Feedback and Feedforward Compressors

Feb 16 / Max Porcelli
Hello, and welcome to HTMEM Blog.
Regarding Compressors, one thing we need to understand is the fundamental difference between the two types of compressors.
We know that the sound usually gets compressed based on the input volume, the attack, the release, the threshold and where present, and the ratio settings. If you want to know more about compression controls, look here.

The Side Chain Circuit

Now, not to be confused with the side-chain input control, which we have already seen here, the so-called side-chain circuit is the detector circuit that tells the compressor to adjust the incoming signal level according to the settings we made.
So, depending on where the sound to be compressed is analyzed in the signal path, we can have two types of compressors: the Feedforward and Feedback Compressors.
Let's understand something more.

Feedback Compressors

Feedback Compression
In a Feedback Compressor, the input signal is initially compressed with the setting we made, so attack, release, threshold, and ratio. Then the same signal is rechecked at the output stage to see if the signal is above or below the threshold.
Some classic Feed-Back Compressors are the Urei or Universal Audio 1176, the Optical LA-2A, the Neve 33609, EMI TG1, and many Tube compressors. 

See compressor types here.

Feedforward Compressors

FeedForward Compression
In a Feedforward configuration, the input signal is analyzed before the side-chain circuit, and the gain-reduction is consequently applied accordingly to the compressor settings.
Classic Feedforward Compressors are the DBX 160, or a classic SSL Compressor, to name a few. For more compressor types, have a look here.
These types of Compressors are great for taming fast transients or hard-limiting applications.
Of course, a Noise Gate has to be absolutely feed-forward.

Conclusions

Generally speaking, in terms of signal processing, feedforward compressors are more accurate than feedback compressors, But depending on the type of audio material, feedback compressors tend to sound more musical thanks to their less exact nature.
Currently, as of the date of this post, the only compressor able to blend a Feed-Forward and Feed-Back compressor type is the Great River PWM 501.

In comparison, the API 2500 and the Neve Portico compressors can be switched between the two modes.

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