Quote:

Which is why digital audio is reframed every frame to the internal oscillator. Again, the part I'm unclear on is how the jitter is supposed to let the stream run long or short with regards to time. If you could shed some light on that - the actual practicality of what occurs to disrupt the waveform - do you suggest that some bits pass other bits on the wire (or in the fibreoptic bundle) to arrive at the receiver before others.
Bren R.



I had to do a little more research to get a better explanation.


Here is a good link which describes the problem and presents the solutions. Professional equipment uses a double phase lock solution, which requires a seperate clock sync line.

http://www.tnt-audio.com/clinica/diginterf2_e.html

As it turns out clock information is in fact transported with the data along an spdif connection. The problem is that the way it was designed, the clock sync information is flawed. What happens is the data is assembled and passed to the spdif transmitter which then grabs the clock info and inserts it into the data stream. He says that due to a structural problem with spdif (hence the stigma of spdif being associated with jitter), the data and the clock sync get messed up.

The first attempts at solving this problem, but also moving into very expensive equipment, the DAC needs to provide it's own internal sync which it then syncs up with the clock from the data (inserted directly into the stream rather than pulled directly by the transmitter). Alternatively, some designs allow for a seperate clock line. This of course means you need a transport that supports clock out and a DAC that supports the input.

In short, spdif by itself is not providing a proper clock. If you have an expensive DAC, it can resync the data with it's own clock. He also claims that cheap timing crystals have been proven to not be accurate enough for high end audio.

He also has a page describing jitter specifically (what I tried to do in my previous post). He has some pretty good pictures showing the overtiming and undertimings. It's worth reading.
http://www.tnt-audio.com/clinica/jitter1_e.html

Last edited by packetlosss; 01/06/07 12:39 AM.