User Manual

User's Guide Digiface USB © RME

25. Technical Background

25.1 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 signal is applied to the Digiface USB, the SYNC input 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]: The Digiface USB

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

nected to the ADAT input. The LED will show LOCK immediately, but usually the mixing desk's

sample rate is generated internally (being Master), and thus slightly higher or lower than the

Digiface USB's internal sample rate. Result: When reading out the data, there will frequently be

read errors that cause clicks and drop outs.

In order to display those problems optically at the device, the Digiface USB includes Sync-

Check. It checks all clocks used for synchronicity. If they are not synchronous to each other

(i.e. absolutely identical) the SYNC LED flashes. In case they are completely synchronous the

LED is constantly lit. In the above example the user would have noticed quickly that the LED

kept on flashing after connecting the mixing desk.

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

Input State the state of the input clock is decoded and shown as simple text (No Lock, Lock,

Sync).

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.