fiberaudio-108

A low-cost USB audio interface with S/PDIF optical output, based on the Cmedia CM108AH ASIC.

View Schematic 📄 or PCB Preview 🌐. Customize using the CM108AH Configuration Tool 💿.

Production files will be made available on the Releases page, once validation is complete.

Key features

• Cmedia CM108AH audio interface ASIC
• 2.0 Stereo, up to 16-bit/48kHz (CD-DA equivalent)
• USB Mini-B input
• Optical S/PDIF output (JIS F05, e.g. TOSLINK™)
• USB Audio Device Class 1.0, uses generic drivers
• Configuration EEPROM

Overview

This USB audio interface provides a low-cost S/PDIF optical output for portable computers and tablet devices. Audio tranmission via optical link - as opposed to a wired line-level connection - eliminates hum caused by ground potential differences, and noise feed-through from switched-mode power supplies.

Design Requirement	Solution Approach
Provide electrical isolation between device and audio equipment	Audio signal transmission via optical link (e.g. TOSLINK™)
Compatibility for wide range of operating systems, with minimal driver maintenance	USB Audio Device Class using OS-provided generic drivers
IP accessibility for PCB design files	Use of free, open-source EDA tooling (e.g. KiCad)
Commonly-used, small form-factor USB connector, robust enough to withstand portable use	USB mini-B receptacle
Option for manual assembly	Passives no smaller than 0603. ICs leaded and no smaller than 0.5mm pin pitch (SSOP)
Low PCB production capability requirements	• FR4 base material • 2-layer Cu, Top+Bot Mask, Top Overlay • ≥0.25mm trace-to-outline • ≥0.25mm trace width and spacing • ≥0.35mm drill size • Gerber, NC Drill and RS274X outputs

Design Review (Prototyping)

Prototyping was carried out using rev. 3 board design files, in a minimum-order batch of 5 PCBs, including automated assembly.

Assembly

• Manual rework of the PCB around U3 (SRV05-4) was required in order to fix the issues found in rev. 3, after production was already started.
• Parts U1 (AT93C46D), U2 (CM108AH), SW1, and both connectors were unavailable for automated assembly, and had to be retrofitted manually using hot-air reflow soldering.

Software Support

• Drivers included with SteamOS 3.0 provide full functionality out-of-box, including volume controls for S/PDIF output.

• Generic Audio Device drivers included in Windows 7.1 (x64) as well as Windows 10 (Version 22H2, x64) only support S/PDIF output in a passthrough mode. Volume control and the mute input are inoperative when using the default driver package (see below for workaround).

Workaround: A vendor-specific Windows driver package for the CM108AH is available for download from the ASIC manufacturer's website: https://www.cmedia.com.tw/support/download_center.

Updated 08/2023: Unfortunately, the ASIC manufacturer have stopped providing any drivers for download. I uploaded the latest signed driver package I could source (Version 8.1.8.2172) as release v0.1.5-driver-only. Please check the file for integrity and verify it includes the correct signed driver binary before installing.

Using the Windows device manager, perform a manual driver update of the Generic USB PnP Sound Device, and select Win8\SoftwareDriver from the extracted archive as the driver installation sources directory.

Updated 08/2023: To install the driver, extract the archive and right-click install the Cm108.INF file.

After installation and a reboot, a new Digital Audio (USB PnP Sound Device) device will show up in the Sound Settings dialog, alongside the old Speakers (USB PnP Sound Device) device.

Disable the Speaker device.

This workaround was tested using Windows 7.1 (x64), and Windows 10 (Version 22H2, x64).

Device Configuration

• A configuration tool for re-programming the external EEPROM was developed and is provided in the sw/ subdirectory.

• EEPROM access sporadically seems to return invalid data. Re-reading or re-programming the affected words can be used as a workaround.

Summary

• The device works correctly in connection with Linux systems. For Windows hosts, a custom driver installation is required for the device to operate properly.

Licensing

If not stated otherwise within the specific file, the contents of this project are licensed under the CERN Open Hardware Licence Version 2 - Permissive. The full license text is provided in the LICENSE document.

    SPDX-License-Identifier: CERN-OHL-P-2.0

This licensing model is compliant with the Open Source Hardware Definition 1.0.