Casiotone CT-410V, circuit-bent!   analogue synth keyboard with complex accompaniment 
& real analogue filter

This is definitely one of the most interesting Casio keyboards, because it features a genuine analogue synthesizer filter with filter envelope and cut- off/ resonance control sliders, lots of accompaniment variations and separate volume sliders. Additionally it has  a wonderful "stereo chorus" rotary speaker/ leslie simulator that produces a great sort of "hemi- sync" mind machine effect with stepless adjustable speed, which makes the thing perfect for brain wave synchronization features in psychedelic meditation or tekkno trance music. A smaller version of this instrument with midsize keys and no built-in speakers was released as Casio MT-400V.

All sliders of this instrument are real analogue potentiometers and thus stepless. The stereo chorus LFO can also modulate the filter in "wah wah" mode, which makes the typical "wokachika" sound known from certain 1970th funk (?) musics. The great accompaniment unit has 4 bass and 4 chord variations and the same unique, dark and sonorously droning squarewave bass tones like the "organ" sound of the Hing-Hon EK-001. In fingered chord mode the accompaniment plays the more notes the more keys are pressed, and this works even perfectly with all non- chord key combinations, which permits very versatile accompaniment sound patterns. (This flexible behaviour is absolute no matter of course, see Yamaha PSS-390 for an annoyingly stubborn example.) And despite this instrument has already many built-in features, there are even still lots of additional keyboard matrix eastereggs to discover, which makes it perfect for circuit bending.

main features:

eastereggs:

There are tons of great keyboard matrix eastereggs; for details see below.

notes:

Although the Casio CT-410V and MT-400V are quite rare, there are many other old Casio keyboards without the synthesizer filter, those are more or less based on the same hardware class and easy to find at eBay. In the following I therefore will explain a bit more detailed than usual how to modify this instrument, because particularly the unique sounding squarewave accompaniment CPU is worth to be explored since many Casio instruments do not remotely make use of all its features. I conclude that the most complete featured specimen of this hardware class was the Casio MT-65/ MT-68, which has the same accompaniment CPU and main voice soundchip like the CT-410V/ MT-400V, but includes 4 arpeggio and 4 envelope variants (but no synth filter). This accompaniment is almost as flexible playable like the Testron one, and thus also accepts any disharmonic note combinations and not just those few ones that establishment has defined to be "chords". When rhythm is off, the rhythm select switches switch the bass and organ chord timbres among 4 squarewave multipulses, and also the chord and bass variation switches change them. Instruments of this hardware class can be recognized by the rhythm and sound preset names (the sound names "funny" and "cosmic tone" are easy to remember) and also the count of 12 rhythms and 20 sounds and the possible presence of locking switches for multiple chord, bass and/ or arpeggio variants hint to it.

The analogue percussion of the CT-410V sound rather unspectacular (no booming tekkno base or the like), but the decay time of all drums can be adjusted with trimmer potentiometers on the main PCB, and like all analogue drums they can be certainly modified in various ways.

The 20 main voices are fixed presets and not user- definable, but there are various mode switches (vibrato, sustain etc.) those modify them. The main voice generally sounds a bit dull when not sent through the synth filter; this has partly to do with dull built-in speakers, but there is also a 1nF capacitor which unnecessarily muffles all timbres - throwing it out is strictly recommended and will improve the sound significantly. (In opposite to this my Casio MT-45 sounds very bright by default, despite it has the same accompaniment CPU.) The main voice sounds of the CT-410V consist of 2 mixed squarewave tone suboscillators with different multipulses and different digital volume envelopes, those are (depending on the preset) muffled by 4 different filter capacitors. In the bass range many sounds turn into a more or less buzzy, sonorous purring drone, which is a characteristic style element of squarewave based instruments. These basses can resemble some of the famous POKEY sound effects on Atari XL homecomputers and are very different from the gradually duller and duller growing sine wave bass behaviour of average Yamaha FM keyboard sounds. When sustain is switched off, all sounds stop almost immediately after releasing the key, and the sound presets itself also contain neither vibrato nor tremolo.

Particularly the piano and the ringing "mandolin" timbres sound really dull, and also the too narrow pulse width of the mandolin makes it sound very unrealistic and buzzy at low notes. As usual with squarewave instruments, the dull "trumpet" sounds very artificial (but a much better trumpet timbre can be generated using the synth filter). Most realistic sound the organ, flute and clarinet presets. The "funny" preset is a sort of rough digital slap bass (creaky picked e-bass) which sounds like "ennng!" (possibly it was initially intended to be called "funky", but sounded to artificial for this?). The "cosmic tone" preset sounds simply like a plain saxophone tone, thus  I guess that its strange name was initially rather related to its 3 switchable envelope variants (those are normally not available on this instrument but can be added by an easy modification), because they turn it into a slowly duller fading spacey synth lead "meow" timbre, which makes audible use of the crossfading between the 2 subvoices. Both sounds belong to the brightest and most interesting ones of this instrument.

Technically the CT-410V is completely mono, besides that the main voice is optionally post- processed by the "stereo chorus" circuit, which makes a pitch shifted version of it alternatingly pan left and right to approximate a rotary speaker effect. Either the main voice ("tone") or rhythm or bass + chord or a white noise source (chopped by the rhythm) can be post- processed by the analogue low- pass synthesizer filter.

