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when do you need an external clock? Dual-Channel Preamps
Old 5th March 2014
  #1
Gear Maniac
 
blackmajik2021's Avatar
when do you need an external clock?

When do you need an external clock? do you need one as soon as you link more than 2 digital devices?

1) apollo
2) adat 8 ch preamp
3) api A2D via spdif

does this setup need an external clock, or can I daisy chain the BCF cables like so apollo (master) > a2d > adat 8ch?

I cant find much info on this.
Old 5th March 2014
  #2
Lives for gear
You can choose one of your devices to be the master clock, and then have your other devices be 'slaves' to that master clock.

It would be best to use which ever device has the best clock installed in it as the master because there is the possibility of it making the conversion in the slave devices sound slightly better.

If you want a better clock than what any of the devices offers - that's when someone would buy an external clock and then have all devices 'slave' off the external clock

(You need to only have one clock in your setup so that all devices are in sync with each other)
Old 5th March 2014
  #3
Gear Maniac
 
blackmajik2021's Avatar
but how could I send the clock from the apollo to two units when it only has 1 output?
Old 5th March 2014
  #4
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Old 5th March 2014
  #5
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MIKEHARRIS's Avatar
For fancier setups there are WordClock distribution amplifiers
Old 5th March 2014
  #6
Gear Maniac
 
blackmajik2021's Avatar
using a splitter like that wont lower the quality of the clock signal?
Old 5th March 2014
  #7
Before the article below made its way around, there was a lot of nonsense kicking around about master clocks -- and you're likely to find plenty of it in older threads here.

This article does a good job of explaining what a master clock does, when you need one, and what to expect, using examples of several product tiers and doing real world objective measure of performance.

Does Your Studio Need A Digital Master Clock?
Old 5th March 2014
  #8
This guy has the best external clock, really.
Attached Thumbnails
when do you need an external clock?-flava-flav-daylight-savings-time.jpg  
Old 5th March 2014
  #9
Lives for gear
 

Using a T piece wont degrade the signal as long as it is terminated correctly.

The last device in the chain needs a 75ohm termination on it.

Some kit has a jumper that can be set internally.

Lot's does not.

The solution on the last device again use a T-piece and one side of the T-piece gets its clock from the previous device the other side is terminated with a 75ohm termination



Old 5th March 2014
  #10
Quote:
Originally Posted by brill bedroom View Post
This guy has the best external clock, really.
Time has come today.
Old 5th March 2014
  #11
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Quote:
Originally Posted by MIKEHARRIS View Post
For fancier setups there are WordClock distribution amplifiers
Which probably makes no difference. Considering that word clock is a 'one way' signal, and that losing 3db of signal on a splitter isn't going to cause you any problems - it would be pretty silly to buy anything more expensive than the basic splitter.
Old 5th March 2014
  #12
Lives for gear
Quote:
Originally Posted by GGreen View Post
Using a T piece wont degrade the signal as long as it is terminated correctly.

The last device in the chain needs a 75ohm termination on it.

Some kit has a jumper that can be set internally.

Lot's does not.

The solution on the last device again use a T-piece and one side of the T-piece gets its clock from the previous device the other side is terminated with a 75ohm termination



Are you sure that the last device in the chain doesn't already do the same job as the terminator? In your picture - it looks like the terminator is only needed because there is an extra unnecessary T-splitter being used....
Old 5th March 2014
  #13
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MIKEHARRIS's Avatar
Termination needed...2x T's considered max
Old 6th March 2014
  #14
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Improved AD/DA Conversion with external synchronization/clock? | rhythminmind.net - so... does a separate word clock matter or not?
Old 6th March 2014
  #15
I wouldn't buy an external clock if you already have a quality clock in one of the units you own. Like someone mentioned, use the highest quality clock (you'll have to determine this by some extensive and critical listening, it may be the Apollo, or the API don't let the price or age of the design be the deciding factor, only the sound) for all the units. If a converter is already high quality, an external clock may degrade the source.

