Compandors as distortions?

[JKowalski]

I was reading up on compandors on wiki after seeing them in delays, and I noticed this:

Original Signal

Companded Signal


Doesn't this waveform conversion look the same as a tube screamer? This is a picture from an excellent analysis of the TS that R O Tiree did a while back, showing the input and output of the logarithmic amplifier "clipping" stage of the TS:



Perhaps a compandor would be an excellent building block of a distortion unit?

[Transmogrifox]

It all depends what kind of compander you're talking about.  In radio transmission terms, then yes.  The compressor on the transmitter is a log amp, and the expander on the receiver is an exponential amp to undo what the log amp did (decompress).  This is quite a bit different bear than what's used in an analog delay line for music effects.

In terms of music effects, companders are applied to a signal envelope via variable gain blocks.  You'll notice with these chips there's a filter capacitor.  The incoming signal is rectified and filtered to DC.  This DC voltage is used to control a VCA.  For a compressor, the gain is reduced as the rectified/filtered DC increases.  For an expander, the gain is increased as the voltage increases. This is an automated way of adjusting the volume knob to keep the music at the same level -- you turn it up when it's quiet, and turn it down when it's loud.

Going into the delay line, you compress the signal to make it all relatively uniform and strong.  This helps improve signal/noise ratio and somewhat reduce distortion.  On the output you expand the envelope to what it was before to restore the dynamics. 

A compander combines the two functions (compression, expansion) into a single IC.

The tubescreamer clipping segment is a compressor in radio transmission terms.  If the FM radio receiver never expanded the signal it gets off the carrier it would sound like you plugged your walkman into a tubescreamer.

[JKowalski]

Ah, okay. I was wondering if they worked on the same principles - I was poking around the datasheets after I posted, and I noticed the diode feedback paths in their block diagrams!

I suppose this is kind of pointless - seeing as how compandors are expensive, and it's doubtful that you can mess with them to get up to or past the quality of a simple TS made-for-guitar type design, which uses a common op amp and diodes. And if you did, I guess the sound won't be that different anyways.