The filter has sliders for cut- off, resonance and an envelope generator with sliders for attack, decay and sustain level. As usual, cut- off removes all frequencies above a certain value (i.e. muffles the sound or even mutes it completely when set very low), while resonance rises the frequencies near the cut- off value and this way a high resonance setting makes the timbre more hollow or metallic. The filter envelope modulates the cut- off value. In main voice ("tone") mode it is started by the begin of each new note (ignoring the key press duration), and because there is only one monophonic filter, during polyphonic play the filter envelope affects all currently held down notes simultaneously, which permits an interesting gate effect because it can make held down notes fade dull or silent (by the filter envelope) and then sound again with full brightness during each new played note by the restarting filter envelope. Unlike this, when the filter is assigned to "bass/ chord", its envelope does not restart with each new chord sound, but seems to ignore new bass/ chord notes so long its envelope is still in the attack or decay phase. Thus with fast rhythm tempo and slow envelope settings it makes rather a sort of "wokachika" sound (like known from 1970th funk(?) music and the oldest "Sesame Street" theme) instead of exactly following the chords. This way the accompaniment pattern gets differently stressed/ phrased by changing tempo or attack/ decay setting, which provides an interesting effect for tekkno. Instead of this envelope generator, the cut- off value can be alternatively modulated by the triangular wave LFO of the stereo chorus (no matter if the chorus effect is currently on or off) by selecting "wah wah", which makes a continuous filter sweep that can (depending on the cut- off slider setting) result in differently strong effects from mild tremolo to "wokachika" or chopping the sound. (Unlike a normal volume envelope, all these filter envelopes fade the sound not only quieter but also make it grew duller by reducing the trebles first.) As well the stereo chorus as the filter of this instrument have a bit thin and slightly harsh sounding cheap op-amp chip timbre. In unmodified state the filter permits no really high resonance level, and at full resonance the signal looses quite much bass intensity. Fortunately some simple circuit bending helps here to make it oscillate nicely and get half- way reasonable acid house sounds out of it.

There is also even a filter envelope input jack, which was originally intended to plug in the Casio BFC-1 breathe filter controller - a small black box with a mouthpiece, with that the filter cut- off value should be controlled by blowing into it like playing a wind instrument. (Likely this feature was intended as a competitive product to a similar breathe input device of Yamaha's famous DX7 FM synthesizer, which also had a special input jack for it.) Unfortunately neither rhythm nor accompaniment are user programmable, and there is also neither a built-in sequencer nor MIDI capabilities.

modifications:

circuit bending details

PCB layout

When the CT-410V is opened (keys facing down, all views at solder side), there are beside various smaller ones 2 large PCBs. The lower one is the main PCB ("M485-MAIM (D)"); from left to right it is divided into the following areas:
  • analogue synth filter (with resonance and sustain preset trimmer)
  • analogue percussion section (with drum decay trimmers)
  • in the middle: main voice sound generator (soundchip NEC D931C) and clock oscillator (below)
  • accompaniment CPU NEC D930G (which also polls the key matrix)
The top PCB ("M487-ASIM (A)") contains mainly the power amplifier, and at a small section to the right the "stereo chorus" effect. It is a good idea to add a polarity protection diode and extend the short battery compartment cables since they tend to tear off very often during work at the electronics. The other PCBs only contain simple helper circuits like key matrix diodes. The PCBs are quite large an look in spite of this like designed for smaller Casio MT series keyboards, thus I conclude that the Casio MT-400V contains likely the same PCBs but has no built-in speakers by the reason that these PCBs take so much space.

accompaniment CPU D930G

The NEC D930G is one of the most interesting squarewave accompaniment chips ever made by Casio, because although it features only 12 rhythms, it has for each of them 4 chord, 4 bass and 4 arpeggio variants and features wonderful dark and sonorously droning multipulse squarewave bass and chord tones with different pulse patterns. All tones have separate output lines (those partly need external envelope capacitors) and it features also separate trigger outputs for analogue drums. The 80 pin SMD chip has own keyboard matrix input lines for control switches and accompaniment section keys, and it apparently also can act as a keyboard matrix decoder to communicate (through a serial port?) with an external main voice soundchip (the NEC D931C in the case of the Casio CT-410V ), while in other instruments (e.g. my Casio MT-45) this feature is not used. Most key matrix pins are at the top, and 2 at the left side of the chip package. The sound output and drum trigger lines are at the bottom side. Bass and arpeggio employ a capacitor envelope for their release phase.

Apparently a direct predecessor of this hardware class was the accompaniment CPU D910G and main voice soundchip D990G, those were used in the Casio MT-60.

keyboard matrix

61
60
59
58
57
56
55
54
53
52
70
71
 
pin
out 1
out 2
out 3
out 4
out 5
out 6
out 7
out 8
out 9
out 10
out 11
out 12
out/ in
 
O.
pipe organ
o
o
o
o
o
o
o
o
o
R.
rock
C.
bass1
in 1
51
O.
flute
R.
synchro
o
o
o
o
o
o
o
o
R.
pops
C.
bass2
in 2
50
O.
trumpet
R.
start/ stop
o
o
o
o
o
o
o
o
R.
disco
C.
chord1
in 3
49
O.
oboe
R.
fill-in
o
o
o
o
o
o
o
o
R.
16 beat
C.
chord2
in 4
48
O.
violin
O.
envelope1
o
o
o
o
o
o
o
o
R.
swing
O.
sustain
in 5
47
O.
bank switch
O.
envelope2
o
o
o
o
o
o
o
o
R.
latin swing
O.
vibrato
in 6
46
 
O.
select
transpose
set
C.
memory
    
C.
manual bass
C.
casio
    
C.
arpeggio1
O.
delayed
vibrato
in 7
45
 
R.
select
transpose
on
octave down
key hold
  
C.
arpeggio on
C.
fingered
C.
memory
mode
  
C.
arpeggio2
O.
reverb.
in 8
44

All unknown function names and in/ out numbers in this chart were chosen by me. The input lines are active- low, i.e. react on GND, thus any functions are triggered by a switch in series to a diode from one "in" to one "out" pin.
 

