A RockBand Drumset to MIDI Hardware Hack!  
   

by Sandy Sims

  Back in the "old" days, I had a real set of drums, some Diamonds, and used to play almost daily. But as time went on, the opportunity to play them became less and less. Mostly the TV (i.e. others watching it) and the internet (in close proximity to the "drum/dining" room! ) were to blame.
So I purchased a $1000 Roland set, but, as usual, the box had been opened and the wrong power supply had been put in there (AC vs. DC) and there was so much trouble returning the thing, I just gave up on the whole idea.

  Later on, I decided to build a table-top unit with 8 pads, which was made of heavy iron and it worked great even though there was some cross-talk between the pads. When we moved onto the  boat, it was just too heavy and dangerous, so I gutted it for the electronics and stuffed it all into a zip lock bag, and there it sits.

  When we sailed into the US, I saw one of those RockBand toy drum sets. It seemed solid enough so now there's one sitting in the studio. I managed to figure out a way to interface it with the PC, as it's recognized as a joystick. I found a nifty little proggie that converts joystick buttons to alpha-numeric keyboard characters, which, in turn, my DAW software (FLstudio in this case) can turn into notes on the synth, sample, or MIDI out in focus. There is very little latency, well except with the "kick" pedal, which is an issue I plan to address.
  The only problem is there's NO velocity sensitivity at all, so the drums don't sound natural, just loud all the time. It's ok for hammering away on in the studio, but I'd feel embarrassed to even include it in a jam with friends.

  The ultimate solution isn't to drop a couple Large on a new set of V-drums, but to do another fun electronics project! 
A few years ago I designed a 16 knob midi controller and tried marketing them, missed the train, but now have lots of experience with MIDI  hardware programming.

Disclaimers: I will make this, the schematic,  and the firmware,  public domain so anyone can have a go at it. So I assume no responsibility on whatever goes wrong with yours!
             Don't take anything you see on this page as fact! I could be a complete idiot, and please don't try to build it until you see a video below of it actually working!

  ..Of course, once I set my sights on something, it generally becomes mine, either by building it myself, or nice people feeling sorry for me and giving me one :)

 
  The Rockband(tm) drums have a little control panel in the center (photo right) which I also plan on hacking, into a PIC 16F57 micro-controller. I chose the '57 not because they are MIDI-ready, or even have a USART, but because I have tons of them (C57's) left over from the 16 knob thingies.

  These buttons will control stuff in FLstudio, as well as programming note# - drum pad data. The big red button could be used to turn MIDI note output on/off, so it can still output MIDI CC data for controlling other things...

  The first step is to draw up a schematic to see how feasible it all is. Here it is below.
To see zoomed (and updated ) schematic, just click on it. The orange area ( blue in the zoomed schematic ) is an add-on for a "bongo mode" that increases the sensitivity so hands can be used instead of sticks. I'm not sure if I should implement it.

  The big empty box is the keypad configuration, and, well I haven't decided how to configure it yet! It may have been written into the completed  schematic by now, so click the image to open in a new window.

 
  The sensors to the right are piezoelectric  cells, just like the ones found inside beepers, cheap stereo tweeters, or talking birthday cards. Besides making a great, albeit tinny, sounding speaker, they can also act as a microphone. They are very sensitive to vibration and, through the piezo effect, can actually create a tiny voltage that is proportional to the amount of vibration.

  Because the voltage is small, it needs to be amplified to microprocessor levels. The quad op-amp LM 324 has been chosen, and is configured to amplify and invert the signal. The input is decoupled with a .1 uF cap, just in case corrosion creates resistance across the piezo. The output is decoupled as well to allow for a AC-DC circuit, made of diodes and another cap, to have it's own voltage floor. The resistor is to bleed off the charge in about 20mS.

  In a non-velocity sensitive circuit, these would just feed into the PIC. To detect velocity, a poor man's ADC is employed, in the way of a 10k resistor, and a .022 uF cap. Acquisition of the voltage present at these pins (PortC,0 to PortC,6) is obtained by setting the port to output, then pulling the pin low for a few uS, then switching back to input to count how long it takes the pin to recover to the minimum logical 1 voltage. The higher the voltage, the faster the recovery. The LM324 on 5VDC can only produce about 3.5 V deviation, but the diodes and cap act like a charge pump of sorts, and without a load to ground can reach higher than 5 volts.
   
 This pulse of voltage is measured, then translated into MIDI note velocity, and tacked on to the end of a note that corresponds to the pad being hit. After the voltage drops below the PIC's input high threshold, a second piece of data has been recorded: the time it took to discharge. This will give a more accurate reading of the velocity, but would take to long and might create 10-20 mS of latency, which is a no-no on any DAW!

  I hate wasting perfectly good data, so this could be used to send a MIDI CC value (control change) which could be used to control a cut-off filter, or perhaps even the level of a sample. It gets even cooler! You may have noticed some of the drums have 2 sensors? The theory here is based on the speed of sound, or shockwave thereof. When I hit the rim of a snare, I want a rim-shot sound not a snare! So a second sensor somehow attached to the rim area or even close, will produce good results.

  I mentioned cross-talk from one pad to the other in the table-top drumpads I built before. There was a few hundred microseconds delay from one to the other, so I was able to cancel the resulting echo of other drum parts in software. This didn't always work well, especially if I was trying to simultaneously hit the pad that was being cancelled. Ugg!
  With 2 sensors, this means the position of the strike can be extrapolated by how long it takes to hit the second piezo after the first. Striking the pad on a line from the center to the edge would create a -1/2 radius to +1/2 radius delay, thus could be fed into another MIDI CC value to control pitch, or a bandwidth center! Cool!

  Another interesting dual sensor would be for the kick. The stock kick is just a magnet and a reed switch to sense it. I good idea if precision isn't a consideration. I am putting a piezo into it, but have decided to keep the switch as well. The switch could be used to measure velocity and provide CC data, but also it can be held longer to act as a muffle, just like a real kick. I really hate the action on this pedal though, having used a real pedal for years, so that may have to be modified to resemble a real action. A spring that flops over? a replica of a real kick pedal?

 
   There is an EEPROM on the schematic, which will keep track of all the note - drumpad settings that I'll be programming in. Because there are only going to be 4 LED's ( One for each Pad, all 4 for the kick ) to select pads to program, the number of "memories" should be 16 ( 2^4 ). Beyond that, using a track in FLstudio to program the Note-Pad configuration as soon as the song starts, could act to facilitate a more universal method of setup. Maybe I'll write a PC software for flash-MIDI to load/save data like a standard MIDI controller board.

Well getting ahead of myself, I haven't even started to pull the thing apart yet!  On the subject of MIDI interfacing; I am fresh out of midi sockets to plug this in to, so I'm going to fork over $40 and buy one of those tiny USB-MIDI interfaces, then built it in. That way the nice long USB cord that comes with the Rockband drums can still be used. Plus the whole thing can be run off of the 5 volt USB power, no more adapters, yay!
 

I will add updates to this page as the project moves along. It's halfway through Nov.2011 so maybe I'll have a start on it before Xmas. I just have 2 other "more important" projects to do first.
Cheers!

Sandy

NOTE: This project has been cancelled in lieu of a Yamaha DD-65! Oh well!
 

DrumpadzVST - flp's - schematic - PC Board - PIC .asm - dp_project.zip - Digikey