Correcting

Exercise nº 3

On the Prelude no. 4 in E minor, Op. 28, by Frédéric Chopin (1839) — public domain.
Exercise material © 2026 Mario Maurano. All rights reserved.

What you will do: open in the Player a MIDI file of the Prelude that has wrong notes on purpose, find them by ear with the score beside you, and fix them with the copyist. At the end you will save your corrected version in three ways: the whole work, one hand only, and a passage of bars.

How long: about thirty minutes. What you need: the files of this lesson (Preludio Nº 4 Op28 - E min (para corregir).mid and the PDF of the score), the COSMIC KEYBOARD open for the sound, and Exercise nº 2 done.

The copyist's moves

In Pythagorean tuning every note has its exact place, and the copyist moves it by true intervals, not by keys:

±T	Tone. Up or down a whole tone (C → D).
±L	Limma, the diatonic semitone: the letter changes (E → F, C → B).
±A	Apotome, the chromatic semitone: the letter stays, the accidental changes (F → F♯).
±K	Comma (23.46 cents): the comma neighbour (A♭ ↔ G♯). They are two different notes.

A tone = a limma + an apotome. That is why «up a semitone» has two paths that do not sound the same: with L the letter changes, with A the accidental changes.

PART ONE — FIND AND FIX

IN THE PICTURES: the orange circles show where to look or click.

1 · Load the MIDI file into the PLAYER

Open the Player from the Cosmic Keyboard and drop the file Preludio Nº 4 Op28 - E min (para corregir).mid on the drop zone. Listen to all of it with ▶, with the PDF score in front of you. Does anything sound odd?

The Player with the MIDI file to correct loaded: the Score Roll with its two tracks (green = right hand, orange = left hand) and, marked, the bar ruler at the bottom.

2 · Move around the Score Roll

Mouse wheel: moves up and down through the pitches.
Shift + wheel: moves forward or back in time.
Ctrl + wheel: zooms in or out, around the cursor.
Click and drag: moves the view when zoomed in.
Double-click an empty spot: back to the whole work.

Zoom first. With the whole work in view, the wheel and the drag have nowhere to go: zoom in with Ctrl + wheel and from then on everything moves.

Below the roll, the bar ruler numbers every barline as in the score (the upbeat is bar 0), and the clock always tells you bar:beat. The green track is the right hand and the orange one the left hand.

One click on a note opens the copyist: its name, its exact frequency and the buttons to move it. With ♪ listen you hear it on its own.

The copyist open on a note: marked, the note name, the pitch buttons in two rows (+ on top, − below) and the chosen note in the roll.

3 · The first mistake

One note of the melody is higher than the score says. Find it by comparing with the PDF.

Hint

It is in bar 5, on beat 4. It is a diatonic semitone: the right note has another letter.

Solution

It says C5 and the score says B4. Click it and −L: it goes down a limma and changes letter. Listen to it.

4 · The second mistake

Another note of the melody lost its accidental. Look for a missing sharp.

Hint

It is in bar 17, between beats 1 and 2. It is a chromatic semitone: the letter stays.

Solution

It says D5 and the score says D♯5. Click it and +A: it goes up an apotome, the letter stays.

Check: listen to the whole Prelude again. If the two corrected notes no longer bother you, it is right. The notes you touched are marked as edited in the copyist.

5 · Discover the comma

This step fixes nothing: it is for hearing something a tempered keyboard does not let you hear. In bar 3, beat 4, the left hand has a G♯3. Open it in the copyist.

Listen to it with ♪ listen.
Press −K. Now it says A♭3: the letter went up from G to A, but the sound went down a comma. Listen to it.
Press +K to go back to G♯.

Which of the two pulls towards the A above? G♯ is the leading note: it is higher and wants to rise. A♭ is another note, a comma lower, that wants to fall. On a piano they are the same key; here they are not the same note. Leave it on G♯, which is what the harmony asks for.

PART TWO — SAVE YOUR VERSION

6 · The whole work

In the Score Roll header, button ⭳ export → Export by instrument…
Leave both tracks checked and the range on Work.
Save MMF. The file downloads with the name of the tracks.

The EXPORT TO MMF window: marked, the ⭳ export button, Range = Work and Save MMF.

7 · One hand only

The same, but uncheck one of the two tracks: only the green one for the right hand, or only the orange one for the left. Save both, one after the other. They are for practising each hand on its own, or for the Score to open them as separate parts.

8 · A passage: bars 13 to 19

Find bar 13 on the ruler. Right-click right on its barline → Set A here.
Find the barline of bar 20 (the end of 19). Right-click → Set B here. The section is shaded between A and B.
⭳ export → Export by instrument… Now the range has one more option: A–B section. Choose it, check the tracks you want and Save MMF.

TIP: zoom in with Ctrl + wheel before setting A and B: the click then lands right on the barline. If it slipped, drag the little A or B flag to its place.

The Score Roll with A at bar 13 and B at the end of 19; marked, the A and B flags, the clock showing A and B, and in EXPORT TO MMF the A–B section button to choose.

Check: your downloads folder now has several .mmf files: the corrected work, each hand and the passage. Open them in the Score (MENU FILES → IMPORT MMF) and see that each one holds what you chose.

Congratulations: you corrected and saved your own copy

In Exercise nº 4 you open the corrected work in the Score and finish writing it: dynamics, slurs, beams and the details a MIDI file never brings.