legend:
"o"
 = keyboard key in accompaniment section
"o"
 = keyboard key in main voice section (i.e. processed by external soundchip)
green
 = processed by external main voice soundchip D931C
underlined
 = function needs locking switch (i.e. stays active only so long the switch is closed)
R.
 = rhythm
C.
 = chord
O.
 = orchestra (main voice sound)
orange
background 		 = easteregg (unconnected feature)

  • main voice sounds ("tone")
    There are 10 main voice sounds selected by 5 individual button inputs in combination with a bank switch input that needs to be pressed simultaneously (e.g. using 2 way push button switches). Although the sound select switches of the CT-410V are locking, the CPU memorizes the last pressed button and thus would also work with normal buttons. A locking "select" switch selects between 2 sets of each 10 sounds, which makes in total 20 different ones. (The CT-410V uses all of them, but possibly smaller keyboards don't.) 

    The main voice sounds are generated by the D931C soundchip. Unlike various older Casio instruments, during trilled notes it does not occupy one new sound channel per repeated note, but re- uses only 1 sound channel per pressed key. This helps to save polyphony, but also makes the sound more boring because it prevents the typical phasing and volume increase effect known from older polyphonic Casios. I am still not sure why the main voice polyphony is reduced from 8 to 4 notes when chord/ bass or accompaniment are active, despite as well the accompaniment CPU as the main voice soundchip contain their own sets of totally independent tone generators. I can only imagine that by a design flaw the keyboard matrix decoder in the accompaniment CPU contains too little memory to handle more than 8 simultaneously held down keys, which makes it cut down the polyphony.
     

  • envelope select
    The Casio MT-65/ MT-68 has 2 locking switches to select 4 main voice envelope envelope combinations, those can be also added here. The O. envelope1 function changes the crossfading of the 2 suboscillator envelopes of a main voice sound, while the O. envelope2 mainly changes the attack rate of both suboscillators. What both switches exactly change depends much on the currently selected preset sound (e.g. with the "mandoline" they also reduce the ringing speed).
       
  • rhythms
    There are 6 rhythms selected by 6 OBS button inputs. Although the rhythm select switches of the CT-410V are locking, the CPU memorizes the last pressed button and thus would also work with normal buttons. A locking "select" switch selects between 2 sets of each 6 rhythms, which makes in total 12 different ones. (The CT-410V uses all of them, but smaller keyboards like the MT-45 don't).
     
  • chord, bass & arpeggio variations
    The 4 chord variations are selected by a locking 4 step switch which switches 2 independant chord select inputs through all 4 possible combinations. The 4 step bass variation switch works the same way with different inputs, and there are also 2 inputs for a 4 arpeggio variation switch, those are not used in the CT-410V by default (see below). Many other Casio keyboards also use only a small subset of the this way possible 64 variations.
     
  • manual bass
    My Casio MT-45 has a "manual bass" switch that turns the accompaniment section of the keyboard into a monophonic organ bass. Unlike the normal bass of the CT-410V (chord mode with muted "chord" volume), this bass is not limited to the scope of one octave, and it can be also played by hand together with rhythm and no accompaniment (the normal bass is chopped by accompaniment when rhythm on). Unfortunately in manual bass mode the main voice polyphony is still reduced from 8 to 4 voices despite the bass needs only 1.
     
  • octave down
    A switch here transposes the main voice 1 octave down (works only in chord mode).
     
  • chord memory
    With the Casio MT-45 setting this switch on makes organ chords (without rhythm) or accompaniment chords (with rhythm) hold when their keys are released; when off, chord or accompaniment both stop when all keys in accompaniment section are released, which permits more flexible play. With the CT-410V instead organ chords simply hold never and accompaniment chords hold always. This behaviour is controlled by a fixed diode soldered into the key matrix.
     
    chord memory
    (in7->out4)
    chord memory mode
    (in8->out9)
    rhythm on
    chord holds
    0
    0
    0
    no
    0
    0
    1
    yes
    0
    1
    0
    no
    0
    1
    1
    no
    1
    0
    0
    no
    1
    0
    1
    yes
    1
    1
    0
    yes
    1
    1
    1
    yes

    The CT-410V has a fixed diode at in7->out4, while the MT-45 has at this location the "chord memory" switch and apparently the diode instead at in8->out9. Thus with the CT-410V solder a switch in series to the diode at in7->out4 and add a fixed diode at in8->out9.
     

  • key hold
    This odd feature simulates that all main voice keys stay held down/ get stuck. Only after the instrument runs out of polyphony, the first pressed keys get freed again with every new key press. Although this was likely intended for a pedal, also a switch here can be musically interesting to experiment with. E.g. with decaying sounds this simply makes a longer sustain, and with continuous tones for tekkno trance etc. you get your hands free to play with the filter, stereo chorus or accompaniment etc. while the main voice stays constant.
     
  • sustain + reverb.
    Despite the soundchip does support sustain + reverb effect simultaneously, the slide switch in the CT-410V only offers them separately. The switch functions by a plastic handle with a leaf spring that presses down a conductive plastic foil through openings in an insulator foil onto PCB trace contacts. Thus I simply cut out the plastic bridge in the insulator foil between "sustain" and "reverb." and bent the spring a little flatter down. This way the switch will connect to both contacts simultaneously when switched into an intermediate position. Unfortunately this caused note mess during polyphonic play by shorting the key matrix lines in5 with in8 by the lack of diodes in between. Therefore I placed a germanium diode into the line from the switch's "reverb." output to fix this. (A silicon diode didn't work here because there are already other diodes in series those caused a too high voltage drop.)
     
