CASIO
Casiotone MT-520		   keyboard with warm timbres, drumpads & greatly versatile accompaniment

This Casio keyboard from 1987(?) has lovely warm semi- analogue sound, sample based percussion and 8 rubber drumpads. Its special feature is the quite versatile "Super Drums" accompaniment with individually mutable tracks, and there are even individual inputs for 8 external drumpads.

The 12 semi- OBS preset sounds and chords employ Casio's classic "Consonant Vowel" synthesis engine (see Casio CT-410V), which mixes 2 stair shaped waveforms with independent volume envelopes and can make nicely warm and sonorous sounds. The 12 semi- OBS preset rhythms are made from woody knocking medium resolution samples (same percussion like with Casio CZ-230S); the rhythms consist of 4 separate tracks, those each have a slide switch for 3 variations and mute, which permits a lot of realtime variations. Also the warm sounding accompaniments have each 3 variations and intro, fill-in and ending and accept beyond establishment chords also wild disharmonic note combinations. The patterns remind to Casio CT-410V; some variations are nicely complex arranged and partly contain arpeggio. The sensitive rubber drumpads mute a running rhythm for a bar; the base drum pad even stops it entirely until a fill-in is started, which can be disturbing. Unusual is that there are even 4 stereo plug jacks to connect 8 external drumpads. There is also a primitive record/ playback sequencer (ignores drumpad sounds). The original German retail price of the Casio MT-520 was 449DM (about 225€, price tag on box).

main features:

8 drumpads can be connected.

modifications:

notes:

The case shape of this instrument resembles much the sample based Casio MT-540 and was likely one of the predecessors. The drumpad jacks could be either connected with each a Casio DP-1 dual drumpad unit or a Casio SS-1 Sound Sticks electronic drum stick (US patent 5157213, 5350881). The obscure Sound Sticks were cabled drum sticks, those vibration sensor sends a signal by hitting any surface; a button on top of each stick selects between the 2 contacts in each jack to switch between both sounds. The sound quality is similarly impressive like with Casio MT-800, although the bass of the accompaniment sounds colder. (Casio MT-500 has warmer accompaniment.)

The preset sounds are based on filtered stair shaped waveforms and thus don't sound perfectly realistic, but they have a nicely warm analogue timbre and sound quite noble. It even layers 2 audio outputs for a thicker main voice in the manner of classic Consonant-Vowel synthesis.

(I am not sure if only advanced filters are used or if possibly even additional waveforms are involved for some timbres, because the bass range sounds smoother, warmer and less buzzy than most other squarewave- based Casio keyboards.) The preset sounds all apparently employs a similar  stereo chorus like with Casio CT-410V, although this one can not be controlled by the player and is always set to a slow 2Hz mode. The "piano" sounds fairly real regarding the technology. The "vibraphone" has a somewhat flute- like ethereal timbre with a dose of chorus and weak 4Hz tremolo; the bass range is fairly dull. The "jazz organ" is a Hammond organ imitation with mildly percussive attack and sounds a little squawky (but not as much as with the digital Casio SA-series keyboards), but generally quite soft and not creaky. The "violin" has a slow attack and sounds quite realistic, although high notes are a little thin. The "trumpet" sounds thin and hollow with delayed vibrato; the sonorous bass range rather resembles a bassoon or oboe. The "funky clavi." is a sort of bassy harpsichord with nicely buzzy bass range and no sustain. Also "elec. piano" resembles a harpsichord, but sounds thinner, has a fast fluttering tremolo and sustain. "elec. guitar" is similar but thinner (more sitar- like) with longer sustain. "pipe organ" is the known sonorous multipulse timbre with slow attack and short sustain that attempts to simulate a metal pipe organ rank, but this one sounds a bit too bright and thin. "human voice" rather resembles a wooden or very muffled metal pipe organ rank with short sustain; it may be a "vox humana" rank, but has very little similarity with a human voice although it sounds nicely warm. "flute" sounds like a softly blown metal flute with fluttering tremolo and mild reverb; the bass range sounds woody. "synth. sound" is a harsh and massive electronic metal pipe organ timbre with percussive attack, fast fluttering tremolo and short sustain; the timbre also resembles a harpsichord and trumpet - only high notes play too soft in relation to its loud and thin buzzing bass range. Unlike other that old Casio keyboards there is neither a sustain switch nor other sound controls. The semi- OBS preset sound buttons respond fairly fast; pressing them retriggers the notes of held keys with the selected preset sound, which can be used for live play tricks.

