[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

[rburnett at richieburnett.co.uk]

I agree "mskala".  Technically the receiving end of a MIDI cable *IS* 
terminated for differential-mode signals (i.e. the wanted MIDI signal,) 
and it's un-terminated for common-mode signals (eg. noise picked up 
along the way.)  It's easy to see this because the load (the 
opto-coupler) is tied between the two MIDI lines and is therefore driven 
by the differential-mode signal.  But any common-mode signal has no way 
of getting back to ground at the MIDI input port, unless there's some RF 
filtering capacitors or something from the MIDI IN lines to the ground.

-Richie,


On 2015-09-11 16:59, mskala at ansuz.sooke.bc.ca wrote:

> A transmission line is two conductors.  The line has a characteristic
> impedance, which will ordinarily be a pure resistance.  It is 
> terminated
> if, at the end of the line, there is a resistance (or something that 
> looks
> like a resistance) equal to the line's characteristic impedance.  If 
> that
> is the case, then at the start of the line, sources will see just the
> line's characteristic impedance and not something more complicated.  
> With
> a properly terminated line, signals will go from the start to the end 
> and
> then be absorbed nicely in the resistance.  In any other case, some 
> part
> of the signal will bounce off the end and propagate back to the start,
> and the impedance seen by the source connected to the start of the line
> may be something other than the line's characteristic impedance, and
> other than a pure resistance.
> 
> Pretty often, especially if it's an unbalanced line, one of the 
> conductors
> will be connected to ground at some point.  But that has very little to 
> do
> with termination.  The termination is between the two conductors of the
> line, nothing to do with DC ground.
> 
> At least, that's how termination works with the transmission lines used
> in radio electronics.  Is this term used to mean something completely
> different in the audio realm?  It wouldn't be the first time.
> 
> Are you just referring to a single conductor as a "line" and assuming
> the other conductor is the unspecified path back through ground?  In 
> that
> case you'd certainly be right that such a path is necessary, but I
> wouldn't call it a transmission line in the first place, and there'd be 
> no
> realistic hope of figuring out what the characteristic impedance 
> actually
> might be.