On 2014-12-21 16:23, Andreas Ladanyi wrote:
Am 16.12.2014 um 18:54 schrieb Tim Wescott:
Hey Andreas:
<< snip >>
Domain = n is for a discrete-time system where the sampling interval
is
defined. It works exactly like domain = 'd', except that things that
depend on the real-world frequency, like Bode plots, will come out
right.
At this point i am a little bit confused. How does syslin works when
the sampling interval is not defined (dom=d) ? And why the bode plots
come out wrong ? What syslin is doing wrong if dom=d ?
Syslin, by itself, is just a structure consisting of three or four
matrices (the 'D' matrix is optional) that describe a system, and a
domain tag. The domain tag just tells the various things that use the
syslin structure how to interpret the description.
When you set dom = 'd', the various bits of code that use syslin either
interpret the structure to describe some sampled system of unknown
sampling interval (i.e., ss2tf, which returns a ratio of polynomials in
z with dom = 'd'), or they implicitly take the sampling interval to be 1
(i.e., Bode plots).
Syslin isn't doing _wrong_ if dom = 'd', unless the system described
actually does have a well-defined sampling interval. There are systems
that do not have a well-defined sampling interval, or at least not in
time -- I have worked on phase-locked loops that sample on every cycle
of the oscillator (or motor), and use the duration of a cycle as the
feedback. In this case the sampling rate is itself being servoed, and
as such is not constant. Sometimes you're always locking to the same
reference, and you can take that reference as your sampling interval --
sometimes not. It's also not uncommon to want to define some signal
processing to be done on data that's sampled at positions on a line or a
grid, in which case the "frequency" is in cycles per meter (or pixels,
or whatever) -- in that case, having a domain that implies a sampling
frequency in Hz is misleading.
Note that I'm not the original author of this data structure, so I may
be missing some finer points of intent. I just do what works for me: I
set the domain to the actual sampling rate if I know and care what it
is, or to 'd' if I just want to play around in the abstract, without
being bound to any particular sampling rate.
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