Making a USB Audio Interface (Part 1)

2025-04-23 | By Uzair Syed

License: None ADCs Amplifiers DACs Headphone / Audio Cables USB Cables

Background

I recently built a new small-form-factor PC, and it's been working great so far! However, one gripe ‎is that since the case was designed to be as small as possible, there is no audio jack. While this is ‎not ideal, it served as the perfect opportunity for me to look into a project I've been thinking about ‎for a long time: a USB-based audio interface. A good place to start when thinking about projects ‎like these are real-life products. There are many USB headsets out there, so let's investigate.‎

How do USB headsets work?‎

If you think about how USB (Universal Serial Bus) works and then about how headphones work, it ‎might get a little confusing: Isn't USB a digital interface, whereas audio (stereo for example) is ‎an analog interface? Doesn't this mean we have to find a way to bridge them? This, in fact, is ‎where all of the complexity lies. Here's a (very) simplified block diagram describing how the ‎process goes:‎

host_1

The host, likely your PC, will engage in communications with the USB device, over that bus. A ‎‎"USB Device" is any peripheral that connects to a host device over a USB connection -- think ‎keyboards, mice, video cameras, printers, or in our case, headsets, or headphones. Upon ‎connection to the host, the USB device will "introduce itself" to the host, importantly, ‎what device class it is. This process is called Enumeration. Speaking on a lower level, a USB ‎device can be implemented in a multitude of ways -- think ASICs (Application Specific Integrated ‎Circuits) or MCUs (Microcontrollers). For instance, a wide variety of the MCUs from the STM32 ‎families have a USB device peripheral.‎

The USB specification defines a dedicated USB Audio Class, which details how analog audio is ‎represented in digital USB packets. Once the device receives these packets, it needs a way to ‎turn the digital packets into analog audio. This can be done with a DAC (Digital-Analog Converter). ‎Interfacing the DAC highly depends on the device. It can be internal or external, it can connect ‎over SPI, I2S, I2C, etc. Note, for the mic input, the same thing goes, just in the opposite direction ‎using an ADC (Analog-digital-converter).‎

What are some potential solutions?‎

The very first thing I thought of was to use a microcontroller, in my case an STM32. They usually ‎have all the interfaces we'd need: USB device, internal DACs, and ADCs, I2S (Inter-Integrated ‎Circuit Sound) if we want to use an external DAC, etc. However, the more I thought about it, the ‎more unappealing it sounded. At the very least, you'd have to write firmware to:‎

‎- Configure the USB Audio Device

‎- Configure the DAC / ADC interface

‎- either internal or over some protocol (I2S, SPI, etc.)‎

‎- Likely configure a DMA (Direct Memory Access) stream between a USB buffer and the DAC

‎- Make sure that the Host and USB device are in sync

‎- Essentially, the USB Device can't "consume" the Host data too quickly or too slowly -- this will ‎cause glitches in your audio stream.‎

‎- If you're interested in this process, look into the different types of USB Audio modes there are ‎‎(Adaptive and Asynchronous).‎

That last task especially requires a good amount of reading of the USB Audio specification. See ‎Section 5.10.4.2 of the USB Specification and Section 3.7.2.2 of the Audio Device Class ‎specification for more info.‎

If only there were a chip that could do this all for us...‎

Enter: Texas Instruments' PCM2906C

Luckily for us, TI has an entire selection of Audio USB Converters, the PCM29xx series! We want a ‎bus-powered application, meaning that the USB host will supply all the power needed for the ‎interface. This is opposed to a self-powered application, which means that we'd have to supply ‎the power with some other interface (battery, barrel jack, etc.).‎

Bus-power simplifies stuff on our side, but with that simplification comes some limitations -- we ‎can only work with 5V (The standard USB voltage rail). If we want to have an amplifier with a ‎higher voltage level, we have to either self-power or boost the 5 volts up somehow. This will be a ‎topic for another day.‎

This lands us on the PCM2906C. Reading the datasheet, we can see the following features:‎

‎- Fully integrated USB Audio Stereo Codec

‎- Uses USB 1.1 as a USB Audio Class

‎- USB is backward compatible, so this will work even though USB 1.1 is... ancient.‎

‎- This will also greatly simplify PCB Layout, as the requirements listed by USB 1.1 are far, far ‎simpler to implement.‎

‎- 16-bit DAC/ADC

‎- DAC Sampling rate options: 32, 44.1, and 48 kHz‎

‎- ADC Sampling rate: 8, 11.025, 16, 22.05, 32, 44.1, and 48 kHz

‎- Also includes digital S/PDIF interface

‎- USB Adaptive Mode for audio output, Asynchronous for input

‎- Includes a HID (Human Interface Device) class function

‎- Volume and mute controls

All of this in one chip, no firmware required! Best of all, it's still in production and available for ‎purchase at DigiKey!‎

Conclusion

Now that we've found a part that will do what we want, join me for part 2 where we'll explore how ‎the PCM2906C works, design a PCB-based prototype for our USB Audio Interface using KiCad, and ‎test it in the lab!‎