Midi and the AVR.pdf

MIDI AND THE AVR

I N T R O T O U S I N G T H E A V R I N M I D I A P P L I C A T I O N S

PAUL M ADD OX

MARCH 2002

TABLE OF CONTENTS

Introduction .............................................................................................................................................2

MIDI, what is it? ......................................................................................................................................2

MIDI specs, a quick guide .....................................................................................................................3

MIDI; the hardware.................................................................................................................................4

MIDI gotcha's..........................................................................................................................................7

MIDI to CV conversion; a quick project ..............................................................................................10

Useful links ...........................................................................................................................................11

INTRO USING THE AVR IN MIDI APPLICATIONS, PAUL MADDOX - AVRFREAKS.NET

Introduction

This guide is intended to help you on the way to useing MIDI with the AVR range

of processors, explaining the concepts and protocols used to some of the gotchas

and common problems people face. Where possible I've tried to kept the guide as

processor independant as possible.

Assumptions

You need a certain amount of knowledge about serial communication, not much,

but a few basic concepts. Also a little understanding of the hardware involved,

AVR, opto-isolators, etc. Also a basic knowledge of what a MIDI system is, e.g.

Sequencer, synthesiser modules, keyboards.

Who is this bean?

My name is Paul Maddox and I'm a keen electronic/synth/micro enthusiast. That

is to say I do this for fun, not for a living, you can see my web site here 

www.wavesynth.com as you can see from the site its a fairly major part of my

life. I wrote this as a guide to get people into useing AVRs for MIDI, Something

that very few people seem to use them for, perhaps they dont know how to? If

not, this will hopefully explain it.

MIDI, what is it?

So, what's all about?

A few years back, quite a few actually, people could see the need for a common

standard for communication between synthesizers. Many manufacturers had their

own standards (V/hz, V/Oct, DCB, PPG BUS, etc) none of which were easily

convertable to communicate with each other. With the increase of sequencers and

affordable computers it was decided to got for a digital system, offering better

accuracy and precision than the tradional CV/Gate type of interface. The standard

they decided on was called MIDI and it stands for 'Musical Instrument Digital

interface'. A webpage published by the organisation that was formed to control

the standard is available at Http://www.MIDI.org/ and is a very useful resource

for anything relating to MIDI.

There are two parts of the MIDI system to consider, the hardware interface and

the protocol used. The main bulk of the issues and problems with useing MIDI are

to related to the protocol, so this document focuses mostly on this, though the

hardware is mentioned.

Protocol

The MIDI system is based on the concept of the user being a keyboard player,

rather than say a wind instrument or guitar payer. In its simplest form MIDI is

the means by which note information is sent, e.g. when a 'key' on the keyboard is

pressed. The type of information that is sent is event information and not audio.

That is to say MIDI is used to describe when a note is pressed, which note it is,

how hard and for how long, but not the sound that is created by this action. MIDI

is also used for a whole host of other events, but the action of pressing a note is

the simplist to describe and emphasise the point at hand. The other thing to note

is that MIDI is a serial data stream and runs at 31250 baud.

Hardware

This is the physical means of connection. Data is sent via a current loop, and

INTRO USING THE AVR IN MIDI APPLICATIONS, PAUL MADDOX - AVRFREAKS.NET

therefore there is no electrical connection between one unit and another via the

midi cable. This prevents Ground loops and removes any chance of hum on the

output of the device. MIDI is a point to point method of connection, ie you can

only connect one device to one other. This can be increased with the use of a

MIDI thru box, which is explained later.

MIDI specs, a quick guide

This section will deal with the protocol standard and explain some of the problems

commonly encountered whilst developing MIDI interfaces.

A 'standard' MIDI word consists of three bytes, though depending on use it may

have more or less, generally though. the first is a Status byte, the second and

third are databytes .

