Using OvertoneDSP FC-70 as limiter

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MaxPerl
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Hello all,

I just wanted to ensure whether I use the wonderful FC-70 (= a Fairchild 670 plugin) in the right way. I found some times ago a wonderful manual (by a different manufacturer of another Fairchild plugin, I think), but I cannot find it anymore.

In this manual was written that the Threshold knob manages threshold and ratio [In the FC-70 this isn't said in this clarity (here it is said that ratio is set by by increasing the input gain, which I don't understand.. On the other hand the it looks as Threshold also sets ratio)]. Because I wanted to use the FC-70 as a limiter with as possible minimal or no compression, my way was, to turn the threshold setting fully clockwise and adjust the amount of limiting alone through the input gain.

Is this the correct way?

Thanks in advance,
Max

mike@overtonedsp
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@MaxPerl: I guess the Ardour forum isn't really the appropriate place for me to do technical support on one of ny products (I hope the Ardour devs don't mind) - but I realise there have also been some 'availability issues' with my site / support too. So here goes:
The FC70 and similar designs are 'soft-knee' limiters, that is there is a gradual transition from 1:1 (e.g. no compression) to limiting as the input level increases. The 'knee' being the part of the input vs output curve around the threshold point. As such, the actual ratio varies dependent on the input signal level. Therefore the input gain, and the threshold control setting will all affect the amount of compression to some degree. I use 'amount of compression' here deliberately rather than ratio, because for the reasons mentioned, the concept of a specific ratio isn't really valid.
The manual refers to a specific use case where you might first set the threshold, which will ultimately govern the point at which you reach limiting, and therefore the maximum output available, and you then increase the input level if you want more compression. In an analogy to dialing in more ratio on a conventional comressor but, importantly, one with an automatic output 'make-up gain' adjustment - as mentioned in the manual.
It is, on reflection, a slightly contrived analogy but it's a little difficult to illustrate all the complexities ('desirable artistic qualities') of the vintage design's behaviour in terms of a modern and more 'clinical' processor which many might be more familiar with.

MaxPerl
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Dear Mike,
Thank you very much for your answer. I didn't want to annoy you with my newbie-understanding question so that I asked the question here. The more thank you very much for your help. By the way, the FC 70 is my absolute favourite plugin. Great work!!!

Ok, I now understand more that the amount of compression correlates with both, Threshold and Input gain. One cannot say, that the FC70 with a certain Threshold level as a compressor with a specific ratio or a limiter. With the same level as compressor with 2:1 ratio, 10:1 or as a limiter, depending on signal input. That means a little bit limitation because I cannot use it as pure limiter with a gain reduction of for example only -2dB. So my question was from the beginning false :-)

On the other hand, I think the handling is much more easier than a "modern" compressor. Because I have only to decide how much amount of gain reduction I want. And if I reach this by increasing Threshold or input gain, is ultimately irrelevant, isn't it?

I now more and more understand why you wrote, one have to forget everything what one have learned about compressors in the manual ;-)

MaxPerl
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Oh the verb was swalloed up..
"One cannot say, that the FC70 acts with a certain Threshold level as a compressor with a specific ratio or a limiter. With the same level it acts as compressor with 2:1 ratio, 10:1 or as a limiter, depending on signal input."

anahata
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You also cannot use it as a brick wall limiter. I often put a Steve Harris LADSPA "Fast Lookahead Limiter" (which is a brick wall limiter) after the FC70 on the master bus. The FC70 does most of the work, but the brick wall limiter acts as a safety net to prevent clipping and is inactive most of the time.

As for manuals, I have a copy of the manual for a real Fairchild 670, which is interesting reading (downloaded for free somewhere).

mike@overtonedsp
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You also cannot use it as a brick wall limiter..

That kind of depends.. You can do, (its original application was to limit modulation of the cutting head when creating master discs from live recordings - you couldn't go back and edit later - one mistake and the recording / disc was ruined) but as I mentioned before, its character is very dependent on the settings. The onset of compression is gradual as the input level increases past the threshold, but ultimately if you hit it hard enough, the behaviour eventually ends up at limiting - assuming you haven't run out of headroom before that point.
There are some graphs in the manual and there are some similar specifications for the original design floating about on the internet. It's difficult to describe the operation in terms of a conventional modern limiter, perhaps it's helpful to outline how it works instead:

  1. The design is a 'feedback' compressor / limiter. This means the side-chain monitors the compressor output and turns down the VCA gain accordingly, as the level increases.
  2. The Input 'gain' control, just sets the amount of signal entering the compressor - it's actually an attenuator right on the input to the unit.
  3. The Threshold control sets the amount of the compressor's output fed to the side-chain. From zero with the threshold at 0 (e.g. no compression happens at all because there is no input to the side-chain, so the unit behaves as a conventional amplifier) to 100% with the threshold at '10'

As adjusting the 'threshold' control also inherently affects the gain of the control loop between the compressor output and VCA, so it influences the shape of the compression curve too. This is why the threshold also affects the 'ratio' or more correctly the amount of compression, as well as the point where compression begins e.g. the threshold.