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Comparisons with other interfaces

Posted by John Swenson (A) on June 15, 2011 at 15:04:24

In Reply to: How can USB performance impact audio quality? posted by mr.bear on June 12, 2011 at 13:13:55:

After reading what has been said on this thread about USB and other interfaces I'd like to make some comparisons based on the same issues I brought up with USB.

First off some definitions, note these are mine, not industry standards:

First order effects: these are jitter and noise from PLLs, frequency sysnthesizers, noisy power supplies etc. They have strong impact on final sound.

Second order effects: Noise on power and ground traces on a board due to devices running on that board and connected to it through interfaces. Note I'm making a distingction here between noise of the power supply itself and noise on board traces due to devices running off the supply.

In many boxes the first order effects are much greater than the second order ones, almost completely swamping them.

So, on to the comparisons. In my first post I was focusing on USB devices that had almost completely eliminated the First order effects, in this case the second order effects are now clearly heard. Its not that things such as asynchronous interfaces are implemented poorly, quite the contrary, they have done their job well and almost completely eliminated the first order effects. Now you can clearly hear the second order effects.

Now compare this to other interfaces. Firewire, ethernet, PCIe, PCI, ALL have very similar second order effects, why are people not screaming about them?

Most firewire implementations (not all, but most) still have very high levels of first order effects. One very popular FW interfece has 700ps of jitter on its local clock, they are even proud of that since their competitor has 1200ps! The second order effects due to timing variations on the bus are completely swamped by the first order effects so you don't notice the dependancies on computer timing.

Ethernet implementations are split, some still have fairly high first order effects and some have quite low first order effects. BUT ethernet receivers are usually implemented with a general purpose processor running linux or some other OS which has a lot more going on than JUST decoding audio ethernet data. The result is quite a bit of second order noise. The contribution of timing variations on the bus is still there but is being swamped by the noise generated from all the other processing going on.

PCIe and PCI sound cards are also affected by second order timing on their busses, but they are so tightly coupled to the noise from the computer itself that they are again being swamped by that noise.

From this analysis it seems to me that far from being the worst as has been postulated by several posts on this thread, USB is now the best of the computer interfaces. The engineers have worked hard and have almost completely gotten rid of the first order effects, and significantly decreased the second order effects. This is bad?

The situation HAS brought to light that things are still not perfect, that there is more work to be done in decreasing the second order effects. But even so one of these devices when plugged into a computer that has not been optimized is still MUCH better than devices that still have significant amounts of first order effects. Yes you can tweak the computer and get even better sound, but how does that mean USB is inherantly BAD? It just means that we are in an intermediate stage of development.

Until designers had gotten rid of the first order effects the impact of the second order effects were not taken into account, now they are getting looked at, some designers are working on ways to deal with them. Others are working on ways to tweak the computer to decrease these second order effects with existing equipment.

Eventually the designers will succeed and these computer tweaks will be unnecessary, until we find out about the third order effects, whatever THEY may be!

John S.

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