Show HN: AutoMIDIFlip – An automatic MIDI flipper

I wrote AutoMIDIFlip based on the #midiflip challenge going around Twitter right now - basically, take a MIDI file, flip it so that the intervals are preserved but in the opposite direction, and see what it sounds like. It actually has some surprisingly good results.

Andrew Huang did the first one manually, as can be seen in the YouTube video linked on the front page. I figured I could write code to automatically do it even for people who didn't have the software to do it themselves.

I plan to add more features to AutoMIDIFlip - in particular, it'd be nice to have an optional feature to auto-shift the different channels up or down octaves in order to have them occupy the same sort of range as they did in the original MIDI, as that'd probably end up with more listenable tunes. (Bass notes become screechy notes of death right now.)

So far, AutoMIDIFlip is the only automatic MIDI flipper I know of that preserves everything about the original song except for the note positions. In other words, if you flipped a tune twice, then removed the 6 extra empty MIDI tracks that AutoMIDIFlip would have added (3 per run, basically just attributing AutoMIDIFlip so that nobody just runs a MIDI file through it and calls it an original), you'd end up with exactly the same file, hash checksums and everything. There are only three exceptions to this rule:

* If the original source didn't utilise MIDI's "running status" feature (which simply acts to reduce the file size by removing redundant information - more info at http://www.midikits.net/midi_analyser/running_status.htm), then the resulting file from AutoMIDIFlip will be smaller than the original. It'll still contain exactly the same information, however.

* For Format 0 MIDI files, AutoMIDIFlip will output a Format 1 MIDI file so that it can insert the empty attribution tracks. These tracks do not contain anything, and AutoMIDIFlip won't attempt to separate the file into tracks; you'll still just have one track containing all the data.

* If for some reason the input MIDI file has more tracks than are indicated in the file header, those extra tracks will not appear in the output file. This scenario should basically never occur; if it does, there's something wrong with whatever program generated the MIDI file.

I'm happy to answer any questions people might have!

I flipped the arpeggio from Stranger Things. Doesn't have the same umpf. Cool tool though.

One of my favorite discoveries, thanks to a suggestion a teacher made, was how inverting a pattern can help you learn something really difficult.

I was learning Beethoven's Appassionata sonata and there's a section where the right hand has a series of five-note arpeggios that gradually decrease in register until the left hand takes over and continues down into the bass (while the right hand takes a climbing melody that originally started in the left).

Those five-note arpeggios were really hard in the left hand. So the technique my teacher recommended was to basically play an inversion of the left hand pattern at the same time as the left hand was playing. So when the left hand started on a Db, the right hand started on an Eb an octave up, and then basically play a mirror version - mirrored by the physical key, so E vs C, Bb vs F#, etc.

It sounded like crap, but it was very effective - since the right hand is stronger, the left hand would basically learn from the right. Pretty quickly, my left hand got very strong at that pattern and it became one of my favorite sections of the movement.

It probably wouldn't sound terribly great; the site doesn't make any attempt to make sure that the flipped version is in the same key as the original (and in fact that would be extremely difficult, if not impossible), so you'd get a rather dissonant-sounding MIDI.

I was curious though, so I tried it out on the Für Elise example. Here's the unedited version: http://automidiflip.com/merge-experiment/furelise-merged.mid

As expected, it's rather horrible. I tried editing it so that they began on the same note but an octave down (as with Andrew's original video), but it wasn't much better: http://automidiflip.com/merge-experiment/furelise-merged-shi....

That sounds like a good idea. Props to your teacher - I might try that myself, as there are a few things I've been interested in learning on the left hand but have had trouble with.