opaqueice;148096 Wrote: > OK, I'm not an electrical engineer, so probably this is a stupid > question - but I still don't get it. As far as I understand, there is > a PLL locked to a crystal oscillator in the CD player. The data > stream coming from reading the disc will be jittery, as you're saying, > and I suppose the output of the PLL could be sensitive to that and > therefore jittery. This is the clock that controls the rate at which > data is read into the buffer. However it's not clear to me why you > would want or need to use this as the clock for reading out of the > buffer
This clock MUST be used for reading the data out of the buffer, and this is the case. Any other asynchronous clock would cause underrun or overrun. It MUST be a PLL based on the actual bit-rate from the spinning disk. This is a real-time system. opaqueice;148096 Wrote: > The point is the AVERAGE frequency of a PLL clock is very very close to > identical to the frequency of the crystal oscillator, or an integer > multiple of it (at least based on the little I know about PLLs). So > why would you use the output of the PLL as a clock for the DAC and/or > buffer readout, when you could simply use the crystal itself? A > problem (with buffer under or overflow) would arise if the AVERAGE > frequencies of those two clocks differed, but I don't see how they > could. I see what you are getting at and it makes sense. It should be possible to do this, however I'm quite certain that this is not how it is done. There are evidently other reasons why the PLL clock must be used. The "raw" clock in the transport or player is generally not used. It is probably because the buffer must be much larger than you think and will add cost and complexity to the device. I know this for sure. If you take steps to reduce the jitter on the CD itself, this makes a marked improvement in the sound quality, particularly the detail in the high-frequencies. This is consistent with reducing jitter. If this were not the case, and only improving the clock itself improved the sound, then I would be inclined to believe that the only the clock generator jitter was important. I have done a number of demonstrations at CES in the past 2-3 years where the listeners could hear the original CD, the copied CD, then a modded Transport and finally the same track only computer-driven. In every case the listeners were able to clearly hear an improvement due to reduction in jitter. This thread has taken enough of my time. I'm too busy to debate this ad infinitim.... Steve N. -- audioengr ------------------------------------------------------------------------ audioengr's Profile: http://forums.slimdevices.com/member.php?userid=8041 View this thread: http://forums.slimdevices.com/showthread.php?t=28621 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
