Casio SK-8 small keyboard with lo-fi sampling, ROM- Pack & key lighting

This quite rare small keyboard from 1987 (embossed case date stamp) is one of the successors of the famous Casio SK-1 lo-fi sampler. It can store up to 4 samples (playable through drumpads or keyboard) and keeps them in battery backed- up memory after power off.

In opposite to the SK-1 it includes a "melody guide" key lighting feature for music teaching (not the keys itself light up but a row of small LEDs above them) and a ROM- Pack music cartridge slot. Unfortunately this instrument is missing the nice drawbar synthesizer and portamento of the SK-1, and the tekkno blip drums were replaced by ordinary percussion samples without accompaniment. Also the sequencer is way simpler than on SK-1. This instrument was also released as Realistic Concertmate-800.

By a keyboard matrix easteregg the SK-8 can be easily modified into an oriental keyboard.

main features:

eastereggs:

modifications:

notes:

Like the Casio SK-1, this instrument has a complex multi- chip hardware that permits many kinds of circuit- bending. Unique is that you can easily modify it into an oriental keyboard with programmable quartertone scales, because for the oriental market the rare variant Casio SK-8A was released, and the contacts and case grooves for its additional "scale" slide switch still exist on the panel PCB of the normal SK-8.

The main voice sounds are based on medium resolution samples and sound quite natural. They sound much like the SK-1, but unfortunately the "human voice", "brass ensemble" and "synth. drums" of the latter are missing. The "trumpet" sound apparently produces an artificial wind noise during its attack phase by a rapidly stuttering zipper noise envelope. "trumpet" and "violin" employ a quite slow (square?) vibrato with about 5Hz. The "pipe organ" resembles a metal pipe organ rank. The OBS sound buttons mute any held notes, which prevents some play tricks. Unlike the SK-1, the SK-8 has no synthesizer anymore and also the selectable envelopes only work with the user samples.

The sampler can store up to 4 short low resolution samples or combine either sample memory 1+2 or 3+4 to double the length. The "reverse" effect takes some seconds (indicated by knocking metronome) computing time. A bit strange is that the 2 pink sample recording buttons are of rubber, while all other buttons are of plastic despite all have the same shape. Possibly the designer intended to make them feel differently to avoid accidental overwriting samples currently kept in the RAM.

The rhythms are based on rather low resolution samples, those knock clean and impulse and (unlike e.g. My Music Center) don't contain disturbing static noise. Only the "clap" sounds dull and too short. Unlike the great electronic blip rhythms of Casio SK-1, the SK-8 has no accompaniment anymore. Unfortunately the 4 rubber sample pads can not be used as drumpads with the internal percussion; when the memory contains no samples, they instead simply play an "A" note on the preset sounds {piano, vibraphone, guitar, pipe organ}.

Unlike other Casio ROM- Pack keyboards, the "melody guide" key lighting feature (see e.g. Casio PT-82) of the SK-8 has no different levels, but always plays with lit LEDs and waiting. Special is here that with ROM- Pack musics not only the sound of the main voice, but also of the obligato (2nd voice) and accompaniment can be selected by the user. Possibly this feature was added because the SK-8 has a slightly different sound set than other ROM- Pack keyboards.

The sequencer records all played sounds (polyphonic), but can not be edited. (The SK-1 had a way more complex sequencer with 3 monophonic tracks.)

So far I remember well, my SK-8 came with the ROM-Pack RO-555 "Home Melodies".
 

circuit bending details

Like the Casio SK-1, the SK-8 has a complex multi- chip hardware that permits many kinds of circuit- bending. The SK-8 hardware is a very close variant of the Casio SK-5 hardware; like the SK-5 it supports 4 additional drumpads, those can be even recorded in the sequencer. But additionally the SK-5 has 4 additional effect preset sounds those don't exist in the SK-8, thus the SK-5 likely has a different or additional ROM. Also the oriental Casio SK-8A seems to have a different ROM, because 3 of its preset sounds are different from the SK-8 and also their order is different. (But possibly this is just some hidden jumper trick.) The PCB of the SK-8 has an empty soldering place for an 8 pin IC and some discrete stuff. The main PCB is labelled "M3123-MAIM(B)". A small diode LN26 glows red like a LED. The IC "CA91A02" seems to be the DAC or main voice sound chip; next to it are 100 kOhm resistors those analoguely mix the main voice, accomp. and obligato channel of the ROM- Pack music; reducing the resistance reduces the volume of the corresponding channel. The main CPU seems to be the "OKI M6283-05" SMD-IC, which also polls the keyboard matrix.

keyboard matrix

73
75
76
77
78
79
80
81
82
83
85
86
 
CPU pin
in7
out 1
out 2
out 3
out 4
out 5
out 6
out 7
out 8
out 9
out 10
out 11
out/ in
 
 P.
clap
o
o
o
o
o
o
o
o
P.
4
R.
stop
P.
clap
in 1
67
P.
hihat
o
o
o
o
o
o
o
o
P.
3
R.
stop
P.
hihat
in 2
68
P.
snare
o
o
o
o
o
o
o
o
P.
2
R.
tempo +
P.
snare
in 3
69
P.
base
o
o
o
o
o
o
o
o
P.
1
R.
tempo -
P.
base
in 4
70
 
