Fixing MIDI Chord Timing Issues In Python

Aug 12, 2025 by ADMIN 42 views

Troubleshooting MIDI File Creation in Python: A Comprehensive Guide

Hey everyone! Today, we're diving into a common issue faced by music enthusiasts and programmers alike: problems with creating MIDI files using Python. Specifically, we'll be tackling a scenario where the first chord in a MIDI file plays all notes simultaneously, while subsequent chords play their notes sequentially. This can be frustrating, especially when you're aiming for a harmonious and well-structured musical piece. If you've encountered this, don't worry; you're not alone! Many developers and musicians have faced similar challenges, and we're here to break down the problem and explore effective solutions.

Understanding the MIDI Structure

Before we jump into the code, let's quickly recap the basics of MIDI (Musical Instrument Digital Interface). MIDI is a protocol that allows electronic musical instruments, computers, and other devices to communicate. A MIDI file (.mid) doesn't contain actual audio; instead, it stores instructions about what notes to play, when to play them, and how (e.g., velocity, duration). Think of it as a musical score that your computer or synthesizer can read and interpret.

A MIDI file is organized into tracks. Each track can contain a sequence of MIDI messages, such as note_on (note start), note_off (note stop), and other control messages. The timing of these messages is crucial for the final sound. If note_on messages for multiple notes are placed at the same time (or tick), they will sound simultaneously, creating a chord. However, if these messages are not correctly spaced, you might end up with the issue we described earlier – the first chord sounding right, but subsequent chords playing notes one after another. This is a critical understanding for anyone working with MIDI files.

Key MIDI Concepts:

Ticks: MIDI time is measured in ticks, which are subdivisions of a beat. The number of ticks per beat is defined in the MIDI file's header.
Delta Time: Each MIDI message has a delta time, which specifies the time elapsed since the previous message. This is how MIDI tracks control the timing of events.
Tracks: MIDI files can contain multiple tracks, allowing for complex arrangements with different instruments or parts.
Messages: MIDI messages include note_on, note_off, control_change, and many others. These messages instruct the receiving device on what to do.

The Code and the Problem

Let's take a look at the problematic code snippet provided:

import mido
from mido import MidiFile, MidiTrack

mid = MidiFile()
track = MidiTrack()
mid.tracks.append(track)

# ... (Rest of the code to add notes and save the file)

The code uses the mido library, a popular Python library for working with MIDI files. The initial setup is standard: we create a MidiFile object, add a MidiTrack to it, and then we would typically proceed to add MIDI messages (notes, timing information, etc.) to the track. The core of the problem lies in how these MIDI messages are added and timed.

The issue described – the first chord sounding simultaneously while subsequent chords play sequentially – usually stems from a misunderstanding of delta time. If the delta times between notes in the first chord are zero, they will all play at the same tick. However, if the delta times for notes in subsequent chords are not zero, they will be spaced out in time, leading to the sequential playback.

Deep Dive into the Root Cause

To truly understand what’s happening, we need to dissect the typical process of adding notes and timing in a MIDI track. When you add a note_on message, followed by other note_on messages for the same chord with a delta time of 0, the MIDI player will interpret this as all notes starting at the same moment. This is exactly how you create a chord.

However, the trouble starts when you move on to the next chord. If you don't explicitly set a delta time before adding the first note of the next chord, or if the delta times are inadvertently set to a non-zero value between the notes of that chord, the notes will not play simultaneously. Imagine you’re writing a musical score – if you don't align the notes vertically on the staff, they won't be played as a chord.

Common Pitfalls:

Forgetting Delta Time: The most common mistake is forgetting to set the delta time to the desired duration after the first chord. For example, after playing a chord for one beat, you need to add a delta time message (typically a Message('control_change', control=64, value=127, time=ticks_per_beat)) before adding the notes of the next chord.
Incorrect Delta Time Calculation: Miscalculating delta times can also lead to timing issues. Make sure your delta time values accurately reflect the desired duration of notes and rests.
Looping Errors: When generating MIDI files programmatically, errors in loops or conditional statements can cause incorrect delta time assignments.

Solutions and Best Practices

Now that we understand the problem and its root causes, let's explore some solutions and best practices for creating MIDI files in Python using mido:

Explicitly Set Delta Times:

The key to fixing the issue is to explicitly control the delta times between notes and chords. After adding the notes for the first chord, ensure you add a delta time message to advance the playback head to the next beat or desired time. This message effectively tells the MIDI player to wait for a certain duration before playing the next note or chord. Here’s how you can do it:
```
import mido
from mido import MidiFile, MidiTrack, Message

mid = MidiFile()
track = MidiTrack()
```

mid.tracks.append(track)

ticks_per_beat = mid.ticks_per_beat

# Add first chord (C, E, G)
track.append(Message('note_on', note=60, velocity=64, time=0))
track.append(Message('note_on', note=64, velocity=64, time=0))
track.append(Message('note_on', note=67, velocity=64, time=0))
track.append(Message('note_off', note=60, velocity=64, time=ticks_per_beat))
track.append(Message('note_off', note=64, velocity=64, time=0))
track.append(Message('note_off', note=67, velocity=64, time=0))


