[sdiy] rev eng -> fwd eng?

[Don Tillman]

   From: Czech Martin <Martin.Czech at Micronas.com>
   Date: Thu, 30 Aug 2001 12:44:44 +0200

   AFAIK the discrete emulations of analog gear are not the result of
   thoroughly analysis of the circuits.

I believe that's a fierce understatement.  Most descriptions of
"emulations" that I've seen don't qualify as emulations.  It seems the
digital keyboard biz has removed all meaning from the word.

   I GUESS that a four pole filter is turned into a four pole (plus
   four zeros) filter by the usual bilinear transform for example,
   which would be a very crude approximation because all the nonlinear
   effects are simply dropped, and new problems are introduced
   due to the frequency warping etc.

Right.  That's not an "emulation" -- that's a basic four pole digital
filter. 

   Now, in the recent years there has been a lot of research,
   also on this list, and computing power has shown no deviation
   from Moore's law so far. I think we're at the edge where a full
   blown circuit simulation with very high internal sampling rate
   is possible, ie. emulation by "SPICE" simulation.

I should point out that SPICE is well known for doing a really bad job
of modeling an analog circuit.  There are number of simple and common
circuit configurations that leave SPICE unable to continue.  Or
certain component values will keep SPICE from converging. Robert Pease
devotes significant space in his book "Troubleshooting Analog
Circuits" to ways to get around SPICE problems and interpretting the
results.

   Are there spice models of tubes?

Sure:
  http://www.duncanamps.com/spicevalves.html
  http://www.lynx.bc.ca/~jc/

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com