A complete scan by the multiplexer
(one revolution of the commutator) produces a frame of the stream of
words containing
the value of each measurand. Every scan produces the same sequence of
words. Only the value of a measurand is captured, not its address (name).
If only the measurand’s data is captured, there is no way to distinguish
the owner of one value from the next. Thus, a unique word called the frame
sync is added at the end of each frame
to serve as a reference for the process of decommutating the stream’s
data (i.e., extracting it into individual measurand values).
The example above shows a frame sync word for only 10 data words. In
practice, the frame may incorporate hundreds or thousands of measurands.
While this reduces overhead, it increases the amount of data lost if the
frame sync is corrupted or bits of the frame are lost and the location
of measurands cannot be guaranteed.In a simple commutator,
each data word is sampled once per revolution at a rate compatible with
the measurand with the fastest changing data. Since the rate of change
of a measurand's value varies tremendously, the sampling frequency rate
must accommodate it. As an example, to characterize vibration requires
many more samples per second (thousands) than temperature (fractions). According to the Nyquist Theorem, you must sample data at twice the maximum
frequency component for the signal to be acquired. Sampling rates of 5
times the maximum frequency component are typical. A low pass filter is
used to eliminate any frequencies that you cannot accurately digitize
to prevent aliasing.
If we were to take a worst-case approach to sampling all measurands at
the highest rate, we could expect much waste in carrier frequency spectrum
and power. Sampling rates should therefore vary with respect to frequency
content and be somewhat independent of other measurands with different
periodic acquisition rates. Highly sampled measurands are super-commutated
with multiple occurrences of the measurand in each frame.
The opposite scheme occurs in subcommutation and embedded
asynchronous data streams, where one
position over time has multiple meanings. More on these commutation schemes
appears later.
Representing the telemetry stream as a continuous string of values in
a diagram, while possible, is very cumbersome as shown below.
In addition to the data words WD2 through WD10, you
will notice the FS for frame
synchronization. Frame syncs mark the
end of a frame so that the original data can be reconstituted in the ground
station. As you can see, it can be cumbersome to visualize the simulated
serial output data in this format.
An easier way to visualize data is presented in the table below and is
defined in Chapter 4 of the IRIG-106 Standard. The standard includes both
naming and numbering conventions of words and frames as seen below.
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