Aphex 1788A Premium Eight Chan Mic Pre - Manual (Page 24)

Page 24
MICROPHONE PREAMPLIFIER
Page 25
Instruction Manual
5. USING THE DIGITAL MODULE
BNC Clock I/O
The sync capability is made even more flexible by providing a feed-through port. When in
external sync mode, the BNC output jack is tied directly to the BNC input jack. This allows
the sync signal to pass through and be sent on to another unit. Depending on the quality
of the driver, up to eight 1788A’s may be looped together from a single clock source.
When the 1788A is set for internal sync, the BNC input jack is ignored and the BNC output
jack is connected to an output driver that supplies a low jitter Word Clock suitable for
sync’ing seven other 1788A’s and additional equipment.
NOTE: The BNC jack input range for good lock is 1V to 5V p-p. This means it can be
used with both terminated and unterminated word clock sources.
Clock In
This BNC jack is provided to receive your master clock source. It also accepts AES/EBU (or
S/PDIF) and Superclock depending on the dip switch settings.
The Model 1788A can operate equally well from a typical “brute force” 5V word clock
output or from a matched impedance master clocking distribution. Please refer to
Appendix F for a thorough discussion of clock wiring and distribution systems.
When the Model 1788A is operating in external clock mode, the Clock In jack is tied
directly to the Clock Out jack through a metal relay contact. The input load is light enough
to daisy-chain up to eight units by looping straight through. The final unit should have
a 50 or 75 ohm (as dictated by your distribution cable impedance) BNC terminator plug
inserted in its Clock Out jack to properly terminate the transmission line. If only one unit is
connected, then simply place the terminator plug on the single unit’s Clock Out jack.
When the Model 1788A is operating in the internal clock mode, the Clock In jack is
isolated and unterminated.
Clock Out
As previously described, this jack is directly tied to the Clock In jack to facilitate daisy-
chaining multiple units to a master clock source when the unit is operating in external clock
mode. You’ll need to decide whether to use the brute force or matched impedance output
mode. We generally recommend impedance matched, as shown in the examples of this
chapter. However, if the unit to be driven is a short distance away, it is more cusromary in
the industry to use brute forced clock. Appendix F tells more about these choices.
When the Model 1788A is operating in internal clock mode, the Clock Out jack is
disconnected from Clock In and reconnected to the unit’s internal clock source. Thus,
any Model 1788A operating in internal clock mode can serve as a master clock source
when required. The internal clock is derived from a low jitter crystal oscillator and is fully
competent as a master clock.
Local Clock Mastering
You can use one Model 1788A as a clock master and slave everything else to it. For
example, to synchronize a group of Model 1788A’s, simply set the first unit to internal
clock mode and all the others to external. Daisy-chain the Clock Out of the first unit
through the clock ins and outs of the remaining units. This way all digital audio outputs
will be locked together to the first unit’s clock reference. You can also loop the clock
output of the last unit in line to your digital recorder or workstation, but remember to
terminate the last unit in line.