MiDi Programming

by MrPink of ReservoirGods

@DrWatson

Published in 

atari

 · 8 Nov 2023

If atari only got one thing right in their entire history, it was the inclusion of MiDi ports in the ST. Whether this was an act of prophetic genius or a total fluke is open to debate. It wasn't long before someone coded a sequencer, and the rest, as they say, is history.

Despite the facts that the power of the top end Macs and PCs totally dwarves the ST, and their Sequencer software has options and features coming out of every orifice, the Atari is still in large number of recording studios. Whilst everyone else seemed to throw away their beloved STs when they saw Doom, the Cubase dads hung onto them with a touching display of affection.

They are still championed by popular beat combos of today. Fatboy Slim, Roni Size, Super Discount and John B all swear by them. And of course Alec Empire wouldn't be seen dead without his beloved 1040.

The lack of Falcon specific MiDi stuff has consequently come as a bit of a surprise. Sure, we have had the big guns like Cubase Audio, but this it out of most people's price range. Where are the patch editors, midi trackers or even midi players? It would be cool if we had some MiDi music demos to go alongside the standard chip and tracker ones.

The MiDi standard has two drawbacks:

1. It is not exactly standard ( but more about this later )
2. The number of channels is quite limiting
3. The data transfer rate is pretty slow

OK, now it is time to unravel some mysteries, like why does MIDI have only 16 channels? How is MIDI information sent between instruments? How do they manage to synchronise with each other?

Anyone with any experience of 'real' network protocols knows what overhead can be placed on just one byte of information. The data is enclosed in a packet (called a frame) which includes header information such as size of the frame, source address, destination address, a number of information flags and some checksums at the end.

MiDi doesn't bother with this. Although the 'packets' of data are of different sizes (e.g. three bytes for Note On, two bytes for program change), there is no frame construct. So how does it work?

MiDi information is divided into control bytes and data bytes. Control bytes are recognised by the fact that they have their highest bit set. Data bytes never have their highest bit set.

This means MiDi data can only range from 0-127. I will explain how it can support bigger values later on in this doc.

So if there are 128 possible data values, there must be 128 different control byte types? Wrong. There are 7. How so?

The control bytes are split into two nibbles.
The high nibble represents the type of control byte.
The low nibble represents the MiDi channel.

Lets look at a simple example, that of a program change.

A program change consists of two bytes:

So a packet indicating we want MiDi channel 8 to switch to instrument 0x32 is:

 0xC7,0x32

Why 0xC7 not 0xC8? Well most people count MiDi channels 1-16 whilst to the computer they are 0x0-0xF. This is also the reason why there are only 16 midi channels - that is the most you can fit in a nibble!

You get the general idea. Here is the list of MiDi controllers

The controllers of the 0xF0-0xFF family generate non-channel specific system wide information. These are mainly used to send System- Exclusive data.

We'll come back to sys-ex stuff later, but first let's look at the more standard commands.
When you press a note on your controller keyboard it will send a 'Note On' command. Let's say you are set to output on midi channel 1, and you tap middle c, this data will be sent from your keyboard:

                        0x90           0x3B        0x40 

                     Note on command, Note Number, Velocity

The first values indicates a 'Note On' command on channel 1. Middle C is defined 0x3B. Thus B is 0x3A and C Sharp is 0x3C. To go an octave up add 12, and for an octave down subtract 12.

The velocity value here, 0x40, it pretty average for most keyboards. If you have a touch sensitive keyboard, this value will depend on how hard you press the key. Remember that velocity is not the same as volume - although it is related. Velocity changes both the sound's volume and its timbre.

When you release the key, a note off commands will be sent:

                        0x80           0x3B        0x00 

                    Note off command, Note Number, Velocity

These note off commands are essential to MiDi programming! If they do not exist

then you will get hanging notes all over the place and your MIDI instrument will run out of polyphony.

The Pitch-Bend commands illustrates a neat expansion to the data byte range. It sends 2 data bytes so the pitch bend range covers 8192 values.

That's all very well, you might be saying, but there must be more Midi commands than pressure, aftertouch and pitch bend? And of course, you are right!. They are hidden away in the 0xBn family of MIDI controllers.

