AVERAGE START (Avg):The unshifted A-key begins a new average in the current mode (wave or spectrum), using the parameters set in the corresponding averager menu invoked by CTRL-A. The status message changes from LIVE to AVG, and the Avg option remains highlighted as long as the average is in progress.
AVERAGE CONTROL:CTRL-A invokes a menu invokes either the Waveform Averager or Spectrum Averager menus, depending on the current mode. If you change modes while the menu is visible, it will change to the proper menu for the new mode. The two menus look and operate very much the same, except the Spectrum Averager allows an extra mode (Peak) not present in Waveform Averager.
AVERAGER MODE:Sweeps parameter. After you start this mode by hitting the A-key, it will run until all requested Sweeps are averaged, then enter the Pause mode and show DONE.
The Expntl (Exponential) mode is an indefinite-length operation, which will run until you hit Pause or exit by toggling the average off with the A-key. Here the Sweeps parameter changes only the effective time constant, not the duration.
The Peak mode is only available for the Spectrum Averager. It will run for the specified number of Sweeps unless you have set that value to 1, which will be shown as 'Cont' for Continuous mode. In that case, the Peak "average" will run until you exit by hitting the A-key again.
AVERAGER SWEEPS REQUEST:This sets the requested sweeps for True mode, or the time constant for Expntl mode. If you increase this after the average is started (but before DONE in True), then the average will just proceed with the new value. If you reduce it, and the average hasn't yet reached the new request, it will also just proceed with the new value. Otherwise, it will restart using the new request.
Note that this is the requested sweeps value. The processed sweeps are shown in the counter above the right end of the trace area.
For Peak mode, if you set the requested sweeps to 1 you will see 'Cont' instead of the value. This indicates a special Continuous peak scan that will run indefinitely when you start the "average", until you specifically exit by toggling the Avg key off. (Note that there is no particular use to an actual Peak setting of only one sweep, since True would give the same result.) In Cont mode, the sweeps counter will count normally up to 65534, then show 'Cont Peak' above that.
AVERAGER ARTIFACT LIMITS:If any sample of a sweep is beyond these limits, that whole sweep will be rejected and the sweeps counter will not change. An 'ARTIFACT' message will appear in the upper right corner of the screen, but the averager trace will not update until the next good sweep comes along. You can view the instantaneous input with the \-key if you want to see just how bad the incoming signal is while the averager continues to run with whatever good sweeps get through.
A limit bar on the right edge of the trace area shows where the limits are set relative to full scale for the input range. This bar does not change with trace magnification, so if you want to compare to the incoming waveform in order to help decide where to set the limits, you should set the trace magnification for full scale.
Artifact rejection works on the incoming waveform data, even if you are doing spectral averaging. The limit bar will thus not be directly comparable to the spectrum display.
The artifact limits are separate for waveform and spectrum averaging, allowing you to keep setups ready for two different acquisition scenarios.
Artifact rejection is always in effect when averaging. If you don't want this, simply set the limits to full scale. Since nothing will ever be above these limits, no sweeps will be rejected. You can easily set full scale values by entering much larger numbers like '1k' and '-1k', and Daqarta will set only the maximums appropriate for the effective range.
AVERAGER REMOTE CONTROL:If you have installed a board / driver that allows digital inputs, you may choose to use remote control to start and stop averaging either electronically or manually via a hand-held switch. If you don't actually have anything connected to the remote input, if you have something else hooked there, or if you just don't want to use remote control, this option must be set Off to disable the input. Otherwise, whenever Daqarta sees a TTL high signal on this input, no sweeps will enter into the average, the display will switch to showing the instantaneous data, and the status will show SKIP.
For tips on construction and wiring of a manual switch, see Remote Control Switch Construction.
AVERAGE ADD (AvAdd) and AVERAGE SUBTRACT (AvSub):For True averages (which run for a predetermined number of Sweeps), there are two special continuation modes. These are not available in Expntl or Continuous Peak modes. Once the original average has reached completion (DONE status), you may extend the average to include additional sweeps. Hitting ALT-A (AvAdd) will extend it by the current Sweeps request setting.
For example, if the original average was 128 sweeps and you hit ALT-A without changing the request value, another 128 sweeps will be added. The sweeps counter will count up from 128 to 256 as the extended average proceeds, and the final result will be as if you had requested 256 sweeps in the first place.
On the other hand, if you instead hit SHIFT-A (AvSub), the second 128 sweeps will be SUBTRACTED from the average. The sweeps counter will still count up from 128 to 256, but any constant features of the signal will average down toward zero.
