what is SC 3db control?

[uglijimus]

just started to use this compressor the other day and I love it.

But I am curious about the 3db HF SC control on the far left. Let's say I set it to 90hz. does that mean that everything below 90hz will not be compressed? and what does the 3dB vs. the 6dB mean? thanks

[EvilDragon]

It's explained in the manual, page 9

[uglijimus]

It's explained in the manual, page 9

I think you mean page 6. I read it, I just didn't understand it.

[David Else]

3 dB is a *very* gentle high pass slope to the side-chain that is intended to make the compressor react in a more 'natural' way to the overall frequency balance.

Think of it like this. If you have it set to 90 Hz and 3 dB then bass sounds under 90 Hz will trigger the compressor a little less than having the side-chain off. If you set it to 6 dB then it will trigger even less still. Its about how much you want bass to trigger compression.

For example, if your mix had a loud bass drum you might not want that to pump the compression. With the SC you can control this.

[uglijimus]

this makes sense, thank you very much!

[FabienTDR]

just started to use this compressor the other day and I love it.

But I am curious about the 3db HF SC control on the far left. Let's say I set it to 90hz. does that mean that everything below 90hz will not be compressed? and what does the 3dB vs. the 6dB mean? thanks

Ok, two separate things:

First of all, the "SC HP Frequency" defines the cutoff point of the high-pass filter at the input the the side-chain. Since no filter's cut can be "total", engineers typically define the cut-off frequency as the point where the filter reaches 3dB of reduction.

And this is exactly how the SC HP Frequency control behaves: It defines at which point the side-chain input gets reduce by 3dB.


Second, there's that "SC HP Slope" control. Now, beware: This is completely unrelated to the 3dB cutof point definition mentioned above. The "SC HP Slope" defines how strong the filter will reduce low frequencies. It is expressed in "decibel per octave". The FB comp II supports 3 differently steep slopes, a 6dB/oct slope (first order filter), a 12dB/oct slope (second order filter) and an exotic 3dB/oct slope (made with a special filter network).

As most audio signals have the tendency to contain higher level of low frequency content than high frequency content, a compressor will also have the tendency to react accordingly: In it's natural form, a compressor will try to reduce LF content by greater amounts that HF content. This doesn't deliver a well balanced sound and creates many negative side-effects down the road.

That's where the idea to reduce a compressor's sensitivity to low freq comes into play: The Side-chain high pass filter. The most obvious and efficient choice would be a simple, first order HP filter. And it works fine in many cases.

However, since most music signals tend to have a pink-noise alike frequency distribution (roughly a constant 3dB slope over the full spectrum) and not a brown noise distribution (6dB slope), the 6dB filter will tend to over-emphasize the low frequencies. Which is often exactly what you want (boosting the lows), but not always. The 3dB/oct mode preserves the original spectral (i.e. "frequency") distribution of typical music better than the 6dB/oct filter.

the 3dB/oct mode also generally leads to less pumping when the low-mid/mid frequency levels grow quickly, such as the case when a vocal part suddenly begins.


@David: Thank you!