Well, to be strictly accurate the input and output from the MIDI device
before the optocoupler is TTL as opposed to +/- 12V. The output is fed
to the optocoupler's diode and the input comes from the sending device's
optocoupler output transistor, pulled up by a resistor to +5V. So in
essence the PIC or other device is seeing TTL. There didn't seem much
point in mentioning this, since I assumed (perhaps wrongly) that the
requirement for a high-speed optocoupler is well known (it is specified
in the MIDI spec, in fact). The reason for optocoupling is to avoid
ground loops, btw, not specifically for speed or noise immunity.
Musicians tend to dislike hum.

> ----------
> From:         Peter van Hoof[SMTP:pvh@MICROSERVE.NET]
> Reply To:     pic microcontroller discussion list
> Sent:         Tuesday, December 09, 1997 3:26 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: MIDI from PIC devices
>
> Negative , midi doesnt use ttl level but rather current loop. this
> makes
> longer cables and higher noise resistance possible
>
>
> -----Original Message-----
> From: Andrew Mayo <andrew@GEAC.CO.NZ>
> To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
> Date: Monday, December 08, 1997 7:56 PM
> Subject: Re: MIDI from PIC devices
>
>
> >Since MIDI is essentially just RS232 async but at a non-standard baud
> >rate (and of course at TTL levels), implementing the transmit/receive
> >logic is pretty straightforward. However, the baud rate is quite high
> >and will require a significant percentage of the processor's
> bandwidth
> >just to acquire/transmit the data. Of course, you could use a UART
> >device or more than one PIC.....
> >
> >> ----------
> >> From:         Bob Shaver[SMTP:bobs@PMDINC.COM]
> >> Reply To:     pic microcontroller discussion list
> >> Sent:         Tuesday, December 09, 1997 9:18 AM
> >> To:   PICLIST@MITVMA.MIT.EDU
> >> Subject:      MIDI from PIC devices
> >>
> >> This message forwarded to the PIC group with permission of the
> sender.
> >> My reply to the original newsgroup msg is at the end of this
> message.
> >>
> >> ----------
> >> From:   A.....
> >> Sent:   Monday, December 08, 1997 2:05 PM
> >> To:     Bob Shaver
> >> Subject:        Re: MIDI from PIC devices
> >>
> >>  Hi.  Thanks for your reply.  I'm developing a MIDI Brass
> controller
> >> that
> >> has sensors on each of the valves to switch between harmonic series
> >> pitch
> >> classes to be stored in an EEPROM and to do freqency counting on
> the
> >> signal from the instrument using a pick-up mic.  Each valve
> >> combination
> >> chooses a set of 16 pitches and the freq. data narrows the pitch
> down
> >> to
> >> one and outputs it via the serial port.  I also will run the audio
> >> signal
> >> through an A/D convertor for Velocity data.  Where I am novice is
> >> putting
> >> it all together and outputting realtime midi data.  There aren't
> too
> >> many
> >> sources for that, so I hope you can help.
> >>  For the prototype I hope to send pitch and volume/velocity data.
> I'm
> >> not worried too much with pitch bend at the moment, but want to
> >> implement
> >> that in the future.  Also, I'd like the option to map an A/D of the
> >> acoustic signal to the breath control feature.  One thing that I'm
> at
> >> odds with conceptually is note on/off with breath control.  Maybe
> you
> >> have some insight.
> >>  I realize that a lot of these are very general questions and
> require
> >> a
> >> lot of explanation in their entirety.  I hope you can bear with me.
> I
> >> really am in need of some basic starting ideas for the MIDI
> >> implementation.  I'm very familiar with MIDI and the spec.  I just
> am
> >> not
> >> so experienced with the PIC.  My mentor in this project is a PIC
> >> programmer, but isn't too well versed in MIDI.
> >>  What is your implementation of the PIC for MIDI?  It sounds
> >> interesting.
> >> Thanks for your interest and I hope you might be able to give me a
> >> starting shove in the right direction.
> >>
> >> Alex Ruthmann
> >>
> >>
> **********************************************************************
> >> ****
> >> *                                    * "Information is not
> knowledge,
> >> *
> >> *                                    *  Knowledge is not wisdom,
> >> *
> >> * Alex Ruthmann                      *  Wisdom is not truth,
> >> *
> >> * sruthman@umich.edu                 *  Truth is not beauty,
> >> *
> >> * http://www.umich.edu/~sruthman     *  Beauty is not love,
> >> *
> >> *                                    *  Love is not music and
> >> *
> >> *                                    *  Music is THE BEST." -Frank
> >> Zappa *
> >>
> **********************************************************************
> >> ****
> >> *         Horn Performance and Performing Arts Technology Majors
> >> *
> >> *   Undergraduate Research Assistant in Frank Zappology and
> Schlepping
> >> *
> >> *          University of Michigan-Ann Arbor -- School of Music
> >> *
> >>
> **********************************************************************
> >> ****
> >>
> >> On Mon, 8 Dec 1997, Bob Shaver wrote:
> >>
> >> > I have just finished a project that sent and received MIDI, using
> a
> >> 16C73.  Ask away...
> >> >
> >> > Bob.
> >> >
> >>
> >
>