Roland MT-32
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The Roland MT-32 Multi-Timbre Sound Module is a MIDI synthesizer module first released in 1987 by Roland Corporation. Along with its compatible modules, it established an early de-facto standard in computer music and was the first product in Roland's ミュージくん (myūjikun) line of Desktop Music System (DTM) packages in Japan.
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-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.
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[edit] 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. It also features a digital reverberation effect. 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.
Being a synthesizer rather than a simple wavetable device, the MT-32 is very programmable. 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.
The MT-32 by default does not play on the otherwise most popular MIDI channel 1 (and by consequence neither do MIDI files targeted for it), but can be reprogrammed to do this, through MIDI system exclusive messages, for example to achieve a certain level of General MIDI compatibility.
[edit] Models
Three major versions of the MT-32 technology can be isolated:
[edit] First generation
No headphone jack. LA32 sound generation chip is an 80-pin PGA. Control CPU is an Intel C8095-90. DAC is a Burr-Brown PCM54 without trimpot; its input signal has a resolution of 15 bits (see below).
- MT-32 with revision 0 PCB, used in units up to serial number 851399.
The PGA LA32 chip is later replaced with a 100-pin flat type.
- MT-32 with "old-type" revision 1 PCB, used in units with serial numbers 851400 - 950499.
[edit] Second generation
A headphone jack is added. Control CPU is an Intel P8098. Same DAC, but with full 16 bits of input signal resolution (see below).
- MT-32 with "new-type" revision 1 PCB, used in units with serial numbers 950500 and up.
- Roland MT-100: A combination of the MT-32 with the Roland PR-100 hardware sequencer.
After this, all compatible modules feature 33 additional sound effect samples.
- Roland CM-32L: Sound module for use with a computer ("CM-" stands for "computer music"), having only a volume knob, a MIDI message and a power-on indicator as external controls. Released in 1989.
- Roland CM-64: A combination of the CM-32L with the sample-based CM-32P, a cut-down "computer music" version of the Roland U-110. The CM-32P part plays on MIDI channels 11-16 which are not used by the CM-32L part.
- Roland LAPC-I: ISA bus expansion card for IBM PCs and compatibles. Includes the MPU-401 interface.
[edit] Third generation
DAC is now a Burr-Brown PCM55, which is factory-trimmed (see below). Vibrato is noticeably faster than on previous models.
- Roland CM-32LN: Sound module for the NEC PC-98 series notebook computers, featuring a special connector for direct connection to the computer's 110-pin expansion port. Released in Japan only.
- Roland CM-500: A combination of the CM-32LN with the Roland GS-compatible Roland CM-300, the "computer music" version of the Roland SC-55. Released around 1992.
- Roland LAPC-N: C-Bus expansion card for the NEC PC-88 and NEC PC-98 series of computers. Released in Japan only.
[edit] Sound quality problems
[edit] 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:
- In first generation modules, this bit shift is performed at the connection between the data bus and DAC[1]:
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.
- In second generation modules, the bit shift is performed at the connection between the LA32 sound generation chip and the data bus[2]:
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.
[edit] DAC distortion
In additional to a constant noise floor found in all electronic equipment, the MT-32 suffers from a "gated" noise that appears as a sound is played, but goes away as the signal settles to zero.
The DAC used in first- and second-generation modules, the Burr-Brown PCM54, although purportedly factory-trimmed, requires an external trimpot to reduce the Differential Non-Linearity Error (DNL) at bipolar zero[3]. To save on cost, Roland did not provide such a circuit, exhibiting an artifact sounding similar to quantization noise.
For third-generation modules, Roland used the Burr-Brown PCM55 DAC instead, which is factory-trimmed and thus has almost no audible Differential Non-Linearity error.
[edit] 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.
[edit] Music for PC games
The MT-32 was used extensively in PC computer games of the late 1980s and early 1990s as a high-end alternative to AdLib or Sound Blaster sound cards for producing game music and sound effects. Sierra On-Line was the first company to support it in their 1988 game King's Quest IV; they also sold the module.
With the popularization of the General MIDI standard and its reference implementation in the Roland Sound Canvas, starting in 1993, support for the MT-32 waned in favor of the General MIDI standard used by many wavetable-based sound cards. Instead of providing custom sounds and music data specifically optimized for the MT-32, later games would simply program the MT-32's sound bank to roughly match the General MIDI Level 1 specification, if they supported the MT-32 at all.
See also: List of MT-32-compatible computer games
[edit] 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 font 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.[citation needed]
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 WDM driver for Windows XP. It is also incorporated into ScummVM, an Open Source Lucasarts adventure game interpreter, as of version 0.7.0. 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.
