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Making Your Own MOD Files
By Tony Morales
INTRODUCTION
One of the most exciting events in the history of IIgs music was
the introduction of utilities to play Amiga-compatible MOD files. These
utilities, with one exception, lacked one important feature--the ability
to edit, and create, MOD files. The one exception, if you could call it
that, was Noise Tracker GS' ability to import MODs and save them under
it's own file format. The disadvantages of doing this were that you
could not load the saved files back into any MOD player, and almost all
of the special effects which MODs support would be lost during the
conversion.
The great thing about MOD files is that many other computers have
utilities to play them. For example, you could download an Amiga MOD,
play it on your IIgs, and take it to work and play it on your Mac or PC.
No other music format on the IIgs is as universal as MOD files are, and
there are thousands of them out there.
Unless you happened to have an Amiga or PC sitting beside your
IIgs, with the appropriate software, you had no way to create MODs, and
become a part of the MOD-mania which is sweeping the various online
services around the world. However, all things must come to an end at
some point.
Obviously, if you are reading this, you are familiar with GS MOD
Monthly, the first IIgs MOD magazine. This magazine has featured new
MODs, not previously available on any other computer. The great part is
that they were created entirely on an IIgs.
The first thing you are saying is, "How is this possible? Is there
a MOD editor I don't know about?" Unfortunately, there is not an IIgs
MOD editor at present, although this may change in the future. It turns
out that with a small amount of effort, MOD files can be assembled by
hand. That's right--by hand. The rest of this article will present
theoretical perspectives on what exactly needs to be done to create a
MOD file.
WHAT YOU'LL NEED
Before a carpenter builds a house, he must be equipped with the
proper tools. The same thing is true for designing MODs. The first
thing you'll need to have around is your copy of the MOD Tools disk,
which you should already have if you are reading this. This disk will
allow you to control the overall sound of your MOD when it is finished.
The second thing you'll need to have is a copy of AudioZap, (not
included on the MOD Tools disk). You can get it from any online service
or your local user group. AudioZap is important because it's the only
IIgs sound utility that can convert IIgs sound samples to the format
used on the Amiga.
The third, and final, thing you should have is a utility which can
take several small binary files and combine them together, (in any
order), into one large binary file. I currently know of only one
program which can do this, Davex. Davex is a ProDOS 8 shell utility
which sports a user interface similar to the MS-DOS shell. Davex's most
powerful feature is it's support for external commands.
Several commands come with the program, including, (you guessed
it), a command to combine small files into large ones. Davex is
available from any online service. It is, without a doubt, the best
8-bit shell for the Apple //e and IIgs.
ABOUT MOD FILES
Assuming you have the above utilities, or some means of performing
the required file operations, we'll start with an overview of the MOD
file format. Note that a much more expanded version of this format is
found in another file on this disk.
MOD files have 3 basic parts; a header, blocks of notes, and sound
samples. The header consists of the title of the MOD, the instrument
definitions, and the order in which to play the blocks of notes the MOD
contains.
PREPARING THE SAMPLES
MODs can have up to 31 instruments. Naturally, you won't need to
use all of them at once unless you choose to--any number of instruments
will work. The first thing you should do is decide on which samples to
use for your particular MOD. Next, you must save the samples as raw
binary files, so there is no extraneous information in the file, (such
as AIFF chunks).
Important! All of the samples should have an even number of bytes
in them. For example, a sample that uses 32000 bytes is fine, while a
sample that uses 21847 bytes is not fine. Also, it is generally a good
idea to round the sample lengths to a multiple of 100. While this last
step is not absolutely required, many MOD-designers do this. When you
have prepared your samples, you need to write down the individual
lengths of each of them. You need this information when defining the
instruments.
Next, you must combine all the samples into one large file, which
we will refer to as a wavebank. When this is done, you must load the
wavebank into AudioZap. If you were to play this file, you would hear
all your samples, one right after the other. Audiozap has an option to
convert a sample into one usable on the Amiga. You need to select this
option, and resave the converted wavebank over the original.
