[sdiy] (OT) Doepfer MIDI-CV oddity

[Jerry Gray-Eskue]

> I think the reason for problems with 3+ stacked THRUs is that the
> couplers may have different delays for rising and falling edges (it is
> with the PC900), so the serial bit timing gets messed up more and more.

Agreed. That was what I was talking about here:

>> > This is true of any communication link that uses an "analog" repeater,

> It's quite different if there is a software THRU involved

Also agreed.

>> > if you use a device that regenerates the digital pulse train using a 
>> > system
>> > clock you get a correction of the distortion at that stage.

> quite certain to introduce delay and jitter.

and again I  agree.

>> Oh I forgot to mention that rebuilding the pulse train Also adds a delay 
>> in
>> the data stream which can be yet another problem for real time stuff like
>> MIDI...

I think we are on the same page here.

Jerry

----- Original Message ----- 
From: "ChristianH" <chris at chrismusic.de>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, January 19, 2011 10:04 AM
Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity


> No, I don't think delay is a problem here, not in a way that real-time
> behavior would suffer.
> We have single-digit microseconds here, at least with the PC900 (which
> is the 'official' MIDI receive coupler, the standard explicitly shows it
> in the suggested circuit).
> I think the reason for problems with 3+ stacked THRUs is that the
> couplers may have different delays for rising and falling edges (it is
> with the PC900), so the serial bit timing gets messed up more and more.
>
> It's quite different if there is a software THRU involved - that one is
> quite certain to introduce delay and jitter. However, as far I have seen,
> those are pretty rare. (But hey - you have perfectly re-generated bit
> timing, isn't that something...)
>
> Chris
>
>
>
> On Wed, 19 Jan 2011 08:44:06 -0600 "Jerry Gray-Eskue"
> <jerryge at cableone.net> wrote:
>
>> Oh I forgot to mention that rebuilding the pulse train Also adds a delay 
>> in
>> the data stream which can be yet another problem for real time stuff like
>> MIDI...
>>
>> ----- Original Message ----- 
>> From: "Jerry Gray-Eskue" <jerryge at cableone.net>
>> To: "Ullrich Peter" <Peter.Ullrich at kapsch.net>; "Colin f"
>> <colin at colinfraser.com>; <synth-diy at dropmix.xs4all.nl>
>> Sent: Wednesday, January 19, 2011 8:39 AM
>> Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity
>>
>>
>> >> The problems get bigger and bigger the more MIDI gadgets are connected 
>> >> in
>> >> series as
>> >> the MIDI pulses get more and more distortions in each thru stage.
>> >
>> > This is true of any communication link that uses an "analog" repeater, 
>> > if
>> > you use a device that regenerates the digital pulse train using a 
>> > system
>> > clock you get a correction of the distortion at that stage.
>> >
>> > ----- Original Message ----- 
>> > From: "Ullrich Peter" <Peter.Ullrich at kapsch.net>
>> > To: "Colin f" <colin at colinfraser.com>; <synth-diy at dropmix.xs4all.nl>
>> > Sent: Wednesday, January 19, 2011 6:17 AM
>> > Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity
>> >
>> >
>> >> Hi!
>> >>
>> >> And the 6N139 switching times can further be optimized by used a bias
>> >> resistor on the
>> >> base of the opto-receiver transistor.  Without this receiver the 6N139 
>> >> is
>> >> quite slow.
>> >> I normally use a 15k resistor from pin 7 to GND.
>> >>
>> >> The maplin MIDI thrubox has this base resistor in it and works very 
>> >> fine.
>> >> I had the same
>> >> circuit without the resistor in my own thrubox and had problems.
>> >>
>> >> The problems get bigger and bigger the more MIDI gadgets are connected 
>> >> in
>> >> series as
>> >> the MIDI pulses get more and more distortions in each thru stage.
>> >>
>> >> Ciao
>> >> Peter
>> >>
>> >> -----Original Message-----
>> >> From: synth-diy-bounces at dropmix.xs4all.nl
>> >> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Colin f
>> >> Sent: Wednesday, January 19, 2011 12:58 PM
>> >> To: synth-diy at dropmix.xs4all.nl
>> >> Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity
>> >>
>> >>
>> >>> As an aside, this is the kind of thing that makes life hard for
>> >>> manufacturers-
>> >>
>> >> Yep, that's why I'm so well acquainted with the range of 6N139s...
>> >>
>> >> Another new factor in the MIDI equation is that some PC interfaces 
>> >> drive
>> >> the
>> >> line with a lower voltage supply.
>> >> MIDI is a current loop, so in theory you just need to reduce the 
>> >> series
>> >> resistors and 3.3v pull up on pin 4 should be no problem.
>> >> But if the forward voltage drop on your opto is higher than usual, you
>> >> might
>> >> find the margins of operation sooner than you'd like.
>> >>
>> >> Cheers,
>> >> Colin f
>
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