I2S, Need some explanation please


#1

Not sure this is the correct place to ask this question, but can someone explain to me why I2S is purportedly a better method of transferring a PCM signal to and from a DAC? And if it truly is why then hasn’t the entire industry not adopted it?

Please don’t misconstrue this as a negative comment about PS Audio by any means, there are quite a few other companies that do use this technology.

Thanks


#2

In theory I2S is better than S/PDIF or AES/EBU (AES3) because it separates the clock and the data onto separate signals (as well as using balanced connections over the HDMI cable.) S/PDIF (etc.) use one wire for both the clock and data and hence they need to be separated on the receiving end. On the other hand I2S was designed for point to point within a case, not between cases and is more general than most people implement so there’s always a chance that two non-complete implementation don’t talk well to each other.

The separation of clock and data is important when you use a PLL (of FLL) with S/PDIF, etc. to recover the clock and separate the data. That recovered clock is subject to jitter because of the very design of the protocol and it takes a lot of work to get the PLL to no be confused by signals going up or down at the key points, there are only a few spots in the protocol that should always have aligned transitions so looking at just those will lower jitter somewhat… The DS doesn’t care about that at all (it doesn’t track the incoming clocks for either I2S or S/PDIF, etc. and doesn’t use a PLL or FLL) so in the case of the DS what really seems to matter the most is the solidity of the ground and how well the cable is shielded.


#3

Ted, I hate to bring this up, but your answer sounds like it came right off of page 2 of “Cloudy with a Chance of Meatballs”. Were you just reading that?


#4

Actually I think the technical guy explained that technical stuff quite well for non-technical people like me. Having followed every word of Ted here in the forum for a few years I’ve found has been helpful to this point in reducing his technical explanations to something that I almost understand.


#5

Thanks for the explanation and response Ted. Not sure what FLL means and perhaps its not all that important, but I’m really surprised by the comment regarding the DS and what is the most import aspect for its performance, that being the ground and shielded cables. Suffice it to say digital audio has become extremely complex over the last few years.


#6

Google is your friend…

PLL is “phase-locked loop” and FFL is “frequency-locked loop”.


#7

Think of the march of technology as the peeling of an onion - each new improvement lets us better hear the remaining shortcomings of the current tech. I took great pains to improve the jitter rejection of the DS, but even a perfect DAC will still cause changes in a system because the shielding of the input and output interconnects will change how much RFI is injected into the signals in the wire or radiated to the rest of the system. Similarly the groundloops formed by proper safety grounding change when you hook up any component. If your system is doing everything else well, these minute changes (in any audio component really) will become apparent. Since DACs by definition have high frequency inputs that means that no matter how you slice it you are subjecting your system to some more high frequency interference than perhaps, say, a turntable might.


#8

Interesting analogy, I get it for the most part. This notion of grounding and how it especially relates to digital might imply why an audiophile might be better off using components from the same manufacturer as much as possible in order to mitigate as many grounding variables as possible. Thanks


#9

I would have thought grounding was universally known common engineering practice implemented by all manufacturers, no? Therefore, I would have thought, a system comprised of components built by different manufacturers would all share grounding as a common design point? Perhaps there are different degrees and level of engineering detail of grounding in audio components.


#10

Perhaps you’ve never had to deal with a ground loop hum? But your right one would think that grounding to be a common practice but from my experience it seems as though different companies have different ground schemes.


#11

Thanks Ted, very useful explanation. I didn’t realize that I2S was intended for point-topoint in a case.

In the context of DSD/DSF given that there is DoP as a transport/wrapping protocol, is there even a need for an external I2S with support from various device vendors?


#12

There’s sort of a standardized different way of using the I2S wires for DSD, you still have the bit clock but now the other two lines simply carry the left and right DSD bits. Of course you can still use DoP which uses up 50% more bandwidth (but is compatible with more hardware.) There’s still the fact that some people have chosen one wire for the left channel and others have chosen the other wire. And some (PS Audio included) invert two of the three I2S wires, so you need to be careful as a user that you don’t end up with channels flipped or abosolute polarity wrong in DSD even tho they are correct for PCM.


#13

Thx Ted. 50% bandwidth is a lot, but better to have a lot more source options.


#14

DSD Channel reversal occurs with those HDMI to I2S boards out of China, discussed in another thread.