wave2blofeld: Building a MIDI thing in C++

Last year’s November I went to the amazing DuraznoConf, which I must emphasize was an awesome experience, and during my free time there I got to finally finish a project of mine which I had sitting around for too long: wave2blofeld.

Basically the Waldorf Blofeld, a lovely synthesizer I happen to own, has something called WaveTable synthesis which means it has literal wave tables (actually arrays though) that it iterates trough and other synth stuff. In the Blofeld, each wave table has 64 waves, and each one of these waves has 128 samples.

Now the fun part: you can load your own wave tables!

The catch is that there is no nice official way to do it 😬

So in order to maximize compatibility, and because why not, I decided to do it in C++ (or Rust, but the only MIDI lib I found was too beta for me).

wave2blofeld takes a 8192 samples WAV file and spits out to a MIDI file which you can transfer to your Blofeld using SysEx Librarian, for example.


Just run. Run as fast as you can (?)

Dependency management in C++ deserves it’s own dystopic novel. I tried a couple of solutions like Buckaroo but guess what happens when you search for midi.

Essentialy, you are on your own, so after losing more hours than I’d want I wrote a 28 lines Makefile which works like a charm. The thing is, since C++ package management is such a complicated issue, library writers usually end up doing their stuff as easy to build as possible, having no dependencies and sometimes even only header files!

Take for example this line from TCLAP’s home:

The library is implemented entirely in header files making it easy to use and distribute with other software.

Having to build libraries without depending on anything is a serious drawback.

Anyway, for this thing I ended up using:


Last year I ended up having to learn CMake because my algorithms class teachers weren’t really good at it, so I went with it. It’s fairly easy to get started and possibly demential to learn in depth (the manual never ends). The whole thing is only a dozen lines long:

cmake_minimum_required(VERSION 3.0)




file(GLOB SOURCES "src/*.cpp" "deps/audiofile-master/*.cpp")

add_executable(wave2blofeld ${SOURCES})
target_link_libraries(wave2blofeld midifile)

It definitely beats having to write a Makefile 😂. Also, it is kind of readable.

With that out of the way, building is as easy as creating a build folder and inside it running cmake .. followed by make.

Actually programming

Building a MIDI file is not complicated, but a SysEx file … that took me a while.

Basically MIDI has this System Extended message format, which manufacturers can use for anything they like. Of course this means it’s all done in binary.

Thankfully for reference I had Jonas Norling’s page which kindly offers the binary format explained and a reference implementation.

For starters, the Blofeld wants 128 samples of big-endian signed 21-bit integers each, that is integers between -1048575 and 1048575. audiofile loads samples as floating point numbers between -1 and 1, so each loop we normalize every sample and then load it in 7-bit triplets [Edit: Jonas’ post says “21-byte little-endian” numbers, which is weird because 21-bytes is a lot and anyway little-endian didn’t work for me, so I guess that’s a typo].

    for (int i = 0; i < 128; ++i){
        mm[8 + 3*i    ] = (samples[i + wave*128] >> 14) & 0x7f;
        mm[8 + 3*i + 1] = (samples[i + wave*128] >>  7) & 0x7f;
        mm[8 + 3*i + 2] = (samples[i + wave*128]      ) & 0x7f;

Then it’s a matter of following the spec, and building a 410 byte array which is the SysEx message. We need to send a message for each of the 64 waves of the wave table.

Finally, a touch of iterators for adding every byte of the wave, which makes a kind of checksum:

    int checksum = std::accumulate(
        mm.begin()+7, mm.begin()+407, 0);

and done

    mf.addEvent(0, 0, mm);

Also don’t forget, you need a lamdba to make it really C++11:

bool isValidName(std::string& s){
    return std::all_of(s.begin(), s.end(),
        [](char& c){
            return 0x20 <= c and c <= 0x7f;

Command line options

This was ok. It’s not Rust’s clap or Haskell’s optparse-applicative but it works. In short you need to build arguments like

    TCLAP::ValueArg<unsigned int>
        slot("s", "slot", "Wavetable to write to. Between 80 and 118.", true, 0, "slot");

and the library’s “template magic” does some validation.

Notice how you need to add the argument to the TCLAP::CmdLine cmd object unless it’s a TCLAP::SwitchArg in which case you pass cmd as an argument. Why? Because software 💦.


In the end, it works like a charm. I also have plans to somehow add this functionality to WaveEdit which is super cool.

For now, you need to export the wave table from WaveEdit, which exports them as 64 waves of 256 samples each (not 128), and convert them using wave2blofeld’s -d switch which will subsample each sample. Not the best solution but hey it works.

January 26, 2018