Part 5 – Asynchronous Sources – USB & Network Audio
Audio sent over an asynchronous format (such as streaming to a smartphone via Spotify, playing content from a NAS via Roon, or playing music from a computer via USB) is, to an extent, the exception to the rules stated in the previous posts, in so much as jitter is not a factor for the audio data, at least until it reaches the endpoint and is converted back to the relevant format (such as PCM or DSD).
With network audio, the interface which is used to send audio data over a network is called TCP (Transfer Communication Protocol). The data which is to be transmitted from one place to another –
in this case a piece of music – is split up into multiple ‘packets’. These packets contain not only the data itself (the ‘payload’), but tags on where it has come from, where it is going, how many packets it is part of and how these packets should be reassembled to get the original data back unchanged.
Take, for example, a track from Qobuz being streamed to a dCS Bartók DAC. If a packet of data is lost or compromised, according to the TCP interface, the Bartók can simply request that packet again. When all the correct packets have been received properly by the Bartók, they are unpacked back to the correct data format (PCM, for example) and buffered before being fed to the DAC. This stage, the unpacking and buffering, effectively removes any timing link between the TCP packets and the resultant audio signal. (Read that sentence again, as it’s very important.)
Once the data has been buffered in the Bartók, the factors discussed above become relevant again. The data is now being directly dictated by the Bartók’s clock and as such, jitter becomes a factor. The accuracy of the Bartók’s clock then controls when the DAC converts the samples back to analogue voltages, so has a direct impact on audio quality. Until it reaches that point, however, jitter is simply not a factor from an audio perspective.
Asynchronous USB audio works in a similar way. There is no timing link whatsoever between the source, such as a computer, and the endpoint such as a Bartók. It does not matter if, while the USB data is being transferred, the bits are not perfectly spaced as a clean square wave. Provided the bits are received by the Bartók correctly (a 1 isn’t misread as a 0, for example) the timing is largely irrelevant. This is because, as with network audio, the data is buffered before being fed to the DAC. It is not until this point that timing becomes a factor, as at this point, it has been converted back from USB format to digital audio (eg PCM or DSD).