Digital Audio Interface Jitter

Abstract

Jitter is the short-term deviation of a digital signal from its ideal position in time. Some common issues know to produce jitter in currently used digital audio interface formats were examined and multiple implementations of a Universal Serial Bus (USB) audio interface were designed with the intention of creating a device free from interface jitter. Using the three standardized clock synchronization mechanisms in the USB protocol for isochronous transmissions and a selection of suitable clock sources, USB audio class devices were created for which jitter measurements then were performed. The results were compared with jitter audibility thresholds from three studies containing listening tests. While all implementations were functionally acceptable, their jitter results did differ. For the two isochronous synchronization modes of USB that require a continuously adjustable clock source on the receiving side of the interface the jitter issue consists of two parts. Periodic adjustments of the clock signal are in itself a source of jitter and the way in which an adjustable clock source is constructed is another. The initial core idea was that a USB audio interface using isochronous transfers coupled with the asynchronous clock synchronization mode and a fixed frequency clock source would be able to provide an interface in which no additional jitter on top of the inherent jitter level of the source clock would be added by the transfer of data over the interface. The two fixed frequency clocks that were used did however not perform any better than the results of the best adjustable clock source and when they were attached to the test system their jitter levels increased even further. Analysis of the jitter measurements point in the direction of asynchronous mode being preferable for lowest possible jitter levels but the results are not completely unambiguous and jitter levels below the lowest recorded hearing thresholds were also achieved with one of the other synchronization modes for isochronous USB transfers.