Audio Processing: Exploring Dynamic Range Compression
Feedforward vs. Feedback: A closer look
If you listened to all six audio samples available on the previous page you may have noticed that the difference between a feedforward and feedback compressor can be difficult to hear. Below we offer a graphical and analytical comparison of the R20 topology in both feedback and feedforward orientation as a way of attempting to make the difference more clear.
Figure 1: Original, Compressed, and Difference signals for Guitar using FFR20
Figure 2: Original, Compressed, and Difference signals for Guitar using FBR20
The best way to see the difference between these two orientations is to take a look at the last half of the difference plot for each configuration. Notice how the feedforward compressor is not compressing as much as the feedback compressor in this region. This is the result we expect - because the feedback compressor is using the previous output samples in the sidechain we see a more laggy response, which translates to more smooth transitions out of compression, especially in less transient material where the input signal is more consistently above or below the threshold.
Spectrogram plot
Figure 3: Illustration of difference between the feedforward and feedback topologies through spectrogram analysis. The first plot is a spectrogram of the difference between the FFR20 and FBR20 when operating on drums, the second is that difference plotted over time. The final plot is the original drum signal
Shown above is a spectrogram analysis on the difference between the feedforward and feedback R20 applied to our sample drum track. We can see that the difference between the feedforward and feedback orientations with the same parameters only exists for transients and disappears for steady state portions of the signal. This is due to the extra delay involved with feedback, as well as the difference between the signals which determine the gain computation, which results in a larger disparity in system response during quickly changing portions of the input signal.