USB Cables, Digital Signals, and YOU! Pt. 1 - Page 3

Originally Posted by oddity

A Quick Note: Bits n’ Bytes

The deficiency in this interface is that it embeds the serial clock in the serial data-stream in order to achieve one-signal cabling. A superior interface would have included both a clock and a data signal, but we don't have this, so we live with S/PDIF. The S/PDIF interface must encode the data and clock into a single signal and then at the destination recover the clock(s) from the data-stream. The process of recovering the clocks can introduce new jitter since there is usually a PLL involved.



Jitter and USB
There is much misinformation on the forums about USB for audio streaming. USB is a fairly jittery interface on it's own. Some of the integrated circuit devices that were created to provide easy plug-and-play USB audio interfaces don't do enough to reduce USB jitter IMO. Many manufacturers adopted these plug-and-play devices to quickly and cheaply add USB to their DAC products. Unfortunately the less-than-stellar reviews that ensued had some of these manufacturers regretting these decisions. This gave USB a bad name in many circles.
Fortunately, there are other low-jitter USB interfaces available now that not only support 24/96, they even compete with the best CD playback devices. In 2009, I believe we will see USB support for 24/192 and even lower-jitter interfaces. USB is IMO the wired audio interface that will be most prevalent in the near future.




The Really Technical Parts: Isochronous Transfers

The Terrors of the Isochronous Mode

We have a problem.

It is a problem with a USB mode: in the adaptive isochronous audio transmission mode, the receiver has to determine the bit rate. This means that the bit rate is unknown prior to the time the data arrives.
The bit rate cannot be known prior to actually observing the packet.

Another terror of USB is that, according to the specification, it would not be unusual for the bit rate to change when the operating system is busy. Since the packets arrive on 1 kHz intervals, the PLL must lock within 1ms. In most PLLs, if we say that 1 kHz fluctuations are clearly audible and decrease the gain, we cannot track! Terror of terrors, we have just bumped into a brick wall.

In reality, fluctuations in the time domain will probably result in an unpleasant listening experience. This is probably because they are delaying the lock-up time in order to reduce the jitter distortion.

Also, for isochronous USB data, a buffer is necessary for the time between the beginning of the packet until PLL lock, so the PLL lock-up time is reflected directly in the chip cost. The more audio quality is pursued, the longer the necessary buffer and the longer the time lag when playback begins. On the other hand, if the time constant of the loop filter is increased, a large RC is necessary and the chip area increases (recent progress in semiconductor technology has brought about minimization of digital devices, but analog devices have not changed).