Takashi Mizuhiki continues to develop his DXi app! This update brings a Retina display for iPad.
DXi started off life as a faithful recreation of the Yamaha DX7, and was infinitely easier to program than the original hardware. The DX7 is the most famous Frequency Modulation (FM) based hardware synth, and the core chip within it was the same one used for all of the sounds in Sega's Genesis gaming console.
These days, DXi far exceeds slavish emulation, as Takashi has modernized the entire instrument. First with the inclusion of a delay and a low-pass filter (uncommon in FM Synths). Now it even sports multiple waveforms instead of the typical sine-only FM synths or yore.
If you've ever thought about getting a DX7, spend all of that money on a solid MIDI controller instead and then spend $2 on this. If you have no idea what FM synthesis is and want to blow $2 this is a good opportunity for that. ECPM will probably be past Lesson #50 before we approach FM though, so you're on your own!