851C What is USB Audio?

USB audio is a digital audio connection used to send digital music from your computer to a digital to analogue converter (DAC). The reason the use of USB audio is preferable when listening to audio from a computer is that the DAC within a computer is usually not very good and of a low quality.

There are two classes of USB audio, Class 1 and Class 2. They can both handle high-resolution music; however, there is a clear distinction between the two.

What is the difference between USB Audio Class 1 and USB Audio Class 2?

The difference is in the resolution of music they can deliver. Class 1 will give you a BIG improvement in sound compared to your computers own headphone output, however, class 2 will take you one step further. Class 1 can send up to a maximum of 24Bit/96kHz hi-res files, but if you want to play those studio master quality files you’ll need to step up to Class 2. This supports up to 24Bit/192kHz resolution files, for those seeking absolute audio perfection.

All current Cambridge Audio DACs support both of these classes. Class 1 is set up as default but this can be switched very easily either in the settings menu or by pressing and holding buttons on the front menu (details of this are in the individual product manuals). By utilising USB audio you are bypassing the internal soundcard of the computer and allowing the USB DAC to perform the Digital to Analogue conversion in much better quality.

Components within a USB Audio System.

There are three main parts to any USB audio system:

First, you have the media player, this is the software you use to play your music e.g. Windows Media Player, Spotify, iTunes

Then you have the driver – there is a USB driver built into either the Windows or Macs operating systems.

Finally, you have the USB device; this is the DAC being used, for example, the CXN or the DacMagic XS.

By default Macs support both Class 1 and Class 2, meaning you can easily send files up to 24Bit/192kHz to your DAC from your Mac. By default Windows will only support Class 1 USB Audio. The maximum bit rate and sample rate this will support is 24Bit/96kHz.

To enable playback of 24Bit/192kHz you will need to download the additional USB Audio Class 2 V3.40 driver from the support site.

Asynchronous, adaptive and synchronous USB DAC.

Further to this, are the benefits of using asynchronous USB audio. A computer and a USB DAC both have their own internal clock which dictates the timing for both devices. One of the problems with USB audio transfer is that these clocks are not running at the same time.

Digital music is sent as USB packets from the computer down the USB cable to the DAC. These are sent in periodic time frames according to the computer's clock.

There are three types of USB audio DAC.

Synchronous USB DAC is the lowest quality of the three. These accept the packets of data whenever the data sends them causing glitches every few seconds due to the differences between the two clocks.

Adaptive is where the DAC constantly adjusts its clock so that it can accept the data being sent from the computer whenever it sends them. The constant adapting of the DAC’s clock means that there is no continuous, accurate master clock in the DAC, which causes jitter in the audio stream.

In both of the above, the computer dictates the timing of the data packets being sent.

Asynchronous – this is the most complex to implement but it is a huge improvement on the other types. This is because it requests the data packets to be sent in time with its own clock’s timing, thus providing the lowest jitter and sounding by far the best. We use asynchronous USB in all of our current DACs, such as the CX and 851 ranges.