Technical Background; Lock And Synccheck - RME Audio Hammerfall HDSPe AIO User Manual

30. Technical Background

33.1 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 HDSPe AIO, the unit indicates LOCK, i. e. a valid
input signal. This information is presented in the HDSPe AIO's Settings dialog. In the status
display SyncCheck, the state of all clocks is decoded and shown as simple text (No Lock, Lock,
Sync).
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 HDSPe AIO
is set to 44.1 kHz internally (clock mode Master), and a mixing desk with ADAT output is con-
nected to input ADAT. The status display will show LOCK immediately, but usually the mixing
desk's sample rate is generated internally (it is Master too), and thus slightly higher or lower
than the HDSPe AIO's internal sample rate. Result: When reading out the data, there will fre-
quently 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 HDSPe AIO 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.
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In order to display those problems, the HDSPe AIO includes SyncCheck . It checks all clocks
used for synchronicity. If they are not synchronous to each other, the status display will show
LOCK. If they are synchronous to each other (i. e. absolutely identical), the status display will
change to SYNC. In example 1 it would have been obvious that the entry LOCK is shown in
SyncCheck 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.
User's Guide HDSPe AIO © RME
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