Operator`s manual

Model VR240 Audio Logging Recorder
January 2000
A-1
APPENDIX A
CLOCK ACCURACY, ADJUSTMENT
AND TIME CODE INPUT
A-1. FUNCTIONAL DESCRIPTION.
Eventide manufactures a standalone, accessory clock/display unit. This clock obtains standard
time from the Global Positioning System satellite constellation, and is usable anywhere in the
world. It has a large, configurable display that shows a time display as setup by the user. The
clock generates a time code for the VR240 that will keep the VR240 accurate with respect to
UTC or local time indefinitely. Further information on this clock is available from Eventide. If
you use this accessory clock, most of the information in Appendix A will be of little interest and
can be disregarded.
The VR240 has a built-in battery operated clock. This clock maintains the time and date
whether power is supplied to the recorder or not. It understands months with varying numbers
of days and leap years, although it does not understand UTC leap seconds. Since knowing the
time at which an event occurs or a signal is transmitted is central to the purpose of a logging
recorder, we feel it appropriate to include this description on the performance of the VR240
clock.
The clock uses a crystal resonator operating at 32,768 Hz. This is divided digitally to create
one-second ticks that are further used to count minutes, hours, etc. The accuracy of the clock is
completely dependent upon the accuracy of the crystal oscillator. When we adjust the
frequency of the oscillator at the factory, it is correct to better than .1 Hz, which corresponds to
an accuracy of about 1 second every 4 days. Unfortunately, the crystal frequency is subject to
drift over time, drift over temperature, and accidental miss adjustment. For these reasons, it is
not reasonable to expect more than about 1 second per day accuracy. If the unit is being
operated at unusually high or low temperatures, it could be significantly worse.
Your need for clock accuracy might range from absolute to unimportant. Of course, the relative
timing between channels will remain essentially perfect regardless of the absolute difference
between the VR240 clock and the Naval Observatory. If all you need is relative timing, errors of
many seconds or even minutes might not matter.
There are several ways of increasing the accuracy of the time on the VR240. The most obvious
is simply to set the clock against an absolute standard as often as necessary. To do this, you
obviously need a standard. Here are several suggestions:
a. If you are affiliated with a network or news service, they frequently provide time marks.
b. Monitor one of the many short wave time standards available. Here are a few:
STATION LOCATION FREQUENCY
WWV Boulder CO 5,10,15,20 MHz (exactly)
WWVH Hawaii 15 MHz
WWVB Boulder CO 60kHz (requires special receiver)