  • arpeggio
My Casio MT-45 has arpeggio, and an MT-65/ MT-68 includes even 4 selectable variations of them. I really love this rhythmical chinking accompaniment effect, which IMO is one of the most typical and interesting style elements of classic electronic keyboards, thus it is a pity that with later Casio and Yamaha keyboards it was omitted because manufacturers apparently considered it out of fashion.

Also the CT-410V is missing this feature, despite its D930G even would have supported 4 variations of it. Fortunately it can be upgraded with it, although this is a little tricky. The easiest part is to add the C. arpeggio on switch and optionally a 4 step slide switch at out11 with diodes to select the 4 arpeggio variations by binary counting through all combinations of {C. arpeggio1, C. arpeggio2}, i.e. 1= open, 2= in7->out11, 3= in8->out11, 4= in7->out11 and in8->out11. (Instead 2 separate switches would work also.) Unfortunately this yet won't produce any sound, because the electronics also needs to be upgraded with a small  analogue circuit to generate the decay envelope for the arpeggio. I copied this circuit from my Casio MT-45, without that I certainly never would have found out how this works.

The squarewave tone for it is outputted at pin 6 of the D930G, and the envelope control signals are pin 79 and pin 12. Due to this is an SMD IC, it needs a fine soldering iron, a calm hand and/ or good luck to solder wires to the pins. Theoretically pin 6 would be sufficient to get an arpeggio, but it would consist of simple continuous organ tones, those keep tooting forever when arpeggio is stopped. To get the classic Casio arpeggio sound, the following components need to be added:

Connect an 1µF electrolytic capacitor from pin 12 to GND and connect a 100 kOhm resistor from pin 12 to pin 6. Connect a diode from pin 6 through a 22 kOhm resistor to pin 79. Connect a 22 kOhm resistor to pin 6 and to its other end a 8.2nF cap against GND and a 10nF capacitor, which open end will become the arpeggio sound output. It is a good idea to mount these components on a small wire wrapped daughter board, that can be hotglued somewhere below the D930G on the main PCB. For arpeggio volume control, connect the arpeggio sound output of the daughterboard through a shielded cable with the right end of a 100 kOhm log potentiometer, its left end with GND and its wiper pin will now become the volume controlled arpeggio output. (I mounted the pot between "main volume" and "rhythm volume" slider.)

The output of the arpeggio volume pot could be theoretically connected with many places to get the sound audible. I added here a 3 position slide switch to route the sound either directly through the output amp ("normal"), or through the main voice ("tone" - i.e. gets processed by "stereo chorus" and timbre filter and synth filter when they modify the main voice) or through the synth filter:

  • The switch output in the the "normal" position connects with the white wire of the shielded chorus cable, which is the left one of 2 such cables down in the middle of the left half of the main PCB, which goes to the output amp.
  • The output in "tone" (main voice) position connects with the rightmost pin of a black resistor array module in the middle of the PCB, which seems to act as DAC for the main voice soundchip D931C. This modifies the arpeggio by everything that actually modifies the main voice (including its muffling capacitors).
  • The output in "filter" position connects through a 2.2 kOhm resistor with a strange looking divided solder pad below the filter resonance preset trimmer pot at the left side of the main PCB. This solder pad is round and consists of 2 semi- circular halves with a small bridge in the middle; likely it was intended as a cuttable jumper for test purposes.
To the upper left you see the new arpeggio mode switches. Below them is the manual bass and chord memory switch.

filter and chorus input jacks

To send an external signal to filter + chorus, connect a cinch jack through a 100nF cap in series to a 1 kOhm resistor with the rightmost pin of a black resistor array module in the middle of the PCB, which seems to act as DAC for the main voice soundchip D931C. This modifies the external sound by everything that actually modifies the main voice (including its muffling capacitors).

To send a signal only through the filter (in any mode), connect a cinch jack through a 100nF cap in series to a 2.2 kOhm resistor with the strange looking divided solder pad below the filter resonance preset trimmer pot at the left side of the main PCB. This solder pad is round and consists of 2 semi- circular halves with a small bridge in the middle; likely it was intended as a cuttable jumper for test purposes. The external signal this way will be sent always through the filter, no matter if the filter processes internal signals at the moment. (So far nothing is currently triggering the filter envelope, the "sustain level" and "cut off" controls must be set high enough to get the external signal audible.)

These jacks can be likely also used as separate outputs.

main voice volume control

Particularly when using the filter, it is annoying that no independent volume control for the main voice exists. To add one, cut the trace from pin 1, 2 at the op- amp "4558-2" in the middle of the main PCB. Connect the the op-amp side with the right pin of a 22 kOhm log potentiometer, and its wiper output with the other trace end. Connect the left pin of the pot through a 10µF capacitor with GND. (This volume control works not when the main voice runs through the filter - apparently the filter gets the signal from another line that bypasses the timbre muffling capacitors.)
 
Here I added 2 potentiometers for main voice ("tone") and arpeggio volume controls.

timbre change & distortion

The main voice sounds of the CT-410V consist of 2 mixed multipulse squarewave tones with different pulse patterns and different digital volume envelopes. (Also the reverb is technically only an envelope modification.) The waveforms are outputted through a resistor array DAC(?) and are post- processed by 4 low pass filtering capacitors those are controlled through a "4066" analogue switch IC. These filter caps have the values {4.7nF, 33nF, 5.6nF, 470pF} and are located to the left of the NEC D931C soundchip. The 4066 is DC controlled by the soundchip, and can be also easily controlled by external switches to modify  the timbres. (This timbre filter circuit is part of the main voice sound generator and has nothing to do with the additional cut- off/ resonance synthesizer filter of the CT-410V).

