Cheap Mods
Casio DH-100 Octave Drop

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I’m a bass player, pretty much, so I’m always looking for new ways of generating sounds on the low end.  I’ve octave dropped my CS01mk1, my MG-1, and my Casio M-10. 


Speaking of the M-10, I learned some nice mods from Robin Whittle’s early 80’s write-up on modding the M-10/MT-30/MT-40, which he sent me last year via snail mail for a small fee.  In fact, my mod here was adapted from one of his mods to the M-10.  Much credit to Robin – he’s the mastermind behind the Devilfish mods of the TB-303 and from the looks of his Casio write-up, has spent WAY too much time playing with Casios!


I’ve found that most Casios are pretty lame when it comes to generating good bass (maybe the CZ’s are OK, maybe the button keys of the MT-40 are decent).  Ask anyone who has an HT-series keyboard about the bass and they’ll say, “What bass?”


However, I think the DH can be made into an exception primarily because of its simple tone generation mechanism.  Most Casios use some crazy multipulse squarewave  waveform straight outta the CPU which is usually pretty anemic.  But the DH just uses a squarewave and this, I think, is the trick. To make the six voices, it uses a squarewave of some footage (16’ for the sax, 8’ for the others) and then a few simple bandpass & lowpass filters, followed by an interesting 4-pole VCF.


This mod is simple:  octave drop the squarewaves and bring that control out to an explicit switch on the casing.  I’ll note here that there’s another way to get an octave drop out of the DH.  Paul Fox mentioned that, while he was changing the 12.0MHz crystal on a DH, he tried a 4.0MHz crystal and noticed that the tones dropped about 1.5 octaves.  I think that’s pretty cool, but my worry is that if you slow the clock to the CPU, the MIDI out will suffer the same slowdown and you’ll lose the ability to use the DH as a MIDI wind controller.  For most users, that’s probably unacceptable (I’ll bet most people aren’t big fans of the DH’s internal sounds in the first place and only use the DH as a MIDI controller).




The Modification

This mod requires only DPDT slide switch, one IC, and two trace cuts.  The IC is a CD4013, a clocked dual D-type flip-flop.


The circuit you’ll be implementing is this:




How it works:  the D-type FF works as a divide-by-two to the squarewave.  The fact that Casio uses a digital squarewave for sound generation makes this possible; if it was a less-than-50% duty-cycle pulse wave, a stepped multipulse approximation of a sawtooth, etc, this circuit would actually turn those waveforms into something digital (maybe interesting, but that’s another mod).  When the DPDTslide switch is in the ‘drop’ position, the signal from the CPU is routed to the CLK input of the 4013.  Every rising edge of the CPU’s squarewave clocks the Q-minus into the D input. The net effect is to create a squarewave of exactly half the frequency generated by the CPU.  When the DPDTslide is in the ‘norm’ position, the mod is bypassed and the DH operates as normal.


Parts List

  • DPDT slide switch
  • CD4013 D-type FF
  • Lots of short  (2-3”) and longer (6-9”) strands of 30Ga Teflon-coated wire-wrap wire with stripped ends
  • 14-pin socket
  • Small scrap of perfboard


The biggest part of this job is the IC wiring.  It’s not an intractable problem to wire up one or two IC’s on perfboard, but any more than that and I start thinking about making a PCB (and at that point, the issues of cost and/or mess usually make me rethink the value of the mod).


Electrical Installation


  1. Cut the piece of perfboard to accommodate the 14-pin IC, with a little bit extra to leave room for a standoff screw hole.
  2. Drill the standoff screw hole on one corner of the perfboard.
  3. Put the 14-pin socket on the perfboard and bend the pins outward, away from the center.  This should give the socket reasonable mechanical stability
  4. Solder up all the connections from the schematic, using the short lengths of wire for the connections on the IC, and the longer lengths for the connections that will go to the DH’s PCB and to the DPDTslide switch
  5. Solder up the connections from the socket to the DPDTslide switch.
  6. Now open the horn.  You only need access to the top side of the PCB for this mod.
  7. Cut the traces as shown on the schematic.  See pics for location of cuts.
  8. Solder the appropriate wires to points on the PCB.  I used VDD and VSS on the board as shown, as well as the points on both sides of the cut traces.
  9. That should be all the soldering you need to do.
  10. Place the IC in the socket.
  11. Turn the horn on and test all the functions to make sure it all works.  When you hit the switch for ‘drop’, all the tones should be one octave lower.


Mechanical Installation

Again, this always seems to be harder to make perfect.


  1. I placed the DPDTslide switch on the side of the horn just below the volume pot.  There’s definitely enough room, though it’s a little awkward drilling and cutting in this area with all the circuitry nearby.  I didn’t remove the top PCB, though it might have made it easier if I had.  I used a drill for the mounting screw holes and to start the main hole for the slide switch.  I carved out the squarish hole for the slide switch with a utility knife and a pair of diagonal cutters.
  2. Unscrew the upper standoff screw from the PCB.  Hold the perfboard in place such that tightening the standoff screw holds the perfboard in place like a diving board, as shown in the pic.  I held the perfboard in place until the standoff screw was fully tightened down and it seems like it’s not gonna move.


My own comments about this mod:

Performance:  I really like this mod.  You get a nice baritone sax “braaap/honk” in the lower register with the sax sound.  The other sounds are also very nice in the lower register, If you use the Casio fingering setting, then transpose down a full octave using the DH’s standard feature, you do start to hit the stutter of very low frequencies.


One motivation for this mod was that, to get good low sounds out of the DH, you really had to use the Casio fingering setting, which is not so bad, but I figure if I’m gonna get any facility on this instrument, why not learn the standard recorder fingering?  That way, if I ever happen to pick up a recorder, my time on the DH will have readied me for the challenge!  ;)


A caveat here is that, with the ‘drop’ in effect, the transmitted MIDI note will be an octave above what the DH internal tone is generating, so you haven’t dropped the octave as far as a MIDI controller goes.  But I would guess very few people mix the DH’s internal tone with a MIDI generated tone and there’s enough MIDI translation equipment out there to have the DH sent a MIDI note ON message for a specific note and have the external sound generator drop that an octave (if not translate it to whatever you want).


Further mods?

Maybe the filters could be modded to match the new low end capabilities, but I’m not sure it’s worth it.  The VCF is quite interesting.  If I do any more on the DH, that’s where I’ll go next.