-Disclaimer: This article goes into technical details that most rational people would never need to know. If you're a technical details person, read on...
USB based audio has been around for a while now. Originally it was scoffed at by discerning audiophiles but quickly it has been realised that can be a fantastic method of digital playback of audio files. When implemented poorly, it can also be a bottleneck for an entire system.
There are a lot of intricacies to the the USB audio standard, that when taken care of, mean USB based dgital audio can sound incredible, in this article we will attempt to explain the how and why.
Even though the USB standard has been around for a long time, it's audio performance has only recently been acknowledged by the wider audiophile community as offering better performance than CD, SACD and even the best analogue sources.
When the first raft of expensive USB digital interfaces were released, they utilised the USB 1.1 standard for USB audio. Unfortunately, this was a very basic standard, designed for wide support, and poor performance. Because the computer was left in complete control of the crucial timing information, jitter levels (a form of digital distortion) were very high.
Jitter has long been recognised as the single biggest performance difference in digital audio. Even a very modest CD player outperformed the best USB DAC's of the time.
This left a very poor taste for anyone who invested in expensive USB DAC's of the time. For many people, USB has never really recovered from this...
The Advent of USB 2.0...Once bitten, twice shy:
The USB 2.0 standard was a godsend to music lovers. For the first time, real industry experts had been involved in the design of the standard to ensure that a widely supported standard allowed for high quality music reproduction as well.
Unfortunately, those who had invested loads in a terrible sounding older USB DAC or digital interface, we pretty much assumed USB 2.0 was unlikely to go anywhere soon. We kept our CD players spinning a little longer...
Slowly but surely, new USB Audio products have been released, each one pushing the boundaries a little further. Today, there are hundreds of excellent sounding USB DAC's on the market... So what makes them tick, why were they such a dramatic improvement from the older standard.
USB - Isochronous Transfer Mode
DCS's USB Interfaces & Clock
No I don't need a tissue, Isochronous transfer mode is how audio is sent over USB. This is USB's "Audio Mode".
There is no error recovery available. In isochronous mode, bandwidth is guaranteed, and the data transmission errors are detected using a cyclic redundancy check (CRC). However this is at the cost of packet acknowledgement or re-transmission in the event that an error occurs.
Basically this is just the name for the section of the USB standard dedicated to audio transfer.
So what does all of this mean?
Essentially, while this is a digital transfer, the signal that is being created can become flawed very easily. Timing information is critical in high-speed digital circuits. When trying to send a signal with 1,411,000 bits of information per second, it is incredibly important this voltage is pulsed with a clean and accurate clock or you will get audible discrepancies.
Isochronous mode has three sub-modes, some of which may be a bit familiar;
Adaptive (Variable source clock)
In adaptive mode, the peripheral / source adapts to the potentially varying sample rate of the host. Generally a detrimental method as most computer vendors main priorities are not on the efficacy of their USB hosts for audio playback. Unfortunately, because it is so widely supported it is also the default mode for many devices, making them sound average out of the box (until the correct drivers are installed)
Luckily however, we have...
Asynchronous - the best options for audio use.
Asynchronous USB is becoming the dominant method of USB DACs and this is a very good thing.
Asynchronous mode allows the USB peripheral, be it a fully fledged DAC or USB interface to take control of the clock. It also means the USB clock is much closer to the ever important DAC chip. This proximity means that Asynchronous should be far less susceptible to jitter. Additionally the DAC's clock is likely to be far superior to that of the computer's thus resulting in lower jitter. Basically, when implemented correctly, this is the mode all DAC's should use. It's the only mode that allows close to transparent sound.
In this mode, all timing information is decided by the USB clock, neither the host or peripheral has any say in the timing. To my knowledge, no consumer USB Audio interfaces use this mode.
It boils down to:
Basically all of this boils down to current model USB DAC's being very competent when implemented correctly. There are still a lot of variables in play. So as always have a listen before you commit to any of these products.