External wordclock (Stupid question!)

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RuudNL

RuudNL

Well-known member
Joined
Apr 26, 2009
Messages
3,113
Location
Haule / The Netherlands
Manufacturers of external clock generators try to convince me that I need an external wordclock generator, to keep my two audio interfaces synchronized.
At this moment I use one interface as a master and the second interface as a slave.
I never had any problems, using them this way!
Of course the master interface has a crystal controlled internal clock.
Now: what could be the advantage to use an external wordclock generator?
(As far as I know a crystal generates a stable and jitter free frequency, there is a clock divider inside, giving a 50% duty cycle)

But as I mentioned in the subject title: this is probably a very stupid question... :p
 
RuudNL said:
Now: what could be the advantage to use an external wordclock generator?
(As far as I know a crystal generates a stable and jitter free frequency, there is a clock divider inside, giving a 50% duty cycle)

But as I mentioned in the subject title: this is probably a very stupid question... :p
Click to expand...

It makes the clock sellers wealthy and you a little less so.  8)

JR
 
The short generalized answer is that most external clocks are a waste of money.  For large complex digital setups they can be useful but for the typical home studio they are no better and sometimes worse than the internal.

External clocks are connected to your converter through a PLL, the quality of that PLL determines much.  Cheap converters unsurprisingly have cheap or poorly designed PLLs..  Good converters already have a decent master clock built in, and short local runs for delicate clock signals are always preferable.  So even though the timing integrity/jitter performance  of your external clock might be better, it might not be by the time it reaches your converter chip.


 
The PLL is in the device receiving the clock, so the quality of the clock is less important than the quality of the PLL.
Generally crystal clocks will outperform the PLL, so it depends on the quality of the receiving unit.  And I agree, in most home studios it would not be an issue. Try thinking up some tests where you can transfer a file with and without external word clock, and then cancel them. Anything left will be PLL error and setup error. Try and use a digital interface to transfer the file, otherwise if you transfer D/A and then A/D, you will show up converter errors.
 
radardoug said:
The PLL is in the device receiving the clock, so the quality of the clock is less important than the quality of the PLL.
Generally crystal clocks will outperform the PLL, so it depends on the quality of the receiving unit.  And I agree, in most home studios it would not be an issue. Try thinking up some tests where you can transfer a file with and without external word clock, and then cancel them. Anything left will be PLL error and setup error. Try and use a digital interface to transfer the file, otherwise if you transfer D/A and then A/D, you will show up converter errors.
Click to expand...
If you have any kind of error with a digital transfer then you have got more problems than a poor quality word clock!

The only reason I can see for using an external word clock source is if you have a huge system with lots of separate converters trying to run in parallel.

As others have already said, the PLL in the receiving device has to be exceedingly good to do a better job at recovering an external clock than simply using the internal clock. Otherwise, the jitter actually gets worse. I heard Hugh Robjohns of Sound On Sound magazine say in a panel session that he'd done experiments with full-on lab equipment and a whole load of external clocks and converters. Only one converter out of the whole lot did not degrade its performance when using an external clock. Note - did not degrade. It didn't get better, it was merely no worse than using its own internal clock.
 
Thank you all for your replies!
It confirms what I already thought.
Of course the PLL is the most important factor in the clock handling.
The clock frequency itself can be extremely stable, but if the PLL causes jitter you are lost!

By the way: is an external clock generator not as simple as a crystal oscillator followed by a couple of dividers and a line driver?
And why do most of those external clock generators cost $1000 ++ ? :p
 
RuudNL said:
Thank you all for your replies!
It confirms what I already thought.
Of course the PLL is the most important factor in the clock handling.
The clock frequency itself can be extremely stable, but if the PLL causes jitter you are lost!

By the way: is an external clock generator not as simple as a crystal oscillator followed by a couple of dividers and a line driver?
And why do most of those external clock generators cost $1000 ++ ? :p
Click to expand...
The most stable ones used in telecommunications are (or were, when I was working in that field) DOCXOs - Double Oven Controlled Crystal Oscillators. So the "simple" crystal oscillator is inside a temperature controlled chamber which itself is within a temperature controlled chamber. Accuracy and stability of parts per billion. I don't know if audio gear goes to such extents, but I can't imagine that it is cheap.
 
As always, it is possible to take things to extremes!
The DOCXO is a very nice concept, but the question is: do we really need this for audio?
In my audio interface there is a crystal timebase, not even in an oven.
But still the quality is excellent and the thing never gives me problems!
(A good crystal oscillator can have a stability of 30 PPM)

Aren't we confronted here with 'creating a market'?
 
There are certain applications where external clocks brings benefits, I don't think any major quality benefit. I don't think I have to name the brand, but is a big one which sells both the converters and external clocks better than the ones inside the converters, plus it allows the setup to do different things, which you couldn't with the converters alone.

