SX-150 Mark II Usage

 (Image taken from the Otono no Kagaku site.)

There are certain Otona no Kagaku kits that have a learning curve, or have a certain number of things you can do with them that involve longer explanations. The main examples being the Japanino, the Denshi mini Blocks, and the EX-150 expansion pack.  We can add the SX-150 Mark II to this list.  The primary reason is that it doesn’t come with an instruction book and the supporting information is spread out across several youtube videos.

If you’re familiar with sound synthesis, then the first concern is with exactly how the controls affect the waveforms, followed by which patches produce which sounds.  On this count, Gakken hasn’t provided much information – 4 patch settings, to be exact. But, by watching the videos you can see what the controls do pretty quickly.

On the other hand, if you don’t know the difference between a “patch” and a “squarewave”, then the Mark II is going to be difficult to figure out.

Waveform Types

(From wikipedia)

Essentially, analog synthesizers start out with a simple sine wave. This gives you a pure note of one frequency.  When you press a keyboard key, the note starts playing at full volume (called “instant on”) and stops playing when you release the key (“instant off”).  It’s a nice concept, but gets boring fast.  What you want to do next is change the frequency, or “pitch” of the note.  With a keyboard, you do this by pressing a different key (where each key is assigned to a different note). With the Mark II, you’re using a solid ceramic strip and a contact wire (the stylus) – to change the note, just touch the stylus somewhere else along the strip.  Or, turn the Mark II over and adjust the Tuning control.  The Tuning control changes the range of notes (the frequency range) of the ceramic strip.

I should mention here that the Mark II doesn’t actually use a sine wave for the starting waveform.  I don’t have an oscilloscope to verify this with, but it’s probably a square wave. If you crank Tuning way down, you get a “boop-boop-boop” sound instead of a continuous low-frequency hum.  If you want a nice, clean bass guitar sound, the closest you’ll get is something like a fart.  But at very low frequencies, along with the VCO Rate and Depth controls, you can get some really cool “house” effects.


The problem with instant on and off sounds is that they’re not all that common in the real world.  You do see them with percussive instruments like the snare drum. But something like a bell is instant on with a diminishing after-note, and a violin can have a slowly rising volume that slowly diminishes.  That is, things like a violin or a flute aren’t instant on or instant off.  This is where the “envelope” comes in.  Early versions of the envelope consisted of Attack, Decay, Sustain and Release settings, also called the ADSR.  Attack is the time it takes for the sine wave to reach full volume from when you press the key.  If Attack is set to 0, it’s instant on.  So, if you want a soft sound like with a flute, you might set Attack to 0.25 seconds or 0.5s.  Decay is the time that the sine wave takes to drop from full volume right after Attack finishes.  Sustain is the level that Decay stops at.  And Release is the time to go from the Sustain level to zero volume when you let go of the key.  So, if Decay is 0, Sustain at 100% and Release is 0, you have instant off.  To get a bell sound envelope, you might set Attack to 0, Decay to 0.1, Sustain to 70% and Release to 3s. (Depending on the synthesizer, and other settings, of course.)

The Mark II has a simplified envelope control that is just AD – Attack and DecayAttack goes from about 0.1s to 1s, and Decay from 0.1s to 5s (roughly).  Regarding Decay, there’s a fixed sustain value and the volume won’t go to 0 until you take the stylus off the ceramic strip.

Additionally, there’s the Power switch.  The Power switch has 3 positions: On, Off and Gate.  The Gate position more or less disables the Attack and Decay controls (not completely, because they do still have some effect with Gate On).

The next section is a bit more complicated.  We need to develop a way of getting a warbling or trill effect, as well as adding additional sine waves to our note to get the range needed to imitate a guitar or cello.  This is where the VCF and LFO controls come in.

The LFO, or Low-Frequency Oscillator, adds a signal between 1 cycle per second (cps) to maybe 2000 cps.  In the case of the Mark II, the LFO is how we get our trill effect. What it does is change the frequency of the note we’re playing by a certain percent plus and minus.  The total effect is controlled by the LFO Wave, LFO Rate and LFO Depth settings.  The LFO Wave switch selects either a Triangle or Square wave shape for going from our minus to our plus values (either instant switch up and down, or straight line).  The value of minus and plus is set by the LFO Depth control (roughly 0.01 to 0.5 – I’m guessing here). So, if we have LFO Depth turned a quarter of the way, this may be a multiplier of 0.2.  If the note we’re playing is 2000 hertz (Hz), then the trill might be jumping between 1600Hz and 2400Hz.  And, the rate of the jumping (the rate of the trill) is set by LFO Rate (roughly once/second to 2000 times/s.)  In combination, you can get a low throbbing beat, or fingernails across a chalkboard.

The VCF, (Voltage Controlled Filter or Voltage Controlled Cut-off) is used with the LFO to limit the frequency of the sound that the LFO makes.  If you look at the Triangle and Square wave shapes, you can see that they make sharp, abrupt changes.  This represents high frequency noise being added to our sound out.  Sometimes we want this noise and sometimes we don’t.  The Mark II has a pressure-sensitive button for controlling VCF.  The harder you press the button, the more high frequency noise gets passed by the filter into our output sound (essentially it’s bypassing the Cutoff control).  The VCF button is instant on, instant off, and adds high frequency components as long as you hold the button down, regardless of what the rest of the sound envelope is doing.

The Mark II adds one more pressure-sensitive button, the LFO.  The LFO button acts as a pitch bender for the ceramic strip, from roughly (0.1 to 0.2 of the note being played). The full amount of the pitch bend depends on the Pitch Env. setting.

When we take the entire waveform produced by the ceramic strip setting, the LFO and the VCF circuits, we have lots of sounds consisting of low-, mid- and high-frequency parts.  We use the Pitch Envelope control to boost the sound amplitude over time, as a triangle envelope.  Depending on the other control settings, Pitch Env. can have no effect at all, or it may soften the output sound a small but noticeable amount. The main impact Pitch Env. has is on the range of bending you get when pushing the VCF button, and the height of the AD envelop for the Attack and Decay controls.

The Cutoff control acts as another filter, limiting the top frequency of the sound out from the Pitch Envelope circuit. That is, it’s a low frequency pass filter. And, it gets bypassed depending on how hard you press the LFO button.

We can argue that Resonance adds a “ringing” sound at the Cutoff frequency. Essentially, it boosts the amplitude of frequencies in the waveform from the Pitch circuit that are closest to the Cutoff frequency. This effectively adds high frequency sine waves to the  sound you’re creating, giving you more of a bell or “chirp” effect.

I’ll repeat it here – the Tuning control on the back of the box HAS A MAJOR IMPACT on the kinds of sounds you can get.  If you don’t mind modding the Mark II, rip out the speaker and put the Tuning control in its place. If you already have this kit in your hands, you’ll immediately understand why I say this.

Now, if you look at the Otona no Kagaku videos, you should have a better understanding of what each of the controls are doing.  If you freeze frame any given video, you may be able to see the control knob settings (the “patch settings”).  However, the first couple videos are accompanied by a patch chart.

If you want, you can add a keyboard pattern to the ceramic strip.


Gakken Instructional Videos

Synth Brass

(Brass patch)

Bell Cricket

(Bell Cricket patch)

Synth Bass Techno

Dog and Cat Voices

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