Digital Channel Number

Joe_Trotter@BD.com Joe_Trotter at BD.com
Tue Mar 9 16:55:34 EST 2004


Hello Leonie,

      In the BD digital systems a log display is just that - a log display
of linear values. In other words, it doesn't matter if you display in
'linear' or 'log', you get the same linear values over the range of the
data. These values, when 'log', are displayed over a 4 decade range from
the top down to 4 decades below (or 5, but that is generally not used - I
won't digress here). That gives, with a maximum 18 bits range (2^18 =
262144), log scaled displays where beginning with the lower limit of the
display you have:

      26, 262, 2621, 26214, and 262143 (values < 26 are displayed on-axis
at 26, but are used for all calculations. I won't digress into other than
log scales here).

      In contrast, the older analog systems used a logamp to obtain the 4
decade dynamic range. As a practical matter, the scale can be considered to
represent  the approximate linear voltage in millivolts that was the linear
input to the logamp. Since the maximum output voltage was 10 volts, in
millivolts the 4 decade display was labeled:

       1, 10, 100, 1000, and 10000

      Now, this was all usually digitized at 10 bit resolution into 1024
channels, 0 - 1023 where channel 0 mapped to 1.0 and 1023 to 10000.0. So,
if what you want to do (OK, I'm guessing here) is plot the digital
equivalent of an analog channel number of logamp compressed data to
calculate something like MESF (relative channel number of analog log data
vs log bead fluorescence), you need to log transform both the bead standard
values AND the digital channel values before doing any linear regression
(easiest way). In other words, use log/log instead of semilog.

      Hope this helps,

      Joe



		      Leonie Gaudry							  		      <L.gaudry at unsw.e	       To:	Cytometry Mailing List
<cytometry at flowcyt.cyto.purdue.edu>			
		      du.au>		       cc:					  					       Subject: Digital Channel Number		  		      03/08/2004 09:08							  		      PM								  



Hi,
Would anyone who is using the BD DiVa option have worked out how to
accurately determine a channel value on a log scale? It was easy to do in
Analogue because it was built into the program. It is also easy to do in a
Digital Linear scale. Any thoughts would be very helpful.
Thank you
Leonie
Haematology
POWH
Sydney
Australia


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