RME Audio ADI-192 DD, Руководство пользователя

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User's Guide ADI-192 DD © RME
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11.2 Lock, SyncCheck and SyncAlign
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, TDIF or ADAT signal is applied to the ADI-192 DD, the corresponding SYNC 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 ADI-192 DD
is set to 44.1 kHz internal clock, and a CD-Player is connected to input AES1. The correspond-
ing LED will start flashing immediately, but the CD-Player's sample rate is generated internally,
and thus slightly higher or lower than the ADI-192 DD'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 ADI-192 DD from internal clock to AES (its internal clock
will then be the clock delivered by the CD-Player). But in case a DAT recorder is connected as
a second source, there will again be a slight difference in the sample rate, and therefore clicks
and drop outs [2]. Another example could be connecting two ADAT machines which are not
synchronous to each other due to wrong clock setup [3].
In order to display those problems optically at the device, the ADI-192 DD 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 example 1 it would have
been obvious that the SOURCE AES LED kept on flashing after connecting the CD-Player. In
example 2, SYNC AES1 would be constantly lit, but the SYNC LED of the input used by the
DAT would be flashing. In example 3, again one LED would be constantly lit, while the other
one keeps on flashing.
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.
A special problem occurs with devices offering several AES or SPDIF inputs. While with ADAT
and TDIF all eight channels share the same clock base, with AES there are several completely
independent receivers with their own PLLs and data buffers. Therefore there can be a random
error of ± 1 sample difference between the stereo pairs. The ADI-192 DD's exclusive
SyncA-
lign technology avoids this effect and guarantees sample synchronicity among all four stereo
channels.