Lock And Synccheck - RME Audio ADI-8 QS User Manual

16.2 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 AES or MADI signal is applied to the ADI-8 QS, the corresponding LED starts flashing. The
unit indicates LOCK, i. e. a valid input signal (in case the signal is 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 ADI-8 QS is
set to 44.1 kHz internal clock (clock mode master), and a mixing desk with MADI output is con-
nected to the I64 MADI input. The OPTN LED will start flashing immediately, because the mix-
ing desk's sample rate is generated internally, and thus slightly higher or lower than the ADI-8
QS's internal sample rate. Result: When reading out the data, there will frequently be read er-
rors 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 ADI-8 QS from internal clock to MADI (its internal clock
will then be the clock delivered by the mixing desk). But in case another asynchronous device is
connected, there will again be a slight difference in the sample rate, and therefore clicks and
drop outs.
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In order to display those problems optically at the device, the ADI-8 QS includes SyncCheck .
It checks all clocks used for synchronicity. If they are not synchronous to each other (i. e. abso-
lutely identical), the LED of the asynchronous input flashes. In case they are synchronous the
LED stays dark, only the LED of the current clock source will be lit (constantly). In example 1 it
would have been obvious that the OPTN LED starts flashing after connecting the mixing desk.
In practice, SyncCheck allows for a quick overview of the correct configuration of all digital de-
vices. This way one of the most difficult and error-prone topics of the digital studio world finally
becomes easy to handle.
User's Guide ADI-8 QS © RME
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