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An exporter converts a chart by reading it as a scanner does, then writing what it read in the other format. Most of a chart converts with nothing lost, but each format holds some things the other can’t. For example, a .chart file gives each difficulty its own star power, and a .mid file gives one instrument’s difficulties the same star power:
A .mid file can hold only one of these phrases, for every difficulty. An exporter writes Expert’s, from tick 768 to tick 2304, so Hard’s star power changes, and with it Hard’s TrackHash.
  • Recommended: a conversion keeps every TrackHash wherever the other format can hold the chart.
  • Recommended: before it converts, an exporter tells the user which tracks will change, and what will be left out.

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What converts as it is

These convert in both directions, each written the way the other format writes it, and every TrackHash stays the same: Forced notes. The two formats decide natural HOPOs with different thresholds: about a third of a beat, 65 × resolution ÷ 192 ticks in a .chart file and resolution ÷ 3 + 1 ticks in a .mid file, both rounded down. A .mid file also has one more rule for a single note after a chord: see Natural HOPOs. So an exporter forces every note whose strum or HOPO the other format would decide differently, and doesn’t force the others. Snapped chords. An exporter writes the notes after chord_snap_threshold has moved them, so the key changes nothing more in the converted chart.

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What .mid can’t hold

A .mid instrument track holds all four difficulties, and its phrases, modifiers and text events apply to each of them. So these can’t be written for one difficulty alone:
  • Phrases that differ between difficulties: star power, solos and drum fills. Recommended: write the phrases of Expert, or of the hardest difficulty the instrument has, and give them to every difficulty.
  • Lanes that differ between difficulties. A lane’s velocity puts it on Expert, on Expert and Hard, on every difficulty but Easy, or on all four: see Lanes. A lane on Hard and not on Expert is written for Expert too.
  • A tom on one difficulty and a cymbal on another, at one tick, on yellow or blue. A tom marker covers every difficulty: see Toms. On green, the tom can be written as its own note, so both stay.
  • A flam on one difficulty and not another, at one tick. A flam note covers every difficulty: see Flams.
  • Text events and versus phrases that differ between difficulties. An exporter writes those of the hardest difficulty. Neither reaches a TrackHash.
The tempo track and the header also have limits:
  • A tempo slower than 3.5763 BPM, a time signature or a resolution over these limits can’t be converted to .mid. Recommended: an exporter refuses the conversion, or with the user’s consent writes the nearest value it can.
  • A tempo faster than 245.316 BPM may move by less than a microsecond a beat. Games play the tempo each file holds, so the timing changes by a fraction of a millisecond, and the TrackHash changes when the tempo moves by more than its rounding: see TrackHash.
Tempo anchors, A in [SyncTrack], and sections a scanner ignores have no place in a .mid file either.

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What .chart can’t hold

  • Vocal notes and harmonies. A .chart file holds one set of lyrics, with its phrases, and no notes: see Lyrics. The pitches, the percussion, HARM1 to HARM3, and the star power of PART VOCALS are left out.
  • Tracks the .chart format has no section for, such as BEAT, VENUE, pro keys, pro guitar and other games’ instrument tracks. See Ignored.
  • Notes a scanner ignores in an instrument track, such as animation notes, and notes 121 to 125.
  • A solo of length 0. A .chart solo covers at least its own tick, so it’s written one tick long, and the TrackHash changes.
.chart instrument sections hold text events, but the text events of a .mid instrument track, such as [idle], mean nothing there.

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The song.ini settings

A converted chart reads the same only with some song.ini keys, because the two formats read some things differently by default. When it converts to .mid, an exporter writes:
  • sustain_cutoff_threshold, when a sustain is no longer than the cutoff the .mid file would be read with. A .mid file cuts sustains up to about a third of a beat by default, and a .chart file cuts none: see The sustain cutoff. The value is one tick less than the shortest such sustain. When no sustain is that short, the key isn’t needed.
  • multiplier_note = 116, when a 5-fret, 6-fret or keys instrument has two or more solos and no star power, so that the solos aren’t read as star power: see Solos.
  • delay, preview_start_time and preview_end_time, from [Song]’s Offset, PreviewStart and PreviewEnd, when song.ini doesn’t set them, since a .mid file has no [Song].
  • A song.ini file, when the song has none, with the values of [Song].
When it converts to .chart, an exporter writes [Song] from song.ini, as [Song] says, and song.ini stays as it is. Edge case: keys that don’t need writing. hopo_frequency and eighthnote_hopo change the threshold the same way in both formats, so the forced notes still cover them. pro_drums and five_lane_drums aren’t needed when the notes give the drum type: in a .mid file, a tom marker that covers a note makes the chart pro, and a 5-lane green note makes it 5-lane. See Drum types.

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How often TrackHashes stay

Converting a .mid file to .chart keeps every TrackHash for almost every chart. Converting a .chart file to .mid keeps every TrackHash for about nine charts in ten. Most of the rest have phrases that differ between difficulties, and some have tempos faster than 245.316 BPM. The TrackHash rounds each tempo to a thousandth of a BPM, so that every tempo of a .mid file, and every .chart tempo up to 245.316 BPM, converts with the same TrackHash. The SongHash changes when a TrackHash changes.