On Thu, 08 Feb 2007 09:27:23 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:


It is all about slew rate limiting - that is the mechanism that gives
rise to transient intermodulation distortion. You do, as you say, have
to make some assumptions about maximum levels and maximum frequencies.
With a digital source you can do that with a high degree of confidence,
but with an analogue one, there is always the opportunity for stuff to
be outside your assumed limits.

In practice, simply putting a passive LPF before the amp input resolves
that problem as it defines the maximum possible slew rate for a signal
whose amplitude would not also clip the amp. This is a trivially easy
'solution' that designers did almost without debate long prior to Otala's
publications and talks.

LPFs are good, but not much loved by manufacturers who play the
numbers game in their sales literature.

Although his ideas made a 'splash' in places like audio mags, my impression
what the most experienced designers thought he was making a mountain out of
a molehill and getting publicity for sticking a new label on a long-known
behaviour.

Out of curiosity I checked my texts by JLH and Self. JLH gives one brief
para to Otala. Self essentially dismisses him with a comment referring to
one of his papers.

I suspect Otala was doing no more than voicing facts that everybody
already knew.

d

--
Pearce Consulting
http://www.pearce.uk.com