Computer Audio Asylum: Re: Confused about jitter and other things. by John Swenson

HI twystd,
yeah this can be a little confusing for those starting down this trail, I'll try and shed a little light on it, there will be some opinion in this so others will come to some different conclusions.

There are several different ways to get music out of a computer:

soundcard analog outs
soundcard digital (coax or optical) to external DAC
USB or firewire to external box, analog outs
USB or firewire to external box, S/PDIF out to external DAC
USB or firewire to external box to S/PDIF to internal DAC
USB or firewire to external box, I2S to internal DAC
USB or firewire to external box, I2S to external DAC
Ethernet to external box analog outs
Ethernet to external box, S/PDIF out to external DAC

As you can see only the first two have anything to do with a soundcard, all the USB or ethernet options do not need a soundcard in anyway. I think the confusion comes in that some people call certain USB input external audio boxes a "USB soundcard" because as far as the software on the computer is concerned they behave like a soundcard, but they really are external boxes connected by USB cable to the computer, there is no card that goes in the computer case. There ARE some hybrid "soundcards" that have a card that plugs into the computer PCI bus, that have their own cable that goes to their own separate box where the analog ins and outs are located. Some of these have the analog circuitry in this external box which radically cuts down on the noise they pickup inside a computer case.

Jitter is a complex subject, far too much to cover in one post (I've posted a lot on the subject here if you want some light after dinner reading (BIG grin!)). The up shot is that there is no such thing as a system that eliminates jitter all together, this cannot be done, anyone that says so is lying or doesn't know what they are talking about. Some methods have more than others and some types of jitter produce different sonic effects. The amount and type of jitter is different for each of the above.

Different people have different priorities as to sound quality tradeoffs so each will have a different preference in the above matrix. There are also VAST differences in implementation in each category. A properly implemented well done version of any of these will sound better than a cheap implementation of any of them. The fun part is: given the BEST implementation of a certain category is it going to be better than the BEST implementation in another? Even that one is touchy because the field is so new that we are nowhere near the BEST posible implementation for most of these categories. The only one that is even close to being "mature" is the soundcard category.

Now as to MY preferences. There are some very good sounding sound cards out there, but for me none of them have come close to the best sound that can be achieved by very well done external DACs. Just because they don't have a "digital cable" does not mean they are jitterless, these still have audible jitter, with the best of breed having quite low jitter. Its their analog sides that I don't think are as good as the best external DACs.

I have a big problem with S/PDIF, every time I have worked on something with S/PDIF, when I get rid of S/PDIF I get better sound. This is why I personally do not prefer any of the above methods that have S/PDIF anywhere in the mix. No I do not believe that anything with S/PDIF is inherantly garbage, but that the S/PDIF is preventing the part from achieving its true potential. It will sound BETTER if it gets its bits some other way.

This led me to USB interfaces, they also have jitter, but its very different than that of S/PDIF, and for me at least I vastly prefer the sound of a very well done USB interface to that of a very well done S/PDIF interface.

This brings up the ethernet approaches, the most prominent being the squeezebox. These have the potential to be extremely good but have not quite reached that level yet. Part of this is due to there not being an official standard for this yet. The slim protocol has become a defacto standard, but it is not implemented in any OS so it needs special server software, it cannot be used as a "soundcard" with any audio application on the computer in the way that a real soundcard or USB can.

The I2S is an interface developed to send audio data between chips on the same board, such as the output chip of a CD drive and a DAC chip or an S/PDIF receiver chip and a DAC chip. Yes it has a separate clock (actually clocks) but that just means that the interface itself is not adding a significant amount of jitter on its own as S/PDIF does. Its still at the mercy jitterwise of whatever is generating those I2S signals. If the I2S comes out of an S/PDIF receiver it doesn't some how clean it up, it just doesn't make it worse.

It sounds like you are planning on geting a DDDAC USB setup, these are very good, no I don't think they are THE BEST its theoretically posible to make, but for actually available interfaces available right now they are right up there in the very top tier. I would not second guess that decision, go ahead and get it and enjoy the music.

I hope that helped some.

John S.