From ucsd!rutgers!gatech!cwjcc!tut.cis.ohio-state.edu!mailrus!cornell!batcomputer!itsgw!steinmetz!ge-dab!ge-rtp!edison!mfs Sun Jan 22 07:21:36 PST 1989
Article 3957 of rec.music.synth:
Path: ucsd!rutgers!gatech!cwjcc!tut.cis.ohio-state.edu!mailrus!cornell!batcomputer!itsgw!steinmetz!ge-dab!ge-rtp!edison!mfs
>From: mfs@edison.GE.COM (Martin @ I was a plant in a previous life)
Newsgroups: rec.music.synth
Subject: The MT-32
Keywords: the guts of the machine
Message-ID: <1780@edison.GE.COM>
Date: 18 Jan 89 22:19:20 GMT
Lines: 152


Ah, the MT-32; a great little box - for the money.  I dearly love
mine, even though it is kind of noisy.  I have gotten a lot of requests
for info on the MT, so I will attempt to share what I know about it.
Most of this info also applies to the D110, except the D110 has more
reverb modes, more PCM samples, battery backup, a card slot, and
killer drum samples. 

First, the obvious.  The MT is a multi-timbral sound module that uses
Roland's now famous (infamous ?) sound generation method called LA
(Linear Arithmetic) synthesis.  This is really just a fancy name for
mixing PCM samples with 'normal' sawtooth/square wave kinds of sounds
While the timbres produced by the LA method sound complex, programming
the sounds is actually rather easy.

Of course, to change any of the MT's parameters, you need a computer
and the proper software; there are only a few things you can change 
from the front panel.

The MT is logically divided into several sections.  First are the
synth parts.  There are 8 of these, each of which has its own midi
channel number, volume and pan setting.  Kind of like 8 synths and a
mixer in one box. 

The next section is the drum section.  This area allows you to assign a
canned rhythm sound or any sound loaded into RAM to a particular note
number.  You can also set the pan, volume and turn the reverb on or off
for each of these sounds.

Now, all of the synth parts and the drum section draw from a 'pool' of
sounds arranged in banks of 64.  There are two banks of 'musical' sounds
in ROM (A and B), one bank of 'drum' sounds in ROM (actually only 30 of
these, abbreviated R) and one bank of RAM sound locations (called M for
memory).

You can get to the ROM A and B sounds in two ways.  The first way is to
select a part button and spin the knob.  The little micro inside the MT
dutifully loads the sounds into the part as the knob goes round.  You can
also use MIDI program change commands to pull up the sounds.  There is
a difference tho, 'cause when you use the program change commands to pull
up a sound, the MT uses an area called the patch map to get to them.

The patch map is an area of 128 locations, each of which corresponds to one
of the 128 possible MIDI program change numbers.  Each entry in the map
specifies a bank (A,B,R or M) and a number (1-64).  When you power up your
MT, this map is defaulted so that program change numbers 1-64 will 'point'
to ROM bank A and numbers 65-128 point to bank B.  Using a computer and
editor, you can change these map entries to point to any sound in any bank.
And, not only can you specify the bank and sound number, each map
entry allows you to specify the bend range, the keyshift (in semitones),
the polymode (whether the next note on cuts off the previous note or
triggers a new one) and the whether the reverb is on or off.  Note that
there is only one patch map.  If you send a program change to one part
on a certain channel and then send the same program change the another
part on another channel, they will both be set according to that particular
patch map entry.

The last group is the system section which controls the global things
such as master volume, tuning and the reverb.  The MT has four kinds of
reverb: room, hall, plate and discrete echo.  You can set the reverb time
from 1-8 and the level from 0 to 7.  Note that the reverb on the MT is
global.  You can turn the reverb on or off for each part or drum sound,
but you can't have one part set to one thing and another part set to
something else. 

So all in all the MT is really very flexible in terms of setting things
up and pulling up sounds via midi.  Now I will try to break down the
parameters that make up a sound.

