Dgarretson: great example of a controlled experiment. I wonder how much the improved clock circuitry reduced radiated emmisions, reduced injected noise into the ground planes, vs the degree it reduced actual clock jitter.
I'll always have a hard time understanding why nanoseconds of clock skew difference matter in most modern cd digital receiver front ends. I mean the digital data is clocked off the substrate then buffered (fifo) in the main cd dsp; inside this dsp, a new clock is generated and sent across the interface to the DAC. This buffer is pretty much crucial to good cdp function. I don't understand how jitter, as long as the set up and hold times of the actual registers are met, can be any kind of problem. I have never done a clock mod; do they instruct the modder to cut the (typical) crystal clock osc and insert the new clock signal there? is this only one point or do they also handle the clock to the actual DAC.
Another key feature to modern cd dsp chips is that the actual word (or frame) is entirely jitter free. Perhaps one could argue if the jitter is so horribly poor that individual bits are mis-sampled as they are clocked into the DSP, (then the overall value of the word is off), but that would have be a seriously poor design. Personally too much marketing emphasis is placed on jitter, I feel. The standard approach by all high end cd player designs is to implement an off the shelf DSP device (the thing that sits very close to the spinning disc and is responsible for servo, and data sampling, as well as microcontroller interface) and an off the shelf DAC. Some play games intermediate stages to decouple clocks and reduce skew, but in the end it's really just designing with building blocks that have existed since the 80s. Jitter is one aspect to all this but I think its contribution to the overall distortion spectrum is very miniscule w.r.t. all the other links in chain from pits and bits to volts.
I don't doubt that anyone hears a difference from a mod, but it's hard to distinguish between REAL differences and perceived ones based on expectations (after all the time effort and money went into it there better be a difference right?!). Anyway these are my personal experiences. It's all good fun.
