Rewritten for e2 (i.e., to be less cold and clinical) from my original public domain writeup at http://library.sound-club.org/wiki/Casio_MT-100.
The Casio Casiotone MT-100 was sold in the mid-1980s as a low-to-mid-range portable home keyboard. It has 8-note polyphony, a 49-key keyboard, 20 instrument patches, and an analog percussion generator (two toms, bass drum, snare, open and closed hi-hats, and click) with 12 programmed rhythms of variable tempo. It also features fingered and Casio Chord accompaniment with three chord tones, four bass tones, and auto-accompaniment with four bass lines and four chord rhythms.
The MT-100 or similar has been used in Homestar Runner cartoons, and the MT-100 has been used by They Might Be Giants, notably the "funny" patch in their song "Put Your Hand Inside the Puppet Head".
The MT-100 has 20 instrument patches (called "tones" in the manual and in the panel legend), selectable through 11 radio buttons (10 patch select and 1 shift) on the front panel. They are:
- Pipe organ
- Elec. piano — A sort of Rhodes simulation very similar to the normal piano sound.
- Elec. guitar
- Funny — This is extremely hilarious and goes "Enng"
- Cosmic tone — zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzznnnnnnnnnnnrrrrr
Sound generation method
(Many, many thanks to Robin Whittle for braving extremely proprietary systems and overpriced service manuals to figure some of the finer points of this out and document it. See his documentation here: http://www.firstpr.com.au/rwi/casio-keyboards/)
The MT-100 and its cousins (such as the MT-68 and the MT-65) employ the NEC D931x chip to digitally synthesize sound. Two short waveforms, stored in a sort of "stairstep" format (instead of being stored as simple quantized sound like PCM, the waveforms are stored in essence as a series of instructions meaning "amplitude goes n steps up/down"), are crossfaded with each other to produce each instrument sound. This was internally referred to as "consonant-vowel synthesis", due to the usual form of each instrument patch — a sharp percussive "attack" waveform segues into a droning sustain phase.
The MT-100 has a wonderfully sharp and clear tone, with no distinguishable aliasing at all — perhaps due to the fact that its internal sample rate is approximately 600kHz. The output of the sound chip has 17-bit depth, with each of the 8 polyphonic voices internally processed at 14 bits.
The analog drums are produced by sine oscillators tuned almost to the point of self-oscillation. The variable resistors controlling the ring time of the sine component of the drums are accessible and may be adjusted. Increasing the ring time of the drums produces a tonal, pitched quality, slightly approximating congas. Increasing it too much causes the drums to self-oscillate, producing a constant sine tone.
Using the MT-100 improperly
Visible on the main PCB are a number of potentiometers to control the ring time of the drums as described above. They look like little slotted discs. Each one controls one drum. Turn them with a screwdriver to mess with the sine component of the drums. Turn them too far for a horrible feedback effect.
As these keyboards are very old by now, aged components in the power supply (notably old electrolytic capacitors) can cause the waveform to be loaded incorrectly on startup as the processor uneasily pulls itself up by its bootstraps. This results in extremely peculiar sounds. Because of the odd waveform format described above, a waveform whose list of "steps" does not sum to 0 can occur when the load process does not complete correctly, resulting in the sound chip getting confused and/or attempting to generate signals not at baseband. Should you wish to explore this behavior, try holding a key down while rapidly varying pressure on the power switch so it switches on and off rapidly. You should eventually hear a sound which is, to some degree, wrong. There is a knack to manipulating the power switch in exactly the wrong way. Finally, note that this is, to some degree, a wrong and bad thing to do, and due to stupid capacitor tricks, it may be possible to blow up your keyboard in this way.
The MT-100 has a number of features which are present in hardware, but not actually connected to a panel switch. These are accessed via unused nodes on the keyboard matrix — for instance, two envelope variation switches can be added to select from 4 available envelopes for each instrument — these are simply wired (with diodes as necessary) from pins 60 - 47 and 60 - 46 of the main NEC D910 CPU. I have not attempted any of these hacks myself, due to sheer laziness, but a friend and bandmate has implemented envelope select in his MT-100 with great success. More hidden features are documented at CYBERYOGI =CO=Windler's page for the similar Casio MT-400V: http://users.informatik.haw-hamburg.de/~windle_c/TableHooters/Casio_CT-410V.html.
When opening the MT-100, it is unadvisable to remove the control panel PCB from its mount unless absolutely necessary, as reassembling the radio button mechanisms is hell.
The Casio MT-68 is merely a more sophisticated MT-100, with extra features (many of which are present but not connected in the MT-100) such as an arpeggiator and selectable envelopes. Similarly, much of the MT series of keyboards uses the same class of hardware; a quick way to identify it is to look for the patches listed above or a very similar set.
My writeup at the SOUND CLUB Library wiki: http://library.sound-club.org/wiki/Casio_MT-100.
Stupid aside: Please sell me your old Casios like this.