Avid Technology Digidesign USD, Руководство пользователя, Страница: 68 / 96

Universal Slave Driver 64
Calibrating the USD
Oscillator
The USD provides a feature for calibrating
the frequency of the on-board crystal oscil-
lator. This allows USD to be used as an ex-
tremely accurate frequency reference while
in Internal/VSO mode.
With normal usage, the USD should never
require recalibrating. Each unit is factory
calibrated to /- 5 ppm (parts per
million).
You may want to recalibrate the USD in the
following situations:
• If greater than 5 ppm accuracy is re-
quired
• If the unit needs to be matched to a
unique (nonstandard) frequency
• To precisely compensate for component
aging
• To restore the original factory setting
✽ Oscillator recalibration does not occur dur-
ing firmware updating or when resetting the
USD to factory defaults (see “Restoring Fac-
tory Settings” on page 65).
Oscillator Resolution and Stability
The USD calibration units are in 1/64th of a
sample period. What this means is that the
USD can theoretically be calibrated to
about 1/3 of a ppm. The unit will maintain
calibration across a wide range in tempera-
ture. Long-term drift should be less than
1 ppm per year due to aging of the crystal.
To put this in perspective, most digital
audio products are accurate to within 20 to
50 ppm and drift with temperature. The
USD’s accuracy is possible because it con-
tains a low-jitter, high-stability tempera-
ture controlled crystal oscillator.
Warm Up USD Before Recalibrating
Before you begin the calibration procedure,
power on the USD and allow it to warm up
for at least five minutes. The temperature
of the room (or chassis) isn’t critical during
the calibration procedure. However, if you
need better than 3 ppm accuracy, it is rec-
ommended that you allow the USD to
warm up for at least 30 minutes and that
the chassis be at normal operating temper-
ature.
To restore the USD’s oscillator calibration to
its factory setting:
1
Press Set, then press Up until Video Sys-
tem (VIdEo SY) is displayed in the LED
readout.
2
With the LED Time Code Display show-
ing VIdEo SY, press and hold the Up switch.
While you continue to hold that switch,
press the Clock Reference switch momen-
tarily, and then release both switches. The
LED Time Code Display will read:
3
Press the Set switch again. The LED Time
Code Display will switch to display the cur-
rent parameter value, which shows a sam-
ple rate frequency deviation from –0999 to
0999.
4
Take note of the Oscillator Calibration
value printed on the factory sticker, on the
USD bottom panel.