But since you have three different converters, one will use its own master clock and the other two the clock of the one using its internal clock. So the added expense of an additional clock along with possibly degrading the best converter you already have there isn't the best thing to do IMHO.
Old 6th March 2014
  #16
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Quote:
Originally Posted by MIKEHARRIS View Post
Termination needed...2x T's considered max
Can you explain why the termination is needed? I always figured that the last unit in the chain was the termination (unless it has a loop-through port). If what you are saying is correct - then it sounds like you need a terminator regardless of whether you are using a T-Splitter
Old 6th March 2014
  #17
Use a time code distribution amplifier. Terminate any loop-throughs.
Termination is necessary to keep the p-p voltage within specs. Unterminating increases the voltage, double-terminating reduces it.
Use 75 Ohm cable and termination.
Old 6th March 2014
  #18
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Quote:
Originally Posted by Gonebytim View Post
Can you explain why the termination is needed?
Short:
The termination is needed to stop the word clock signal from "bouncing back" and interfering.

Long:
The word clock signal is a quickly rising edge of a square wave signal. The receiver will listen on the cable and try to trigger as closely to the same place as possible on rising edge. It will then use this trigger point to synchronize an internal oscillator, most often using what is called a PLL (Phase Locked Loop) although there are other technical solutions. The internal clock is set to allow the AD converter to run at a higher frequency than the work clock, often 256 times as fast (which is a long story in itself).

The word clock signal will move down the word clock cable at slightly lower speed than the speed of light in vacuum. When it comes to the end of the cable it will reflect back as an echo. This echo will then move back through the cable and if you are unlucky it will trigger the word clock at a receiver at the wrong time.

The termination is used to stop the signal from "bouncing back" as an echo -- instead the energy is converted into heat (not a lot though). To be exact, the word clock cable should be terminated in both ends. But the "sending" end is usually terminated inside.

From knowing the specs and doing the calculations this "echo" will only matter if the total cable lenght is somewhat above 2 meters,
--edit: but there are yet another reason for having termination and that is to keep maximum voltage within specs. The simple recommendation is to always include termination. Period.
-- edit: note that some AD-s or DA-s has a switchable termination built into the card. Check as example most of the RME cards.

The echo as well as how PLL circuits are made will result in a small amount of uncertainty in exact when the analog signal should be sampled. This uncertainty can be measured as jitter.

Contrary to common ad buzz, every AD converter yet produced by any man will have a lower jitter when run on the internal oscillator as compared to running on word clock. Now, it seems, some people like the sound better...

// Gunnar

Last edited by ghellquist; 6th March 2014 at 01:12 PM.. Reason: Added info, see --edit
Old 6th March 2014
  #19
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Quote:
Originally Posted by Gonebytim View Post
Can you explain why the termination is needed? I always figured that the last unit in the chain was the termination (unless it has a loop-through port). If what you are saying is correct - then it sounds like you need a terminator regardless of whether you are using a T-Splitter
It really does depend on the gear. Just because a device is last in the chain and is reading the word clock signal doesn't mean that it also terminates the line correctly. Some do, some don't, some are switchable between terminating/non-terminating. It's always worth checking the manual/spec for this kind of thing.
Old 6th March 2014
  #20
Gear Maniac
 

If I dont use the 2nd T-spilter, the cable goes out directly from the 1st T-spilter to the last machine, will the singal be terminated??
Old 6th March 2014
  #21
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Did anyone check that link that I posted? I'm wondering what your thoughts on it are.
Old 7th March 2014
  #22
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Quote:
Originally Posted by kei_siuip View Post
If I dont use the 2nd T-spilter, the cable goes out directly from the 1st T-spilter to the last machine, will the singal be terminated??
IF the last machine has the option of terminating as a switch or internal jumper. Otherwise, get the second T and terminating.

What brand and type?
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