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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Soft Drummer (Music)
Soft Drummer 1.0 Device: iOS Universal Category: Music Price: $14.99, Version: 1.0 (iTunes) Description: Soft Drummer is the closest thing to a pro session subtle drummer in your pocket. Easy to use and fast, it's much more than a... | Read more »
Is GO Gear the Pokemon GO map app you...
Now that we've settled into something of a Pokemon GO status quo, the number one desire of most players can best be summed out by modifying a quote from Rod Tidwell of Jerry Maguire: "Show me the Pokemon!" [Read more] | Read more »
Rodeo Stampede update: Mountains, new an...
The Savannah and Jungle were just the beginning in Rodeo Stampede. Get ready to head for the Mountains. I think I heard that in a beer ad once. [Read more] | Read more »
COSMOS RINGS (Games)
COSMOS RINGS 1.0.0 Device: iOS iPhone Category: Games Price: $5.99, Version: 1.0.0 (iTunes) Description: This game cannot be played without the Apple Watch.Released anniversary sale until August 31,2016 PST! A tragic tale of time's... | Read more »
How to get started selling on Mercari
As far as ecommerce has come over the last decade or so, there's still a tremendous opportunity to make it easier for people to buy and sell goods. That's especially true when it comes to shopping apps, which should only continue to increase in... | Read more »
Human Anatomy Atlas 2017 Edition - Compl...
Human Anatomy Atlas 2017 Edition - Complete 3D Human Body 1.0.24 Device: iOS iPhone Category: Medical Price: $24.99, Version: 1.0.24 (iTunes) Description: | Read more »
Heroes of Normandie (Games)
Heroes of Normandie 1.5 Device: iOS Universal Category: Games Price: $14.99, Version: 1.5 (iTunes) Description: The game does not support iPhone 4s and below | Read more »
Why you should never power up Pokemon in...
There's no question that candy is dandy in Pokemon GO. You need big quantities of it to evolve your Pokemon, and when combined with stardust, it can be used to power up your favorite pocket monsters as well, making them more formidable for the gym... | Read more »
Webzen launches 3D MMORPG MU Origin on i...
Mu Origin is featured time and time again at the very top of App Stores in China, and within the top five worldwide top-grossing charts on Google Play.Its popularity in Korea and China, featuring more than 120 registered players in China and 6... | Read more »
Severed (Games)
Severed 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: LAUNCH DISCOUNT ON NOW!! ENDS AUGUST 4! ==== Take control of a one-armed warrior named Sasha, wielding a living sword on her journey... | Read more »

Price Scanner via MacPrices.net

9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9″ 32GB WiFi Space Gray Apple iPad Pro on sale for $50 off MSRP including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 32GB WiFi iPad Pro: $549 $50 off... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1849 $150 off MSRP - 15″ 2.5GHz Retina... Read more
Second-Quarter Tablet Shipments Fell 4.8% –...
The latest report from the global market research firm TrendForce finds that worldwide tablet shipments for this second quarter totaled 33.54 million units, representing a quarterly drop of 4.8% and... Read more
Global Smartphone Sales Volumes Mark Second S...
According to preliminary results from the International Data Corporation (IDC) Worldwide Quarterly Mobile Phone Tracker, vendors shipped a total of 343.3 million smartphones worldwide in the second... Read more
Apple TVs on sale for $20-$40 off MSRP
Best Buy has 32GB and 64GB Apple TVs on sale for $20-$40 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store... Read more
Mac minis on sale for $50-$100 off MSRP
B&H Photo has Mac minis on sale for $50 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini: $949... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $300 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple certified refurbished iPad mini 4s avai...
Apple has certified refurbished iPad mini 4s now available for up to $120 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free. The following models... Read more
Notebook Makers In No Rush To Adopt USB-C – R...
Digitimes’ Cage Chao and Joseph Tsai note that while the USB Type-C interface is enjoying growing popularity among smartphones and tablet makers, notebook and all-in-one (AIO) PC vendors (other than... Read more
iMacs on sale for up to $250 off MSRP
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $250 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2049 $250 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more

Jobs Board

*Apple* Solutions Consultant - APPLE (United...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
Lead *Apple* Solutions Consultant - APPLE (...
Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer across a number of Reseller locations. The Lead ASC's role Read more
*Apple* Retail - Multiple Positions, Charles...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions, Willow...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Evangelist - JAMF Software (United S...
The Apple Evangelist is responsible for building and cultivating strategic relationships with Apple 's small and mid-market business development field teams. This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.