The number of records whose sound was shaped by this unit cannot be overstated. The detection circuit is similar to the lesser-known Urei LA-4 in some regards, which also put its control over attack and release at the end of the detection circuit. The SSL bus compressor is designed so that faster attack release times will deliver smaller amounts of reduction while slower settings provide lots more, making for more even gain reduction as you cycle through the attack settings.
The interesting thing about the SSL bus compressor is that even at a high ratio of , the mix still is punchy, yet breathes. A major reason for this is that the design also varies the compression knee according to the set ratio. More about that some other day. The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage.
As the input exceeds the threshold, the compressor kicks in with its given attack time. As the input drops back down below the threshold, the compressor disengages according to its release time. What is the ratio of a compressor? The ratio of a compressor compares the number of decibels the input signal is above the threshold to the number of decibels the output signal is above the threshold.
In other words, it is the relative amount of attenuation the compressor will apply to the signal. To attenuate the audio signal, a compressor must have a gain reduction circuit. The sidechain signal path will also manipulate the signal in order to achieve some or all of the aforementioned parameters threshold, ratio, time controls, etc.
This is where the design concepts of feedforward and feedback come into play, respectively. As was mentioned previously, feedback and feed-forward compressor topologies refer to where, in the circuit, the sidechain signal comes from.
A feedback compressor feeds the audio signal into the sidechain just after the gain reduction element. A feed-forward compressor feeds the audio signal into the sidechain just before the gain reduction element. This compressor type anticipates the signal amplitude and attempts to adjust the sidechain signal in advance.
Consider, for a moment, an ideal compressor: one that does not affect the audio signal whatsoever except by a defined amount of gain reduction is and when the sidechain signal surpasses the threshold. In this ideal compressor, a feed-forward and a feedback design would act the same below the threshold. The input signal, output signal and sidechain signal would all be at the same level, and no gain reduction would take place.
Even with no gain reduction, the sidechain taken before the gain reduction circuit feed-forward would be different, even if only slightly, from the sidechain taken after the gain reduction circuit.
Now comes the more important question. What happens when the sidechain signal level surpasses the threshold and causes the gain reduction circuit to attenuate the signal?
It would seem that in the case of the feedback design, it would be the already compressed signal that would cause the compression in the first place. This is correct. As the program audio signal surpasses the threshold, it passes through the gain reduction circuit and the sidechain, in that order. The signal is also, of course, passed through to the output. The sidechain then tells the gain reduction circuit to attenuate the signal, which is then fed back into the sidechain and output.
You may be thinking that this would cause a significant delay in how a compressor will react. It gives smoothness to the front end of the sound. But it also risks shaving off the initial impact the transients making it more dull and lifeless. So adjust the attack to suit your sound. The release is how quickly the compressor lets go—in other words, the time it takes for the compression to stop working. A fast release ms or less means the compressor stops compressing quickly after it started. A slow release greater than ms gives you more dynamics and smoothness.
But overdoing it can suck the life and impact out of the next sound. It can also mess up the groove and feel of your track. So Listen and adjust to suit your tastes! Set the release so it goes back to zero the top before the next hit. Before you slap that compressor on your track ask yourself: what am I trying to achieve or solve? To tame the peaks on your drums, set your compressor to:. Why it works : The compressor reduces the gain of the peaks and lets you bring the level of your whole drum track higher.
You get a more even and tight drum sound. Why it works : Sidechain compression lowers the bassline when the kick comes through. That leaves space for the kick to get heard. Set the ratio to medium say to duck some of the bassline when the kick hits. Set the ratio to high like to cut out the bassline almost completely when the kick hits—this creates that noticeable pumping effect.
Why it works : The first compressor will be slow and natural-sounding. Hot Tip : Sometimes having several compressors doing a little bit of compression much less gain reduction sounds more natural than one doing a bunch of gain reduction.
How to do it : Record your drum kit using both direct mics and a room mic. Place the direct mics in front of each piece of kit. Place the room mic overhead. If non-linear amplifier were omitted, the compression would have a "hard-knee" characteristic, which might be preferred in some applications such as protection limiting, where the compressor is ordinarily operated below threshold level and only produces gain reduction to protect against level-setting errors prior to the compressor.
If the non-linear low-pass filter were omitted, the audio would not be as smooth or clean, but the system would retain the advantage of permitting the compression ratio to be changed easily, and producing a "soft-knee" characteristic.
In the present invention, the gain of the pilot VCA i. This forces the feedback loop to increase the gain reduction in the pilot VCA and applies additional gain reduction to the VCA in the main audio path. By correctly choosing the amount of gain increase as a function of release time, one can force the additional gain reduction signal to approximately cancel the effect of the increased output level in the feedback compressor caused by a faster release time.
Another way to achieve the goal of preventing the output level from increasing when the release time is made fast involves controlling the threshold level within rectifier The release time control signal is coupled to rectifier such that speeding up the release time decreases the threshold level of this rectifier, and thus, the threshold level of compression.
This provides more compression for any input above the threshold level, increasing the control voltage on line and achieving essentially the same goal as increasing the gain of VCA Thus we see that the present invention has all of the advantages of a feedforward compressor while retaining the dynamic smoothness of an optimized feedback compressor.
Further, it greatly reduces the tendency of the output level to change with changes in the release time control. All rights reserved. Login Sign up.
Search Expert Search Quick Search. Audio compressor combining feedback and feedfoward sidechain processing. United States Patent An audio compressor having both a feedback compressor and a feedforward compressor. The feedback compressor operates so as to provide envelope detection.
A sidechain processor which receives an input from the feedback path includes a non-linear low-pass filter and a non-linear amplifier. The output of this processor provides the gain-control signal for the feedforward compressor.
The main audio path is through the feedforward compressor. Orban, Robert A. Belmont, CA. Click for automatic bibliography generation. AKG Acoustics, Inc. San Leandro, CA. Download PDF EL-5, UDC I claim 1.
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