Project Description

Device for connecting an electric guitar or bass to a computer via USB-C. It is basically just a glorified A/D converter, although with a twist. It can emulate proprietary commercial devices for use with applications such as Rocksmith. The name comes from the official device for Rocksmith, which is named Realtone Cable.

Goal for this Hackweek

My goal is to create a free implementation of such board using a CM108 audio chip in KiCad. Anybody can make their own board by sourcing provided Gerber files. The board is also designed to be soldered by hand, despite being primarily SMD.

I'd also like to write a detailed blog post about the development process.

Resources

• CM108 datasheet

TBD.

Working Prototype

This prototype is based on a cheap sound-card, which is built around the CM108 chip. This sound-card is basically an implementation of the reference diagram in CM108 datasheet, albeit with some filtering removed, to probably save up on parts and cost.

Several modifications were made, including:

• Removal of R6 to eliminate piezoelectric microphone power
• Removal of LED
• EEPROM with flashed data from the original device
• Replacement of audio input Jack

It works OK, but the custom board needs better noise filtering.

Schematic

This schematic is based on the reference implementation of CM108, but has many modifications, mainly:

• Better power and signal filtering
• EEPROM
• Simple connectors for interfacing with the EEPROM
• USB-C

Board design

I've managed to design a complete board during this Hack Week. This is the first complete iteration of the board which will go intro production and testing.

Post Hack Week

A second iteration has been successfully tested. It has some changes, mainly a correct D+ and D- USB orientation (oops), two possible mounting styles for the EEPROM and some minor layout fixes. This version will go into a larger production. Great success.