What are the differences between the compression models found in the ChannelStrip MK2, MultiComp and TrackShaper?
As you may know the ChannelStrip MK2, MultiComp and TrackShaper are all built around the same compression technology, three different models: VCA, FET and OPTO.
Each model has different compression characteristic and is inspired by the inner workings of some famous compressor, and even the analog emulation is different from model to model.
Let’s start with the VCA model which is the most “modern” sounding one. This model is inspired by the one found in some large console of the 80s, at it’s core there is a linear Voltage Controlled Amplifier (VCA) in fact when the sidechain signal crosses the set threshold, compression start to act linearly with a hard knee. This is pretty simple stuff, the trick is that since it’s a feed-forward compressor if it acted on peaks it would pump really too much so the sidechain driving the VCA is derived by an RMS calculator. It’s quite easy to calculate the perfect RMS of a signal in the digital domain, but here we must emulate what’s done in the analog world so the calculation is not precise, and this is what gives the compressor “a sound”.
When you turn on the analog emulation you actually engage a waveshaper that simulates the saturation of a AB class amplifier (the one found in operational amplifiers) and DC blocking stage.
The FET model is build after a very famous boutique compressor of the latest 60′ it’s completely different from the VCA one because it’s not linear at all and it’s a feed-back compressor. This in addition to the attack and release times is probably the main contribution to the peculiar sound of the model. In a feed-back compressor the sidechain is derived *AFTER* the gain reduction stage so it’s very program dependent, this signal is then fed to a model of a FET transistor acting in its linear region. What happens to the compression curve is that after you cross the threshold it start to get more drastic as the signal increases, this also affect the knee, in fact using higher ratios result in a harder knee.
When you turn on the analog emulation you actually engage a waveshaper that simulates the saturation of a A class amplifier and DC blocking stage.
The OPTO model is inspired by a classic vintage compressor that is build around a electroluminescent panel. This gain reduction unit is very peculiar since it has a frequency dependent response (compress more the bass frequencies as a result) and a strange release time, it drop off quite quickly but then after around 50% of the luminescence it lasts longer, up to ten times the first phase. This gives a sort of two staged envelope detection to the sidechain signal, adding it to the frequency dependence of the gain reduction module you obtain a very pumping compressor.
When you turn on the analog emulation you actually engage a waveshaper that simulates the saturation of a A class amplifier and DC blocking stage.
I hope this article explains a little more what’s going on behind the scenes of the compression models, if you have more question just add a comment and I will answer!