The percussion has a woody knocking style; the samples sound almost identical with Casio SK-8 and also the rhythms sound very similar; some of these (samba, reggae) sound quite oriental. With the "Super Drums" slider each of the 4 tracks in a rhythm can be switched to 3 variations or muted. Starting with all tracks muted, you can build up a gunk structure, which is nice for tekkno- like meditative musics. The semi- OBS preset rhythm buttons respond quite fast during rhythm and thus can be also used for live play tricks. (The Super Drums concept and algorithm is described in US patents 4757736 and 4972755.) The accompaniment has 3 switchable variations and with standard chords many patterns are similarly complex like with Casio MT-540, but unlike MT-540 this one fortunately also accepts non- chords and the fingered accompaniment changes depending on how many keys are pressed. Some variations feature arpeggios, walking bass lines and other complex ornaments. Unfortunately a running accompaniment can not be muted without disturbing the rhythm, because the "casio chord" switch always makes the rhythm stutter when moved.

There is also a simple record/ playback sequencer that records any played notes with chords/ accompaniment, but it can not be edited and records no drumpad hits.
 

hardware details

The Casio MT-520 is based on the CPU "HD61702A03" with external 8KB ROM "NEC D23C64EC 029" and 2KB SRAM "NEC D449C-3". The CPU controls the percussion IC "OKI M6294-02".
The pcb much is smaller than in MT-500. There are unused solder holes for an additional IC.
It contains a fairly complex hardware with much analogue stuff, but the PCB is small compared with its predecessor MT-500. The CPU is close relative of PT-100 and MT-88 with different software, but because the MT-520 has a percussion IC and so doesn't need the CPU percussion channel, it uses the surplus polyphony to layer 2 audio outputs for a thicker main voice in the manner of classic Consonant-Vowel synthesis and even can do a stereo chorus within the CPU. For its complex Super Drums accompaniment it needs an a 8KB ROM and has a 2KB SRAM for sequencer. Unlike MT-500, with ROM removed there is no sound besides drumpads (LEDs behave normal, but it completely locks up when I try to start a rhythm).
 
 These are the main voice filter and gain control outputs of the HD61702A03 CPU shown in Casio CT-450 service manual. A drawing hints that the melody alpha envelope is used for attack and beta for the rest. But the fixed filter sections of alpha and beta subvoice seem to be the same (also control lines wired parallel) which contradicts the original Consonant-Vowel approach. Thus they seem to be rather used for stereo effects than timbre changes.
 
preset sound:
FC4 (gain)
 FC3 (gain) FC2 (filter)
FC1 (filter)
piano
H
H
H
H
vibraphone
 
H
H
H
jazz organ
H
H
  
H
violin
 
    
 
trumpet
 
  
H
 
funky clavi
 
H
 
 
elec .piano
 
 
 
 
elec. guitar
 
 
 
 
pipe organ
 
 
 
 
human voice
 
H
H
H
flute
H
H
 
H
synth. sound
 
H
 
 
The accompaniment filters uses output pin FC5 (bass) and FC6 (chord) for the chord and bass preset sound, depending on the selected rhythm and bass/chord switch setting. Apparently hi means duller, but the resulting complete table (not shown here) is rather large, so I only include here the sound relations.
 
bass:
FC5 (filter)
piano
H
guitar
 
organ
H
synth. sound
H
chord:
FC6 (filter)
piano
H
guitar
 
organ
H
synth. sound
H
celesta
 

Said table also lists an obligato voice (can be {mute, organ, strings, brass, human voice} - output pin unknown) for each accompaniment, which doesn't seem to affect them.

The stereo chorus is generated inside the CPU, which outputs the melody voice at 2 pins to be mixed externally. Quote from CT-450 service manual:

  1. Melody alpha enters negative terminal of Mixer 1 via 180k resistor and positive terminal of Mixer 2 through 33k resistor. Thus, the right and left channel's phases are opposite.
  2. Melody beta is heard from the center as it enters the negative terminal of Mixer Amp's 1 and 2 via 12k resistors.
  3. When Stereo Chorus switch is ON, either of signals alpha or beta is delayed by frequency modulation in CPU.
The meaning of item 3 is unknown, since neither CT-450 nor MT-520 contain a stereo chorus switch. Possibly it is part of the internal preset sound definitions (software) or hints to an unidentified easteregg.

keyboard matrix

I haven't analyzed this huge keyboard matrix by myself, but only read the service manual and schematics of CT-450 (same hardware class with jacks instead of drumpads), thus there may be still unknown eastereggs. But because many other variants are more restricted and e.g. lack Super Drums, this still should be useful. I expect e.g. the here absent off-positions of Super Drums sliders in row KI4.
 
