Roland MT-32

Alt text
Front view of MT-32

The Roland MT-32 Multi-Timbre Sound Module is a MIDI synthesizer module first released in 1987 by Roland Corporation. It was originally marketed to amateur musicians as a budget external synthesizer with an original list price of $695. However, it became more famous along with its compatible modules as an early de facto standard in computer music. Since it was made prior to the release of the General MIDI standard, it uses its own proprietary format for MIDI file playback.

Within Roland's family of LA synthesizers, the multitimbral MT-32 series constitutes the budget prosumer line for computer music at home, the multitimbral D-5/D-10/D-20/D-110 models the professional line for general studio use, and the high-end monotimbral D-50/D-550 models for sophisticated multi-track studio work. It was the first product in Roland's Myuujikun (ミュージくん) line of Desktop Music System (DTM) packages in Japan.

Features

Like the Roland D-50 Linear Synthesizer, it uses Linear Arithmetic synthesis, a form of sample-based synthesis combined with subtractive synthesis, to produce its sounds. Samples are used for attacks and drums, while traditional synthesis assures the sustain phase of the sounds.

The original MT-32 comes with a preset library of 128 synth and 30 rhythm sounds, playable on 8 melodic channels and one rhythm channel.[1] It also features a digital reverberation effect.[1] Successors (see below) added a library of 33 sound effects. Because of the absence of a piano attack sample, it cannot play a convincing acoustic piano sound.

Sounds are created from up to 4 partials which can be combined in various ways (including ring modulation). With 32 partials available overall, polyphony depends on the tonal complexity of the music, and 8 to 32 notes can be played simultaneously.[1]

The MT-32 by default assigns its parts 1~8 and R(hythm) to respond on input MIDI channels 2~9 and 10 respectively. By consequence, MIDI files using the popular channel 1 or the other channels 11~16 cannot have those parts played on the MT-32. However, the MT-32's melodic parts can be shifted down to respond to channels 1~8 using a button combination or through MIDI system exclusive messages, enabling improved compatibility with non-MT-32-specific MIDI sequences.

Additionally, in 1993 Roland released the "GM2MT" SysEx pack, which can be used to reprogram the MT-32 and compatibles to match General MIDI specifications as close as possible.[2] 64 of the 128 patches (the limit of possible variations) are completely new or modified sounds, with additional sounds having been added to drum channel 10. Despite this, compatibility with GM is still limited by the lack of parts (9 on the MT-32, 16 per GM specification) and reversed panpot compared to MMA MIDI specifications. The utility was predated by a pack called "MT32GS", released by Mike Cornelius in 1992.[3]

MT-32 models

Two major revisions of the MT-32 were produced. Roland refers to them as MT-32 (Old / Without headphones) and MT-32 (New / With headphones).

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Back view of MT-32 (Old)
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Top view of MT-32 with cover removed (Old, Rev. 1 PCB)

MT-32 (Old)

LA32 sound generation chip is an 80-pin PGA. Control CPU is an Intel C8095-90 in ceramic DIP48 package. DAC is a Burr-Brown PCM54; the input signal having a resolution of 15 bits (see below). Line outs are unbalanced 1/4″ TS phone connector (separate left and right channels.) No headphones jack.

The PGA LA32 chip is later replaced with a 100-pin QFP type.

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Back view of MT-32 (New)

MT-32 (New)

Control CPU is an Intel P8098. Same DAC, but with 16 bits of input signal resolution (see below). A stereo 1/4″ TRS headphones jack is added.

MT-32 compatible models

To target computer users, Roland released a number of CM (Computer Music) modules. They came without a LCD display and had most buttons removed. CM modules are compatible with MT-32, but feature 33 additional sound effect samples which many games took advantage of. These sound effects cannot be heard on a MT-32. Early models share a similar design to MT-32 (New). Control CPU is an Intel P8098 and DAC is a Burr-Brown PCM54.