For those who are curious, the method of encoding the bytes used on
the Amiga is to exclusively OR each byte with $80. For you
non-technical folks, 8-bit sound samples consist of volume intensities
in the range $00 to $FF, (0 to 255 decimal). If you are unfamiliar with
hexadecimal notation, you can think of the exclusive OR with $80
operation as the following.
If the volume intensity is greater than or equal to 128, subtract
128. If the intensity is less than 128, add 128. For example, $7D
would become $FD, (125 would become 253). As explained before, each
byte in an 8-bit sample is a number from 0 to 255 that represents the
volume level at that moment in the sample.
After you have saved your wavebank samples in the Amiga format,
it's time to define the instruments which correspond to the samples.
For the following examples, you need to be in BASIC.System and in the
Monitor. To get to the Monitor from BASIC, type:
CALL -151
All of the following examples will use memory locations starting
with $1000. It might help to have a printed copy of the MOD file format
note to guide you through the examples. First, you need to prepare your
MOD header. Each header is 1084 bytes long, so start by zeroing 1084
bytes of memory. Type:
00<1000.143BZ
The first 20 bytes of every MOD contain the MOD's title. If you
can't think of a title for your MOD right off hand, you can skip this
step. To title your song, first clear bit 7 of the ASCII filter mask,
(all MOD ASCII strings must have their high-order bit off). Do this by
typing:
7F=F
In the next example, "My first MOD," is an example title for the
MOD. Type:
1000:"My first MOD"
DEFINING AN INSTRUMENT
Each instrument definition takes 30 bytes. All 31 definitions come
directly after the MOD's title. The first definition starts at offset
20, the next at 50, and so on. Convert each base into hex, and add
$1000, and you will have the address where the corresponding instrument
definition begins. The following would calculate the address for
instrument #1:
(((1*30)+20)+$1000) = $1032
You could also look at the offsets in the file format note, convert
these to hex, and add $1000 to get the instrument address. The first 22
bytes of an instrument definition is the instrument title. Some MODiots
like to make a message, using all 31 instrument titles as a few words of
the message. Some simply title the instrument with a description of the
instrument. Of course, you can also skip titling your instruments if
you wish.
If you choose to title any or all of the 31 instruments, repeat the
processes discussed above for titling the MOD. Be sure to use the
correct addresses. This example would title instrument #1:
1032:"Instrument 1"
After the title comes the length of the instrument/2. Here is a
very important point which you need to be familiar with. Almost every
other microprocessor, other than the 65xxx, stores byte pairs, (words),
in memory with the high-order byte first, followed by the low-order
byte. The 65xxx stores the low-order byte first, followed by the
high-order byte.
Suppose your first instrument was 32000 bytes long. Divide this in
half to get 16000. Convert this to hex to get $3E80. Flip the bytes
around to get $803E. The following shows how to enter this in memory:
1048:3E 80
The next byte is the semitone for that instrument. Values here
range from $00 to $0F. I've never seen a MOD that uses anything other
than $00, so you might wish to stick with this value.
The next byte is the volume level for the instrument. Range is $00
to $40. Most instruments use $40, but you can use lower values if you
want to.
The next word is the loop offset for the instrument/2. For our
32000-byte instrument, if we wanted to loop at offset 12000, we would
use a value of $7017, ($1770*2 = 12000). If you don't want to loop, set
this field to 0.
The last word in an instrument definition is the loop length/2.
This value is the number of bytes to repeat, following the loop offset.
To loop 100 bytes over and over from offset 12000, you would enter
$3200, ($0032*2 = 100). If you don't have the instrument looping, you
need to set this field to $0100, ($0001). To define the first
instrument with all of the above information, you could enter the
following:
1032:"Instrument 1"
1048:3E 80 00 40 17 70 00 32
COMPLETING THE HEADER
Before we move on, two more fields must be added to the MOD header.