40 KI1
41 KI2
42 KI3
43 KI4
44 KI5
45 KI6
46 KI7
47 KI8
 
CPU pin
in 1
in 2
in 3
in 4
in 5
in 6
in 7
in 8
in / out
 
o
C1
o
C#1
o
D1
o
D#1
o
E1
o
F1
 O.
piano
R.
rock 1
out 1
83 KO1
o
F#1
o
G1
o
G#1
o
A1
o
A#1
o
B1
 O.
vibraphone
R.
pops
out 2
84 KO2
o
C2
o
C#2
o
D2
o
D#2
o
E2
o
F2
 O.
jazz organ
 R.
disco
out 3
85 KO3
o
F#2
o
G2
o
G#2
o
A2
o
A#2
o
B2
 O.
violin
 R.
swing
out 4
86 KO4
o
C3
o
C#3
o
D3
o
D#3
o
E3
o
F3
 O.
trumpet
 R.
samba
out 5
87 KO5
o
F#3
o
G3
o
G#3
o
A3
o
A#3
o
B3
 O.
funky clavi
 R.
march
out 6
88 KO6
o
C4
o
C#4
o
D4
o
D#4
o
E4
o
F4
 
 R.
select
out 7
89 KO7
o
F#4
o
G4
o
G#4
o
A4
o
A#4
o
B4
 
O.
select
out 8
90 KO8
o
C5
 
 
 
 
 
 
 
out 9
91 KO9
R.
bass drum I
R.
bass drum II
R.
bass drum III
 
 
 
 
R.
synchro
out 10
92 KO10
R.
snare drum I
R.
snare drum II
R.
snare drum III
 
 
 
 R.
start/stop
 R.
fill-in
out 11
 93 KO11
R.
hand clap I
R.
hand clap II
R.
hand clap III
 
 
 
R.
tempo +
R.
tempo -
out 12
94 KO12
R.
cymbal I
R.
cymbal II
R.
cymbal III
 
power off
power on
memory
play
memory
record
out 13
66 LO1
C.
variation I
C.
variation II
C.
variation III
 
C.
off
C.
fingered
C.
casio chord
 
out 14
67 LO2
P.
snare
P.
base
P.
handclap
P.
rimshot
P.
low bongo
P.
high bongo
P.
cymbal
P.
hihat
out 15
77 LO12

The input lines are active-high, i.e. react on +Vs. Any functions can be triggered by a non- locking switch in series to a diode from one "out" to one "in" pin.

Because drumpad hits can be faster than the keyboard matrix scan cycle, for buffering the 8 drumpad inputs each set a flipflop. To read their contents, the CPU sets LO12 high, which makes the flipflop bank appear on the KI lines of the keyboard matrix. Afterwards it sets LO13 high to clear the flipflops to get ready for sensing new drumpad hits.
 

legend:
"o"
 = keyboard key
underlined
 = function needs locking switch (i.e. stays active only so long the switch is closed)
R.
 = preset rhythm
O.
 = preset sound ('orchestra')
C.
 = chord
P.
 = drumpad
orange
background 		 = easteregg (unconnected feature)

A smaller Casio MT-520 variant with only 4 drumpads was released as Casio MT-220 and without drumpads as Casio MT-205/ MT-207 (aka Realistic Concertmate-700 | both without rhythm tracks mute). The same hardware class was also used in Casio MT-110 (with neither Super Drums nor drumpads). A fullsize version of the MT-520 was released as Casio CT-510 (with additional pedal jack (for bass drum??)) and without drumpads as CT-450 (additional volume- and bass drum pedal jack, lacks drum switch mute positions) and CT-360 (mono, no drum jacks). The direct predecessor of the MT-520 was the Casio MT-500, which had only 4 drumpads (each switchable to 3 sounds) but even 20 preset sounds; the MT-500 is also a nice keyboard but has other nasty flaws, thus do not believe false claims that it would be better. A smaller Super Drums keyboard with semi- analogue percussion (but no drumpads) was the Casio MT-52 (all seen on eBay). Another midsize drumpad keyboard was the Casio MT-640 (based on MT-540 hardware, 6 rubber drumpads, 4 effect pads, bigger speakers | I own one).
 

 removal of these screws voids warranty...    
WarrantyVoid
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