1 Nov
2009
1 Nov
'09
9:20 p.m.
On Mon, 2009-11-02 at 10:52 +1100, David wrote:
Another thing about the that MIDI spec is where it
says "optoisolators
... rise and fall times should be less than 2 microseconds" which is
amusing because the total time of one midi bit is 3.2 microseconds. So
don't imagine you have nice square bits driving the system even if
your cable is zero length. So your optoisolators *might* be a limiting
factor depending on their speed (ie age, cost).
I think you make a factor of 10 error! One midi bit is 1/31250 = 32
microseconds, not 3.2 which makes a difference.
On transmission line effects, lets see, 31,250 baud lets say you need
the first ten harmonics to give a reasonable eye pattern, so say 300Khz
bandwidth, and that transmission line effects become important at one
tenth of a wavelength (reasonable rule of thumb), and that velocity of
propagation is 0.6C, then:
Wavelength = 300,000,000*.6/300,000 = ~600M, so transmission line
effects can be totally ignored out to at least 60M or so.
Lumped constant models look to be quite good enough, and it will almost
certainly be a 5mA sources ability to charge the cable capacitance that
will impose the ultimate limit.
Current loops tend to have excellent interference rejection, and the
fairly tight twisting in cat 5 can only help with this.
Regards, Dan.