Roland 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 linear arithmetic synthesizers, the multitimbral MT-32 series constitutes the budget prosumer line for computer music at home, the multitimbral D-5, D-10, D-20 and D-110 models constitute the professional line for general studio use, and the high-end bitimbral D-50 and D-550 models are for sophisticated multi-track studio work. It was the first product in Roland's Myuujikun line of Desktop Music System 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. It also features a digital reverberation effect. Successors 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. 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 assigns its parts 1~8 and R 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 closely as possible. 64 of the 128 patches 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 and reversed panpot compared to MMA MIDI specifications. The utility was predated by a pack called "MT32GS", released by Mike Cornelius in 1992. The CM-Panion, by Gajits Music Software, was an Amiga editor which worked with the MT-32.

MT-32 models

Two major revisions of the MT-32 were produced. Roland refers to them as MT-32 and MT-32.
Image:MT-32 f gen.svg|thumb|300px|Back view of MT-32
Image:Roland MT-32 Oldtype Revision 1 PCB View.jpg|thumb|300px|Top view of MT-32 with cover removed

MT-32 (old)

The LA32 sound generation chip is an 80-pin PGA. The control CPU is an Intel C8095-90 in ceramic DIP-48 package. The digital-to-analog converter is a Burr-Brown PCM54; the input signal having a resolution of 15 bits. Line-outs are unbalanced 1/4″ TS phone connector No headphone jack.
  • 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 QFP type.
  • MT-32 with "old-type" revision 1 PCB, used in units with serial numbers 851400–950499.
Image:MT-32 s gen.svg|thumb|300px|Back view of MT-32

MT-32 (new)

The control CPU is an Intel P8098. Same Digital-to-analog converter, but with 16 bits of input signal resolution. A stereo 1/4″ TRS headphones jack is added.
  • MT-32 with "new-type" revision 1 PCB, used in units with serial numbers 950500 and up.
  • Roland MT-100: Combination of MT-32 and PR-100. While it uses a MT-32 PCB, the chassis is different.
MT-32 that have had the mainboard replaced by Roland because of a repair can contain the MT-32 mainboard, with the headphone jack removed.

MT-32 compatible models

To target computer users, Roland released a number of CM modules. They came without an LCD display and had most buttons removed. Most of these CM modules aside from the CM-32P and CM-300 are compatible with the MT-32 but feature 33 additional sound effect samples which many games took advantage of. These sound effects cannot be heard on an MT-32.
Early models share a similar design to MT-32. Control CPU is an Intel P8098 and DAC is a Burr-Brown PCM54.
  • Roland CM-32L: Released in 1989, this Roland CM has only a volume knob, a MIDI message and a power-on indicator as external controls.
  • Roland CM-64: A combination of the CM-32L and the CM-32P, a cut-down "computer music" version of the Roland U-110. Like the CM-32P, the CM-64 can be expanded with a Roland SN-U110 sound library card 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.
In later models, the DAC is a Burr-Brown PCM55, and vibrato is noticeably faster.
  • 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-98 series of computers. Released in Japan only.
  • Roland RA-50: LA unit with CM-32L ROM : Requires software work around or hardware modification to work 100% as a MT-32.

    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.
Despite having the capabilities of a professional synthesizer 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 E-20 keyboard internally contains a partially unpopulated MT-32 motherboard, while the RA-50 Realtime Arranger uses a highly modified one. 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.

Digital overflow

The MT-32 and compatible modules use a parallel 16-bit DAC at a sampling rate of 32,000 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 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.
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:
Original data bit # Connection
--------------------------------------------------------------------------------
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 Output of LA32 synthesizer chip
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 Input to reverberation 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 reverberation 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:
Original data bit # Connection
--------------------------------------------------------------------------------
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00 14 Output of LA32 synthesizer chip
15 13 12 11 10 09 08 07 06 05 04 03 02 01 00 14 Input to reverberation 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 reverberation 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 must be kept low instead.