I soldered a 4 channel alternating switch into the control lines to the 4066 to make the 4066 inputs (pin 5, 6, 12,13) switchable between their original soundchip outputs and 4 external controls. Instead of 4 simple switches I connected here the wipers of 4 10 kOhm potentiometers to permit analogue control over these inputs (use shielded cables against hum). The 4066 in this instrument switches its load fully "off" at voltages below about 2.8V and fully "on" at about 2.87V, thus the pots need about 2.8V at the left and 2.87V at the right end. To generate these voltages, I built 2 voltage dividers those each consist of [a chain made from a 330 Ohm resistor, a 100 Ohm trimmer and a 220 Ohm resistor in series] from GND to +5V. The trimmer wiper of the 1st voltage divider is connected with the "left" pins of the 4 pots, and the trimmer wiper of the 2nd with the "right" pins. The trimmers have to be adjusted until the potentiometers block all sounds when fully turned left, and let it pass undistorted when turned a bit less than fully right.

The filter capacitor #1 seems to be a pop noise protection or the like, because when its pot is turned fully close (left), the main voice sound is muted, independant from the other pot settings. Cap #2 and #3 simply modify the timbre. With these pots are in an intermediate position (between left and right) the sounds turn gradually quieter and distorts in interesting ways. Cap #4 also changes the timbre, but it behaves special, because when its pot is in an intermediate position, there is much static, humming, and depending on the other pot settings the amp even tends to toot loudly by self- oscillation. There is also a popping noise while pot 4 is turned (unless the sound is muted with pot 1).
 
To the right you see the new timbre potentiometers with bypass switch for normal mode. 
Left next to them are the switches for envelope control, key hold, octave and transpose.
In the upper left corner you can see the added intermediate setting "sustain + reverb" for the effect switch.

note: When the main voice is routed through the synth filter, these pots do nothing since the main voice apparently bypasses the normal timbre filter caps for this.

remove dullness

In comparison to a Casio MT-60 or MT-45, the CT-410V sounds quite dull. One reason for this are the big speakers with poor trebles, but the main reason is a totally useless and permanently active 1nF capacitor near the main voice filter caps in the middle of the main PCB. This annoying cap makes all sounds dull, which e.g. makes the characteristic harpsichord attack click almost inaudible. It is therefore strictly recommended to disconnect this muffler.

extended tempo range

Unlike with older Casios, the maximum rhythm tempo of the CT-410V is a little slow. A 10nF capacitor at the right upper end of the main PCB controls the tempo oscillator. Replace it with 1.7nF to speed up the tempo a lot. (The still functioning minimum value may vary among instrument specimen, since with a slightly smaller cap my CT-410V stopped the rhythm entirely.) Due to this also rises the minimum tempo a little high, connect the open end of the tempo potentiometer with a 680 kOhm resistor in series with a trimmer pot (e.g. 300 kOhm) against GND. Adjust the trimmer in a way that the rhythm barely stops (arpeggio can make a continuous tone here) when the tempo pot is moved to minimum. This will provide you the maximum possible tempo range. (With full tempo arpeggio sounds great.)

filter resonance improvement

The synthesizer filter of the CT-410V sounds a little boring because it refuses high resonance values. Despite there is a resonance preset trimmer at the left lower half of the main PCB, high settings of it eat much bass. Even worse, instead of gently tweeting near the cut- off frequency, at high resonance settings the filter op- amp tends to run amok and suddenly squeak without warning terribly loud (about 10 times of the rest) like a police whistle at a far higher frequency than the cut- off value. To make the filter behave nicely, solder at the white wire of the grey shielded cable from the resonance pot (upper left main PCB area) a 56nF capacitor in series to a 6.8 kOhm trimmer pot (as adjustable resistor) against GND. By alternatingly tweaking the resonance preset and the new trimmer, you can now adjust the filter to friendly tweet around the cut- off frequency at low volume without blasting away your ears. The filter can this way even safely howl in self- oscillation by its automatic envelope at maximum resonance and sustain level without squeaking unbearable loud, and it will stop as soon sustain level or the envelope drops below the necessary value. This permits also nice "acid house" sound effects, although the timbre is a bit different from a TB303. (Unfortunately there is no sequencer, thus you can only send the accompaniment through the filter.)

Attention: Someone e-mailed me that this mod does not work with Casio MT-400V due to different hardware. I am not sure if both keyboards are indeed technically different or if there is even an error in my description. This  is what he wrote:

A couple of notes. On the 400v there's the resonance pot, right next to it is the cutoff pot. By tweaking these two it's possible to get the filter sounding pretty good. Also the wiring's different so I couldn't get the mod to work without having a drastic drop in volume on the filter output. 

On the arpegiator, the main voice in is the same, but the dry and filter ins are differerent. Particularily, I couldn't find the filter-in. The best I could do is get it routed through the bas/chord input. It does strange things to the amp and envelope of the bass, but it sounds pretty cool. The chorus distortion is a little bit different as well. If you want me to send you sound samples or pictures of the boards then let me know.

Thanks for being so intrepid with the circuit, the arpegiator adds wonders to the sound and the chorus distrotion is great fun to play with.

bass circuit

The bass output of the D930G employs a very similar envelope capacitor circuit like the arpeggio. It consists of an 1µF electrolytic capacitor (for decay duration) from pin 11 to GND and a 82 kOhm resistor from pin 11 to pin 5. There is a diode from pin 5 through a 22 kOhm resistor to pin 80. The sound is outputted through a 1µF electrolytic capacitor from pin 5.

