Don Pearce wrote in message . ..
On 10 Jan 2004 01:33:35 -0800, (Svante)
wrote:
Nowadays, it is quicker and easier (less typing) to enter a
transmission line model, than to type in a componet (L,C) model. Use
whatever way floats your boat.
OK, in my boat there are no transmission line models, but a lot of R,
L and Cs.
You would have thought that given enough Ls and Cs (ie dividing the
lumped model very finely) you would end up with a perfect equivalent
of the transmission line model, but you don't. You just end up with an
ever steeper lowpass filter. Up to the cutoff point, this filter does
indeed behave remarkably like a true cable, though.
Yes, so the trick would be to use enough Ls and Cs then.
For much audio work a single L and C seem to do just fine, but there
are problems - like what order should you put them in? In theory you
can put the shunt C at either the start or end of the network, but if
you are modelling a very unmatched situation this won't work. For
example, if you are looking at an amplifier to a loudspeaker, the C
must be put at the speaker end - it has no effect on amplitude at the
amplifier end. In a matched scenario - equal impedances both ends, the
capacitance must be split and placed both ends if the model is to
work. So you must be careful in the application of a lumped model
cable, and understand the significance of the impedances at both ends
before you use it.
I'd think that either is OK as you increase the number of Ls and Cs.
This number will determine a highest valid frequency, and below that
frequency it does not matter much if analog starts with an L or a C.
When dealing with acoustic tubes I ususlly do it like this:
------L/2------*-----L/2-------
|
C
|
---------------*---------------
but one could also do:
-------*-----L------*----------
| |
C/2 C/2
| |
-------*------------*----------
Now, given that I have a high number of these sections, each of the
components will be small, and it will not matter much which model that
is used.
The true transmission line model has the advantage that all this is
taken care of, there is no anomalous lowpass filter effect to worry
about and it is really easy to change lengths - you just alter the
length term. It also works at any frequency. It is a sledgehammer to
crack a nut, though, and representing a cable as Ls and Cs (given the
caveats above) is perfectly proper, particularly if you are having to
hand-crank the results, or just doing a back-of-an-envelope
calculation. If you are using Spice, or something similar that
possesses native transmission line models, then why not use them? They
are easier to use, just as accurate for audio, and vastly more
accurate outside the audio band.
I can understand that this COULD be the case, but I don't understand
it (yet). I guess I'll just have to learn it.