The smallest unit of sound generation is called a partial.  Four partials
are grouped together to form a timbre.  You can make a timbre using all
four partials if you want, or you can use just one or two.  There is a
section of the timbre refered to as the common parameters. This area 
contains the 'switches' to turn on or off any of the four partials and
the partial structures (see below)  All together the MT can generate
32 partials at any one instant.  These are all dynamically allocated
between the synth and drum parts.

Let's look at a partial.  Each one of these is made up of
a Waveform Generator (WG), Time Variant Filter (TVF) and a Time Variant
Amplifier (TVA).  Each of these blocks has several parameters and a five
stage envelope generator associated with it.  The WG has controls which
affect the coarse and fine pitch, select the waveform (sawtooth, square
or PCM #) and control the amount of envelope effect.  The TVF parameters
include cutoff frequency, resonance, velocity sensitivity, several keyfollow
parameters and a five stage envelope generator (the last stage of the
envelope is fixed as the sustain level and release time).  The TVA has an
overall level adjustment, velocity sensitivity controls, a five stage
envelope and two programmable keyfollow controls.

Partials can be one of two types: Synth or PCM partials.  In the Synth
partials, all of the above mentioned blocks are active.  You can set up
a sawtooth on the WG, filter it with the TVF and adjust the volume with
the TVA.  In a PCM partial, instead of using the standard square or
sawtooth kind of waveform, a sampled waveform is selected.  Some of these
samples are looped so they sustain, while others only sound during the
initial attack of a note.  Also, in a PCM partial, the TVF has no effect. 

As mentioned above, partials are combined into blocks called timbres. 
Each timbre is made up of two pairs of partials.  These pairs are combined
by setting a parameter called a structure.  The structure tells the partials
what kind they are  (Synth or PCM) and how their output is combined. 
Depending on the structure setting, the partials can be set up as 2 synth
partials, a PCM partial and a synth partial, 2 PCM partials, stereo PCM/synth
partials (one partial out the left and one out the right) and ring modulated
synth and/or PCM partials. (13 all total) Very flexible.  I should note here
that the MT firmware has a bug which does not allow the volume to be 
controlled properly if structures 10-13 are used. (Maybe this is just a H/W
limitation)

Now for the MT insides:

The MT-32 uses an Intel 8097 microcontroller as the main CPU.  It has
a total of 64K of ROM and 32K of RAM.  The RAM is not battery backed, so
you have to reload all of your sounds and the patch map when you power up. 
The CPU (and the operating system in the ROMs) controls the custom LA32
chip.  The LA chip has connections for the PCM sample ROMs (two 256Kx8 Toshiba
Mask ROMs on board rev 0 or one Hitachi 512Kx8 ROM on board rev 1) and the
digital reverb chip.  Contrary to popular opinion, the LA chip sends all it's
data to the reverb chip in digital format, not in analog form.  The
data is routed to the D/A converter (a BurrBrown PCM54 chip) and out
to 6 sample/hold chips.  With a few exceptions, this is the same circuitry
as in the D110.  (And probably the D10 and 20, but I don't have them so I
can't look inside)  For some reason, one of the data
lines from the LA chip to the D/A is LEFT OFF, killing the S/N ratio.  Also,
(as near as I can tell with the trusty 1240 logic analyzer) the sample data
in the ROMs has no data for this bit.  Note that the D110 runs ALL 16 bits
to the D/A which explains its better S/N ratio.

-------------------------------------------------------------------------

Oh well, I'm busy trying to find a way to quiet my MT down by running the
extra line and using a slightly better D/A, but so far, no luck.  I am also
gonna try putting a D110 PCM sample ROM in the MT to see if that will work.
I've managed to dissassemble all the code in the ROMs and I hope to get
some simple front panel editing functions in there.  (Not that easy!)
And, I'm workin' on a battery backup circuit and a 500+ EPROM sound bank.

I also have new EPROMs available wich have 128 new sounds in them.  Unlike the
aforementioned products, these are done and work quite well. (The sounds are
MUCH better than the originals too)  Of course you will have to install
sockets in your MT, as only the first 20000 or so were sold with socketed
EPROMs.....  Oh well.   Write or Email for more info -


Martin 

mfs@edison.ge.com

BLUE RIDGE MUSIC SOFTWARE
Rt 8, Box N55,
Charlottesville, VA 22901