Surely a better clock (more precise, lower jitter) to the silicon converters is a good thing, I guess that could bring noticeable differences, but as mentioned adding an external clock doesn't mean the chips are getting any benefits.

JS
 
A lot of people seem to think that using an external clock (like a Black Lion for example) on lower end gear improves the sound. Is that not correct then? They seem to be convinced to have an audible difference...
 
Bomper said:
A lot of people seem to think that using an external clock (like a Black Lion for example) on lower end gear improves the sound. Is that not correct then? They seem to be convinced to have an audible difference...
Click to expand...
Expectation bias and post-purchase rationalisation.
 
I remember reading  a discussion about this some time ago. It was Apogee vs Lavry. 
I think it was at a mastering forum, maybe it´s still online.
What I kept  after the reading  is that more than 96Khz is useless marketing bullshit. Also external clocks for better sound .
As many have said it has to do with the difficult design of a great PLL. It is much easier to have a great sound from an internal clock than slaving to an external one.
I remember one quote saying that if your converter sounds better slaving to your external clock then your converter have serious design flaws.
Great reading and a funny heavy weights fight
 
For years I was under the assumption that external clocking is more marketing than reality.
But few weeks ago I was troubleshooting some bad BNC cables, and since I have a setup consisting of 4 converters and a soundcard, I tried slaving all of them to one particular converter that is used when recording mixes from my console. And it was night and day difference!!
Usually I had my soundcard generate clock and all other converters were slaved to it. That way I could use one converter on adat sync instead of BNC wordclock. But when I changed to AD converter that is used to record my console output, the stereo image changed into more convex soundstage. It was mostly noticable on reverb tails and generally how reverbs spread into stereo.
So I would say that I sort of believe in this whole clocking story. At least that it makes sense to have main AD converter serve as a master clock to other converters.
Btw, my setup is now considered semi-pro. Years ago it was great but now it could be even called obsolete. So clocking in this system is not par-excellence by today's standards. It's two Swissionic converters (AD and DA) and two Creamware AD-DA units. All on Creamware Scope system... Great stuff if it is year 2002, but not in 2015 anymore...
 
This makes sense. The wordclock coming from your sound card is probably the worst quality one you had.
shot said:
For years I was under the assumption that external clocking is more marketing than reality.
But few weeks ago I was troubleshooting some bad BNC cables, and since I have a setup consisting of 4 converters and a soundcard, I tried slaving all of them to one particular converter that is used when recording mixes from my console. And it was night and day difference!!
Usually I had my soundcard generate clock and all other converters were slaved to it. That way I could use one converter on adat sync instead of BNC wordclock. But when I changed to AD converter that is used to record my console output, the stereo image changed into more convex soundstage. It was mostly noticable on reverb tails and generally how reverbs spread into stereo.
So I would say that I sort of believe in this whole clocking story. At least that it makes sense to have main AD converter serve as a master clock to other converters.
Btw, my setup is now considered semi-pro. Years ago it was great but now it could be even called obsolete. So clocking in this system is not par-excellence by today's standards. It's two Swissionic converters (AD and DA) and two Creamware AD-DA units. All on Creamware Scope system... Great stuff if it is year 2002, but not in 2015 anymore...
Click to expand...
 
Multiple converters here, Apogee AD-16X, DA-16X  (32 I/O's) and a AVID digtal interface, all slaved to a single external clock, using high quality cables of the exact same length. The cables made a big differnce in the soundstage after upgrading from a hodgepodge of different lengths and differing quality clock cables.

The Apogee Big Ben offers an additional advantage of being able to do vari-speed which comes in handy from time to time.

In your situation, I don't see any advantage to an external clock per say.

I have found that different clocks affect the sound in different ways as borne out when you switch your clocking between devices and maybe worth some experimentation.

Mark

 
Have you considered using S/PDIF signals to sync your system?
As it's running at a faster rate, the PLL's can update, lock and stabilize much quicker.

Most S/PDIF receivers will lock and provide a wonderful set of MCLK, BCK and LRCK with such a signal.

cheers

/R
 
Bomper said:
A lot of people seem to think that using an external clock (like a Black Lion for example) on lower end gear improves the sound. Is that not correct then? They seem to be convinced to have an audible difference...
Click to expand...

It is almost never correct.
 
First: It is not a stupid question.

Second: You have a performance boost with a master clock if you have a lot of different digital equipment in you're setup. So syncing 1 machine with an external clock is not a good idea for you're wallet.....

 
there is a nice bench test made by the guys at Sound on sound. They test the frequencies with external clock, internal, etc. etc. etc. And the external clock didn't worked as good as sellers say. So keep your setup as it is. :)
 

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