MIKE ROSS wrote in a message to Roy J. Tellason:

MR> "Roy J. Tellason" wrote to "MIKE ROSS" (02 Feb
03  20:01:43)  ---
MR> on the topic of "Monitors"

MR>  "Roy J. Tellason" wrote to "Jasen Betts" (02
Feb 03  04:06:16) 
MR> ---  on the topic of "Monitors"
JB> MIDI is also balanced,

RJT> No it's not.  Typical driver circuit is one resistor to +5 and another
RJT> tied to the output of an open-collector TTL gate.

MR> MIDI is 5mA current loop, logic 0 is current ON.

RJT> Right.  But it's still not balanced...

MR> I didn't say it was, Jason did. 

Jasen,  yeah.

MR> I was only pointing out it was a current loop similar to the old 
MR> IBM 20mA spec, except it is 5mA.

Did that originate with IBM?  I thought it was a teletype thing.  I know
that before that they used to have a 60mA current loop,  too.

JB> but AFAIK most midi line driver circuits arent up to driving the
JB> signals long distances.

RJT> Yeah,  a couple of 220 ohm resistors at the driving end and one more
RJT> at the receiving end don't give much margin when run off a +5
RJT> supply...

MR> MIDI spec is 50 ft max but 100 ft is not unusual.

RJT> So they used 74xx in stead of 74LSxx parts on the driving end,  or
RJT> CMOS instead of 74LS on the receiving end...

MR> Nothing special done, just regular 74LS jelly beans.

Hm,  maybe a cable quality issue,  then?

MR> Also note that each MIDI IN usually has a MIDI THRU which is a 
MR> repeater so that more boxes can be daisy-chained for even longer 
MR> distances. Of course the problem may then arise having to deal with 
MR> delays. For example some MIDI OUT's/THRU's mix the local data with 
MR> the received data so that there are processing delays involved.

Thru isn't supposed to do that.

MR> A normal THRU however has very little delay other than the 
MR> opto-coupler/gate and the negligible 1ns every 9 inches through 
MR> space or wire conductor.

That's how it's supposed to work.

MR> When delays get into the 1ms range then it's troublesome. Kind of 
MR> like a beginning pianist where a chord will sound with a tiny bit
MR> of delay between each tone. It's usually not that bad however.

I've known of some musicians at least that had to change their setups
around or even buy multi-way boxes to avoid this problem.  Of course, 
these guys had a fair amount of equipment...

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