Asynchronous USB Audio / Asynchronous Digital to Analog Converters

Posted by HiFimeDIY 24/09/2013 1 Comment(s) Articles,DAC's,

USB transfer modes has received a lot of interest and attention recently and "Asynchronous DAC's" has been praised and hyped for it's superior performance. However there seems to be a lot of confusion around asynchronous USB Audio. 

There is a good and explaining article at EDN network about USB Audio. 

Here's a short explanation:

There are 4 types of USB transfers,:  Isochronous, Interrupt, Control and Bulk

Bulk transfer is used for other types of USB devices like harddrives etc. USB Audio uses isochronous, interrupt and control transfers. All audio data is transferred over isochronous transfers. Interrupt transfers are used to relay information regarding the availability of audio clocks; control transfers are used used to set volume, request sample rates, etc.

Illustration from

In audio we need to maintain a constant flow of data to keep the music playing continuously without any stops. Isochronous transfers provides a guaranteed bandwith and give priority above any other USB transfers. For example if you are making backup copies to your external hard drive, the data stream going to the DAC would have priority over the data sent to the hard drive.

When sending the data (music), to the DAC, the time needs to be defined. We know how many samples should be played pr second, but how long is a second? 

This is where the master clock comes into play. Normally the computer (host) controls the master clock defining the second. this is called synchronous transfer

Asynchronous transfer is when we include another oscillator (clock) into the external DAC and make this the master clock.  The master clock is then moved away from noise sources in the computers (CPU etc) which avoids jitter(noise) to be added to the USB signal. It also makes it possible to use more suited and high end oscillators as master clock than those on the computer motherboard.

Adaptive transfer means that the data flow controls the timing. This avoids having to use additional oscillators and greatly improves the performance compared to synchronous transfer.

Short summary:

  • With Asynchronous transfer the DAC controls the timing
  • In synchronous the computer controls the timing
  • In Adaptive transfer the timing is controlled by the data flow


1 Comment(s)

25/09/2017, 10:11:37 PM

Hi, I've just discovered and I think it is a really cool portal, congrats! I've read carefully (I mean, at my best) this post about USB modes, I liked it, but there are two points not clear to me. First: what does it mean "the timing is controlled by the data flow" in the case of the adaptive mode? When you write "the clock is on the DAC's side" or "the clock is on the host's side" it is clear who answers the question "how long is a second": one clock or the other. But who says how long is a second in the adaptive mode? How can "the data flow" contain that information? Second: it seems to me that the asynch mode is the best approach, since it does not rely on the host's clock, that could be of low quality. Though, in the description of your product "HiFime Sabre 9018 USB DAC" you state: "After careful experimentation we have chosen USB adaptive mode for this DAC". Does it mean that you've gone for a good compromise or that you have actually registered the best performance through that mode respects to the others? Thank you in advance for your attention and for the great job you've done!

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