Technical Background; Lock And Synccheck - RME Audio Digiface AES User Manual

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38. Technical Background

Digital signals consist of a carrier and the data. If a digital signal is applied to an input, the receiver

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 frequency of

the incoming signal, it is locked. This Lock state remains even with small changes of the fre-

quency, because the PLL tracks the receiver's frequency.

If an ADAT or SPDIF signal is applied to the Digiface AES, the corresponding input field starts

flashing. The unit indicates LOCK, i. e. a valid input signal (in case the signal is also in sync, the

filed 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: The Digiface is set to

44.1 kHz internally (clock mode Master), and a mixing desk with ADAT output is connected to

input ADAT. The corresponding field 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

Digiface'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 Digiface 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 Digiface includes SyncCheck. It

checks all clocks used for synchronicity. If they are not synchronous to each other (i. e. absolutely

identical), the SYNC field of the asynchronous input flashes. In case they are completely synchro-

nous, all fields are constantly lit. In the above example it would have been obvious that the field

ADAT 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 becomes

easy to handle.

The same information is presented in the Digiface's Settings dialog. In the status display the state

of all clocks is decoded and shown as simple text (No Lock, Lock, Sync).

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