Interesting is that the Casio MT-45 produces a much softer, duller and more pressureful bass which timbre resembles a triangular wave or Roland TB303 (without resonance), while the CT-410V bass is a more sonorous droning squarewave tone. The MT-45 has instead of the 1µF envelope cap a 0.47µF one, and behind the 1µF output cap follows a 220 kOhm resistor against GND, a 120 kOhm resistor against +5V and a 47 kOhm resistor to the amplifier, but I doubt that this causes the difference. The CT-410V has after the 1µF output cap a black cable that leads to one input of the bass volume potentiometer, and its wiper (?) output is connected with a 22nF muffling cap against GND (located 3cm below the bass volume pot cable near the corner at the top of the middle of the main PCB) and resistors lead the signal to the amplifier. Bridging the muffling cap with a bigger one makes the bass indeed duller, but in spite of this it won't really sound as smooth and pressureful as the MT-45; a too big additional cap here only reduces the bass volume too much. I guess that the MT-45 circuitry has a much higher inner resistance and thus doesn't damp the bass as much as the bass volume pot stuff of the CT-410V does. (When I experimented with adding the bass potentiometer to the Testron I discovered similar behaviour.) Increasing the envelope cap value makes the bass decay slower (and thus sound less dry), but this also doesn't imitate the MT-45 bass sound. (The MT-45 has even a smaller instead of a larger cap, but this is likely rather caused by the higher inner resistance of the rest of its circuit, because it doesn't seem to fade the bass silent much faster. Certainly potentiometers can be added here to make the envelope and muffling capacitor values adjustable etc., but I didn't modify this.)

Possibly the CT-410V has even intentionally a less dull bass with more overtones to sound stronger when processed by the synth filter. Which bass sounds "better" is a matter of situation or personal taste and there is no objective answer for this.

stereo chorus modification

The stereo chorus is a separate unit that can post- process the main voice. It was intended to approximates a rotary speaker effect by panning a pitch shifted version of the signal left and right, that is mixed with the original signal, but in fact it resembles much rather the hemi- sync signal of a mind machine, and thus can be greatly used for meditation music, because the chorus speed is stepless adjustable by a slider. The stereo chorus circuit is located at the right side of the amplifier PCB; it contains 3 small ICs (each 8 pins, from top to bottom: "MN3209", "MN3102", "4558DD-1") and various discrete stuff with 2 (?) transistors. I am still not sure how the pitch shifting functions, but the entire thing seems to be analogue.
  • intensity control
    At the "4558DD-1" IC there is a 82 kOhm resistor at pin 7, which connects through a 47 kOhm resistor with pin 6. To make the effect intensity controllable, remove the 82 kOhm resistor and replace it with a 100 kOhm potentiometer (left end at pin 7, wiper at the trace to the 47 kOhm resistor).
     
  • effect sounds
    Connect the wiper of the intensity pot with the one of another 100 kOhm potentiometer. Connect its right end with one end of the transistor T6, and its left end through a capacitor with the right end. This way in middle position the sound will stay clear, while by turning right, the continuous panning will gradually turn into short blips and finally produce a helicopter- like throbbing. When turned left, the same effect will be milder and instead turn into a sort of squeaking, which resembles Hammond organ distortion. The timbre of the squeaking depends on the capacitor value; 47nF sounds most interesting. With the throbbing blip effects a lot of mind- machine like psychedelic sound effects for meditation music or tekkno trance can be produced, and this potentiometer also changes the synth filter envelope in "wah wah" mode.
    The stereo chorus can be modified in a lot of other ways and e.g. turns into stuff like an American police siren or makes strange phasing effects when shorted with resistors or capacitors at certain places. But the siren effects are independant from the input signal, which is rather boring thus I didn't install them.
Caution: For sound reasons I did not install here any protective resistors in the lines to the potentiometers; although the measured current through the pots is only a few mA and nothing runs hot, there might be still a small remaining risk of long term damage to the stereo chorus ICs. (I had no means to check e.g. the peak current.) And because these ICs are possibly special parts those are not made anymore, replacement would be likely difficult. (But the same ICs were certainly also used in other Casio stereo instruments of the same era, those are less rare than the CT-410V and MT-400V and thus may be easier to find in the case of spare parts.

analogue drums

The percussion sounds are fully analogue and have trimmers for decay time adjustment, thus they can be certainly modified in a lot of ways. Because they are triggered by pulses on individual output lines of the D930G, it would be certainly possible to modify the preset rhythms a lot by muting individual drums or even connecting them through a switch matrix in different orders (the "Super Drums" rhythm variation slide switch feature of certain old Casios does possibly the same). (I didn't modify this.) Different Casio keyboards with the D930G also can be equipped with varying percussion; my MT-45 has e.g. some drums (2 toms) less than the CT-410V. 

pitchbend

This instrument has already a built-in tuning potentiometer, thus here certainly easily one or more pitchbend pots could be added. (I didn't add one yet.) But be warned that the clock oscillator is AC controlled, thus do not connect simple sensor contacts here, because they would flood your nervous system with maladjusting and potentially cancer causing high frequency crap.

shitshot

By re- plugging the power supply plug, the main voice sound envelopes change a lot. Most of these shitshot sounds distort much during polyphonic play. While sustain and vibrato behaves as usual, the operation of the added "envelope" switches always reset to a default sound. Also selecting another sound clears these sounds (as expected), thus I conclude that much like with Yamaha FM keyboards, the D931C soundchip is programmed with synthesis parameters a single time by the main CPU D930G when a sound is selected, and it keeps these parameters in internal registers until a new sound or envelope is selected (or a shitshot messes them up). 

