USB vs S/PDIF: in simple terms, what does the computer do differently?

My (perhaps misplaced) assumption has always been that the computer is 'doing less' to the audio data when it sends it out over USB as opposed to optical or coax.

The two basic aspects I'd guess are:

1. changes of state, eg. to convert from electrical to optical

2. changes of format, eg. WAV to S/PDIF, S/PDIF to i2s etc.

I'd also guess that both of the above can be done entirely transparently.

Coming before both of the above is what the OS does before it gets sent out which perhaps would be identical irrespective of the output method, or not?

 

This is misguided 'analogue' thinking, imo.

Data is data. - not music.

Computers don't care whether it's audio or not. Data can be transmitted and translated thousands of times with hundreds of different propagation methods, formats, and systems, without any error or corruption, viz internet distribution of lossless audio files.

Data is data, till in the case of a music file, it gets to the DAC, - then it becomes analogue,

- that's it !

JC

 

I think you would have to examine a little more closely the form of the USB and the type of synchronisation ,technically Async offers advantages given that everything else is implemented properly.
Keih.

 

So how do normal external audio interfaces communicate the data to and from the computer? There have been multi-channel in/out pro audio interfaces for years before the hype about async for UBS DACs. Where all these external interfaces using the standard synchronous methods or did the audio interfaces buffer the data a little so they can send and receive from the computer without issues of jitter?

 

The difference between the two is SPDIF is an entirely dumb one way system and it throws data out one end. The receiving end has to determine what it is receiving and lock on and process accordingly.

USB in its simplest form does communicate both ways and can simply say "yes I am connected, go ahead" or, in the case of asynchronous "yes I am connected, go ahead, too much, too little, just right". The difference is that in SPDIF and the first form of USB, the source is the clock. In a PC this might not be a great situation. In asynchronous the clock is at the distand end and controls the data flow so it is optimised.

Laymans terms so forgive me if it blurs a few points.

 

Thanks for that (happy with the blurring).

But, I was really asking about what happens before the data leaves the PC.

 

Technically Async protocols whether USB or FireWire are the best method of sending timed data, as CM states, the DAC clock is in sole control of the flow of data, nd with the proviso of everything else being competently designed should result in extremely low interface jitter.
Keith.

 

This post from Dominik, a USB software programmer, over on PFM is very informative I think:

http://www.pinkfishmedia.net/forum/showpost.php?p=1280789&postcount=1521

 

While we're on this subject, is FireWire always async?

 

Does USB 3 change anything to the advantage of those wanting to stream audio?

 

USb 3 is a specification for software and interface, how to connect to the PC, there's currently nothing in the USb spec for audio that wasn't in the 2.0.

USB 2.0 is capable of sending data faster than any current USB dac can convert it.

There is a new connector format under 3.0 with extra pins and a tighter manufacture spec for the cable, better tolerances, RF rejection and a closer impedance matching due to the higher data rates. None of which matters to USB audio as they should have no effect on the slower singals going to a USb dac (384khz max currently)

At some point someone might design a clever USb dac that needs 3.0 to send higher data rates, but frankly I can't hear the difference between 192 and 384khz- so why bother.

 

And where will Copper Peak or Light Peak take us?

http://en.wikipedia.org/wiki/Light_Peak

It could push all else aside.

 

Wherever that my be it won't be for a while!
Keith.

 

Less than five years?

Better get those USB to S/PDIF thingies moving Keith.

 

Brian Hi, I dont believe I will be selling many converters in two years time let alone five, the rise of the Async DAC will see to that!
Vb Keith.