Dcs 992 Technical Information; Clocking - DCS 992 User Manual

dCS 992 User Manual
dCS Ltd
dCS 992 T
ECHNICAL

Clocking

Operating Modes
Master clock
Manual part no: DOC063992 iss 2A1
Contact on + 44 1799 531 999
dCS
(inside the UK replace + 44 with 0)
I
NFORMATION
The sample clock quality significantly determines the performance of a digital
audio system.
The highest quality clocks that are available are crystals, so we use these. The
dCS 992 uses one of two on-board voltage controlled crystal oscillators in a
temperature compensated (TCXO) configuration, as a clock source – one for
48 kS/s related outputs and one for 44.1 kS/s related outputs. These TCXOs are
aged to allow the crystal to relax from manufacturing stresses then specially
selected for predictable behaviour over temperature. After assembly, the unit is
subjected to repeated temperature cycling, during which calibration data is
collected. This data is unique to each unit, and is stored in non-volatile memory
in the unit. A final calibration run confirms the accuracy of the unit.
When an external clock is applied for Slave operation, the correct VCXO is
chosen and synchronised to it by a phase locked loop (PLL). The PLL uses
substantial DSP to achieve ultra low bandwidths (in the mHz area) without large
resistors or capacitors. These components can add substantial amounts of
flicker noise. If synchronising to an external source is needed (a house sync, for
example), the PLL provides a high degree of "clock cleaning". Even a poor
quality source may be cleaned up and used by a dCS 992 to re-generate an
excellent one, if necessary. A consequence of the narrow (possibly very narrow)
bandwidth is that it takes quite a long time for the PLL to lock to a new clock
frequency.
The unit can operate in Master mode, or in Slave mode.
In Master mode the TCXOs can be controlled to operate from their calibration
data, or the frequency can be offset by ±5 ppm. The temperature compensation
still operates in this Offset Master mode.
In Slave mode, the unit can just follow its sync'ing input, or it can learn the
sync'd input and remember and use it if the input disappears. Thus, a unit may
be left "learning" the frequency of a house sync, until it is required to be taken
out on location. It can then recall the house sync, still temperature
compensating it. Or, a GPS signal might be used as an accurate reference –
but GPS signals can disappear for short periods. The dCS 992 will continue to
run at the last learned frequency if this happens. If the reference reappears, the
dCS 992 will very slowly and gradually adjust its frequency and phase to align
with the reference again – there will be no switching glitches. The re-alignment
can take 10 minutes or more, and is independent of PLL bandwidth setting.
Accuracy at 6 months
Accuracy when shipped
Temperature range
Warm up time
x
< ±1 ppm
typically ±0.1 ppm. See
-10°C to +60°C (+14°F to +140°F).
See Figure 19.
< 30 mins from turn on - see
< 4 mins when changing from one TCXO to the
other
Page 44
Manual for Software Version 2.0x
January 2001
Figure 18.
Figure 17.
file 063992ma2a1.pdf available from website
email to: more@dcsltd.co.uk
web site: www.dcsltd.co.uk