Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
Science with your Sound Card!
The following is from the Daqarta Help system:

Decimate Envelope

Controls: X-Axis Dialog >> Slow >> Envelope
Macro: X_Env

When the Envelope button is active in Decimate mode, the waveform display becomes a "fat trace" that includes both the maximum and minimum data values at each time point, with the vertical space between them filled with the trace color.

To understand why this might be useful, first consider a normal decimated trace, where the Envelope, Demodulate, and Signed buttons are all off.

Suppose you are monitoring seismic activity, with a Decimate Rate of 1 sample per hour. (You'd typically enter this as 3600 sec per sample.) If the raw sample rate is 48000 samples per second, then Decimate Factor will be set to 48000 * 3600 = 172800000.

At this slow effective sample rate, the 1024-point display will show 42 days of activity. But in normal Decimate mode, each 1-hour display point will be the average over the hour-long sample period. A brief event that only lasts a few seconds or minutes would thus be averaged together with a lot of inactivity, greatly reducing the apparent size of the event.

Instead, Envelope mode will show both the positive and negative peak values over that same hour interval. The event will thus stand out clearly from the inactive background.

Note that Envelope mode does not reflect the total energy of the sample period, because it doesn't look at the area under a peak, nor does it distinguish between a single brief peak and multiple peaks during the same interval.

For very slow rates (anything below an effective rate of about 500 Hz or so), it is usually best to run with Trigger off. This will result in a scrolling waveform to emulate a conventional chart recorder or data logger. Otherwise, the display will remain stationary until a new trigger event arrives.

However, if you do use Trigger with Envelope (presumably at higher effective sample rates), note that by default the triggering operation is applied to the raw data, not to the decimated data. The Trigger Source label becomes a button marked 'Source Raw', which can be toggled to 'Source Decimate' in order to trigger from the decimated data. However, unlike with normal Decimate mode, the peak-hold action of Envelope mode provides no low-pass filtering to reduce the effect of noise and transients on the trigger operation.

Envelope mode compresses the time axis by the Decimate Factor, showing you more on a single screen. This can be helpful for analysis of long data files, such as recordings of speech or music, to find portions of interest. (Spectrogram mode is another way to do this.)

The Envelope dataset consists internally of 1024 data points per screen just as other Daqarta modes, but here they are arranged as pairs of alternating maximum and minimum waveform values. Instead of the usual single trace of 1024 time points, there are really only 512 time points, interpolated to cover the normal 1024-point time span.

When you move a waveform cursor (solid or dotted vertical bar), it can still go to each of the possible 1024 screen positions, including interpolated points which are midway between actual time points. On the non-interpolated actual points, the X readout shows the correct time, and the Y readout shows the maximum value for that time point.

On the interpolated points (midway between each pair of true points) the X readout still shows the true time, which is the same as for the maximum-value point to its left, and the Y readout shows the minimum value for that true time. Because the readouts don't match the apparent cursor, they are highlighted in pink on these minimum positions.

In other words, as you move the cursor from left to right, the readouts will alternate between white and pink backgrounds. When the readouts are white, they match the maximum value under the cursor. If you then move the cursor one step to the right, the X readout doesn't change value, but both readouts change to pink. The Y readout then shows the true minimum value that goes with the X readout, but does not match the apparent X or Y values under the cursor.

When Envelope is active and a trace is saved as a .TXT file the file consists of 512 pairs of data lines... two lines of data for each true sample time. The first line shows the time, followed by the maximum values for each channel, while the second line shows the same time and the minimum values.

If you save a file using the .DQA option, the Envelope mode status is saved with the file so that it will be properly interpreted when later opened by Daqarta.

If you want to extract the envelope as an unsigned waveform, such as to apply to another underlying "carrier" waveform, see the Demodulate option.

Spectrum and Spectrogram (Sgram) display modes are not recommended for use with Decimate Envelope, because the raw 1024-sample dataset consists of alternating maximum and minimum values. This is not likely to give useful results; consider either Demodulate, or plain Decimate.

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