RE: Extract a column of data from 2D Waveform Record

[Mark Rivers <rivers@cars.uchicago.edu>]

Hi Peter,

The EPICS areaDetector module supports an object called an NDArray, which is an N-dimensional array.  EPICS drivers generate these arrays from the underlying hardware library, and pass them to plugins.  Plugins do things like save the arrays to disk, send the data to EPICS waveform records, perform statistics calculations on them, and extract regions-of-interest (ROIs).  The ROIs can be any dimensions, so they can extract single rows, single columns, etc.  The ROI plugin is itself a source of new NDArrays, and plugin chains can be constructed at run-time.

In your case, if your driver generated NDArrays (N=2) you could do the following:

Driver -> ROI plugin (extract 1 row or column) -> waveform record plugin

This processing all happens below the level of EPICS records, so it is very fast and efficient.  Data rates of hundred of megabytes per second are not a problem.

Although the NDArrays were designed originally to be used in imaging-type detectors, I have used them for a spectroscopy detector somewhat similar to yours, where the data contain spectra, not images.

Mark


________________________________________
From: tech-talk-bounces@aps.anl.gov [tech-talk-bounces@aps.anl.gov] on behalf of Peter Milne [peter.milne@d-tacq.com]
Sent: Wednesday, March 28, 2012 4:47 AM
To: Tech-talk
Subject: Extract a column of data from 2D Waveform Record

The ACQ196 multi-channel digitizer includes an embedded microprocessor
and EPICS IOC, and in normal operation outputs a Waveform Record per
channel on a relaxed schedule, 1Hz typical.

The data from the hardware is a 2D array in [sample][channel] order, and
embedded software takes care of extracting ("channelizing") the data so
that each waveform record is a 1D array   eg CH01[sample].

Now we have a requirement to run this at higher pulse rates, up to 60Hz.
The embedded software will not be able to channelize the raw data at
this rate, although it should be possible to present a single Waveform
Record per pulse containing the 2D data.

My question is how to handle this 2D record in a client and/or soft IOC
running on another host?.

The client software needs to be able to take a slice [:][ch].

Maybe it's quite easy to do this outside the IOC (eg with Python Channel
access), but ideally it would happen inside an IOC so that it's
completely transparent to a client. The Subarray record is an obvious
choice, but it appears to take a contiguous block of data, there doesn't
seem to be a concept of a stride value.

eg assuming the Waveform Record refers to data
[samples][channels] with dimension (typical) [1024][96], the extract
routine needs to run through the data set with a stride of 96.

It looks like subarray could be used to extract [sample-N][:] but not
the required [:][channel].

Any suggestions welcome.

Thanks


Peter.




--
Peter Milne                     Peter.Milne@d-tacq.com
D-TACQ Solutions Ltd            www.d-tacq.com