[LAD] [semi-OT] midi snakes using CAT5?

Dan Mills dmills at exponent.myzen.co.uk
Mon Nov 2 01:20:08 UTC 2009

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.

More information about the Linux-audio-dev mailing list