Sampling Rates
The SAMPLE RATE window offers a choice of button-selected sample
rates. You can also define your own sample rate by entering the desired
rate (in seconds) in the edit field and then clicking the 'Use
other interval' button.
Note that not all of
the standard rates are always usable, depending on recording mode, the machine
type, and the number of channels. The fastest continuous rate possible
is about 60 samples/sec on a Mac IIx or IIcx; considerably greater speeds
are possible on faster computers -- up to 500 hertz for a single channel
on 200 megahertz PPC-604 machines (and much faster on G3s and G4s).
The maximum sample rate drops as the number of channels increases.
Note: with internal A-D cards and ADC-1s, high input voltages (absolute value > 2.5 volts) are read slightly faster than lower voltages. This is because the automatic gain function starts with the lowest gain, and then -- if necessary -- adjusts gain to achieve maximum resolution and makes a second reading.
Also note that it's possible to select any sample rate from the 'Use other interval' button. If the selected rate is too fast for the number of channels used, the program shows a warning message. If you ignore the warning and proceed, the program will attempt to keep up but may not be successful. Data will be recorded and the computer will calculate the actual sample rate (which may be slower than desired) and display it at the end of sampling (see below).
When you select
a sample rate, LabHelper calculates the total trace time (elapsed
time for the number of samples in a complete screen). This is displayed
in the box at the bottom left of the window.
When selecting sampling rates for Chart
channels, you may adjust the desired recording time (i.e., the duration
of sampling) using the buttons on the right side of the window. Click
one of the available present recording times, or increment the number of
samples with the appropriate buttons ("<<" and ">>"
decrease or increase the sample count by screenwidth
samples; "<" and ">" increment by screenwidth/2
samples, and "max" sets the sample count to the maximum possible
value. In this example, the sampling rate is 2 samples/second, and
by clicking on the "1 hr" key the total number of samples has
been set to 7200 to yield a total sample duration of 1 hour (3600 seconds).
Note that some preset recording times are not available because they would
require more than the maximum possible samples,
or they are too short for effective recording with the selected sample rate.
LabHelper will not allow you to specify recording durations of less
than 10 samples.
For Oscilloscope channels, you can use a similar (but smaller) set of buttons below the trace duration box (in the lower left of the window). You can set the recording time for the minimum possible (one screenwidth) to the maximum possible (32,670 cases), in increments of 1 screenwidth.
In Multichannel
Oscilloscope mode, or for most sampling rates of 200 hertz to 20-100
kilohertz in 'regular' Oscilloscope or Scope plus Chart mode,
LabHelper automatically shifts to discontinuous sampling.
These rates are shown on the right edge of the scope sample rate window.
In Scope plus Chart mode, the chart sampling rates should be no faster than
every 2 to 4 seconds on Mac II, IIcx, or IIci computers. Buttons for
these sample rates are on the left.
When selecting an Oscilloscope sample rate,
you also need to specify the scope gain (1, 2, 5, 10, 20, 50, or
100 - fold amplification of the signal with 12-bit boards; 1,2, 10, or 100
with 16-bit boards). These gains are applied to the standard input
voltage range of plus or minus five volts (see next section for details
on polarity and scaling), to yield a range of acceptable input voltages
that vary from plus or minus 5 volts to plus or minus 0.05 volts (plus or
minus 10 V to plus or minus 0.1 volts for 16-bit boards). Use the
highest gain possible with your input signal in order to produce the maximum
resolution.
You also need to set the scaling (polarity) of the oscilloscope screen (zero to maximum volts, negative maximum volts to zero, or plus or minus maximum volts). Both gain and scaling are selected from pop-up menus in the lower right corner of the window, as shown here.
If you are using
an ADC-1 or DataTaker, the maximum realistic sampling rate is about 3 to
4 samples/second, depending on the input voltage and machine type.
You can set the program to sample faster, but the internal voltage conversion
of the converter will probably not keep up. If you use an external
A-D converter the SAMPLE RATE window will show all the higher sampling
rates as activated -- but in reality an external converter can't sample
that fast.
If you have set
the alarm function (see the MENUS page for details), the maximum
possible sampling rate decreases to some extent (particularly if large numbers
of readings are averaged). However, for most sampling rates, use of
the alarm function has no negative effects.
When data are gathered
and subsequently saved, LabHelper will keep track of the total number
of samples and the starting and ending times, from which it calculates the
average sample rate. If the average rate differs by more than 3% from
the requested rate, the average rate is recorded in the data file instead
of the requested rate. This is displayed at the end of recording;
see the options window page for details.
You can adjust this precision factor
from the EDIT MENU. In discontinuous
sampling for oscilloscope channels, the computer does not check sample rates
(they are 'hardwired' into the circuitry of the A-D board).
In Oscilloscope
plus Chart mode, you are presented with the SAMPLE RATE window
TWICE. The first time is for selection of the Oscilloscope sampling
rate. The second time is for selection of the sampling rate for the
Chart channels. The title bar at the top of the window indicates which
sampling rate is being selected.
For Chart channels, and continuously-sampled Oscilloscope channels, LabHelper relies on software to keep track of time. Unfortunately, it is possible to disrupt the timing functions (this is not true of discontinuous oscilloscope sampling, which is handled by hardware on the A-D board). To maintain a constant and consistent sampling rate, avoid 'distracting' the computer during data acquisition periods, particularly at high sampling rates. 'Distractions' include anything that causes the Mac OS to temporarily switch away from running LabHelper to a different task. Some hints:
In general, it's a good idea to shut down all other programs -- especially screen-savers, which will DEFINITELY cause problems -- when LabHelper is gathering data.
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