How Melodics Stores Song Data

Overview: "Self Esteem" by The Offspring

This document explains how the Melodics app stores songs like "Self Esteem" on macOS.

1. Directory Structure

~/Library/Application Support/Melodics/Melodics/42VGQ2OQOL01/
├── data/drums/songs/
│   ├── lessons.json          # Index of all drum songs with metadata
│   └── lesson/
│       └── DJ3LZP3NV45N.json # Individual lesson file for each difficulty
├── data/drums/instrument/
│   ├── selfesteem_dr_t.json  # Backing track instrument config
│   └── samples/
│       └── selfesteem_dr_t_sample.json # Sample mappings
├── midi/                     # 837 .mpk files (encrypted MIDI packages)
└── samples/                  # 733 .spk files (encrypted sample packages)

2. Data Organization

lessons.json

Contains an index of all songs with metadata:

Song title, artist ("Self Esteem" by The Offspring)
Difficulty levels (Beginner, Intermediate)
Grade, genres, tags
Preview URLs, album art
UIDs for each lesson version

Individual Lesson Files

Each lesson file (e.g., DJ3LZP3NV45N.json) contains:

Musical data:

BPM (105)
Time signature (4/4)
Key signature

Download links:

URLs for MIDI (.mpk) files
URLs for sample (.spk) files

Song structure:

Array of "steps" (Intro, Verse, Chorus, Bridge, etc.)
Each step includes:
- Number of bars
- Description
- User progress/scores

MIDI references for each section:

backing_midi: Background track (e.g., "B1.mid")
user_midi_left: Left-hand part (e.g., "LH1.mid")
user_midi_right: Right-hand part (e.g., "RH1.mid")

Instrument configurations:

References to JSON files for drum kits and backing tracks

Sample mappings:

MIDI note numbers → audio file paths

Download secret:

Field download_secret (e.g., "8sa78sa786jk352") - possibly for decryption

3. File Formats

.mpk files

Melodics Package files containing MIDI data
Encrypted with proprietary encryption scheme
Stored with MD5-like hash names for caching/security
File size: ~10-20KB for typical song

.spk files

Sample Package files containing audio samples
Encrypted with proprietary encryption scheme
Stored with MD5-like hash names

4. How It Works Together

For "Self Esteem - Beginner" level:

Metadata in lessons.json points to lesson UID DJ3LZP3NV45N
Lesson file DJ3LZP3NV45N.json defines the song structure with 7 steps (sections)
Each step references MIDI files:
- B1-B7 for backing tracks
- LH1-LH7/RH1-RH7 for user parts (left/right hand)
Sample configs map MIDI notes to audio files (drums, backing track stems)
The .mpk and .spk files are downloaded and cached with hash names

5. Architecture Benefits

The system cleverly separates:

What to play (JSON configuration)
How it sounds (sample mappings)
The actual data (encrypted .mpk/.spk files)

This allows Melodics to:

Update songs without redistributing large files
Share samples across multiple lessons
Protect content while keeping it locally cached for performance
Organize lessons by difficulty while reusing the same backing tracks

6. Encryption Investigation

Attempted Decryption Methods

The following methods were tested to extract MIDI from .mpk files:

❌ Failed Attempts:

Simple XOR with download_secret
XOR with hashed secret (MD5, SHA256 of secret)
AES Encryption:
- AES-128/256 ECB mode
- AES-128 CBC mode (with IV from file header)
- Keys derived from MD5/SHA256 of secret
Combined keys:
- secret + file hash
- secret + lesson UID
Compression:
- zlib decompression
- gzip decompression

Findings

The .mpk and .spk files use a proprietary encryption/obfuscation scheme that:

Is not standard XOR, AES, or simple compression
Likely serves as DRM to protect licensed content
Would require reverse engineering the Melodics binary to decrypt

File Analysis

File structure observations:

No standard magic bytes (not MIDI "MThd", not ZIP "PK")
File starts with: 384321e7... (varies by file)
Appears to be encrypted or heavily obfuscated
The download_secret field may be part of the encryption scheme but not in a straightforward way

7. Options for Extracting MIDI

Option 1: Reverse Engineering (Advanced)

Use disassemblers to analyze the Melodics binary:

Tools: Hopper, Ghidra, IDA Pro
Target: /Applications/Melodics.app/Contents/MacOS/Melodics
Goal: Find the decryption routine in the C++ code

Option 2: Runtime Hooking (Intermediate)

Intercept decrypted data at runtime:

Tool: Frida or similar dynamic instrumentation
Method: Hook the process and capture MIDI data after decryption
Advantage: Don't need to understand the encryption

Option 3: Community Resources

Search for existing tools: "melodics mpk decrypt"
Check GitHub for reverse engineering projects
Look for community forums discussing Melodics data extraction

Option 4: Official Export (Recommended)

Check if Melodics offers MIDI export feature
Contact Melodics support about exporting practice data
May be available as a paid feature or for educational use

Example: Self Esteem Lesson Data

{
  "uid": "DJ3LZP3NV45N",
  "song_title": "Self Esteem",
  "artist": "The Offspring",
  "title": "Beginner",
  "grade": 5,
  "bpm": 105,
  "beatsperbar": 4,
  "genres": ["Rock"],
  "tags": ["1/8 notes"],
  "download_secret": "8sa78sa786jk352",
  "download_midi_link": "https://api-cdn.melodics.com/assets/lessons-download/4161bb4287d84d0cf3cad8605f1ebfa3.mpk",
  "steps": [
    {
      "title": "Intro",
      "bars": 7,
      "backing_midi": "B1.mid",
      "user_midi_left": "LH1.mid",
      "user_midi_right": "RH1.mid"
    }
  ]
}

File Locations

Application: /Applications/Melodics.app
User data: ~/Library/Application Support/Melodics/Melodics/
User ID directory: 42VGQ2OQOL01 (varies by user/session)
Cached MIDI files: ~/Library/Application Support/Melodics/Melodics/42VGQ2OQOL01/midi/*.mpk
Cached samples: ~/Library/Application Support/Melodics/Melodics/42VGQ2OQOL01/samples/*.spk

Conclusion

While the song metadata and structure are stored in easily readable JSON format, the actual MIDI and audio data are protected by proprietary encryption. Extracting this data would require either reverse engineering the application or using runtime interception techniques. For legitimate use cases, contacting Melodics for official export options is recommended.

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