Non-linear techniques with filters

[Grant Richter]

> 
> Now that Terry and Juergen have brilliantly dissected the mechanism at work
> in the transistor ladder, it suggests another class of filter which
> deliberately exploits this perceptual effect.

To clarify:

It appears the ladder filter design introduces distortion at the crossover
point. The ladder introduces non-linearities at the input transistor, which
are compensated for by complimentary non-linearities on the output
transistor, except at the cross over point. It is generally agreed this
sounds good (perceptual advantage).

Since we know the mechanism, can it be applied to other filter topologies?
If a non-linear process is applied to the audio before filtering and a
complimentary process used after filtering to re-linearize, will a
perceptual advantage be gained?

One example immediately springs to mind. A CA3080 has a very limited linear
range. To extend the range, diodes are used as compensation on the input
side. But what if the compensation were done on the OUTPUT side? Use a large
signal into the 3080, then use an inverse of the input non-linearity on the
output side to make the whole stage linear. This is effectively input
compression and output expansion, a technique long used to improve signal to
noise ratio.

If an integrator is formed like this, the capacitor may produce the same
effect as the ladder filter. Now cascaded 3080 designs would have that
"ladder" sound. But other topologies are possible such as biquads and
Sallen-Key, which may have the perceptual advantage of the "ladder" sound.

Thanks again Terry!