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Well one result that could come out of studying this phenomenon is that it's all (or mostly) about the converter's PLL, and (some? many?) converters will have the same performance on any external clock. In which case, you could get the same effect with the $100 Hosa as the $1300 Apogee. Or just use the clock out of your old digital reverb.
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From what I've noticed in my experiments, the clock signal's harmonic content has an effect on the converter's internal PLL response. Whether that means a converter's internal PLL sucks, I don't really care. Apogee, if I'm correct, uses a synthesized waveform for the C777 clock signal that's derived from two master clock sources, and because of this Apogee has greater control over what the initial content of the waveform is. That means they can sort of 'tailor make' a particular sonic response from a conversion process (to my ears, a Big Ben has a very big, smooth midrange similar to some vintage piece of gear).
The Hosa, I would wager, takes little to no precaution in preventing deterministic jitter from occuring in the division stage.
While deterministic jitter can certainly be added during signal injection into a given system, I found that it's not
nearly as much deterministic jitter as is created during signal division. When I say 'deterministic jitter,' I'm referring to that content which gives a sonically undesirable result, such as high frequency harshness or loss of image. There are other types of 'deterministic jitter,' such as artificially boosting or exciting harmonics withing the clock's frequency spectrum that I feel shouldn't be necessarily counted if they give sonically pleasing results. Perhaps that falls in line with some of the ideas presented in Digi's white paper.
Listen to the sound that an external clock gives--each one is different because of differing spectral content. Then, manipulate that spectral content in some way by altering some element of transmission, and you'll alter the tonal character of the conversion process itself.
My opinion is that we want to keep the phase characteristics of the spectral content in question as pristine as possible. We also want to prevent unwanted harmonic elements from entering the signal.
But I also think we should have some fun with this--if we can get different sonic results by artificially manipulating harmonic amplitudes, let's play around with it. After all, experimentation is a good part of what music creation is about!
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We had to have the BL clocks take OUT of some of a clients MOTU stuff due to WC issues. Not being able to lock up with other gear.
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Yes--that's why we stopped doing the internal clock for MOTU. Variations within internal clock management caused problems with skew on some revisions.
Matt
Black Lion Audio