The first of these is at offset 951, ($3B7). This byte should be set to
$7F. This is for some old MOD players, which use this byte as the
number of patterns to scan when loading. Setting this byte to $7F tells
the players, (who care), to scan all patterns. This will enter the
correct value:
13B7:7F
The last thing that needs to be added is the Protracker signature.
This signature tells MOD players that they are dealing with a
Protracker-compatible MOD file. The Protracker signature is made up of
4 letters. The following will enter the signature, which begins at
offset 1080, ($438):
1438:"M.K."
That is all there is to it. Note that we haven't defined any music
patterns yet. These will come later. You must save the header for
future reference. This example will do it:
BSAVE HDR,A$1000,L1084
While we are at it, it might be a good idea to rename the file that
contains your wavebank samples to, "INST." This is because we will be
using this name later, when we put the MOD together.
CREATING A PATTERN
A pattern is a 1024-byte chunk of the MOD, which is made up of 64
positions. Each position consists of 16 bytes, and up to 4 notes can be
playing at each position. A note takes 4 bytes, so this effectively
allows for 4 tracks. The first 4 bytes in a position correspond to
track 1, the second 4 correspond to track 2, and so on.
Each note has information for the note value to be played, the
instrument corresponding to the note, and the effects/parameters, if
any. See the file format note for a pictorial representation of the 4
bytes that make up a note. As shown in the figure, the note value takes
12 bits, the instrument number takes 8, the effect takes 4, and the
parameter takes 8 bits.
The first byte of the 4-byte note consists of the upper 4 bits of
the instrument, (in the high-order nibble), and the upper 4 bits of the
note value, (in the low-order nibble). The high nibble will always be
$0 or $1, as instrument numbers range from $00 to $1F. The low-order
nibble of the instrument number is stored in the high-order nibble of
byte 3 of the 4-byte note.
The low-order nibble of byte 1, and all of byte 2, combine to be
the note value. See the file format note for allowable note values.
There are 3 octaves possible, with 12 notes in each octave. The
low-order nibble of byte 3 contains the effect number, ($0 to $F). The
last byte contains the parameter for the effect. If no effect is
desired, both the parameter byte and the effect nibble should be set to
0.
A more descriptive list of the allowable effects, with their
required parameters, is located in another file on this disk. The
following example demonstrates the 4 bytes needed to play a note C in
the 3rd octave, with instrument #1, and a fine slide up value of 2:
00 D6 1E 12
It is rather difficult to tell you which notes and effects to enter
in your MOD. Experiment with different values for a while. To begin
creating a pattern, zero 1024-bytes of memory:
00<1000.13FFZ
Then begin entering your notes. When the pattern is done, save it
to disk:
BSAVE P1,A$1000,L1024
Go ahead and make 2 more patterns, repeating the above steps, each
time saving the new pattern with a higher number, (P2, P3). When this
is done, we need to reload the MOD header and define the pattern play
list. Type the following to reload the header:
BLOAD HDR,A$1000
Then, you must decide how many patterns you would like to play.
You can repeat any pattern as many times as you wish. Suppose you
wanted to play patterns 1, 2, 3, 3, 2, and 1. There would be 6 total
patterns. At offset 950, ($3B6), you must store the total number of
patterns:
13B6:06
Then, at offset 952, you must enter the patterns to play-1:
13B8:00 01 02 02 01 00
Now resave the header as follows:
BSAVE HDR,A$1000,L1084
Now, we have constructed a very primitive MOD file. Combine the
files together in the following order:
HDR, P1, P2, P3, INST
Now load this file into one of the IIgs MOD players, and test the
results. It may take some time to understand all of the concepts
presented in this note. If you are serious about creating your own
MODs, don't give up--it can be done. Probably the best thing to do if
you are confused, is to look at other MOD files, and see the way the
various effects are utilized.
If you do manage to produce a very good MOD, and aren't too afraid
to show it to the IIgs community, I would appreciate it if you sent it
to me, to feature in GS MOD Monthly.
Happy MODing!