Custom firmware is needed for this to work
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If your 3ds has Luma3DS 10.3 or higher use this
Using Luma3DS:
Press (Left Shoulder) + (D-Pad Down) + (Select) at the same time to open the Rosalina menu
If one of these buttons is broken, download config.ini
https://3ds.hacks.guide/assets/config.ini
And put it in your luma folder, replacing the existing one. This will change the Rosalina menu key combination to (X) + (Y)
Select Miscellaneous options
Select Dump DSP firmware
Press (B) to continue
Press (B) to return to the Rosalina main menu
Press (B) to exit the Rosalina menu
https://3ds.hacks.guide/finalizing-setup
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If your 3ds has Luma3DS 10.2 or lower use this
Using DSP1:
Download DSP2.cia from this link: https://github.com/zoogie/DSP1/releases/tag/v1.0
Copy DSP2.cia to the cais map on your micro sd card of your 3ds.
Open the FBI homebrew app on your 3ds.
Press A
Go to your cias map and press A
Select DSP2.cia and press A
Press A
Press A
Press A
Press the home button on your 3ds.
Open DSP1 homebrew app.
Press B
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You must have CFW on your 3ds!