User`s guide

User’s Guide HDSP System HDSP 9652 © RME
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9.2 Clock Modes - Synchronization
In the digital world, all devices are either the ‘Master’ (clock source) or a ‘Slave’ synchronized
to the master. Whenever several devices are linked within a system, there must always be a
single master clock. The Hammerfall DSP’s intelligent clock control is very user-friendly, being
able to switch between clock modes automatically. Selecting AutoSync will activate this mode.
In AutoSync mode, the system constantly scans all digital inputs for a valid signal. If this signal
corresponds with the current playback sample rate, the card switches from the internal quartz
(AutoSync Ref displays 'Master') to a clock generated from the input signal (AutoSync Ref
displays 'Slave'). This allows on-the-fly recording, even during playback, without having to
synchronize the card to the input signal first. It also allows immediate playback at any sample
rate without having to reconfigure the card.
AutoSync guarantees that normal record and record-while-play will always work correctly. In
certain cases however, e.g. when the inputs and outputs of a DAT machine are connected
directly to the Hammerfall DSP, AutoSync may cause feedback in the digital carrier, so
synchronization breaks down. To remedy this, switch the HDSP’s clock mode over to 'Master'.
Remember that a digital system can only have one master! If the HDSP’s clock mode is set
to 'Master', all other devices must be set to ‘Slave’.
All the ADAT optical inputs in the
Hammerfall DSP as well as the
SPDIF input will work simultaneously.
Because there is no input selector
however, the HDSP has to be told
which of the signals is the sync
reference (a digital device can only
be clocked from a single source).
This is why the system has been
equipped with automatic clock source
selection, which adopts the first
available input with a valid digital
signal as the clock reference input.
The input currently used as sync
reference is shown in the AutoSync
Ref status field, together with its
sample frequency.
Via Pref. Sync Ref (preferred
synchronization reference) a
preferred input can be defined. As
long as the card sees a valid signal
there, this input will be designated as
the sync source, otherwise the other
inputs will be scanned in turn. If none
of the inputs are receiving a valid
signal, the card automatically
switches clock mode to ‘Master’.
To cope with some situations which
may arise in studio practice, setting ‘Pref Sync Ref’ is essential. One example: An ADAT
recorder is connected to the ADAT1 input (ADAT1 immediately becomes the AutoSync source)
and a CD player is connected to the SPDIF input. Try recording a few samples from the CD
and you will be disappointed. Few CD players can be synchronized. The samples will inevitably
be corrupted, because the signal from the CD player is read with the (wrong) clock from the
ADAT i.e. out of sync. In this case, 'Pref Sync Ref' should be temporarily set to SPDIF.