Ronny wrote on Thu, 27 April 2006 06:06 |
Chuck, bear with me please, I'm not clear on how to plot a tranfer function on one of my comps.
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Set up one of your compressors
as you normally would use it. You MUST follow this step if any of this is going to make sense. Just get your favorite source spanked up with a compressor, popping it in and out of bypass till you like the setting, unpatch it and then then simply send a signal to the unit and measure and record the output signal as you increase the input level. You could use the posted graphs as a model, and run the input level from -50dB to +20 dB. Just put a dot on the graph at the intersection of each point, so you have a line comprised of these measurement points, input along the Horizontal axis, and measured output along the vertical axis.
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I have over 50 different compressors and every one of the manuals that shows compression graphs has the same type graph that I displayed. The data represents exactly what is happening at the compression stage. On Morgan's graph the signals are not at unity gain.
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Yes they are at unity gain, look at the reference level, which in this case is: "0dB" The curves
all intersect at this point. Input Level (horizontal axis) =0dB, output Level (vertical axis) =0dB. This is the definition of unity gain.
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The signal enters the compressor from the bottom in gain. Unity in my explaination means "input gain" All compressors have input gain at unity.
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Unity simply means that the signal level in = signal level out. The word unity means "one", a gain of "1" is called unity gain. Input signal multiplied by '1' produces the same level at the output.
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If the threshold is not reached it passes the same input gain through output "at unity".
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Not in most cases...(real world). Look at the graph again. at the 2:1 ratio the input signal of -50dB produces an output signal of -40dB. These low level signal are increased in level by 10dB. Further examination of that curve shows you that the 2:1 ratio at a -20dB threshold will require 10dB of gain to be added so that there is no insertion loss at the nominal operating level of "0dB"
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I understand that some comps restructure the gain as in the LA-2A but before therehold is met, input and output levels are the same. The signal "has to enter the compressor where the X and Y lines read the same decibel levels" as it's before threshold and before the signal is processed, thus input and output levels must be the same. What I'm having a problem with is Morgan's two lines show input at -40dB on the 2:1 line and input near -30dB "output" on the 20"1 line, the input gain at compressor input in reality is the same as the output gain "until the compression threshold level is reached" from the bottom left. So no this is not crystal clear to me. I think I understand what you are saying how the rotation point is showing makeup gain, but looking at it this way is new to me because I just can't understand how an input signal can be input at -40dB or -33dB output when the graph shows unity gain at -50dB. My blue lines represent every compression graph that I've seen up until you showed Morgans. Are you telling me that you don't understand what I'm relating? I admit that I don't understand fully what you are relating as the graph's just don't represent real gain going into a compressor. Perhaps they must do this to show make-up gain, but that's still not clear to me.
There is no -19dB loss at insertion on my graph, signal enters at unity gain -50dB input -50dB output, when threshold is met at -20dB, the 20:1 ratio "attenuates" the signal by -19dB for every 20dB that exceeds the threshold level.
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Look at the graph you've posted: When the input signal reached 0dB, the output is only at -19dB.
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Output gain structure, not showing on my graph can be set at any level to make-up for the attenuation that is happening after threshold of the compression stage.
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The graph is used to define the transfer function of the
system. Input versus output. Makeup gain is a part of every compressor design, manual or automatic makes NO difference.
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On your graph the 20:1 line shows input at -50dB and output at -33dB, "before the threshold level is met and before the compression stage is activated" and threhold applied at -3dB output, this is why it's hard for me to understand this. I'm not clear on how you are seeing a gain rise after threshold is met, because even on Morgans graphs, the lines go to the right of the input gain, signifying what I see as gain reduction "from input", same as my graph if he moved his two lines down to unity gain at the mid center 45 degree line that represents unity gain on his and all graphs that I'm familiar with and the line that divides the X-Y access, so that "output level of the compression stage" shows on right side of the mid center diagonal line as gain reduction. Compressors reduce gain when threshold is met if ratio is more than 1:1, therefore the reduction line will always be below input (unity) gain level to the right side of the diagonal mid center line when ratio is applied. The amount of ratio applied determines the level the signal will be when it reaches the ouput gain structure. In this case the 2:1 ratio is reducing the signal by -10dB when the signal leaves the compression section and goes to the output gain structure.
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Ronny, just plot some of your favorite compressor settings and this will all be much more clear...