RME HDSP 9652 [71/75] Technical background

User's Guide HDSP System HDSP 9652 © RME

30. Technical Background

30.1 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 HDSP 9652, the unit indicates LOCK, i. e. a valid

input signal. This information is presented in the HDSP 9652'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 HDSP 9652

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

nected to input ADAT1. 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 HDSP 9652'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 HDSP 9652 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, the HDSP 9652 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 right 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.