Data AcQuisition And Real-Time Analysis
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The following is from the Daqarta Help system:



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Trigger Delay

Controls: Trigger Dialog >> Delay
Macros: TrigDelay, SmplSec

With Trigger Delay set to 0, the display shows the samples immediately following a trigger event. The waveform display shows up to 1024 samples (with X-Axis eXpand off). Think of that as a 1024-sample window onto the set of all samples; Delay allows you to slide that window later in time (positive Delay) or earlier in time (negative Delay) relative to the trigger event.

Why would you want to use Delay? Consider that sometimes the region of interest in a signal is not near the trigger point in time. With an impulsive response, for example, you probably will want to set Slope and Level to trigger near the initial peak of the response for stable triggering.

The non-Delay display will then show the next 1024 samples, which only runs out to about 23 milliseconds at a 44100 Hz sample rate. What if you want to study low-level phenomena that take place as the impulse response dies down farther than this? Sure, you could reduce the sample rate, but that only goes so far, and may compromise time resolution and introduce aliasing.

The ideal way to handle this is to slide the trace area later in time, to look at events that happen well after the trigger while still maintaining a high sample rate. This is done by setting a positive trigger Delay value. After the trigger is detected, Daqarta will wait for the equivalent number of Delay samples before starting to collect the 1024 samples for the trace, thus sliding the "viewing window" to later times.

The waveform display X axis will always show the correct time since the trigger sample. With a Spectrum display, there is no change to the frequency axis since Daqarta is analyzing the same number of samples at the same sample rate... they just start at a different time.

But what if you want to see events that happened before the trigger sample? If you set the trigger Level to a high value to get reliable triggering from a response peak, you will miss seeing the leading edge of that peak. Or consider the case of a normal electrocardiogram (EKG), where there are low level P and Q waves that precede the large R wave trigger event.

CAUTION: Do not connect any electrical equipment to a living subject without proper signal isolation techniques. A lethal shock could result.

You can look ahead in time with a negative Delay value. This is not really precognition, since you don't get to actually see these events before they happen... you get to see events that happened before the trigger happened. In reality, incoming data is simply stored to a large circulating buffer, and when the trigger arrives it is an easy matter for Daqarta to count back the proper number of samples in the buffer to get data before that trigger.

Daqarta can operate with delays of up to +/- 32767 samples. But note that the acquisition time includes all of the samples, even those you don't see. For example, at a 22050 Hz sample rate a positive Delay of 32767 samples adds nearly 1.5 seconds to the total time between trace updates. (Negative Delay values don't have this effect since the data has already been collected.)

The small button to the left of the Delay control toggles between 'Smpls' (samples) and 'sec' (seconds). You can enter or view Delay in whichever units you choose, and switch back and forth at any time. The units setting also applies to the Holdoff control below Delay, as well as several other controls throughout Daqarta.

Note that Delay should only be adjusted when the display is not Paused. If you try to change Delay during Pause, the trace and X axis will not change; instead, you will see a 'PARAM CHANGE' message on the trace. The axis will jump to the proper range once you unPause.

If you need to be able to scroll back and forth over a single contiguous set of samples, consider recording a DDisk file that you can later scroll though.

Macro Notes:

The Smpls/sec state determines the units used when setting Trigger Delay, as well as Holdoff. SmplSec=sec sets the units to seconds, as does SmplSec=1. SmplSec=Smpl or SmplSec=0 sets Samples, and SmplSec=x toggles between the two.

TrigDelay=10m sets Trigger Delay to 10 msec if the units are seconds, else 0 if the units are Samples (10 millisamples = 0.01 sample = 0).

Alternatively, TrigDelay=>1 increments Delay by 1 sample and TrigDelay=>-1 decrements by 1 sample. The step size is the same for seconds mode, but the display shows the equivalent seconds.

Regardless of Smpls/sec state, you can set the delay directly in samples via TrigDelay#N=UD, where UD can be any value or expression. You can read the current delay in samples via UD=TrigDelay?N.

See also Trigger Toggle - Introduction, Trigger Control Dialog


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