Hey guys

Was wondering what the difference really is between clocks. Things like the drawmer m-clock for example. What difference would this make to me or most project studio owners as opposed to our MOTU/M-Audio etc interfaces clocks?

thanks

--------------------
www.myspace.com/michaeldow www.myspace.com/portasoundband

if you use multiple devices (i mean lots, not just 2) they're essential.


if you use just one half decent one, they're not

if you use 2 half decent ones, they're still not usually needed...




the Drawmer M clock is a nice bit of kit, and if you need to synchronise multiple interfaces on multiple machines, using assorted standards, and maybe do some hardware unit SRC to sync in standalone devices that have no external clocking features, then it's a perfect tool

so for example to sync a MOTU logic rig, with an older TDM Protools rig (x256 superclock) , 3 external 2 track digital machines, and a pair of PC's running Gigastudio, and all the computers and DAW's connect via ADAT , and the 2 tracks over AES and SPDIF...

then the M-clock is perfect

if you just need to sync a Motu to an M-audio, use a BNC cable and be done with it.

AHh looks like i dont need it then, thanks for the info

Im just running a dual motu 2408 system. Looking to get a third though.

--------------------
www.myspace.com/michaeldow www.myspace.com/portasoundband

Funnily enough, I'm just finishing a major feature article on clocking, comparing a variety of clocks from the extraordinarily cost-effective M-Clock all the way up to the astonishingly expensive atomic clocked Antelope products.

The bottom line is that practical clock quality is actually more influenced by the design of the box being clocked rather than the box providing the clock.

As has been said, if you need to clock a lot of different things from a central clock source, and particularly if you have to work with external video clock references, then a master clock is a useful box to install.

But if you're just clocking an interface and some other digital unit, local clocking using embedded clocks of a wordclock cable is perfectly adequate and can't be bettered in most cases.

Hugh

--------------------
Technical Editor, Sound On Sound

Thanks Hugh.

Reading adverts and peoples ideas of products can sometimes make one think "oh wow that sounds great and looks amazing, it surely must make me better" When really, all you really NEED is the bare minimum most of the time

--------------------
www.myspace.com/michaeldow www.myspace.com/portasoundband

Quote Hugh Robjohns:

---

The bottom line is that practical clock quality is actually more influenced by the design of the box being clocked rather than the box providing the clock

---

I'm so pleased you said that Hugh, since that ties in with my experience as well. So many musicians get sidetracked onto buying better quality clocks without realising that simply by switching from internal to external clock, many audio interfaces exhibit significantly increased jitter, HOWEVER GOOD THE EXTERNAL CLOCK YOU'RE PLUMBING IN


Martin

--------------------
YewTreeMagic

Quote Martin Walker:

---

...simply by switching from internal to external clock, many audio interfaces exhibit significantly increased jitter, HOWEVER GOOD THE EXTERNAL CLOCK YOU'RE PLUMBING IN

---

Absolutely, and that article I mentioned (which will be in the mag soon) includes numerous test plots with a variety of well known converters to prove exactly that point once and for all!

Hugh

--------------------
Technical Editor, Sound On Sound

I'm very pleased about that Hugh, but can i ask that you remind people that in the scenario type that I outlined.... a decently equipped master clock is the correct solution.... i can see myself having all sorts of fun trying to explain to people why the article doesn't apply once you're syncing multiple devices....

and a goodly number of people still do have that scenario... composers especially..

or studios like ours, with multiple logic rigs, protools rigs, and guest rigs....


cheers



incidentally.... did you get my ansafone message.... new toys to hear....

Quote Hugh Robjohns:

---

Funnily enough, I'm just finishing a major feature article on clocking, comparing a variety of clocks from the extraordinarily cost-effective M-Clock all the way up to the astonishingly expensive atomic clocked Antelope products.

---

I hope you included the Audio Design SyncroGenius it's supposed to be superb and at a very reasonable price too.

