On Wed, 07 Feb 2007 19:43:55 +0000, Serge Auckland
wrote:
Bill Taylor wrote:
On Wed, 7 Feb 2007 17:20:28 +0200, "Iain Churches"
wrote:
As an ex-patriot, I am probably often far more patriotic than
someone actually living in the UK. (Maybe distance lends
enchantment to the view:-)
The situation here in Scandinavia is much more healthy, it seems
to me.
In Sweden, everyone who knows anything at all about audio
can tell you about Per Johansson and the Swedish group of
researchers and experts from the renowned German Fraunhofer
Institute, who invented MP3.
Any Finn, who knows anything about audio is justly proud of
the achievements of Matti Otala, the man who first realised the
significance of intermodulation distortion in audio amps in 1973
ITYM transient intermodulation distorion; plain old IM had been known
about for decades.
His achievement was only significant in terms of publicity. TIM was
just another name for clipping inside the loop due to poor design and
was, AIUI, understood by competent designers well before Mr Otala
produced his obfuscations.
Indeed. I was reading his article in Wireless World of 1975 or
thereabouts just the other day. His assertions about TIM just don't
stand up if the input conditions are defined properly. Audio input
signals have a defined bandwidth and defined level. If those are taken
into account in the power amplifier design, then TIM doesn't happen.
S.
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.
Of course slew rate limiting will only be a potential issue in an amp
containing a dominant pole cap which must be charged by the current in
the input stage. So poorer designs - no, or too small a dominant pole
cap - which might exhibit HF instability are less likely to suffer
TID.
d
--
Pearce Consulting
http://www.pearce.uk.com