Technical Background; Lock And Synccheck - RME Audio ADI-2 Pro User Manual

34. Technical Background

34.1 Lock and SyncCheck

In the analog domain one can connect any device to another device, a synchronization is not
necessary. Digital audio is different. It uses a clock base. The signal can only be processed and
transmitted when all participating devices share the same clock. If not, the signal will suffer from
wrong samples, distortion, crackle sounds and drop outs.
!
A digital system can have only one master! If the ADI-2 Pro uses its internal clock, all other
devices must be set to 'Slave' mode and be synced to the ADI-2 Pro's clock.
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 SPDIF signal is applied to the ADI-2 Pro, the State Overview screen shows LOCK, i. e. a
valid input signal. Unfortunately, lock does not necessarily mean that the received signal is cor-
rect with respect to the clock which processes the read out of the embedded data. Both sample
rates have to be fully identical, not only in their frequency, but also in their phase relation. This
state is called Sync and also shown in the State Overview screen if present.
Example: The ADI-2 Pro is set to 44.1 kHz internal clock, and a CD player is connected to its
input. The State Overview screen will display the input signal and LOCK state. The CD player's
sample rate is generated internally as well, and thus slightly higher or lower than the ADI-2
Pro's internal sample rate. Result: When reading out the data, there will frequently be read er-
rors that cause audible clicks and drop outs.
In order to display this problem the ADI-2 Pro includes SyncCheck. It checks all clocks used for
synchronicity. If they are not synchronous to each other (i. e. absolutely identical) the State
Overview screen will show LOCK. In case they are synchronous the screen shows sync.
In the example above the CD player can not be set to be clock slave, it will always use its inter-
nal clock (master). There are two solutions:
• Set the Clock Source selection of the ADI-2 Pro to SPDIF. The ADI-2 Pro will now precisely
follow the input signal's clock, the State Overview screen will show a stable sync for the
SPDIF input.
• Set the SRC (sample rate converter) to SPDIF In. The SRC operates as clock decoupler, so
the ADI-2 Pro can stay on internal clock. In this case the State Overview screen will show
sync for the SPDIF input as well as SRC SPDIF.
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.
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User's Guide ADI-2 Pro © RME