R/E/P > Dan Lavry

Proper word clock implementation

<< < (2/45) > >>

Ozzie Bostic:
danlavry wrote on Sat, 02 October 2004 16:54
If
I am no fan of distribution amplifiers either. You can not beat:
Driver to point A (with a BNC T), than to point B (with BNC T)… at the end the BNC T is terminated with the proper line impedance (if the cable is 75Ohm, so is the termination). It is a cost effective solution that yields the best results.

BR
Dan Lavry      



Hello and Welcome Dan,

I am currently enjoy the wonderful sound and clarity of your Blue series converters great product.

My question is since your not a fan of distribution clocks; what is the maximum number of devices that should be connected in series via the BNC T method mentioned above before the clock source is degraded?

Thanks in advance.

Albert:
Another question: Would you use a BNC t-bar and terminator on gear that is already internally terminated? Would that yield better results, or would it damage the gear?

Also, would you use a BNC t-bar on gear that has a WC in and out, or would you pass through the gear, using it's own circuitry? And if you did use the BNC t-bar to bypass the gears internal routing, would you then use a terminator on the units WC output or rely on its own termination?

Thank you, this is a great thread.

bobkatz:
Albert wrote on Sun, 03 October 2004 11:49
Another question: Would you use a BNC t-bar and terminator on gear that is already internally terminated? Would that yield better results, or would it damage the gear?





It wouldn't damage the gear, but eventually it would load down the source wordclock so that the system would not function or would not function well.


1) I suggest you start with an oscilloscope on the wordclock generator. Measure its output, which should be 4 v p-p if possible. Any lower and some following devices will not like it, but most following devices are comfortable with as low as 1 v p-p---
TERMINATED!!! Hey, there's no standard! It's a wild wild west.

2) Then, put a terminator on it in parallel with the scope input. The level should drop by 1/2. If not, then the source is not truly 75 ohms. This is not a big big deal with wordclock frequencies, which are not so high generally as to be subject to reflection problems. I'm sure Dan can explain the compromises there, I don't know. It would cause some limit as to the length of the cable and how many devices you can attach.

3) Using this level as your goal (2 v p-p terminated), run the signal into each of your wordclock devices and use the scope to see if the device terminates the line. Chances are it does and you'll have to go inside the device and remove the 75 ohm resistor that's there, so you can then use the BNC-T.

If the wordclock generator is truly 75 ohm, 4 v p-p (unterminated), 2 v p-p terminated, then I don't see a problem using low loss coax for even 20-40 feet and having 3 to 6 high impedance (unterminated) dropoffs in between. Scope the end of the chain. If it's still a clean square wave at approximately 2 v p-p into the terminator at the end of the chain, then I'd say it's a win-win!

Quote:


Also, would you use a BNC t-bar on gear that has a WC in and out, or would you pass through the gear, using it's own circuitry? And if you did use the BNC t-bar to bypass the gears internal routing, would you then use a terminator on the units WC output or rely on its own termination?




The problem is that most word clock equipment has internally terminated inputs and will have to be modified (real real pity, since Dan's approach is absolutely valid). If you remove the 75 ohm resistor, then you will be able to use a BNC T, as this effectively bypasses the gear's internal routing. All you are concerned with at that point is the impedance of the gear's loading. Usually it's a small coil or transformer loaded by a 75 ohm resistor. Remove the resistor and (hopefully) it becomes high impedance, that is, negligible load on the rest of the circuit.

By the way, if the equipment to be slaved is NOT a converter, but simply a digital processor or DAT machine or whatever, then a wordclock distribution amplifier will do no harm, as the interface jitter is irrelevant. I use a Lucid Clk-X to feed wordclock to my Digital Timepiece, which is NOT feeding converters, to my DAT machine, to my TC Electronic System 6000, whose converters I am not using, etc. etc.

You could take the Clk-X box, remove the input terminator, and feed using Dan's BNC-T method through the input of the Clk-X, and feed (modified unterminated) critical converters like the Digidesign, and so on, through  the BNC T system, and use the Clk-X wordclock distribution box to feed the rest of things.

My Tascam DAT machine has a Wordclock through jack and some kind of automatic termination. How does that automatic termination sensing circuit work? I don't know!!!! I don't trust it, either, but I suppose I could sniff it out with a scope...

punkest:
danlavry wrote on Sat, 02 October 2004 22:54



The best way to clock a converter is with internal clock,    


A mediocre internal crystal implementation is going to outdo even a good external clock implementation.



It is true that the PLL does better when fed a less jittery clock, but that is just a tiny portion of the overall issue. As Bob stated, most of the burden is on the PLL. A Good PLL, inside the AD chassis should clean most of the jitter out.

Dan Lavry      




    I thought this was only the case with older PLL that does not have reclocking circuits. I heard that many new converters used the PLL to sync and then regenerated the clock to get rid of jitter.

    Anyway, I always try to slave all to the converter?s clock, as you pointed out, Dan.

     Bob pointed out that AES sync is more jittery that WC, how would you rate the jitter when synced over ADAT optical?

     I have apogee AD. Optical cable to feed converted signals and clock to computer interfaces. Word clock cable to feed clock to Yamaha 03d console that also receive signal but no clock optically from computer interface. Would it be a better way???

     Is there potential problem with feeding parallely two sources instead of the usual serial A to B, B to C ???

Hans Mues

bobkatz:
[quote title=punkest wrote on Sun, 03 October 2004 13:53]danlavry wrote on Sat, 02 October 2004 22:54




    I thought this was only the case with older PLL that does not have reclocking circuits. I heard that many new converters used the PLL to sync and then regenerated the clock to get rid of jitter.






Good PLL design really separates the men from the boys. Now Dan's not going to mention brands but I don't have an afiliation so I'll just tell you that Apogee's claims of low jitter just did not bear out. The performance did not match their claims.

All PLL's "reclock" so that's not the issue. It's "how well they do it", and how well the output performance actually works that counts. I have measurements of converters' jitter performance, both good and bad, in my book.

Quote:


     Bob pointed out that AES sync is more jittery that WC, how would you rate the jitter when synced over ADAT optical?





Doesn't ADAT really require a wordclock anyway?  If that's the case, then ADAT sync would be irrelevant, except to the extent that clocks and power and grounds can interact in a poorly-designed box.

Quote:


     I have apogee AD. Optical cable to feed converted signals and clock to computer interfaces. Word clock cable to feed clock to Yamaha 03d console that also receive signal but no clock optically from computer interface. Would it be a better way???

     Is there potential problem with feeding parallely two sources instead of the usual serial A to B, B to C ???




Hans, can you please repeat the above, more slowly  .  

1) What is the block diagram of your interconnections
2) What functions as the clock master?
3) What A/D and D/A converters do you use in this block diagram and what are their functions (monitoring, mixdown, etc.)

I know that many people feel this jitter "nonsense" is overrated. But if it weren't important, why would so many people be complaining about getting "great imaging and sound with one clock and real flat sound with another". So, "the sound" must be important to a lot of people, and discovering that clocking makes a sonic difference is enough to educate people as to why and how.

That's why getting jitter right is important! However, it is important to know when jitter makes a difference and when it does not. It does not make a difference with digital to digital transfers or digital processors (EQs, compressors, etc.).

BK

Navigation

[0] Message Index

[#] Next page

[*] Previous page

Go to full version