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USB Audio Class, what it is for and where it is used

The USB Audio Class is highly popular and is also used because it is supported by all operating systems and therefore platforms. We can see it used in Windows, Linux, macOS, etc. So connecting a USB Audio Class compliant device is as simple as plug and play, with no additional worries of any other kind.

The standards of the USB Audio Class

We have two different standards, the first is USB Audio Class 1.0, which was proposed in 1999 and is based on version 1.0 of the USB standard.

  • Specifications indicate; 24-bit stereo, two-channel, sampling up to 96 KHz.
  • In every millisecond a packet is sent through the USB port, the size limit per packet in the USB standard is 1024 bytes. USB Audio Class 1.0 sends 576 bytes per millisecond, but much higher sample rates cannot be used because of the per-packet limit.

A decade later, USB Audio Class 2.0 was proposed, intended for USB 2.0 standards onwards. Since the bandwidth had gone from 12 Megabits per second to 480 Megabits per second. The limitations of USB Audio Class 1.0 disappeared.

  • The audio quality goes from 24 bit to 32 bit and it is possible to obtain sample rates for both playback and recording of up to 192 KHz.
  • The difference in bandwidth from USB 1 to USB 2 is such, 40 times more, that it is possible to record 60 simultaneous channels with the same quality as in USB Audio Class 1.0 when in the previous version of the standard only 2 channels are possible.
  • It is backwards compatible with version 1.0 of the standard.

The high-end gaming headphones, with one speaker per channel, make use of the second version of the standard, which is designed not only to reproduce audio with higher quality, but also for a greater number of simultaneous channels.

The DAC Types in the USB Audio Class

DAC

Although sound is treated digitally when it is manipulated within our computer as data. When it is transmitted to the speakers, it does so in the form of an analog signal that has to be converted through a converter that converts the digital signal to an analog, or better known by its English acronym DAC.

So the audio output must be synchronized to the speed of the audio tracks themselves, which are not always symmetrical. This means that there are several different DACs for the different USB Audio Class.

  • First type, USB synchronous DAC: It is the worst quality and is used in low-end products, accepts data packets as they arrive and is the most prone to audio quality failures.
  • Second type, USB adaptive DAC: Here the clock speed of the DAC is adjusted, in such a way that there is complete coordination between sending and receiving data.
  • Third type, USB asynchronous DAC: The most advanced of all and the best, it has a unit that marks the timing of the data flow, which leads to greater precision and avoids the CPU having to coordinate. It is the type of DAC that is used in high-end products.

As a curiosity, if you have bought headphones with USB you will see that many of them are not connected by USB but with a small box with a minijack at one end and a USB output on the other side. Well, that box is nothing more than a very cheap DAC, since usually PC USB ports do not usually have an integrated DAC, so special devices that fulfill this function are necessary.

In high-end headphones with multiple channels, because it is not normal for the motherboard of the PC to include a USB that can transmit audio, they usually have the DAC inside them. In any case, if you have a sound card it is possible that several of its USB ports include an integrated DAC.

Not only for computers

Android USB C

Smartphones and tablets also have support for USB Audio Class through their USB ports, but it is especially through USB-C and the like that USB Audio Class 2.0 has reached these types of computers. Making possible not only the playback, but also the recording of several simultaneous channels through this type of input and output port.

Little by little the minijack port is disappearing in favor of using USB ports and the like. Which means that eventually all USB ports on tablets and smartphones will have built-in DACs. Moreover, due to the high demand for the USB type C port with DAC, it is possible that the USB port with integrated audio DAC is also standardized on PCs.