The set of a MIDI controllers is a good example of how the standard of MIDI is actually non standard. Ironically, the fact that the standard is loose has helped strengthen the standard. I'll explain more...

The MIDI controller packet is in this format:

 Controller Command, Controller Type, Controller Data

As the controller Type byte occupies the space normally taken by data, this means it can have 128 different families - so this effectively gives us 128 different controllers!

A lot of these controller types have been set in stone by the MIDI standard. So, for example, every synth should recognise a controller type of 0x0A as PAN.

But a lot of controller types are 'free', and each synth can respond to them as they want. This gives the MIDI standard a lot of flexibility and also the ability to expand.

Here is a list of the standard controllers:

Not all these controllers will be recognised by every synth. As you can see there are lots of unused values for new synths to use for extra controllers.
Finally, let us go back and look at Sys-Ex data.

Sys-Ex differs from standard midi data in so much as it invariably it involves shifting a great deal more data around.

Sys-Ex is usually used to dump edited information from your synth to your sequencer package. You might have fiddled around with lots of patch parameters for a particular song and you want these changes saved.

Samplers also use Sys-Ex to dump samples.

The format of Sys-Ex packets varies from machine to machine. I will give you an example sys-ex packet, this one is from the Roland Super-JV 1080:

 0xF0  Begin System Exclusive 
 0x41  Manufacture ID (0x41=Roland) 
 Dev   ID status of Unit 
 0x6A  Model ID (0x6A = JV 1080) 

 aa    Address MSB 
 bb    Address 
 cc    Address LSB 
 ee    Data 
 : 
 ff    Data 
 sum   Checksum 

 0xF7  End of System-Exclusive

In the case of the Roland, each Sys-Ex packet has a checksum. But not all instruments use checksums in their sys-ex, and if they do they use different ways of calculating the value.

OK, you know have a general grounding in MIDI commands. Now onto to actually sending data to your synth...

The BIOS gives you simple access to your MIDI ports.

To read a byte from your MIDI device, you must first check that there are bytes waiting to be read before reading them.

This is achieved using the bios commands Bconstat and Bconin. They check and read from a specified device, respectively.

The format of Bconstat is:

This returns 0 if no characters are waiting, or -1 if there is something there.

The dev number for the MIDI interface is 3. This should be defined in most C libraries as DEV_MIDI.

To read a character, we use Bconin:

So the logic we would use is..

char get_midi_byte() 
{ 
 char c; 

 while(!Bconstat(DEV_MIDI)); 
 return(Bconin(DEV_MIDI)); 
}

To send midi information, we first check if the MIDI device is ready to receive data, then we send it:

The trap calls for output are Bcostat:

 move.w #dev,-(a7) 
 move.w #8,-(a7) 
 trap   #13 
 addq.l #4,a7

Again this returns 0 if the device is not ok, -1 if everything is hunky dory.

To send a character, you can use Bconout:

 move.w #char,-(a7) 
 move.w #3,-(a7) 
 trap   #13 
 addq.l #4,a7 

void send_midi_byte(char c) 
{ 
 while(!Bcostat(DEV_MIDI)); 
 Bconout(DEV_MIDI,c); 
}

If you want to send a lot of data then it is preferable to use the Xbios call Midiws() which sends a string of data to the MIDI port. The format of this call is:

 pea     buffer 
 move.w  #count,-(a7) 
 move.w  #$C,-(a7) 
 trap    #14 
 addq.l  #8,a7

count+1 characters are transferred from the buffer to the MIDI device.

If you want to bypass the operating system completely and access the hardware, here is how to send data direct:

rlkb_send_midi: 
        btst    #1,$fffffc04.w          ;is data reg empty? 
        beq.s   rlkb_send_midi          ;no, wait for empty 
        move.b  d0,$fffffc06.w          ;write to MIDI data reg 
        rts                             ;return

I hope this has given you a useful insight in the wacky world of MIDI!

Keeps your eyes (and ears!) peeled for GodTracker, our forthcoming MidiTracker for the STe and Falcon!