Time varying Transfer Functions

[Vivek]

Most guitar pedals use a fixed Transfer function for the distortion stage. That transfer function does not change in shape or bias point.

The diodes have a fixed transfer function curve. The bias is fixed irrespective of the level of signal that came before.


The next step up are circuits that "bias shift" the signal up or down the static Transfer Function curve. The curve remains the same shape, but the centre point of the output can be made to vary up and down based on build up of a bias voltage. This is a Time Dependent Bias point.

This supposedly adds some "movement" and feel since the harmonics created depend on the strength of plucking etc



I know nothing about Tubes, except that sometimes, you can travel from one part of the city to another using them.

But I understand that triode stages exhibit static transfer function curve with dynamic bias point

and I was (today years -2) old when I heard that Pentodes have Time Dependent Transfer Function Curves along with time dependent bias point due to bias point shift by DC build up on caps.


Source: Guitar Amplifier Overdrive
By Ulrich Neumann, Malcolm Irving
Pages 140-145


Curve on the left is transfer function for very small signals or at start of a large signal. Curve on right is the transfer function for continuous large signal. In practice, the transfer curve keeps on mutating in a series of curves in between these two extremes.

When a stage has time dependent Transfer function that changes with signal amplitude, even non-distorted signals change character based on player's style and technique, increasing the sensitivity of the Amp to the player's touch.


If most pedals have not implemented time dependent transfer function curves, it either means

A) There is not sufficient advantage to implement it

B They are relying on a downstream tube amp to provide this "movement"


But then it raises the question: Suppose one is designing an "Amp in a Box" meant for direct to desk applications, does it add significantly to the user experience to implement time dependent transfer function curves ?

PS: The ORIGIN EFFECT Amp in a Box have "Sag control" which supposedly approximately approximates this.