--------------------
John - Sound-Link ProAudio
President - Federation Internationale des Chasseurs de Sons

As it happens I did include the A+D SynchroGenius -- it is a very nice bit of kit that punches way above it's cost.

And yes, I have tried to explain when and why it is a good idea to use a master clock, as well as when and why it isn't. And what a master clock can and can't do for the system as a whole.

Hugh

--------------------
Technical Editor, Sound On Sound

This is why I love Sound on Sound! Good solid reasoned reviews backed up with science, and written by sound engineer.

--------------------
madtheory creations
Synths and pianos for Kontakt

Yes, and now we know the conclusion, we don't even need to read it!

I want to see the graphs. Hugh, in the article do you explain how you made the measurements?

--------------------
madtheory creations
Synths and pianos for Kontakt

Of course

Hugh

--------------------
Technical Editor, Sound On Sound

............and on the 7th day he chilled.

--------------------
Samplecraze
Stretch That Note

Quote Martin Walker:

---

Quote Hugh Robjohns:

---

The bottom line is that practical clock quality is actually more influenced by the design of the box being clocked rather than the box providing the clock

---

I'm so pleased you said that Hugh, since that ties in with my experience as well. So many musicians get sidetracked onto buying better quality clocks without realising that simply by switching from internal to external clock, many audio interfaces exhibit significantly increased jitter, HOWEVER GOOD THE EXTERNAL CLOCK YOU'RE PLUMBING IN


Martin

---

That is indeed true. But there are some companies that use jitter to their advantage with their external clocks. I believe the idea is to use audible jitter in ways that are perceived as sounding pleasing to the listener.

So often people get obsessed with eliminating jitter at all costs, never considering the possibility of jitter not always being a bad thing. Just as analog distortion isn't always a bad thing.

--------------------
MIRRORMIX STUDIO
blog

Quote EnlightenedHand:

---

That is indeed true. But there are some companies that use jitter to their advantage with their external clocks. I believe the idea is to use audible jitter in ways that are perceived as sounding pleasing to the listener.

---

I know which 'A' you are talking about, but there is a fundamental problem with the approach (Even if we allow that it is legit in the first place (dubious)), the clock generator CANNOT know what the loop filter of the receiving device is going to do to the jitter spectrum!

Pretty much any device with an external wordclock input will use the wordclock to lock a PLL running at 256Fs or so in order to generate the Sclock and the bit clock for the serial data stream to the converters. This PLL has a loop filter (it is a feedback mechanism that tries to lock the phase of a local oscillator to the phase of the wordclock (there is usually a divide by 256 in there so the oscillator runs at 256 times the wordclock), this loop filter typically has a bandwidth of less then a few Hz.

Now the interesting thing about noise in PLL systems is that below the loop filter cutoff the noise is controlled by the noise on the PLL reference input (the word clock source in this case) while above the loop filter cutoff it is the noise of the local oscillator.
Thus above the typically few hz bandwidth of the PLL loop filter the external clock has NO EFFECT on jitter.

There is some evidence that it is close in phase noise that is audible (Makes sense, it amounts to frequency modulation and so will produce bessel function sidebands), but I am hard pressed to see that there is much that is meaningful that a clock generator can do deliberately within the very small loop filter bandwidth that is going to have much effect.

Of course there is some right crap out there that has way too wide loop bandwidths and poorly thought out PLLs, but the fix there involves buying proper gear, not gluing a bag on the side to 'fix' it.

Quote:

---

So often people get obsessed with eliminating jitter at all costs, never considering the possibility of jitter not always being a bad thing. Just as analog distortion isn't always a bad thing.

---

I want jitter, I can emulate it in software trivially, I don't want a general purpose converter forcing audible distortion on me with everything I record, any more then I want a tapedeck having more flutter then I can avoid (it is actually the same thing!).

IMHO Jitter is largely a solved problem in any decent gear, it just is not something to worry about unless you are designing (not just using) digital audio.