You might want to use AvAdd in cases where the result from the original average is not quite clear enough, and you don't want to waste the original sweeps. This might be the case where you have a limited time to collect data before conditions change, or where averaging is inherently slow because of some limit on how fast you can induce responses from the system being measured.
But the most powerful uses of AvAdd and AvSub are in special cases where the response you want to observe is contaminated by an undesired response or stimulus artifact. If you can arrange to produce the same desired response while obtaining an inverted undesired response (or vice-versa), you can use these modes to cancel the response you don't want.
For example, suppose you are recording electrical responses of the ear to acoustic tone bursts. (Always be sure to use proper electrical isolation techniques to prevent lethal shocks!) The raw response will contain a component called the Compound Action Potential (CAP) that represents the summed response of many neurons firing in synchrony with the start of the tone burst.
Many of these neurons fire regardless of the polarity of the tone in the burst... they just respond to the overall onset of the tone. But the response will also contain a Cochlear Microphonic (CM) component that has a waveform similar to the stimulus itself, which is thus inverted when the polarity of the stimulus is inverted. The overall signal is contaminated with plenty of noise that needs to be removed by averaging.
So if you average half of the desired number of sweeps normally and then flip the stimulus polarity before you average the remaining half with AvAdd, the CM portions will cancel and you will be left with only the CAP portion. Or if you used AvSub for the second half, the unchanged CAP portion would be subtracted away, while subtracting an inverted CM that would be equivalent to averaging the original CM alone.
Similarly, you could be looking at thermal effects in a loudspeaker driver at high power levels. The polarity of the driving signal doesn't change its heating ability, so if the drive signal is contaminating the temperature measurement, you can use the above trick.
Note that these options may not always be the best way to do this job. For example, if conditions are slowly changing during the average, the second half may not cancel the first half properly after you invert the polarity. This can easily happen in physiological systems due to electrode contact deterioration, anaesthesia wearing off, etc. Or you might be testing your loudspeaker outdoors and the weather changes.
In these cases AvAdd and AvSub may be helpful during the development phases of your experiment, but you way want to use something fancier for the real data collection. If you can arrange things so that the stimulus alternates polarity on each sweep, then all you may need is a normal average to get rid of the stimulus-like component of the response. This has the advantage that the two sweep polarities are near each other in time and thus more likely to cancel properly since the system has had less time to change.
AvAdd and AvSub may be used in spectrum Peak mode only when the Sweeps request is a specific number, not Continuous. They both function only to continue the same peak scan action for an additional number of Sweeps.
JOINT AVERAGE START (JntAv):The unshifted J-key starts both waveform and spectrum averagers simultaneously, using parameters set in the Joint Average menu invoked by CTRL-J. The JntAv option will be highlighted instead of Avg, and hitting the J-key will cancel the JntAv, but otherwise operation is the same as for a regular single average, including operation of Pause, Skip, Remote, and instantaneous view (\-key).
During the JntAv you may toggle freely between waveform and FFT modes to view those respective averages.
JOINT AVERAGE SPECTRUM \ WAVE TOGGLE:There is one additional option you might occasionally wish to use while in JntAv mode: If you are viewing the FFT average, ALT-Backslash (ALT-\ key) will toggle between that and the FFT of the wave average. Since the wave average reduces noise and the FFT average shows the average noise, you can view two entirely different pictures of what is happening.
JOINT AVERAGER CONTROL:CTRL-J invokes a special menu that includes both waveform and spectrum modes. It looks just like the normal averager menu for the current mode, except that the title is always Joint Averager. This is really two menus that are cross-linked to each other, but completely separate from the Waveform and Spectrum menus... changing a parameter in the Waveform menu has no effect on the corresponding Joint menu for waveform mode, and vice-versa.
To set up a Joint average, you must set the Joint parameters in waveform and spectrum modes separately, which you can do simply by toggling between then with the F-key after you invoke the Joint Averager menu with CTRL-J.
Since a Joint Average involves two averagers running at the same time, certain compatibility issues are enforced. If both the waveform and spectrum Joint Averager menus have True modes set, then the Sweeps parameters are simply locked together: Changing Sweeps in one menu changes it in the other to match. In all other cases, the Sweeps settings are independent and function as for normal averaging.
The overall averaging operation will stop when either waveform or spectrum mode reaches its Sweeps request, although as usual in Expntl mode the Sweeps value is a time constant and not a specific count request. If both waveform and spectrum Joint menus are of indefinite duration (Expntl or Continuous Peak modes), then averaging will continue until specifically toggled off by hitting the J-key again.
The Artifact Limits for both Joint menus are locked together, so you can change them from either menu.
Similarly, Remote Control is enabled or disabled from either Joint menu.
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