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about sync
Old 20th January 2013
  #1
Gear Head
 

about sync

you can sync two interfaces with ADAT/SPDIF/WORDCLOCK
what's the difference?
does it matter quality/stability wise?

thanks
Old 20th January 2013
  #2
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UnDeFiNeD's Avatar
 

Quote:
Originally Posted by tuna View Post
you can sync two interfaces with ADAT/SPDIF/WORDCLOCK
what's the difference?
does it matter quality/stability wise?

thanks
It all depends on who does what in your particular setup.

The basics are very simple, each time you interface two audio devices with each other over a digital connection, you have to establish sync between them.

This is because both devices have an internal clock signal, running at the chosen samplerate (or multiples of, but thats not the point here). While samplerate can seem equal between the two devices (eg. both are set internally at 44.1kHz), in reality there are two major problems when you try to make them interface with each other:
1. Both rates will never be exactly the same (ex. one might have 44101Hz, and the other 44099Hz)
2. Even if they would, by miracle, be exactly equal, the exact timing when a sample is either captured or playbacked, will never be the same with both machines.

The result of interfacing these two devices without sync will be (strong) jitter. Resulting in heavy distortion on high frequency material and clicks and pops.


As an example, if you have an ADAT capable A/D converter, and an ADAT in port on your computer interface and you want to make a recording, you will need to chose one of both as either clock master or clock slave.

Option 1 would be to tell the computer to instead of listen to it's own build in clock, to listen to the clock of the A/D converter. This option is usually set in either the interface settings, or DAW settings.
Now to your question, ADAT is a very nice interfacing format since it transports a clock signal over it's own cable. You could tell the computer in this case to go and listen to the incoming ADAT clock signal, and sync to that to guarantee the exact same samplerate inside the DAW as the converter uses to capture the audio.

Option 2 would be to tell the converter to listen to the PC instead. In this case we could have a problem, since only an ADAT cable is wired from the A/D to the PC, but not vice versa, and so there is no incoming clock signal to sync to. Thats where for example a wordclock cable might be helpful. If the PC's interface has a WC out, you could wire the BNC cable to the A/D's WC-in and establish sync by slaving the A/D to the WC.


Which one here is better? Technically speaking option 1, since A/D conversion is the most destructive process in the chain, and if the clock signal isn't stable at capture, there is no way to fix it afterwards, and playback will be flawed too.

However, it could be possible that your A/D's built in clock is so bad, that an external one gives better results. This could be used to have all other digital devices (in our example the A/D and the PC interface) slaved to. But results in this case vary widely and are subject to a lot of controverse also.

Along the same lines, you could be working in a media company where they have an in house clock where everything should be slaved to. In that case all your devices need to be slaved to that one master clock to retain sync (wordclock as well as timecode) throughout the building.


Regarding your question wich one of the formats is better, depends on your particular setup:
S/PDIF, AES/EBU and ADAT carry their own clock signal. So if you have decent cables to interface your devices with (remember, 75ohm RCA for S/PDIF and 110ohm XLR for AES/EBU!), then there is no immediate need to hook up an additional wordclock to sync to. Just sync to the incoming interface itself, and judge if the connect seems correct. You could also try to mesure the jitter of your interface to see if everything is correct.
If not you can always opt for the extra wordclock connection to sync to, but benefits here are usually preserved for larger distances. Changing to a better digital cable will in most cases be sufficient.

Alex
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