# Advance time to the next beat
# track.append(Message('control_change', control=64, value=127, time=ticks_per_beat))  # Wait for one beat

# Add second chord (D, F#, A) - with correct timing
track.append(Message('note_on', note=62, velocity=64, time=ticks_per_beat))
track.append(Message('note_on', note=66, velocity=64, time=0))
track.append(Message('note_on', note=69, velocity=64, time=0))
track.append(Message('note_off', note=62, velocity=64, time=0))
track.append(Message('note_off', note=66, velocity=64, time=0))
track.append(Message('note_off', note=69, velocity=64, time=0))

mid.save('my_midi_file.mid')
```

In this example, we explicitly set the delta time (`time` parameter in the `Message` constructor) to `ticks_per_beat` after the first chord. This tells the MIDI player to wait for one beat before playing the next chord. Notice how the notes within the chord have a delta time of 0, ensuring they play simultaneously.

Use Functions for Note and Chord Creation:

To make your code more readable and maintainable, consider creating functions for adding notes and chords. This encapsulates the logic for creating MIDI messages and setting delta times, reducing the risk of errors and making your code easier to understand. This is a fundamental principle of good programming.

import mido
from mido import MidiFile, MidiTrack, Message

def add_note(track, note, velocity, time):
    track.append(Message('note_on', note=note, velocity=velocity, time=time))
    track.append(Message('note_off', note=note, velocity=velocity, time=time))

def add_chord(track, notes, velocity, time):
    for note in notes:
        track.append(Message('note_on', note=note, velocity=velocity, time=0))
    for note in notes:
        track.append(Message('note_off', note=note, velocity=velocity, time=time))

mid = MidiFile()
track = MidiTrack()

mid.tracks.append(track) ticks_per_beat = mid.ticks_per_beat

# Add first chord (C, E, G)
add_chord(track, [60, 64, 67], 64, ticks_per_beat)

# Advance time to the next beat
# track.append(Message('control_change', control=64, value=127, time=ticks_per_beat))

# Add second chord (D, F#, A)
add_chord(track, [62, 66, 69], 64, ticks_per_beat)

mid.save('my_midi_file2.mid')
```

These functions abstract away the details of creating individual MIDI messages, allowing you to focus on the musical structure of your composition. They’re also a ***game-changer*** for complex compositions.

Leverage Mido's High-Level Abstractions:

mido provides higher-level abstractions that can simplify MIDI file creation. For instance, you can use the Message.copy() method to duplicate messages and modify their attributes, or you can use the MidiTrack.extend() method to add multiple messages at once. Exploring these features can significantly streamline your code.
Debugging Techniques:

When troubleshooting MIDI timing issues, it's helpful to print the MIDI messages you're adding to the track. This allows you to verify the delta times and note values. You can also use a MIDI editor or sequencer to inspect the generated MIDI file and visually confirm the timing of events. Debugging is an essential skill for any programmer or musician.
```
import mido
from mido import MidiFile, MidiTrack, Message

mid = MidiFile()
track = MidiTrack()
```

mid.tracks.append(track) ticks_per_beat = mid.ticks_per_beat

# Add first chord (C, E, G)
track.append(Message('note_on', note=60, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_on', note=64, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_on', note=67, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=60, velocity=64, time=ticks_per_beat))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=64, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=67, velocity=64, time=0))
print(f"Adding: {track[-1]}")

# Advance time to the next beat
# track.append(Message('control_change', control=64, value=127, time=ticks_per_beat))

# Add second chord (D, F#, A)
track.append(Message('note_on', note=62, velocity=64, time=ticks_per_beat))
print(f"Adding: {track[-1]}")
track.append(Message('note_on', note=66, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_on', note=69, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=62, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=66, velocity=64, time=0))
print(f"Adding: {track[-1]}")
track.append(Message('note_off', note=69, velocity=64, time=0))
print(f"Adding: {track[-1]}")

mid.save('my_midi_file_debug.mid')
```

Conclusion: Mastering MIDI Timing

Creating MIDI files programmatically can be a rewarding experience, allowing you to generate music algorithmically or manipulate existing MIDI data. However, it requires a solid understanding of MIDI concepts, particularly timing and delta times. The issue of the first chord sounding simultaneous while subsequent chords play sequentially is a common pitfall, but by explicitly controlling delta times, using functions for note and chord creation, and leveraging mido's features, you can overcome this challenge and create beautiful, well-timed MIDI compositions. Remember, practice makes perfect!

So, the next time you encounter this issue, remember these tips and dive into your code with confidence. Happy composing, guys! And if you have any more questions, feel free to ask. Let's make some awesome music together!