We all come across Transfer Curves, they are the graphs dynamics processors use to show what particular settings will do to sounds at different levels. It’s not the whole story as they don’t take time into account so don’t show the effect of attack and release settings but they give a good indication of what different thresholds, ratios and makeup gain settings will do.

The most familiar form these take is like the one found in the old Dyn III dynamics plug-ins. In these input level is shown across the x axis and output level is shown across up the y axis. A 45deg line means input equals output. This is the same as bypassing the compressor. If the line above the threshold tips down towards the horizontal then the output of signal over the threshold is less than the input. This is downward compression (i.e. the loud bits are getting turned down…).

There is an alternative way to display this information and the Avid Channel Strip plug-in and the Pro Series Compressor and Expander all use this method. Instead of plotting input against output they display input (still on the x axis) against gain reduction, on the y axis. In this situation no compression (previously a 45deg straight line) would be a straight horizontal line.

How Is Plotting input Against Gain Reduction Useful?

I didn’t really see the point of this method when I first came across it. It struck me as a system which was OK but had no advantage over the old in/out system. Why is it useful?

One benefit it has is that it is visually similar to the gain reduction meters in many compressor plug ins and DAW mixers which show gain reduction as a downward firing meter next to an upward firing level meter, and histograms which are more and more common in dynamics processors. Plug-ins like Maxim have had static histograms for a long time but modern compressors and limiters from the likes of iZotope and FabFilter use scrolling gain reduction history displays showing time on the x axis and gain reduction on the y axis. If you see the downward action of gain reduction in your DAW mixer in scrolling histograms and in transfer curves it helps visualise the relationship between level and compression.

Whether or not visualising this kind of thing is desirable is different discussion…