In later models, the DAC is a Burr-Brown PCM55, and Vibrato is noticeably faster.

Sound quality problems

Given the MT-32 was intended to be a relatively low-cost prosumer product, many corners were cut in the design of its DAC output. For example, the circuitry needed to properly calibrate the DACs was omitted, resulting in distortion of the analog signal.[4]

Despite having the capabilities of a professional synth module, the noisy output of the MT-32 caused it to be generally considered unsuitable for professional studio use, although it was considered sufficient for use as the sound engine within other Roland prosumer products of the period - for example the first generation of E-series home keyboards produced by the company, the first being the E-20 (and its associated modular version the RA-50 arranger), use a highly modified MT-32 motherboard. However an aftermarket modification was available from Real World Interfaces to improve the MT-32's sound quality and generally increase its suitability for professional use.[4]

Digital overflow

The MT-32 and compatible modules use a parallel 16-bit DAC at a sampling rate of 32000 Hz. In order to improve the signal-to-noise ratio without investing in higher-quality components, the volume of the digital signal fed into the DAC is doubled by shifting all 15 non-sign-carrying data bits to the left, which amounts to multiplying the amplitude by two, or making the signal twice as loud while keeping the noise floor constant at the analogue output.

However, if this doubled amplitude exceeds the amount that can be represented with 16 bits, an arithmetic overflow occurs, audible as a very loud popping or cracking noise that occurs whenever the original signal crosses +16384/-16384 (the value of bit 14 lost in the bit shift).

This bit shift is implemented differently between module generations:

Original (non-shifted) data bit #                Connection
--------------------------------------------------------------------------
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00  output of LA32 synth chip
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00  input to reverb chip
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00—input to DAC
|  |
|  +- most significant data-carrying bit
+- sign bit

This means that the reverb chip will not "see" the overflow noise and thus not reverberate it. However, since bit 14 is dropped completely, the effective resolution is reduced to 15 bits, and since the DAC's least significant bit is not connected at all and thus not changing with the sign, additional one-bit noise is produced, audible at low signal levels.

Original (non-shifted) data bit #                Connection
--------------------------------------------------------------------------
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00 14  output of LA32 synth chip
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00 14  input to reverb chip
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00 14  input to DAC
|  |
|  +- most significant data-carrying bit
+- sign bit

This means that the reverb chip will "see" the overflow noise and thus reverberate it. However, since the DAC's least significant bit is connected and does change with the sign, the sound quality is improved slightly over the earlier implementation.

To prevent digital signal overflow and its audible result, the digital output volume must be kept low enough so that bit 14 will never be used. On the first generation MT-32, this can simply be done by selecting a lower main volume on the unit's front panel, which directly controls the software main volume setting, which in turn directly translates into the amplitude of the digital output signal. On later generation units, this does not work, as the main volume knob and the software main volume setting only modify the volume of the analogue output using voltage-controlled amplifiers and have little effect on the amplitude of the digital signal. To prevent signal overflow, each individual part's volume (controller #7) must be kept low instead.

A third party solution

In the period of 1989 to 1993,[7] Robin Whittle of Real World Interfaces offered aftermarket modifications to the MT-32 to address its sound quality issues, as well as improve the functionality of the reverb unit, provide discrete analog outputs for the internal reverb send and reverb return, and provide battery backup of the MT-32's settings.[4]

According to documentation written in 1990,[4] these modifications were only available for the first-generation MT-32, and not the later "headphone" model or any of the other MT-32 derivatives.

Note that the RWI modifications were intended for those using the MT-32 professionally, and may cause some minor compatibility issues with video game soundtracks intended for a stock MT-32. In particular the changes to the reverb unit functionality will likely cause an RWI modified MT-32 to render reverb differently from what was intended, with possibly detrimental effects.

