If your John S is John Swenson, please provide a link.
The I2S specs:
http://www.nxp.com/acrobat_download2/various/I2SBUS.pdf
Thanks for the link.
Yes, it is from John Swenson.
Link
Quote from John
"As to how this is important I'll cover a scenario with USB, I'm assuming this is already an extremely well done async interface.
A packet comes in over the cable and goes in the receiver chip. This chip does a lot of stuff with this packet and eventually puts some bits in a buffer that down stream circuits extract using its super stable ultra low jitter clock. But what does all that processing in the receiver do? It generates noise on both the positive supply rail AND the ground plane. Any good design is going to have separate voltage regulators that prevent the noise on the positive supply rail from reaching any other circuits (such as the ultra low jitter clock, DAC chip, analog stage etc). BUT those don't work for the ground. If you are not EXTREMELY careful, some of that ground noise from the receiver is going to be on the ground pins of the other parts. If you just use a common ground plane you are almost guaranteed to have this. No its not huge, but its still there.
The interesting aspect of this noise is that if you look at the spectrum of it you see a strong 1KHz component, which is the packet rate on the USB bus. This falls right smack dab in the middle of the audio range. It gets even worse. In most circumstances the exact time at which packets are sent is controlled by software, NOT a low jitter hardware clock. This means that things like kernel scheduling policies, interrupt latencies etc have a HUGE impact on the exact timing of those packets. It comes out to close to 1KHz all the time, but there is significant variations on the exact time between packets. This causes that 1KHz component of the ground spectrum to spread out, it has "skirts". This "modulation" is in the low end of the audio range. There is a fair amount of evidence that this type of low frequency jitter can make subtle changes in the listening experience even when it is fairly low level. "
Unfortunately this can make USB DACs quite susceptible to all kinds of things that happen on computers. The player software used, what audio stack is used, kernel parameters, timing on memory chips, all kinds of things can cause subtle changes in the packet timing on the bus."
