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APPENDIX B
50
If another type of streaming is enabled while memory streaming is enabled, memory streaming is
disabled unless the
MemoryChannelList
property is set.
Disk Streaming
Disk streaming is intended for high performance applications and/or long acquisitions. When
enabled by setting a filename, the driver optimizes the data path by not converting the data from the
native VRT data format, but, instead, delivering it as directly as possible from the network socket
to the disk. A streaming file library has been made available so that post-processing can be done to
turn this data from VRT format into any of several other formats.
Disk streaming has an optional maximum file size. When this is set, an error will occur and
streaming will be disabled if the maximum file size is reached. This may be useful if the user has a
disk quota or other limiting size they wish to avoid.
Another option provided is a maximum file count. When paired with the above option, the user-
provided filename will be appended with a number (“Datalog” would turn into “Datalog0”), up to
the maximum file count. This may be useful if the user can only process a file of a certain maximum
size, but wishes to continue to take data after that size is reached.
A file rotation option has also been provided. When the
FileRotate
property is set to true and paired
with the two options above, the driver will create
FileCount
files of
FileMaxSize
bytes, and then
begin overwriting the earliest file created. This may be useful for infinite acquisitions, but assumes
the user will be able to process the completed files (or copy them to another location) before the
driver overwrites them.
S
AMPLING
R
ATES
The properties in the
Sampling
interface configure the A/D converter and decimation filters to
specify the sampling rate and frequency span of the data to be acquired. Some properties in this
interface are interrelated. Changing one property value can affect the other.
There is a
Sampling
property in the
Measurement
interface as well as the individual
Channel
interface. The global
Sampling
parameter that applies to all channels can be configured from the
Measurement
interface, while the
Channel
interface allows the user to configure each channel
independently.
ClockFrequency
The
ClockFrequency
specifies the A/D conversion rate. This property determines the highest
SampleRate
of the data acquisition session. The highest rate can be achieved by bypassing all
decimation filters. There are discrete sets of
ClockFrequency
values that the user can specify
depending on the instrument model. See instrument specification for the list of
ClockFrequency
values.
ClockFrequency
is a global parameter. It cannot have different values for each channels.
While it is possible to have different
ClockFrequency
values between cards in a single driver, it
would be more convenient to have multiple driver instances for each
ClockFrequencies.
Prescaler
This is the sampling rate divider before the ADC data is decimated by the ½ decimation filter stages.
When a value of 5 is specified, the output from A/D converter is sent to 1/5 decimation filter first
before going into ½ decimation filter stages. Otherwise, the
Prescaler
value is 1 and the data goes
directly to the ½ decimation filters.
SampleRate
This is the effective data rate. The inverse of
SampleRate
specifies the interval between data samples
that the user can obtain. The
ClockFrequency
,
Prescaler
, and
SampleRate
determines the number
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