I will be interested to see how Hugh went about measuring the jitter, it is notoriously tricky to do in a repeatable way.

Regards, Dan.

Edited by dmills (11/04/10 04:32 PM)

I imagine it would be of paramount importance to pro recordists not to introduce any unnecessary jitter distortion. - if they could avoid it, that is.

--------------------
.. is this thing on?

Edited by Stan (11/04/10 05:15 PM)

As ever it is not that simple.....

Firstly there are a half dozen of so ways to give a single number jitter level, and NONE of them are any real use (And depending on how you measure you can get 3 orders of magnitude difference in the number for the same clock circuit)!

As I say there is some evidence that what really matters is close in phase noise (within a few hundred hz or so of the carrier), but measuring that (let alone measuring it with good repeatability) is **hard** even with a good RF lab.

What is actually informative is a phase noise spectrum, but nobody includes this on their datasheets for complete systems (and it is missing on a surprising number of clock generator data sheets).

Now the effect of all this is that it is almost impossible to produce a meaningful set of comparison data between products and thus jitter is seen as some sort of voodoo. Kit that specifies a high level of jitter (because of how they measure) sometimes sounds excellent, while kit that specifies a low level (which they cribbed from some clock modules app note) sometimes sounds horrible.

What would be nice would be for all the specification sheets to include both a clock jitter spectrum (for both free run and external WC at each sample rate, with specified video and resolution bandwidths)) and details of the converters inherent aperture jitter, this would allow meaningful comparison by the purchaser (but would give marketing fits!).

Actually, before worrying about jitter there are more important things to put on the spec sheet, common and differential mode input impedance, what source impedance you used when you measured the noise floor (it really matters), what the word clock inputs and outputs are electrically (this is horribly badly specified, as in there is NO standard) even the basics like what the alignment level is would be helpful, and most of these things are usually missing.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

Quote dmills:

---

I will be interested to see how Hugh went about measuring the jitter, it is notoriously tricky to do in a repeatable way.

---

I didn't bother to measure jitter because it isn't particularly relevant to the case I was presenting. If I tried to measure actual jitter figures the clock manufacturers would end up complaining that I didn't do it the way they did it anyway.

What I wanted to show was that in almost all cases, using an external clock made the performance of a range of A-Ds worse than when they were running on their internal clocks. The evidence was very clear and repeatable, and in the form of changes and additions to the low level spectral artefacts in the noise floor -- as measured on a Prism D-Scope III system.

Whether those changes would be audible under all situations is open to more debate... In most cases I suspect most people almost cettainly wouldn't be able to hear any difference. But that doesn't change the fact that the often repeated claim that a master clock can improve the performance of a converter is clearly not true.

However, the use of a master clock might well improve the usability and reliability of a complex digital audio system or digital audio-for video system -- and that is often true, as discussed earlier.

Hugh

--------------------
Technical Editor, Sound On Sound

Quote Hugh Robjohns:

---

I didn't bother to measure jitter because it isn't particularly relevant to the case I was presenting. If I tried to measure actual jitter figures the clock manufacturers would end up complaining that I didn't do it the way they did it anyway.

---

Wise of you, but I was looking forward to the bun fight in the letters page, I have seen that squabble and it gets **really** ugly.

Quote:

---

What I wanted to show was that in almost all cases, using an external clock made the performance of a range of A-Ds worse than when they were running on their internal clocks.

---

I thought that was pretty much a given, especially at the cheap end where VCXOs are seen as something to be 'value engineered' out.

Quote:

---

The evidence was very clear and repeatable, and in the form of changes and additions to the low level spectral artefacts in the noise floor -- as measured on a Prism D-Scope III system.

---

Got to be a bit careful there Hugh, If I was a clock manufacturer I could claim that those sidebands 100+db down on the signal were there by design and made things more 'musical' or 'sweetened the top end' or some other such merde. It would all be horse pucky of course, but wrap it in enough sciencey sounding words and I am sure I could shift some clocks.

