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 RMS (or vector) summing, 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.

major mkIII update for Density bus compressor released

preFIX 1.0 – out now!

preFIX – getting those alignments done

preFIX – final teaser and release info

preFIX - gate and expander section with detailed sidechain fitering options

preFIX – getting those alignments done

getting those alignments done - finally is easy and fun

The upcoming preFIX audio plug-in was originally designed to meet the recording engineers needs and is all about audio alignment tasks: fix all that stuff that can’t be fixed outside the box (anymore) and do that fast and with highest quality right before the mixing starts itself.

preFIX is a pre-mixing and audio alignment tool which typically takes place upfront the mixing process. It provides a clever tool set to clean-up, fix and align audio tracks (typically taken from recordings) concerning overall frequency correction, phase alignment, spatial stereo field corrections and routing. It contains a complete gate/expander solution with a dedicated and comprehensive sidechain filtering path as well.

Though, preFIX is not only tailored to the recording engineers environment but delivers a truly great performance when shaping and lifting poor sample sources, sculpt electronic softsynth instruments or just by dialing in that super tight drum bus sensation as well.

NastyDLA – final teaser and release info

NastyDLA – technical architecture

simplified technical architecture

Internally, NastyDLA consists of quite a bunch of DSP processing building blocks which as a whole are summing up to an authentic signal path simulation of it’s analog models. The blocks and the according signal flow are shown in the diagram above. Basic signal flow goes from left to right except the feedback path which goes in the opposite direction.

With NastyDLA, signal path coloration already starts in the input stage which provides a complete model of both, frequency and phase response as well as dynamic saturation. It’s located in the dry path but all nonlinear processing and coloring can be disabled on demand so it remains as a simple input volume control then. But while switched in, the input stage can greatly contribute on getting the processed signal to fit right into a mix.

the gate/expander in use

written by susiwong

A basic gate has a single parameter, the threshold – when the level is above the threshold the signal passes unchanged, when the level drops below the threshold the signal gets switched off, simple as that. Attack time ideally should be as fast as possible without causing clicks or distortion, so it’s preset to a sensible compromise with most gates, a few good gates even offer you a choice of two settings. Knee, hold and release determine shape and speed of the fade out, release is responsible for the overall decay time, knee changes the behaviour around the threshold level, helping you avoid the dreaded “motorboating” effect where the gate switches on and off rapidly. Think BSS or Drawmer gate vs Alesis compressor …

Hold simply specifies the “reaction time” from the moment the signal passes the threshold till the begin of the gain reduction – critical to preserve as much meat as possible from drums or keeping guitar decay intact. This is mostly what separates the good from the bad and the ugly. Last is the “range” or “floor” parameter, it sets a certain minimal volume to which the signal gets attenuated when dropping below the threshold, instead of being muted completely. Very helpful when you need to reduce the background noise between a singer’s phrases for example, much less obtrusive than muting the track completely. Set the floor so the background noise gets masked well enough by the music, often 3dB or 6dB are enough. This technique is also known as downward expansion, paired with a longer release and soft knee it’s often used for distorted guitars (with slow decay), too.

Some good gates offer sidechain filters allowing you to “zero in” on the important part of a complex signal, take a tom mic of a multi-miced drumset for example, where a lot of similar signals (bleed) are fighting for control. Difficult even with sidechain, impossible without. Worth noting that these filters do NOT influence your audio signal, only the signal used for detection, hence the name sidechain. And finally an external sidechain allows you to even borrow a signal from another channel to trigger your gate – the creative options are huge.Unfortunately not all hosts have this implemented in a user-friendly way. One popular example is tightening up the bass by triggering its gate from the kick.

NastyVCS – released today

NastyVCS - Virtual Console Strip

NastyVCS – I can has color

Similar to the EQ’s and filters in BootEQ mkII, NastyVCS stays on the musical (and not the surgical) side of the audio source. It is more kind of coloring toolbox (especially in the combination with the PHASE option) rather than allowing to shape audio disfigured beyond recognition. Some classic technical principles have been carefully selected and replicated and the overall combination and attention to detail makes NastyVCS stand out in the crowd of equalizers today.