TweetFollow Us on Twitter

Midi
Volume Number:1
Issue Number:11
Column Tag:Sound Lab

The Midi Connection

By Kirk Austin, San Anselmo, CA.

This article will describe one way to interface a Macintosh with the MIDI standard for communication with music synthesizers and related equipment. By making a few hardware and software changes to the existing Macintosh serial ports, MIDI ports can be created. A Macintosh equipped with MIDI ports can act as a very powerful controller for musical equipment. By sending and receiving MIDI data the Mac can do things like record synthesizer music, edit it, play it back, and do an automated mixdown to stereo. The implications of this capability are great. Many of the functions of a modern recording studio can now be brought into the Macintosh owner's home.

MIDI stands for Musical Instrument Digital Interface and is a standard that was agreed upon by the various major manufacturers of electronic music synthesizers. The MIDI standard is a protocol for transferring data between music synthesizers and computers. It allows equipment made by different manufacturers to communicate with each other. Originally it was designed to let one keyboard instrument control another one. You could play the keyboard on synthesizer "A" and the sound would be produced by synthesizer "B". But the originators of the MIDI concept gave it enough flexibility to allow it to do other things as well. Now it is possible to buy MIDI gear that will control lighting equipment and mixing boards as well as keyboard synthesizers. An important point to remember is that MIDI recorders don't actually record the sound that is produced by the synthesizer but just the control signals that the synthesizer needs to produce a sound. For instance, when a key is depressed on a synthesizer keyboard at least three bytes of information are sent out over the MIDI cable: the code describing a note down event and MIDI channel, the number corresponding to the key (middle C=60), and the velocity that the key was struck with (used for dynamics). A note up event is recorded in a similar fashion. So, altogether it takes six bytes to record a note of any duration. This makes for a very compact data structure compared to actually recording the sound itself.

In order to use the MIDI ports on the Mac you will have to write your own serial drivers. As far as I know the driver routines for configuring the serial ports must be written in assembly language. If you have a high level language compiler it should allow inline assembly code in order to take advantage of the information presented in this article. If your compiler doesn't allow this you may want to read on just to further your understanding (you may also want to consider getting another compiler). This months coverage will be of the hardware interface necessary to convert the Mac's serial ports to MIDI specifications. The software driver routines will be covered in a future article.

The most important consideration in converting to the MIDI way of doing things is the baud rate. MIDI is clocked at 31.25K which is a nonstandard rate. The Macintosh uses a Zilog 8530 chip to handle the serial ports. In order to get the Mac's 8530 chip to handle data at the MIDI clock rate you have to provide an exernal clock signal for it to use as a timing reference. I have used an oscillator circuit and a divider in order to derive a 500K clock signal. This is again divided by 16 in the 8530 to produce the necessary clock rate of 31.25K. Note that you may optionally use other pins on the divider chip to get a frequency of 1 Meg or 2 Meg. Either of these rates will also do the trick if you select the appropriate divisor in the 8530's write register #4 (more on this in the forthcoming software description). Other than the external clock the interface consists of some level shifting and isolation circuity in order to make the Mac's signals conform to the MIDI specification.

The circuit is pretty straightforward. First we need to generate the clock signal which is done with the circuit built around the crystal. The two inverters, two resistors, capacitor and crystal form a 4 Meg oscillator that drives the 74LS93 binary divider chip. One of the outputs of the divider (depending on which frequency you want to use) is routed to a capacitor and then a resistor to ground that is used to offset the pullup resistors internal to the 26LS32 receiver chip in the Mac. The reason this is necessary is that the negative input of the 26LS32 is referenced to ground, so we need to swing at least 200 millivolts below ground in order for the 26LS32 chip to recognize it as a legitimate clock signal. This particular idyosyncracy of the Mac is rather annoying, but not unworkable.

The transmitted data signal line is the next consideration. The TXD+ signal from pin 4 of the Mac is fed through a couple of inverter stages to provide the TTL output required by MIDI. The resistor and diode in front of the inverters protects them from the negative voltages produced by the Mac. This is the easiest part of the interface.

The MIDI input port uses an optoisolator to avoid a condition known as ground loops. This happens in audio systems when there is a slight voltage potential between the ground lines of two different pieces of equipment. The result being an irritating 60 cycle hum in the audio output. The optoisolator keeps the equipment from reacting this way by eliminating the common ground connection. The Sharp PC900 is a MIDI specified part, although other optoisolators with Schmitt trigger outputs can be used. The output of the opto is routed to a couple of inverter stages to provide the RXD+ and RXD- signals that the Mac wants to see.

All that's left is to provide the power and ground connections. According to the hardware information I got from Apple the +5 volt pin on the serial connectors can provide as much as 200 milliamps output current. This interface draws about one tenth that amount.

The clock signal may be wired to a three way switch in order to switch between the three different speeds, or it may be hardwired if you are only going to be using one speed. So far all three speeds have been used by one piece of software or another.

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.