Quote:

---

But that doesn't change the fact that the often repeated claim that a master clock can improve the performance of a converter is clearly not true.

---

I doubt that you will hear that from many engineers or even all that many technicians (Now marketing types are possibly another matter), the operating principle behind a PLL is fairly well understood these days by those who need to care.

Quote:

---

However, the use of a master clock might well improve the usability and reliability of a complex digital audio system or digital audio-for video system -- and that is often true, as discussed earlier.

---

And THAT is a good reason to use one!

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

Quote dmills:

---

I was looking forward to the bun fight in the letters page

---

You're a cruel man!

Quote:

---

I thought that was pretty much a given, especially at the cheap end where VCXOs are seen as something to be 'value engineered' out.

---

To those with engineering knowledge I agree, but the SOS readership is far broader of course, and many will believe the hype on face value.

Quote:

---

Got to be a bit careful there Hugh, If I was a clock manufacturer I could claim that those sidebands 100+db down on the signal were there by design and made things more 'musical' or 'sweetened the top end' or some other such merde.

---

Yes... they could, and critical listening might confirm or deny that... But then what we would be talking about is introduced distortions, in a similar way to the numerous tape recording artefacts, perhaps. Some might think such things add a certain musical character and that's fine. All I'm trying to show is that external clocking can't improve the sound of an A-D converter. It might change it... but it can't improve it at a technical level.

Quote:

---

I doubt that you will hear that from many engineers or even all that many technicians

---

The reason I did this feature was precisely because I keep hearing it from SOS readers and students. You'd be amazed how often I'm asked by people about which master clock they should by to upgrade their converters...

Hugh

--------------------
Technical Editor, Sound On Sound

fly in ointment moment, of course, a large number of readers aren't using the latest technology either.... or even it's most recent parents..... and perhaps it's fair to say that in older, less effective designs, it's an immediately obvious phenomenon that , for example, my 888/24's sound better clocked from my Apogee , than from their own clock....

swapping round the clock chaining and interface order in that old TDM system produces fairly obvious changes in the sound of the digidesign units.

so perhaps things have changed over the last 10 years..... ??


any chance you tried out your tests on some very much older low to mid range kit Hugh?

Going back ten years or more there were certainly some systems that had very ropey internal clocks and external clocking sometimes did appear to improve things -- I found that myself too.

Today most if not all gear is using much better technology and designers have much better knowledge and experience so internal clocks are usually now very good indeed.

I tested a small but reasonably represenative range of budget through to high end converters and clocks, some old, most current.

Hugh

--------------------
Technical Editor, Sound On Sound

Is there a consistency with clocks with regards to sample rate. Do any converters behave better or worst at a particular rate?

There was actually some sense in the 888 having a wide lock range and a wide loop bandwidth....

Locking to timecode from external analogue tape transports in the days before good arbitary ratio resamplers!

Having a system that will chase a master clock that is 10% out and changing at the sort of rates that wow on a tape transport causes makes chasing LTC or VITC **MUCH** simpler, and the data sheets for delta sigma parts at the time did not make a big deal out of the jitter sensitivity (They still dont unless you read real carefully).

Now there are much better ways to acheive the same ends, but at the time it probably felt like good engineering given the expected use cases for the 888, as ever things are not as simple as they appear.

Personally I think that Apogee made their name courtesy of the 888s suceptibility to external clock influence and have been slightly milking it ever since.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

Quote dmills:

---

Personally I think that Apogee made their name courtesy of the 888s suceptibility to external clock influence and have been slightly milking it ever since.

Regards, Dan.

---

I would concur, although it's interesting that their first product, the improved filters for the Mitsubishi, was designed by none other than Dan Lavry. That gave them a good rep too...

--------------------
madtheory creations
Synths and pianos for Kontakt