Hi Yosuke

In a digital mic split system 1 system has to be the clock master and the other system will have to be a slave system that recieves its audio clock from the master.

I am assuming you are using ACE to connect each mixrack via PORT B?

you set the audio clock source in the mixrack setup – mixer preferences audio – sync page.
set the system you want to be the Audio clock master to: Internal
Set the system you wish to be the slave as: Port B

if you cannot see port B available in the drop down menu this means that nothing is fitted to the Port B Slot or there is a fault with the Port B Module. In this case please contact A&H Technical support who will help you resolve your issue.

Hope this helps
Sam A&H

https://en.wikipedia.org/wiki/Word_clock

Audio sync is the source were your WC (word clock) is coming from.

It is simular to a army that marches.
One guy sets the rythem, if he doesn’t do this it becomes a mess!

so if you digitally link 2 devices (thru ACE) (both of the machine’s have a guy who gives the rythem) you have to tell the devices wich one is setting the rythem.
This is what you select in audio source!