Especially after these modifications, the CT-410V constitutes a respectable sounding analogue synthesizer that can do many similar effects like known from early Jean Michael Jarre works; with the arpeggio mixed into the main voice and routed through the synth filter, you can e.g. get the typical style of chinking atmospheric melody clouds known from these records. Unfortunately nothing is programmable, which limits the instrument to its standard rhythms and accompaniments, although they permit many variations. (Without any sequencer you also soon wish a 2nd pair of hands to crank at the knobs while playing on it, but early Moog synthesizers also had none anyway, and Jarre played live in spite of this. ;-) )

I read in a forum that when Casio developed their first soundchips, they were urged to transmit their digital waveform data between CPU and soundchip in an odd format based on incremental values to prevent conflicts with a digital pipe organ patent by the church organ manufacturer Allen, who had introduced the banal concept of adding a digital envelope to digital waveform samples. I am not sure if the simple squarewave sound of the D931C is really affected by this, but the shitshot distortion may have to do with improper increment sums those cause a DC offset on the output waveform that exceeds the DAC range. The forum article was written by Robin Whittle and I have included it here for educational purpose because it mentions the D931C chip and may help to bring more light into the sound synthesis technology of historical Casio keyboards:
 

Re: [music-dsp] patents (the softsynth was patented 1997)

To: music-dsp from shoko calarts edu 
Subject: Re: [music-dsp] patents (the softsynth was patented 1997) 
From: Robin Whittle <rw from firstpr com au> 
Date: Sat, 14 Oct 2000 22:56:37 +1100 
Organization: First Principles 
References: <Pine.LNX.4.21.0010131840120.5467-100000 from shoko calarts edu> <00101412325300.00991 from linuxhost localdomain> <39E832AD.FA73C0CD from cableinet co uk>
Reply-To: music-dsp from shoko calarts edu 
Sender: owner-music-dsp from shoko calarts edu 


Regarding digital musical instrument patents, around 1981 when I began modifying the Casio M10, I met a chap who was the agent here for Allen digital organs.  These were serious "pipe organ" like things for churches, full of Rockwell LSI chips with round metal chip covers, black plastic packages with quad inline staggered leads.

He assured me that Allen had licensed a patent, by one Ralph Deutsch (if I remember correctly, and I haven't thought of it for 18 years or so) which was for a musical instrument which worked by storing a digital representation of a waveform in its memory.  He showed me the patents - I may still have copies somewhere.

He was intrigued by the Casios - the first production digital keyboards.  He was convinced they used a curious form of synthesis, the name of Walsh Functions - some mathematical abstraction of little obvious importance - because he was sure that Casio would not want to be caught out on Allen's worldwide patents.

The Casio M10 chip and its siblings in the MT-30, MT31, CT-201 (the very first Casio - 4 octaves, full size, two chips with different waveforms) work by generating complex 16 step waves (I assume this from what I know about a later chip I describe below), eight notes at a time.  The waves are made of two component waveforms and there is crude envelope control over each.  The sum of the 8 waves appears as a 14 bit binary number at a sampling rate of about 500 kHz.  This means nice, crisp, non-aliased high tones!  The DAC was inverters driving a R-2R resistor array for 12 bits and a few resistors for the least significant 2 bits.  There was no sample and hold - just an op-amp - so timing anomalies in the bits from the chip and the inverter slew rates caused spikes when the wave went from 10000x to 01111x.

The waveforms were pretty crude and of course made of stair-steps.  The signal went through a switchable analogue LPF - but my mods bypass that.  I even made a super-low glitch tweaked dual 14 bit DAC mod board for the CT-202, using the standard Casio R-2R network and some extra resistors, trimpots and judiciously clocked HC174 latches so the rising and falling edges were symmetrical.

Later, in 1982, Casio produced a similar sounding MT-65 and its full-size equivalent.  The sound chip in this is a 42 pin custom LSI which has its note playing and waveforms loaded into it by an external CPU and software.  Around then, I figured out the protocol for talking to the chip and wrote a C program (BDS-C for Z80 CPM) to write waveform data to the chip via the parallel port of a Big Board I.

The reason I mention the Casio is that the chip (the NEC D931) did not actually receive, or apparently store, waveforms.  It received and stored *increments* - and used these steps to generate the waveform.  If your increments did not add up to 0 then all sorts of trouble occurred!

Let me look into my archives . .  In less than a minute I found the patent!

US Patent Office 2 June 1970  Patent 3,515,792

Ralph Deutsch, Sherman Oaks Calif. assignor to North American  Rockwell Corporation.

A digital electronic organ wherein a digital representation of an organ pipe wave shape is stored in a memory. A frequency synthesiser activated by a manual or pedal key produces a clock frequency at Nf, where f is the frequency of the note selected, and N is the number of sample points of the stored wave shape. The digitized wave shape is read out repetitiously at the generated clock frequency and converted to analog form to produce a musical note having a waveshape corresponding to that stored in memory. Circuitry is provided to sum digitally notes which are played simultaneously; to shape each note in attack and decay using digital operations; and to read out stored multiple wave shapes to implement harmonic and mutation stops. 

There is no mention in the prior art of computer software generation of music - though I guess no-one had used a computer to make an *organ*.
 

Using increments (albeit simple +/- 1, +/- 2 +/- 4 +/- 8) was probably harder than simply storing the waveform, and led to less flexibility than a stored waveform - but I figure that Casio did it so they didn't have to worry about the Allen patent.  So the dull force of patent law made a popular instrument more awkward, or at least more idiosyncratic.