O.
pipe organ
O.
pipe organ
O.
clarinet
S.
1
S.
effect
S.
sample long
R.
swing4
R.
march
R.
tango
T.
normal
  
in 5
71
 
O.
guitar
O.
guitar
O.
flute
S.
2(?)
S.
effect
S.
sample
R.
swing2
R.
beguine
R.
rock2
T.
arabic
  
in 6
72
 
O.
vibraphone
O.
vibraphone
O.
violin
S.
3(?)
 memory
start
S.
sample
R.
disco
R.
bossanova
R.
waltz
T.
set up
  
in 7
73
 
O.
piano
O.
piano
O.
trumpet
S.
4(?)
 R. 
stop
S.
sample
R.
rock
R.
samba
R.
slow rock
T.
set down
  
in 8
74

All unknown function names and in/ out numbers in this chart were chosen by me. I am not sure if there are some mistakes in it, because some entries exist multiple times; it is also incomplete because others are missing. The input lines are active- high, i.e. react on +Vs, thus any functions are triggered by a switch in series to a diode from one "out" to one "in" pin.
 

legend:
"o"
 = keyboard key
R.
 = preset rhythm
O.
 = preset sound ("orchestra")
S.
 = sampling
P.
 = pad
T.
 = tone scale
orange
background 		 = easteregg (unconnected feature)

  • drumpads:
    • Solder each a button with diode in series from out 11 to {in1..in4} of the keyboard matrix to get drumpads with the sounds {base, snare, hihat, clap}.
    • additional rhythms:
      Solder each a button with diode in series from out 9 to {in 5, in 6} of the keyboard matrix to get the "tango" and "rock2" rhythm.
    • tone scale switch: (oriental keyboard mode)
      On the panel PCB is a place for an additional 4 position slide switch, that even has a corresponding rail in the case top. The switch positions are marked {NORMAL, ARABIC, SET [glyph 1], SET [glyph 2]}. (The first glyph look like a mirrored unequal sign.)

      Saw (or carefully melt by mini soldering iron) a slot for the slider into the case top. Make a slider modelled after the given ones, that fits into the rail. I made mine of a small wood block with a contact made from a piece of thin sheet steel (e.g. from a broken PC diskette) hammered into its bottom (sharp edges bent up to prevent scratching the PCB) and a handle made from thicker sheet metal piece hammered into its top (both should not touch each other inside the block to avoid possible chip damage by static electricity when operating the handle). Also a missing diode needs to be soldered on the panel PCB into the line from out 10 to the common contact of the tone scale switch to make it work.

    percussion pitch control

    The trimmer "VRI 5KB" on the main PCB adjusts the percussion sample pitch. You can add a potentiometer here.

    shitshot

    When pins at the ICs D23C256EAC (ROM?) or D43256C (RAM?) are shorted with each other through a 100 Ohm resistor, the "Auto Play" mode makes wild melody mess. Shorting pins at the M6294 makes the individual parts of the music play out- of - sync. (I recommend rather to interrupt traces by switches than to short things with low resistors to avoid possible chip overload damage.)

    Attention: I have only incomplete draft schematics of my modification, thus this description might be partly inaccurate or even wrong since it is based on them.

    Casio SK-8A and the oriental keyboard mode

    The Casio SK-8A was an obscure variant of the SK-8, that was apparently released in Arabian language countries and likely constitutes the world smallest oriental keyboard. Additionally to the functions of the normal SK-8 it had the "scale" slider with that any of the 12 octave semitones can be shifted up or down by a quartertone. The SK-8A apparently was shipped with the ROM-Pack RO-456 "Beautiful Arabian Music" (seen on eBay).

    To edit the tone scale, set the "scale" slider to the 1st "SET" position and press the keys for all octave tones you want to shift a quartertone up (indicated by a bongo sound), or slide it to the 2nd "SET" position and do the same to set octave tones a quartertone down. Then set it to "ARABIC" to play the edited scale. In this mode also the "slow rock" and "waltz" rhythm is replaced by the 2 oriental rhythms "adani" and "romba junubi" (adani sounds like the typical oriental dance/ belly dance rhythm). All these features also work on the upgraded normal Casio SK-8.

    The upgraded instrument is not only good for oriental music, but sounds very anti- establishment and partly reminds (depending on your own warped tone scale) to Atari XL POKEY music. (Remember, you can sample what you want and play it in that scale.) Also the 2 oriental rhythms may be useful for tribal- like tekkno. In opposite to the upgraded SK-8, the real Casio SK-8A has a different preset sound set {piano, violin, pipe organ, qanun, trumpet, oud, clarinet, nay} that has the sounds in a different order and includes 3 changed preset sounds. Likely it contains a different ROM, but it may be also just a hidden bank switch jumper anywhere buried on the PCB. I don't know if any oriental music ROM- Pack cartridges were released; with my normal ROM- Packs the "arabic" mode does nothing.

    A close variant of the SK-8 was the Casio SK-5 (with 4 additional drumpads and 4 effect preset sounds instead of key lighting and ROM- Pack slot). Also the upper keyboard on the Casio DM-100 (the likely world- only dual- midsize- keyboard instrument) and the crippled Casio SK-2 (only 5 sounds, 6 rhythms) seem to be members of this hardware family. (Perhaps the above keyboard matrix chart may also help to upgrade the SK-2.)
     

    Questions: Has the Casio SK-8A a different ROM than SK-8, or is the chip set identical? Did Casio release oriental versions of any other keyboards? Or are any other cheap and small oriental keyboards known? So far I know, at least in Europe only professional fullsize General MIDI keyboard workstations were advertised as "oriental keyboards", but the cheapest thing I remember was a fullsize keyboard with MIDI and retail price about 350€. Are there any really cheap oriental keyboard tablehooters with hardware resembling My Music Center or MC-3? (Regarding how many that cheap tablehooters end in turkish shops, there would be undoubtedly demand for them.)
     

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