[sdiy] MIDI notes filter which is also a MIDI converter for Junos ?

[Dave Brown]

I appreciate your feedback and the time to answer.  It certainly "pushes"
the MIDI spec but I disagree on the opto-isolator point.  This is a
"floating" processor directly attached to the transmitting device.  It will
only operate if the transmitting device supports pin 2 of the connector
which is ground.  I think I have seen at least one commercial product which
did not have this connection.  I think of it as an "extension" of the
transmitting device ... just a bit more circuitry on the output.  There are
no other connections to any other ground since it is self-powered.  The
receiving device will still have an opto-isolator so there ground isolation
is maintained between the two instruments.

The MIDI spec does not sanction providing power; neither does it prohibit
it.  It does say that the transmitting device must provide up to 5 mA of
current.  On the receiving side it says the device can consume up to 5 mA.
Since I am not driving a an opto-isolator 5 mA is plenty of current to power
the device.  The voltage level will fluctuate, hence the need for a crystal
to maintain the baud rate within specifications.  I don't remember what the
typical operating voltage is, but it is lower than 5 volts which is why the
output resistors are reduced from the typical 220R values.  This is not a
product nor was it intended to be so, so I'm not worried about the MIDI spec
in itself but whether it actually operates.  The value is in the software so
anyone concerned could augment the hardware with an external power supply
and an opto-coupler if they wish.  That is the beauty of DIY.

I designed this for two specific issues I had.  One was a Roland setup with
a lot of non-Roland gear.  I don't know if it is Roland specific, but the
JV-1010 volume responds to the expression controller and not the volume
controller.  I used this processor in-line with the Roland to convert volume
controllers to expression controllers.  It works great.  The other was to
fix an issue that I created with my 88 note keyboard.  I repurposed a
Kurzweil Ensemble Grand Mark III digital piano key bed and controller and
didn't include the transpose switch.  With no switch the firmware transposed
C to F.  I **could** have torn it all apart and jumpered the switch but I
chose to just include this processor on the output.  It had the advantage of
adding some code to invert the two pedals from NC to NO.

Your self-powered observation is a good one.  MIDI Solutions makes a lot of
stand-alone gadgets that are powered by the MIDI.  I've tried this on a
number of different units and they all have worked (except for that one that
did not have the ground pin 2).  I've done some specific code for it to
implement a keyboard split for some people in the UK who have some
multi-keyboard setups and I did caution them that there could be issues but
it worked perfectly.

I will say that the keyboard reversal in the standard firmware is fun to
play with.  It takes about 10 minutes to get used to the idea that your
hands are reversed and then (at least I) can start thinking that way.  I
also have better bass riffs with my right hand:)

Thanks again for the comments.  I should have mentioned the self-powered
caveat in my original post.  This is a great forum with an exceptional
knowledge base.  Take care.

Dave

-----Original Message-----
From: rsdio at sounds.wa.com [mailto:rsdio at sounds.wa.com] 
Sent: Saturday, December 28, 2013 5:26 PM
To: Dave Brown
Cc: synth-diy at dropmix.xs4all.nl List
Subject: Re: [sdiy] MIDI notes filter which is also a MIDI converter for
Junos ?

Dave,

I prefer to encourage DIY efforts, but this design violates the MIDI
specifications in multiple ways.

First of all, there is no opto-isolator on the MIDI input, which risks
ground loops between your synthesizers. Is there any reason you did not use
the standard chip?

Second, powering a circuit from the MIDI cable is a hack that was never
sanctioned by the MIDI Manufacturer's Association (to my knowledge - if it
has been added to the spec, please let me know). In this age of 3.3V
devices, there's no guarantee that a MIDI output will actually be 5V. As
long as a device delivers the required 5mA of the MIDI spec, then  it's
theoretically possible for the output to be less than 5V. Of course, the
example MIDI schematics do show 5V, but that supply voltage is never
mentioned in the text as a requirement.  
Fortunately, your circuit does provide the option of external, regulated
voltage.

Brian Willoughby
Sound Consulting


On Dec 20, 2013, at 17:03, Dave Brown wrote:
> This is a real DIY solution but I had similar needs for filtering 
> stuff, transposing, etc. so I designed a small MIDI-powered processor 
> system.  The MIDI received stream is parsed to each of the base 
> commands which then reassemble them for transmission.  You can do 
> whatever you want to the MIDI stream with it.  The schematics and code 
> is public and on my site at 
> http://modularsynthesis.com/processor/processor.htm
>
> There isn't much to the schematic if you just want a filter:  
> processor,
> crystal and caps, a couple of resistors, and a couple of diodes.   
> You can
> leave off the LED, reset circuit, and of course the switches.  I have 
> a variety for whatever specialty I need.  This is probably more DIY 
> than you are interested in.
>
> I use them for changing expression messages to volume messages, 
> filtering active sensing, transposing midi channels, keyboard splits, 
> and just messing around - hence the keyboard reverse.
>
> Dave