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User's Guide Fireface 800 © RME
95
37. Technical Background
37.1 Lock and SyncCheck
Digital signals consist of a carrier and the data. If a digital signal is applied to an input, the re-
ceiver has to synchronize to the carrier clock in order to read the data correctly. To achieve this,
the receiver uses a PLL (Phase Locked Loop). As soon as the receiver meets the exact fre-
quency of the incoming signal, it is locked. This Lock state remains even with small changes of
the frequency, because the PLL tracks the receiver's frequency.
If an ADAT or SPDIF signal is applied to the Fireface 800, the corresponding input LED starts
flashing. The unit indicates LOCK, i. e. a valid input signal (in case the signal is also in sync, the
LED is constantly lit, see below).
Unfortunately, LOCK does not necessarily mean that the received signal is correct with respect
to the clock which processes the read out of the embedded data. Example [1]: The Fireface is
set to 44.1 kHz internally (clock mode Master), and a mixing desk with ADAT output is con-
nected to input ADAT1. The corresponding LED will show LOCK immediately, but usually the
mixing desk's sample rate is generated internally (also Master), and thus slightly higher or lower
than the Fireface's internal sample rate. Result: When reading out the data, there will frequently
be read errors that cause clicks and drop outs.
Also when using multiple inputs, a simple LOCK is not sufficient. The above described problem
can be solved elegantly by setting the Fireface from Master to AutoSync (its internal clock will
then be the clock delivered by the mixing desk). But in case another, un-synchronous device is
connected, there will again be a slight difference in the sample rate, and therefore clicks and
drop outs.
In order to display those problems optically at the device, the Fireface includes SyncCheck
®
. It
checks all clocks used for synchronicity. If they are not synchronous to each other (i. e. abso-
lutely identical), the SYNC LED of the asynchronous input flashes. In case they are completely
synchronous, all LEDs are constantly lit. In example 1 it would have been obvious that the LED
ADAT 1 kept on flashing after connecting the mixing desk.
In practice, SyncCheck allows for a quick overview of the correct configuration of all digital de-
vices. So one of the most difficult and error-prone topics of the digital studio world finally be-
comes easy to handle.
The same information is presented in the Fireface's Settings dialog. In the status display Sync-
Check the state of all clocks is decoded and shown as simple text (No Lock, Lock, Sync).