User’s Guide HDSP System Digiface
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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 'Internal')
to a clock generated from the
input signal (AutoSync Ref
displays 'SPDIF' or 'ADATx').
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 causes 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 A
utoSync Ref
status field, together with the
current 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 sync 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.
