unique design and soundwise also different but appealing in its very own regard.

They employed very similar concepts to their vibrato/chorus box “Shallow Water” featuring also random delay modulation and a low pass gate but this time a little bit more prominent on the face plate. On top, their JFET op-amp adds some serious grit to any kind of input signal. All in all, I did not expect such a bold but niche product to exist. If I ever will own such a thingy, there will be a much more detailed review here for sure.

TesslaSE mkII released

TesslaSE mkII – All the analog goodness in subtle doses

TesslaSE never meant to be a distortion box but rather focused on bringing all those subtle saturation and widening (side-) effects from the analog right into the digital domain. It sligthly colors the sound, polishes transients and creates depth and dimension in the stereo field. All the analog goodness in subtle doses. It’s a mixing effect intended to be used here and there where the mix demands it. It offers a low CPU profile and (almost) zero latency.

With it’s 2021 remake, TesslaSE mkII sticks to exactly that by just polishing whats already there. The internal gainstaging has been reworked so that everything appears gain compensated to the outside and is dead-easy to operate within a slick, modernized user interface. Also the transformer/tube cicuit modeling got some updates to appear more detailed and vibrant, while all non-linear algorithms got oversampled for additional aliasing supression.

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

The TesslaSE Remake

There were so many requests to revive the old and rusty TesslaSE which I’ve once moved already into the legacy folder. In this article I’m going to talk a little bit about the history of the plugin and its upcoming remake.

The original TesslaSE audio plugin was one of my first DSP designs aiming at a convincing analog signal path emulation and it was created already 15 years ago! In its release info it stated to “model pleasant sounding ‘electric effects’ coming from transformer coupled tube circuits in a digital controlled fashion” which basically refers to adding harmonic content and some subtle saturation as well as spatial effects to the incoming audio. In contrast to static waveshaping approaches quite common to that time, those effects were already inherently frequency dependent and managed within a mid/side matrix underneath.

(Later on, this approach emerged into a true stateful saturation framework capable of modeling not only memoryless circuits and the TesslaPro version took advantage of audio transient management as well.)

This design was also utilized to supress unwanted aliasing artifacts since flawless oversampling was still computational expensive to that time. And offering zero latency on top, TesslaSE always had a clear focus on being applied over the entire mixing stage, providing all those analog signal path subtleties here and there. All later revisions also sticked to the very same concept.

With the 2021 remake, TesslaSE mkII won’t change that as well but just polishing whats already there. The internal gainstaging has been reworked so that everything appears gain compensated to the outside and is dead-easy to operate within a slick, modernized user interface. Also the transformer/tube cicuit modeling got some updates now to appear more detailed and vibrant, while all non-linear algorithms got oversampled for additional aliasing supression.

On my very own, I really enjoy the elegant sound of the update now!

TesslaSE mkII will be released by end of November for PC/VST under a freeware license.

What I like about the Behringer 2600

What I really like about the Behringer 2600 is that it’s not just a plain copy but introduces some real useful improvements over the original concept. Most important to me is the 19″ form factor which not only reduces the originals size and fits in the rack but also remains big enough to enjoy a great user experience while cabling and tweaking things. And they got rid of those speakers! Instead it offers 2 filter revisions to choose from, two of the oscillator sections are now fully featured, the LFO is part of the main chassis now and new additional timing options for the envelopes has been added as well.

On the other hand, the Behringer 2600 sticks to CV gate voltages following original levels which limits full integration in todays modular world quite a bit. However, this is currently not a big deal to me. I only wish they would have made a true analog delay instead of the spring reverb (emulation). Offering audio in, the device also doubles as an excellent analog effect unit which seems to be a little bit underrated in this regard. Given it’s pricepoint, this feature is already something to consider if one is just looking for an outboard analog filter box or an alternative for something like a MS-20.