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The Common Clock Concept
Understanding the concept and limitations of independently counting timecode.
We have seen that the use of a 24 hour real time stamp has many benefits, one
of which is the non-repeatability of the timecode. There is and always will be
only one value for any given date and time. This unique time reference thus
becomes the common denominator that synchronizes all Aaton and SMPTE devices on
set.
On paper, the concept is simple and elegant: if all devices on set contain
highly accurate clocks recording the identical date and time, then the
associated image and audio can be synced to one another by referencing the time
stamp rather than the clap of a slate.
In the real world, however, it is important to take a few precautions to enable
the system to work as such.
For the purposes of audio sync, Aaton specifies that cameras, recorders and
other devices remain accurate within 1/2 a frame to one another at all times.
Today's technology allows for the development of extremely stable temperature
compensated electronic crystal systems to control the internal timecode clocks
to a high degree of accuracy.
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A ppm (parts per million) rating is a measurement of the accuracy you can
expect from a particular timecode device over time. The lower the ppm number,
the more accurate the device. The more accurate the device, the less timecode
drift will occur over time.
Here is a chart listing the manufacturers' specifications of the stability of
their internal timecode
crystals:
Denecke Ambient GMT 1 7-8 hrs Any device w/ GMT 1 7-8 hrs
As you can see from the chart, the accuracy of the crystal used in all Aaton
cameras and timecode generators, as well as the crystal used in a growing
number of DAT recorders (such as the HHB with MS1000 timecode module) is rated
at 1ppm. This translates to an accuracy of approximately 1/2 a frame drift or
less over 7 hours.
Ideally, the better the ppm rating of all equipment on set, the more time you
can roll before the need to re-initialize. In a timecode environment, remember
that your estimated time between re-initialization will be dictated by your
weakest link.
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