It's rare that a simple format description can give so many hints about the evolution (or lack thereof) of this system: - The MP3 file encoding a stereo signal

It's rare that a simple format description can give so many hints about the evolution (or lack thereof) of this system: - The MP3 file encoding a stereo signal with one being audio-rate encoded digital data makes this just the latest layer of a barely updated system. - The two channel format inside the MP3 must itself be evolved from another audio storage medium that didn't have a way to interleave digital data and audio. Furthermore, it must have been limited to only two channels, so they had to sacrifice one channel for the MIDI data. - An audio CD would fit the specifications, but then, a mixed audio/digital CD with a player that buffers the MIDI from the data partition and plays the audio in real time wouldn't have been hard to create. Only the memory for the MIDI data could have been a potential cost issue in the mid-80s. - Encoding the digital data on a carrier wave, cassette tape style, is really odd as MIDI was released in 1982 and mixed audio/data CDs were standardized in 1983. PianoDisc as a company was founded in 1988, so all-digital decoding tech would have been reasonably easy to build. The really mind-boggling part of all of this to me is that it would be extremely easy to create a modern, higher quality recording format (e.g. proprietary container with MIDI and compressed audio) and the decoding hardware would still be dirt cheap.