Sidechain Revisited

With the building blocks of compressor design in place we will now begin the development of our compressor implementations. We will show how the ordering of our sidechain main componants (gain computer and level detector) along with a few additional operations can produce perceivably different compression characteristics. Our analysis begins by introducing our first signal flow variable - sidechain orientation.

Feedforward vs. Feedback sidechains

The choice between a feedback sidechain system and a feedforward sidechain system is essentially a selection of sidechain input signal. In the feedforward case the sidechain input is a copy of the raw input signal - this is exactly how we have presented the sidechain thus far. In a feedback design, the input to the sidechain is the previous output sample of the system. This is illustrated at a high level in the figure below.

Figure 1: Feedforward (top) and Feedback (bottom) sidechain configurations

The tradeoffs between the two configurations in the digital realm are primarily computational. We found our feedback designs to be significantly more expensive due to the need to delay the sidechain by one sample for every iteration. However, historically speaking feedback designs have been favored for two reasons:

Because the sidechain is working on a signal which has already been processed it can operate over a smaller dynamic range
Imperfections from any Output Gain stage could possibly be filtered out by level detector in the sidechain.

For these reasons and in the interest of completeness, we have chosen to implement compressors with both feedforward and feedback sidechain configurations.

Easy enough - next!