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Re: FANTASTIC, Tweak of the Year!

64.0.142.226

Posted by wildmonkeysects on March 14, 2001 at 15:04:44

In Reply to: Re: FANTASTIC, Tweak of the Year! posted by Elso Kwak on March 14, 2001 at 03:54:02:

Well hey, thanks for the good news, glad to share.

Yes, I found that reducing the loop corner/bandwidth/rolloff by half and adding the 3300pF cap was a [ not the one true ] sweet spot. The 84xx vco is probably not the cleanest one around, so there are diminishing returns when reducing the corner further. Also there are issues with loop stability and peaking. Have not twizzled the 8420, which includes a datasheet 1 kHz loop, and may well have a cleaner vco, but that is conjecture at this point. Anybody out there try one yet? Some revisions of the data sheet had the "extra" 3rd order loop filt cap, I seem to remember, but crystal has not revised the data sheets for the "older" chips, to my knowledge. They should. Probably the bigger factor is the 3300pF cap, as the loop filt pin will translate probably tens of nanovolts of noise into audible jitter.

One can "ignore" [ well almost, there will be a _small_ amount of jitter feedthrough between two oscillators sharing the same ground plane, but not enough to cause injection locking in this case ] the mclk generated by the 84xx DIR when placing the new master clock and register/reclocker at the DAC. The new master clock should be whatever the transport requires, probably 384 fs, maybe 256 fs. If 256 fs, you should see the "ignored" mclk pin of the 8412 in synch with the "new master". If 384 fs, the "ignored" mclk pin of the 8412 will be at 2/3 freq, but in synck, ie not wandering; and there should be sufficient setup and hold times avail at the reclocker/register next to the DAC. If not, invert the polarity of the new master clock at the transport to shift a half clock cycle.

It's a mixed bag, genlocking, or master/slave clocking is more elegant, but more complex...

Now, there is one more way to "attack" the issue of correlated jitter that the SPDIF (sub)standard is soooo adept at generating that is not really commercially viable, but fair game in the arena of DIY:

A significant factor is the *low* frequency rolloff of the transmission medium shifting the level of the transition points. This is in some ways worse than jitter generated by high frequency roll off in the transmission medium, and contains components correlated down to the lowest frequencies of the audio data, including the envelope, which is typically subsonic ie the rhythum or phrasing of the audio.

Here is the fun part [well, to me at least]: by deliberately introducing a high pass / low roll off at least an order of magnitude higher than the highest anywhere else in the transmission chain, we can make it behave like a simple first order high pass, meaning critically damped, minimal undershoot. In other words, a differentiator. This is a big factor in keeping it from being commercially viable, preconceptions about noise sensitivity...maybe Peter Q who has both ears and b***s, and knows how to use them both would have success...
Anyhow, we have essentially removed sensitivity to low frequency roll offs in the transmission chain by transmogrifying the SPDIF [ also applies to AES with higher voltage levels ] waveform suchly: what was a positive going transition from -0.25V to +0.25V is now a positive spike with a clean decay lasting 10s of nanoseconds, and conversely what was a negative transition from +0.25V to -0.25V is now a negative spike with a similarly clean decay "upward" lasting 10s of nanoseconds. Then, we drive two fast comparitors set as a window comparitor who drive a set/reset flipflop, or a dflop via the set and reset pins producing a clean zero to vdd SPDIF signal which can directly [ no coupling caps needed ] drive the 84xx inputs from q and qbar.

I need to dust off and reverse engineer the part values I used [am I the only one who is lax about taking notes...] and _will_ post a schematic, unfortunately in about a month or so after work settles down a bit.

The bad news is that there is sensitivity to radiated noise, ie AC motor start surges, piezo stove starters will cause some bits to be dropped, and may cause secondary low frequency loops to slip for a moment. The good news is that it removes yet another layer of grunge and makes it all more solid and tight with more defined space around musical events.

The golden handcuffs call, back to work.

Share and enjoy...

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