What is Asynchronous USB 2.0 audio class?

Great Audio Quality Requires More Than USB 2.0 Support

Now that listening to music on computers is as commonplace as listening to music on the radio—and maybe more so—Audio Interfaces have also become commonplace. Today, anyone can enjoy high quality sound via a single “music server device,” which is a PC or Macintosh computer. But what are the technical limitations of these audio interfaces? Is it enough that an interface be USB 2.0 compatible?

The numerous USB Audio Interfaces, which can be thought of as external sound cards, all contain Digital-to-Analog Converters (DACs). Regardless of the capability of the DAC, it must rely on USB to receive and deliver data—and USB was not designed for audio. Early USB DACs came with limitations and could not provide “real” hi-fi audio quality.

The audio community recognized the problem and gathered to work on the “USB Audio Class” standard, which was finalized in 2006. The new standard should have solved the problem—but it didn’t. By the time the finalized USB Audio Class 2.0 was released, the USB 2.0 specification was already complete. And it used the earlier audio spec from USB 1.1.

The saddling of USB 2.0 with the legacy USB 1.1 audio spec is one reason so many Audio Interfaces on the market claim support for USB 2.0, but are limited to 24bit/96kHz audio. Just because the USB jack supports USB 2.0, does not necessarily mean that the product supports the Asynchronous USB Audio Class 2.0.

What, then, are the limitations of the USB 1.1 audio spec? USB 1.1 supports a maximum throughput of 10Mbps (megabits per second). For audio, you need to fix, or maintain, a particular frequency, and doing so prevents you from using the full 10Mbps. That is why USB 2.0 and USB 1.1 devices are limited to 24bit/96kHz audio.

What are the advantages of  Asynchronous USB Audio Class 2.0? The Asynchronous USB Audio Class 2.0 supports 480Mbps of audio data (in contrast to 10Mbps which cannot be fully utilized), delivering 24-bit audio with sample frequencies up to 192kHz (in contrast to 96kHz).

You can see that while USB 2.0 or better support is a must, it is not an indicator of audio quality. When comparing Audio Interfaces and DACs, look for Asynchronous USB Audio Class 2.0 support (such as the AMI MUSIK DDH1), and check the Audio Interface delivers up to 24bit/192kHz audio.

In fact, these are the types of audio interfaces that we are developing here at AMI International, Inc., such as the AMI MUSIK DDH-1 and the AMI MUSIK DS5.

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