RME Fireface UCX II 40-Channel USB-B Audio/MIDI Interface - Manual (Page 47)

User's Guide Fireface UCX II © RME
47
!
22.2 Technical Description and Usage
In the analog domain one can connect any device to another device, a synchronization is not
necessary. Digital audio is different. It uses a clock, the sample frequency. The signal can only
be processed and transmitted when all participating devices share the same clock. If not, the
signal will suffer from wrong samples, distortion, crackle sounds and drop outs.
AES/EBU, SPDIF and ADAT are self-clocking, an additional word clock connection in principle
isn't necessary. But when using more than one device simultaneously problems are likely to
happen. For example any self-clocking will not work in a loop cabling, when there is no 'master'
(main clock) inside the loop. Additionally the clock of all participating devices has to be synchro-
nous. This is often impossible with devices limited to playback, for example CD players, as these
have no SPDIF input, thus can't use the self clocking technique as clock reference.
In a digital studio synchronisation is maintained by connecting all devices to a central sync
source. For example the mixing desk works as master and sends a reference signal, the word
clock, to all other devices. Of course this will only work as long as all other devices are equipped
with a word clock or sync input, thus being able to work as slave (some professional CD players
indeed have a word clock input). Then all devices get the same clock and will work in every pos-
sible combination with each other.
A digital system can only have one master! If the Fireface's clock mode is set to 'Master', all
other devices must be set to ‘Slave’.
But word clock is not only the 'great problem solver', it also has some disadvantages. The word
clock is based on a fraction of the really needed clock. For example SPDIF: 44.1 kHz word clock
(a simple square wave signal) has to be multiplied by 256 inside the device using a special PLL
(to about 11.2 MHz). This signal then replaces the one from the quartz crystal. Big disadvantage:
because of the high multiplication factor the reconstructed clock will have great deviations called
jitter. The jitter of a word clock is typically 15 times higher as when using a quartz based clock.
The actual end of these problems is offered by the
SteadyClock technology of the Fireface UCX
II. Combining the advantages of modern and fastest digital technology with analog filter tech-
niques, re-gaining a low jitter clock signal of 22 MHz from a slow word clock of 44.1 kHz is no
problem anymore. Additionally, jitter on the input signal is highly rejected, so that even in real
world usage the re-gained clock signal is of highest quality.