Reverse Engineering SysEx Protocols

Reverse Engineering Proprietary Synthesizer SysEx Protocols

When integrating undocumented vintage synthesizers, rare boutique sound modules, or obscure expanders with bipluk.com, reverse engineering the MIDI System Exclusive (SysEx) protocol becomes necessary. This masterclass document outlines the mathematical and empirical methodologies used by the bipluk.com engineering team to dissect unknown byte streams.

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1. Reverse Engineering Methodology Workflow

flowchart TD
    Capture["Phase 1: Differential Packet Capture (State A vs State B)"] --> Bitwise["Phase 2: Byte Isolation & Hamming Distance Mapping"]
    Bitwise --> Range["Phase 3: Parameter Value Range Boundary Testing"]
    Range --> ChecksumCrack["Phase 4: Checksum Algorithm Discovery & Proof"]
    ChecksumCrack --> Adapter["Phase 5: Python Adapter Implementation in sysex_adapters/"]

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2. Phase 1: Differential Packet Capture

Tip

Hardware Loopback Capture: Use a hardware MIDI monitor or Web MIDI loopback in Chrome DevTools to sniff raw SysEx bytes without interrupting the hardware MIDI stream.

To decode parameter addresses inside an unknown synthesizer:

1. Dump Baseline Patch (State $S_0$): Trigger a single-voice dump or edit buffer dump from the instrument. Save as $\mathbf{B}_0$.
2. Mutate Exactly One Parameter (State $S_1$): Increment a single synth parameter (e.g., Filter Cutoff from 50 to 51) via the front panel. Dump again. Save as $\mathbf{B}_1$.
3. Differential Comparison ($\Delta$): Compute the byte-by-byte XOR differential:

$$\Delta[k] = \mathbf{B}_0[k] \oplus \mathbf{B}_1[k]$$

flowchart LR
    subgraph Stream0 ["Baseline Dump S0"]
        B0["F0 41 10 23 12 00 14 02 32 38 F7"]
    end

    subgraph Stream1 ["Mutated Dump S1 (Cutoff + 1)"]
        B1["F0 41 10 23 12 00 14 02 33 37 F7"]
    end

    Stream0 --> DiffEngine["Differential Comparator"]
    Stream1 --> DiffEngine
    DiffEngine --> Result["Payload Byte 8 changed: 0x32 to 0x33 / Checksum Byte 9 changed: 0x38 to 0x37"]

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Because byte index 8 changed from 0x32 to 0x33, index 8 represents the Filter Cutoff parameter address. Notice that the trailing byte dropped from 0x38 to 0x37, immediately suggesting an additive two's complement checksum relationship:

$$\Delta_{\text{data}} = +1 \implies \Delta_{\text{checksum}} = -1$$

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3. Phase 2: Checksum Cracking Heuristics

Synthesizers historically implemented one of six primary checksum archetypes. Given an unknown packet $\mathbf{P} = (p_0, p_1, \dots, p_{N-1})$, test the trailing byte $p_{N-2}$ against these mathematical models:

Model 1: Roland 7-Bit Two's Complement Modulo 128

$$\sum_{i=k}^{N-2} p_i \pmod{128} \equiv 0 \implies C = (128 - (\Sigma \pmod{128})) \land 0\text{x}7F$$

Model 2: Yamaha DX 7-Bit Modulo 128

$$C = (128 - (\Sigma \land 0\text{x}7F)) \land 0\text{x}7F$$

Model 3: Longitudinal Redundancy Check (XOR Sum)

$$C = \bigoplus_{i=k}^{N-3} p_i \land 0\text{x}7F$$

Model 4: Kawai Block Sum Modulo 128

$$C = (\Sigma + 0x5A) \pmod{128}$$

Model 5: 7-Bit Cyclic Redundancy Check (CRC-7)

Using generator polynomial $G(x) = x^7 + x^3 + 1$ ($0\text{x}89$):

$$R(x) = M(x) \cdot x^7 \pmod{G(x)}$$

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4. Automated Checksum Discovery Script

The following automated Python script tests an unknown packet against all standard synthesizer checksum algorithms:

def crack_sysex_checksum(raw_bytes: bytes):
    """
    Empirical solver for unknown synth SysEx checksum algorithms.
    """
    header = raw_bytes[0]  # Expects 0xF0
    trailer = raw_bytes[-1] # Expects 0xF7
    candidate = raw_bytes[-2]
    
    # Try different starting payload offsets (typically index 4, 5, or 6)
    for start_offset in range(2, min(8, len(raw_bytes) - 3)):
        payload = raw_bytes[start_offset:-2]
        
        # Test 1: Roland two's complement
        roland_chk = (128 - (sum(payload) % 128)) & 0x7F
        if roland_chk == candidate:
            return f"MATCH: Roland 7-bit two's complement from offset {start_offset}"

        # Test 2: Standard additive sum & 0x7F
        sum_chk = sum(payload) & 0x7F
        if sum_chk == candidate:
            return f"MATCH: Simple 7-bit sum from offset {start_offset}"

        # Test 3: XOR reduction
        xor_acc = 0
        for b in payload:
            xor_acc ^= b
        if (xor_acc & 0x7F) == candidate:
            return f"MATCH: 7-bit XOR check from offset {start_offset}"

        # Test 4: Yamaha two's complement
        yamaha_chk = (-sum(payload)) & 0x7F
        if yamaha_chk == candidate:
            return f"MATCH: Yamaha 7-bit check from offset {start_offset}"

    return "No standard heuristic matched. Check for nibblized multi-byte CRC or manufacturer offset."

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5. Integrating with bipluk.com

Protocol Reverse Engineering Checklist

• Capture pristine baseline dump ($S_0$) via Web MIDI monitor
• Mutate one parameter incrementally by $+1$ and capture mutated dump ($S_1$)
• Perform byte-by-byte differential XOR check ($\Delta[k] = \mathbf{B}_0[k] \oplus \mathbf{B}_1[k]$)
• Isolate active parameter index and bitfield width ($2, 4, 7 \text{ bits}$)
• Run crack_sysex_checksum() automated solver against the trailing bytes
• Write Python parser inheriting from GenericAdaptation in sysex_adapters/
• Verify against known implementations in Hardware Compatibility Matrix
• Submit Pull Request to https://github.com/maxcomperatore/bipluk.com

For deeper insight into Roland, Yamaha, and Korg encoding strategies, see SysEx Specifications and Checksums. If experiencing dropped packets during raw captures, consult Hardware MIDI Troubleshooting Guide.