Let me express my agreement with the nice choice of subject: the
simulation of tube amps. Of course during and before the advent of solid
state systems, some people may have laughed about the idea alone
(because tubes sound so annoying after while), but in the context of
guitars, it's usually nice.
Of course there's the question as to what is nice to simulate about
them, such as it is often suggested that a tube stage will mostly add
second harmonic distortion, but of course certain transients, multi-pole
filter and multiplicative effects are there as well. Probably for a
guitar, the added harmonics, which are a function of the tones and
volume you play, can be best generated by a mild (non-resonant) band
pass with at least a true high-pass (zero DC component remaining), and
possibly you want the average the result, maybe with a potent at least
multi-coupled impulse cabinet simulation.
About delay: clearly, it's important to acknowledge the existence of
delay in just about every digital system, while it is possible to get
rid of it altogether (except for the speed of light) like I've worked on
with digitally controlled analog parts, in a full digital system and
current clock rates: it's always there. The character of the delay has
been mentioned, sure that's important too, and it's not a ad idea to
think that in most current practical cases, there's a certain form of
limited reconstruction filter in the DA converter, that certainly has a
character (try feeding the sound round and round a few times, you''ll
get a good idea of that character). To be sure to know the difference
between a general delay and a digital one, use a speaker at big
distance, or a tape echo. The constancy of the delay is another factor
that is important to players and singers, with as possible exception of
naturally sounding equalization (which also introduces some phase
delay): the more constant the delay is, for my personal preference to
the accuracy of a mid frequency wave length easily, the better. Finally,
there's the "digital hole" in the sound you get when a (reconstruction
error containing) digital system amplifies a microphone and is heard
back "live": probably it can be instantaneously spotted for it's
unnaturalness, compared with an analog system, which is something to
think about.
Concerning the sources having created the advent of the h(z) things,
from what I recall from my university text-books, the main idea was
simply the congruence of the h9z0 with the h(s) or H(1/(2 PI jW)). Of
course the undergrad EE student will have the understanding that the
digital signal will be a sequence of impulses, which in principle can be
reconstructed into and analog signal, and of course we also know the
idea of a z-1 delay is in principle an infinite order system, so tht
it's a bit different transform than a perfect digital form of the Linear
System Theory and its very complicated complex plane analysis.
T.
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