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TFI is the format in which TFM Maker instruments are stored.

TFI is a format designed to be easy to parse by editors, rather than by
hardware. Each TFI file is 42 bytes long, where each byte specifies a
different parameter.

The first two bytes are the parameters global to the entire instrument:

   * Algorithm (0..7)
   * Feedback (0..7)

After that, there are four groups of ten bytes each, where each group
contains the information for a different operator. The data for each group
is as follows:

   * Multiplier (0..15)
   * Detune (0..6, see below)
   * Total level (0..127)
   * Rate scaling (0..3)
   * Attack rate (0..31)
   * Decay rate (0..31)
   * Sustain rate (0..31)
   * Release rate (0..15)
   * Sustain level (0..15)
   * SSG-EG (0..15)

The detune parameter may range from -3 to 3. The TFI file will contain the
detune value incremented by 3 (i.e. a value of 0 means a detune of -3, while
a value of 6 means a detune of 3).