So, with a stream of data comming in, how do you know which byte is which? and

which byte is for you? lets take the action of pressing a NOTE on the keyboard,

this is called a Note Event . Three bytes are sent; NOTE_ON , NOTE, VELOCITY

if we asssume we're working on Midi Channel one the data would look something

like this

0x90, 0x3C, 0x7F

lets go through the bytes one at a time.. first the statusbyte, 0x90 if we translate

this into binary we get ;- 10010000 The upper nybble (1001) shows we have a

NOTE_ON event, the lower nybble (0000) is the MIDI channel, MIDI device can

have a channel number between 1 and 16, because People work better with

numbers starting at 1 rather than 0, so simply subtract one from the channel

number to get the expected value here, e.g. if we wanted MIDI channel 8 we

would expect to see 0111 (7). The next byte is the NOTE value, in this case 0x3C

which is note 60, which is in fact middle C. The final byte is 0x7F , which is the

velocity ie how hard the key was pressed, in this case maximum.

Now, many of you will have noticed that I said the maximum velocity was 0x7F,

which in binary is 01111111. But surely, you ask, isnt the maximum 0xFF ie

11111111? The answer is not for MIDI, a Status byte is defined by the fact that it

has the MSB set, all other bytes that are data must NOT have the MSB set, so the

valid range is 0-127 for any data. This may seem very limiting at first but given

that this gives nearly 12 octaves of note data and also given that very few people

can press a note with an accuracy approaching seven bits, its not as bad as it

may seem. Remember we're just sending note information, not audio, so

resoloution isnt so much of an issue. This gives us the added advantage that

status byte are easily seperated from data bytes, how? Easy all status bytes have

the MSB set to 1, all data bytes have it set to 0. You can see this in the table

below.

So here is a list of the basic commands, first column is the Status byte, second

coloumn is the second byte and what it relates to,third coloumn the same,

thirdbyte and what its use is. The fourth column is a description.

STATUS

2nd Byte

3rd Byte

Description

1000cccc 0nnnnnnn

0vvvvvvv

Note off,'cccc' is channel, 'nnnnnnn' note

value''vvvvvvv' is velocity value

1001cccc 0nnnnnnn

0vvvvvvv

Note on, 'cccc' is channel, 'nnnnnnn' note

value''vvvvvvv' is velocity value

1011cccc 0nnnnnnn

0vvvvvvv

Control Change, 'cccc' is channel,'nnnnnnn' is

Controller Number,'vvvvvvv' is value

INTRO USING THE AVR IN MIDI APPLICATIONS, PAUL MADDOX - AVRFREAKS.NET

1110cccc 0LLLLLLL 0mmmmmmm

Pitch Wheel Change,'cccc' is channel, 'LLLLLLL' is LSB

of value 'mmmmmmm' is MSB of valuecenter (no pitch

change) is 0x2000

MIDI Clock pulse, 24 pulses per quarternote, 96 to the

bar (assuming 4/4 time)

11111000

none

11111010

none

Start,sent at start of sequence, followed by MIDI clock

pulses

11111011

none

Continue,sequence carries on from where it was

stopped

11111100

none

Stop,sent when sequence is stopped

11111110

none

Active sensing

This is a brief (very brief) list of the events, these are the events you will see

most of when useing MIDI data, there are more and there are some odd quirks

which I will explain. I've also shown some of the MIDI status bytes that are only

one byte long. MIDI clock for example, this ensures that if you have, say, a drum

machine and sequencer that they stay in time with each other. active sensing is

mentioned in the 'gotcha's' section of this guide, but its one you will see a lot of,

so its in this list. a full list of Status bytes and message can be found on the

midi.org page ;-) http://www.midi.org/about-midi/table1.htm

MIDI; the hardware

So, why use the AVR for MIDI?

Why not use any of the many other micros available?

There are some good solid reasons for my choice (and many others) of the AVR

when useing MIDI. Firstly, it has a built HARDWARE UART, this means no messing

with 'bit bashing' and constantly reading ports. Secondly, it's interupt driven! This

means the AVR can be doing other things, and not worry about MIDI data until it

arrives. Thirdly, it's quick! At 8MHz (on an 8515 for example) you can get nearly

8 MIPs. Processing of MIDI data can get quite complex, and with a large system

you can have an almost constant stream of data to work with, so it needs to be

quick and decide which bytes it wants to use and which it does not care about

(data on other channels for example).

Below is a schematic showing how to connect the AVR to various MIDI ports.

INTRO USING THE AVR IN MIDI APPLICATIONS, PAUL MADDOX - AVRFREAKS.NET

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