Compatibility problems

First generation units, having control ROM versions below 2.00, require a 40 millisecond delay between system exclusive messages. Some computer games which were programmed to work with the compatible modules (see above) or later ROM versions that do not require this delay, fail to work with these units, producing incorrect sounds or causing the firmware to lock up due to a buffer overflow bug, requiring turning the unit off and on. However, some games were designed to exploit errors in earlier units, causing incorrect sound on later revisions.

Also, some games were written to use instruments not found in the MT-32 models, and require a compatible module, such as a CM-32L, for proper sound playback.

Music for PC games

Despite its original purpose as a companion to other professional MIDI equipment, the MT-32 became one of several de facto standards for PC computer game publishers. Sierra On-Line, a leading PC game publisher of the time, took an interest in the sound-design of its PC games. Sierra secured a distribution deal to sell the MT-32 in the US, and invested heavily in giving its game titles (at the time) state-of-the-art sound by hiring professional composers to write in-game music. King's Quest IV, released in 1988, was the first Sierra title with a complete musical soundtrack scored on the MT-32.

The MT-32 with a necessary MPU-401 interface cost $550.00 to purchase from Sierra when it first sold the device. Although the MT-32's high price prevented it from dominating the end-user market of gamers, other PC publishers quickly followed Sierra's lead, expanding the role of music in their own game titles with Roland supporting the industry by releasing CM modules for computer users. The MT-32 remained the gold-standard for musical composition well into the early 1990s, when the game-industry began to shift toward General MIDI and later CD Audio.

The proliferation of the General MIDI standard, along with competition from less expensive 'wavetable' sample-based soundcards, led to the decline of musical soundtracks using the MT-32's proprietary features. Games that played General-MIDI tracks on the MT-32 initialized the MT-32's sound bank to approximate the General MIDI Level 1 (GM1) specification, but avoided any of the MT-32's hallmark music-synthesis features, adhering to GM1's rather limited set of controllers.

Emulation

Due to the popularity of the MT-32 as a music playback device for PC games, many modern sound cards provide a simple "MT-32 emulation mode", usually realized by way of a sound mapping comprised either of General MIDI instruments rearranged to roughly represent the MT-32's preset sound bank, or of samples directly recorded from the original unit. Results are often considered poor, as the sampling technology used can not reflect the pitch- and time-variable characteristics of the original synthesizer technology, with the programming of custom sounds (see above) not being supported at all. One exception is the Orchid SoundWave 32 card released by Orchid in 1994, whose on-board DSP allowed for a more faithful reproduction of the original sound characteristics.

More recently, there have been attempts at emulating the LA synthesizer technology in software using images of the original PCM and control ROMs. The most notable of these emulators is the Open Source project, Munt, which currently tries to emulate the MT-32 hardware by way of a virtual device driver for Windows. It is also incorporated into ScummVM, an Open Source adventure game interpreter, as of version 0.7.0, and into SVN versions of DOSBox. Munt is based on an earlier MT-32 Emulation Project, which was the source of a short-lived legal squabble over distribution of the original ROM images with Roland Corporation, who manufactured the MT-32 and claims copyright on the ROM's data.

Roland has recently been offering its own emulation of classic synthesizers, notably of the D-50.

References

  1. 1 2 3 , MT-32 Owner's Manual.
  2. ↑ http://cms.rolandus.com/assets/media/exe/MT2GM.EXE
  3. ↑ http://ftp.monash.edu.au/pub/midi.songs/unsorted/M/mt32gs.doc
  4. 1 2 3 4 http://anotherunknowntime.com/music/rwi-mt-32-mod.pdfhttp://www.pdf-archive.com/2012/12/18/roland-mt-32-rwi-mod/
  5. ↑ Roland Corporation: MT-32 Service Notes, Second Edition. January 1988.
  6. ↑ Roland Corporation: MT-32 Service Notes, Third Edition. October 1988.
  7. ↑ http://www.firstpr.com.au/robin/cv.html
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