I just found my doco file for the chip in the MT-65 - the D931.  All the guff is there on how to talk to it.  I have C-code as well - and 8 D931 chips, seven unused.  I was able to load novel waveforms into the D931 in my MT-65 it and then play it from the MT-65's CPU via its keyboard, or the MIDI interface I added.  I also made up a second D931 on an external board with an independent clock source so I could have two waveforms and detuning.

A web search for:

"Ralph Deutsch" and patent
leads to:
http://www.allenorgan.com/book/jbook.htm
where there are two blank pages, entitled:
Honoring the Intellectual Property of Others
Ralph Deutsch and the Dark Side
amongst a lot of other similarly empty pages referring to patent litigation.  The author is Allen Organ founder Jerome Markowitz, who died in 1991.  He had been dabbling in electronic organ patents since the 1930s.  I have some patents of his here, from 1973, on internal plumbing inside digital organs.
 

- Robin

dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links
http://shoko.calarts.edu/musicdsp/
 

I had downloaded a sound demo record of the Casiotone 202, which was the direct successor of the Casiotone 201, but the 202 there seemed to have far more natural (non- squarewave) sounding timbres than my CT-410V (D930G/ D931C chip set like MT-65). But now I think this was likely rather caused by either post- processed studio recording or clever use of limited octave ranges, because my (later bought) own specimen of both keyboards sound fairly similar, beside that the 201 can even detune its 2 suboscillators against each other to produce a chorus effect. My Casio MT-70 otherwise produces all of its sounds by mixing instead of squarewaves only 2 very dull tones (apparently digital sine waves), despite it outputs the sounds the same way digitally through a resistor array DAC like the CT-410V. My Casio MT-60 otherwise sounds much like the CT-410V, thus Casio apparently produced multiple differently sounding variants of this early sound synthesis hardware, which (at least with the Casiotone 202) had the fancy advertisement name "Consonant- Vowel" synthesis.

By space reasons and because I can not play well on these huge piano keys, I originally had intended to buy the smaller Casio MT-400V instead of the CT-410V, but much like with the insanely overpriced Casio CZ-101, at eBay they were sold only for moon prices (200€ etc.), thus I bought the fullsize CT-410V instead (which seems to be even more rare). But by the lack of internal speakers, a MT-400V would have likely taken as much space, and regarding the large PCBs inside my specimen, it would have been likely much more difficult to install here all the switches and potentiometers I added to my CT-410V. Also don't confuse the MT-400V with the Casio MT-800, which looks similar but has no synthesizer, a stubborn and banal accompaniment and only 12 preset sounds.

Casio MT-400V (This is just an eBay picture; I don't own one.)

There are plenty of other instruments in the D930G/ D931C hardware class, those are usually cheap and easy to find at eBay, but they are all missing the synth filter and often many other features. Such instruments can be recognized by the rhythm and sound preset names (the sound names "funny" and "cosmic tone" are easy to remember) and also the count of 12 rhythms and 20 sounds and the possible presence of locking switches for multiple chord, bass and/ or arpeggio variants hint to it.

The CT-410V/ MT-400V is by my knowledge the only Casio instrument with a genuine analogue synthesizer filter and analogue realtime sliders. Someone told me by e-mail that also the Casio HT series synthesizers HT-700, HT-3000, HT-6000, HZ-600 had analogue filters, but these are MIDI synths without realtime sliders. The HT-700 and HT-3000 had basically quite similar sound generation like the CT-410V, but with different and fully programmable accompaniment and PCM rhythm. The HT-6000 was the flagship of these instruments and had for each polyphony channel even an own filter and per channel 4 detuneable suboscillators those can be set to different velocity curves. I initially thought that the HT "analogue modelling" series would be virtual analogue and internally based on Casio's digital FM variant phase distortion (there were many such rumours), but he told me that the PCB of his HT-6000 contains indeed analogue filter hardware with "NJM2090" chips and many trimmers.

Other keyboards with the same chip set like the CT-410V (but no synth filter) and 49 midsize keys include the Casio MT-65/ MT-68, the CK-500 (MT-65 with 2 cassette decks, radio, only 12 preset sounds) the Casio MT-100 (with equalizer) and the Casio MT-45/ MT-46 (only same accompaniment, not main voice). Fullsize keyboards include the Casiotone 404/ 405 (fake woodgrain case like Casiotone 401), the Casiotone 610 (or CT-610?, similar with 61 keys and stereo chorus, woodgrain and silver version exists), the similar CT-620 , likely the Casio CT-605 (61 keys, mono), the Casio CT-430 (49 keys, stereo) and the Casio CT-310 (mono, 49 keys, only 12 sounds but same accompaniment - all seen on eBay). Likely also the Casiotone 7000 (beige metal case with 61 fullsize keys, digitally programmable stereo chorus effects, sequencer with tape storage, LCD display, same 12 preset rhythms) is based on this hardware class, although its 20 semi- OBS preset sounds include some different names - possibly because these are controlled through an external CPU. Also the Casio MT-210 seems to be member of this hardware class (49 midsize keys, stereo chorus, functions like MT-65 without the envelope variation buttons); only few of the 20 preset sound names are changed (likely by hardwired envelope variation settings) and on eBay someone claimed it had PCM rhythm, thus possibly the drum trigger outputs of its accompaniment CPU control an external sample based rhythm IC. I first also though that Casio MT-800 and MT-85/ MT-86 (with ROM-Pack and key lighting) would be in this hardware class, but its CPU seems to have a different software number and thus is only the same family. A successor of the CT-410V/ MT-400V was the Casio HT-700/ Hohner KS 49 midi (has MIDI but unfortunately sounds cold and lacks realtime controls).
 

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