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from mido import Message

def set_track_instrument(track, program):
    """Insert a program_change message at the beginning of a MIDI track."""
    # Find position after all meta messages (track_name, etc.)
    insert_pos = 0
    for j, msg in enumerate(track):
        if not msg.is_meta:  # Insert before first non-meta message
            insert_pos = j
            break
    else:
        # If all messages are meta, insert at end
        insert_pos = len(track)
    track.insert(insert_pos, Message('program_change', program=program, time=0))

**Step 1: Compose with music21 (use placeholder or skip instrument)**
```python
from music21 import stream, note, chord, tempo

score = stream.Score()

**Step 2: Inspect MIDI structure and assign instruments with mido**
```python
from mido import MidiFile, Message

mid = MidiFile(midi_path)

**Step 3: For drums, ALSO set channel to 9 (channel 10)**
```python

**Step 4: Render to audio**
```python
from midi2audio import FluidSynth
from pydub import AudioSegment

fs = FluidSynth('/usr/share/sounds/sf2/FluidR3_GM.sf2')
wav_path = # define a path here
fs.midi_to_audio(midi_path, wav_path)

audio = AudioSegment.from_wav(wav_path)
mp3_path = # define a path here
audio.export(mp3_path, format='mp3', bitrate='192k')

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from music21 import instrument

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from mido import MidiFile, Message

mid = MidiFile(midi_path)

**Common MIDI Programs:**
- 38: Synth Bass 1
- 80: Square Lead
- 81: Sawtooth Lead
- 88: Pad 1 (New Age)
- 25: Acoustic Guitar (Steel) - loud, cuts through
- 33: Acoustic Bass

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# Bass note
bass_part.insert(offset, note.Note('A2', quarterLength=0.5))

offset += 0.5  # Advance time

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bar_length = 1.5  # 6/8 time signature
offset = 0.0

for bar in range(8):
    # Only fills first 1.0 quarter lengths, leaving 0.5 silent!
    guitar_part.insert(offset, note.Note('D3', quarterLength=0.5))
    guitar_part.insert(offset + 0.5, chord.Chord(['D3', 'F#3', 'A3'], quarterLength=0.5))
    # Missing: offset + 1.0 to offset + 1.5

    offset += bar_length  # Advances to next bar, but gap remains

**Solution Pattern** (complete coverage):
```python
bar_length = 1.5  # 6/8 time signature
offset = 0.0

for bar in range(8):
    # Fill the ENTIRE bar with notes
    guitar_part.insert(offset, note.Note('D3', quarterLength=0.5))           # 0.0 to 0.5
    guitar_part.insert(offset + 0.5, chord.Chord(['D3', 'F#3', 'A3'], quarterLength=0.5))  # 0.5 to 1.0
    guitar_part.insert(offset + 1.0, note.Note('D3', quarterLength=0.5))     # 1.0 to 1.5 ✓ Complete!

    offset += bar_length

**Best Practice**: Before advancing `offset += bar_length`, verify that your last note ends exactly at `offset + bar_length`. For example:
- If `bar_length = 1.5`, your last note should start at `offset + 1.0` with `quarterLength=0.5` to reach `offset + 1.5`
- If `bar_length = 2.0`, your last note should start at `offset + 1.5` with `quarterLength=0.5` to reach `offset + 2.0`

from mido import MidiFile

def set_track_velocity(track, velocity):
    """Set velocity for all note_on messages in a track."""
    for msg in track:
        if msg.type == 'note_on' and msg.velocity > 0:
            msg.velocity = velocity

mid = MidiFile(midi_path)
for i, track in enumerate(mid.tracks):
    if i == 1:  # Drums
        set_track_velocity(track, 75)
    elif i == 2:  # Bass
        set_track_velocity(track, 80)
    elif i == 3:  # Lead instrument (sax, guitar, trumpet)
        set_track_velocity(track, 95)
    elif i == 4:  # Background (organ, pads)
        set_track_velocity(track, 55)
mid.save(midi_path)