Checking out iYM2151 for the iPad, I noticed that although it's eight part multi-timbral, each part is monophonic. I contacted the developer to find out if there was a way to play a single voice polyphonically. It turns out the app has a somewhat quirky method for polyphony. You can have up to eight voices of polyphony by setting all eight voices to the same patch in the Main screen. You can actually set up various combinations, for example 4 voice strings and 4 voice organs. Whichever is selected indicated by the little sideways triangle is active in the various editing screens including keyboard for playing. There is a limitation with the implementation though. The envelope release stage is cut off when triggering a new note or notes. It essentially gives the impression of polyphony only being active for the immediate triggering of notes. I mentioned this to the developer and he verified this is how it is currently implemented, and that he would like to change the behavior to match what you would expect. As this app is based on "a single-chip FM synthesis implementation, an eight-channel four-operator sound chip. It was used in many 80's arcade games and home computers," the current behavior is somewhat expected. See my original post here for further details and videos. Be sure to forward to the track demos in the video playlist to get an idea of what this synth is capable of. After looking at some of the editing screens, this is a fairly editable FM synthesizer. Check out the user manual at http://www.detune.co.jp/detune/iym2151
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