Home Computer Audio Asylum

Music servers and other computer based digital audio technologies.

RE: Ok Far Enough

"What IF both the Dac and the transport were slaved to the same word-clock?"

If the word clock were external to both the DAC and the transport and separately buffered outputs were used then the jitter on the cable from the transport to the DAC would not affect the audio quality. However, the quality of the output would depend on the quality of the clock signal received by the DAC. In other words, the sound might not vary much with changes in the transport but it wouldn't be good unless the word clock was very high quality since the word clock would definitely be in the signal path. If the word clock is a separate wire that is isolated from other wires it won't have any signal dependent jitter, which is a problem with systems where the same wire is used for both clock and data. However, there will still be noise of various sorts, so it is unlikely that the word clock as received by the DAC will be as clean as the word clock at the actual oscillator. This problem can be avoided by locating the oscillator in the DAC.

Unfortunately, there is another problem with using an external word clock that affects delta-sigma DACs and other upsampling converters. These do the actual digital to analog conversion at a higher sampling rate than the word rate. If an external word clock is used then the received word clock must be multiplied up to the final master clock rate which will be some multiple of the word clock rate. There is no way to do this frequency multiplication without employing heroic measures (expensive, not completely effective).

The correct way to implement a system synchronized by a word clock is to run a high quality fixed crystal oscillator at the master clock frequency, and send this clean signal directly to the actual digital to analog converter. At the same time, buffer this oscillator signal and send it to a divide circuit (a digital counter) to divide the high master clock rate down to the output word clock. Then send this word clock out of the DAC to the transport. A DAC that works this way will have a word clock output, but not a word clock input.

I don't believe it is possible to tell whether a given DAC has implemented the clock architecture correctly from reading spec sheets and manuals. At the very least one has to open up the device and do a little reverse engineering, something that can be difficult these days due to a high degree of integration, programmable gate arrays, etc. One can look for crystals and ascertain their frequencies, etc. (I haven't reverse engineered any audio devices, but I have reverse engineered data comm devices as part of patent lawsuits, which involved a combination of various system tests, measurements, reading of chip numbers with a flashlight and magnifying glass, Googling, reading various company literature, technical, legal, and marketing.)


Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar


This post is made possible by the generous support of people like you and our sponsors:
  Parts Connexion  


Follow Ups Full Thread
Follow Ups

FAQ

Post a Message!

Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.