Wordlength – (Best Bit Depth) and Sample Rate

Robert Katz Leave a Comment

From: Stephan Cahen
1.) I understand that the A/D converter shouldn`t be clocked via an external Masterclock using its PLL. My Lake People F27 has an AES sync input. Does the AES sync format affects the devices` PLL ? If I want to clock the A/D with the rest of my equipment, I have to use the AES sync in – right

Anytime you clock an A/D converter externally, a PLL is involved. In simple terms: AES sync is potentially much dirtier than Wordclock sync. If you must use AES sync, do not feed program down that sync line. Try to use an AES black generator direct from the generator (e.g., Aardvark, DCS).

2.)  Are AES sync interfaces better than clocking via common BNC connectors ? My SADiE also has one…

Answered above. Wordclock sync is the next most stable (after A/D on internal sync). AES sync is dirtier.

3.)  Master-WC units like the dCS 992 have a clock accuracy of 1 ppm. This depends on the temperature and age of the device. Is there a noticeable difference between devices with such an accurate clock and others with slightly other settings?

Clock accuracy and clock jitter are only slightly related. I’d rather have a unit which has extremely low clock jitter and a little less clock “accuracy”. In other words, if one clock is centered at 44,100.001 but jittering all over the place and another clock is at 44,100.1 and extremely stable, I’ll take the latter any time. Many many people confuse clock accuracy with clock stability.

4.)  John Etnier of Studio Dual suggested the Aardvark Aardsync Master WC unit (thanks, John !). The german distributor wasn`t able to tell me exactly how this device operates, but said, that there wouldn`t be any VCO/VXCO in it, but “something more stable and accurate, like a DSP or a frequency generator for very high frequencies”. It sounded like bullsh… Who knows more about the Aardsync?

All the sync generators will use a stable crystal oscillator for generation. VCXO’s are methods of PLL; they are required for low jitter (phase noise….) when clocking externally. So if the sync generator has an external sync input, it will probably be a VCXO. Kind of defeats the purpose of low jitter, though… the whole idea is to have a very stable, and accurate (non-PLL) crystal sync generator.

I understand that it can generate very rare frequencies I don`t need (like 45.937, 48.048, 42.336,…), so perhaps there lies the difference in generating the signal.

Even a divide-down circuit, if implemented poorly, can create jitter, so I’d much prefer multiple crystal oscillators, each of which is turned off by a careful switching or grounding arrangement when not in use (to prevent interference). But I’m not an expert on that, and if Julian Dunn is reading this, or someone else with his credentials, he could tell you his preferred method of creating a sync generator which has multiple sample rates.

5.) OK. SDIF-2 is preferred in the pro`s world. But if we handle AES signals, why not use AES-ID that has significant advantages over that bloody XLR interface?

There is no technical difference between “regular AES” and AES-3ID except the level, the impedance and whether it’s balanced or not. AES-3ID may actually encourage some amount of line frequency jitter passed over ground loops between equipment because it is an unbalanced interface. But so is wordclock! The main difference is wordclock is not a modulated scheme, and AES is a biphase-modulated scheme with an imbedded clock, so the chance of creating complex jitter-causing situations is far less with wordclock.

Personally, I’d try to find an integrated A/D/A converter that runs on internal sync and whose sound I like. Confirm that the D/A clock is connected directly to the A/D’s crystal oscillator. Then run the wordclock output of the A/D to a wordclock distribution system, and that’s that.

In theory, that would be the lowest possible jitter situation. “All other things being equal”. (My favorite copout phrase).

Best Wishes,
Bob Katz

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