Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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The following is from the Daqarta Help system:



Spectrum Analyzer

Signal Generator

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Engine Simulator

LCR Meter

Remote Operation

DC Measurements

True RMS Voltmeter

Sound Level Meter

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Histogram (PSTH)

THD Meter

IMD Meter

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Macro System

Multi-Trace Arrays

Trigger Controls


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Direct-to-Disk Recording



Frequency response

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DDisk Read Step Size

Controls: DDisk Dialog >> Read Step Size
Macro: DDiskReadStep

This control has no effect on DDisk recording, only subsequent reading. When you first open a DDisk file for viewing, the display is Paused showing the first screen of data. If you stay in Pause mode, you can move through the file manually. The <> file navigation buttons or the unshifted <> keys move by steps of one sample, and the << or >> buttons or shifted <> keys move by 512 samples. The ALT+<> keys move by the Read Step Size. You can also enter a desired value directly into the position readout below the right end of the trace.

When Decimate is active, all steps are multiplied by the Decimate Factor.

Hitting unPause allows the file to "play" on the screen, showing the same sort of display you would have seen while recording. This is useful for rapidly reviewing the file.

The Trigger controls work normally in this situation (except Gen Sync, since the Generator doesn't run during file viewing), which allows you to set a given level, slope, and hysteresis to get a stable trace during the playback. However, triggered operation implies that some data is skipped until the proper level and slope are met for the next display.

On the other hand, if you toggle Trigger off then all the data will be displayed, one screen after the other, but it may go by too fast to really see. You could resolve this by setting a slower Trace Update interval, but there is another way:

The Read Step Size tells Daqarta how far to move through the data before each display update, when Trigger is off. The value you set is always relative to the start of the screen; if you set it to 1024 Samples, then the next screen would start with the next unseen sample. The situation would be just as described above, one screen after another.

But you can set the Read Step much lower. If you set it to 1, then each new screen only advances 1 sample through the file, so that the first 1023 samples of each waveform are old data and only the right-most sample is new. When you do this, the waveform scrolls smoothly across the screen from right to left, in slow motion.

As you increase Read Step, the scrolling appears faster and faster. But if you set it to some multiple of the period of a wave, that wave will appear stationary. Set it slightly beyond that and the wave will appear to scroll backwards due to a "strobe" effect. So you will probably want to set Read Step to a very low value if you want smooth, natural-looking scrolling.

Alternatively, suppose you have a really long file but the events you are searching for are themselves many screens long, and hard to trigger on. You can set Read Step to show only every other screen by setting it to 2048, or up to every 64 screens by setting it to the maximum of 65536. Then when you see a likely-looking signal flash past, you can Pause and back up for a closer look.

But probably the most useful application of Read Step is with Spectrogram or Pitch Track (Sgram/PT). Each time point on the Sgram is a vertical column that is a spectrum obtained from 1024 data points. Typically when viewing the Sgram of a file, you will want to toggle Trigger off so that each spectrum is spaced according to Read Step.

If you want fine temporal resolution, such as for viewing the syllables of speech or details of the attack transients of a musical instrument, you can set Read Step to a small value. At the minimum of 1 sample on Read Step, each Sgram time point is effectively only 1 sample long. That will "zoom in" the spectrogram so that the screen is filled with less than 12 milliseconds of data.

Note that if you need still better temporal resolution, you must be willing to give up some spectral resolution. You can do this by setting the Spectrum Window Width to values smaller than the default 1024, giving an effect similar to using a shorter FFT for spectral calculations. See the Spectrogram page of the Screen Shots section of the Daqarta Website for examples of wide and narrow Width applied to a speech spectrogram.

The default units for Read Step are Samples, but you can use the DDisk Units control in this same dialog to toggle the units among Bytes, Samples, Seconds, or time in HH:MM:SS format. In the above case, Seconds may be especially useful if you know the time resolution you want. (Local or UTC Time modes are for working with absolute file positions; HH:MM:SS relative time will be substituted here.)

Macro Notes:

DDiskReadStep=10 will set the Read Step to 10 bytes, 10 samples, or 10 seconds, depending on the DDisk Units setting. (HH:MM:SS mode is regarded as seconds here.)

See also Direct-To-Disk (DDisk) Toggle, DDisk Controls Dialog


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