Trace Update Interval

Controls: X-Axis Dialog >> Trace Update
Macro : TraceUpdate

The Trace Update control sets the interval between display updates. Note that this only affects the rate at which data is displayed; it has nothing to do with acquisition of data.

A small cursor-colored square is shown in the upper right corner of the trace area on alternating updates, so you can tell that the trace is "live" even with a waveform or spectrum that repeats exactly on each update.

The Trace Update interval works together with the Trigger process. At each Trace Update interval, Daqarta tests to see if the specified trigger event has occurred since the last display update and shows the new trace if so. If not, it waits for the next Trace Update.

If Trigger is toggled off (free-run) then traces will be shown at the Trace Update interval unless Trigger Holdoff is set to a higher value.

The Trace Update value defaults to 10 msec (100 updates per second), but can be set as low as 1 msec (1000/sec). Win9x systems default to 55 msec (18/sec), and can be set as low as 10 msec.

You might think that faster is always better, but that's not always the case. For waveform and Spectrum displays, update rates faster than the eye can follow may be simply wasting processing time, possibly causing poor control response on overburdened systems. There are also limits to how fast Windows can draw a full trace on your system, which can prevent you from actually getting the specified update rate.

You may sometimes want to deliberately slow the updates, such as to produce a Spectrogram that shows a long time span on a single screen. (That assumes, of course, that the signal is not changing so rapidly that you miss interesting events.) You can set Trace Update as high as 10000 msec (10 sec), but it is usually better to keep it at the default value and use Trigger Holdoff for longer intervals. This works even when Trigger itself is not active.

On the other hand, for maximum speed of untriggered live spectrograms (best time resolution), you may want to set the update interval as low as your system allows. Note that spectrograms can use much faster update rates than waveform or Spectrum displays, since a spectrogram update consists of just a single column of pixels that Windows can draw quickly.

You can tell if you are getting the specified update interval by considering that a full spectrogram consists of 512 time points (pixel columns). So if you set Trace Update to 1 msec, the horizontal axis of the spectrogam should be 512 msec wide. At this high speed it is normal to find that the axis values jump around a little, since the timing is more sensitive to competing Windows activities.

In particular, note that Windows takes longer to draw large characters for the Frequency Counter, Sound Level Meter, or Voltmeter. You may see gaps or discontinuities in the spectrogram that repeat at 100 msec intervals, which is the meter update rate. If you have multiple large meters open, the gaps will be more frequent. (The meter updates are staggered so they don't all happen at once.)

Spectrograms of file data aren't affected by the Trace Update control: They are controlled by Read Step Size instead. However, when you unPause the file to let the spectrogram "run" as if live, the Trace Update value is used to pace the drawing process. The resulting display shows the same image with the same time axis regardless of the Trace Update value; the only thing Trace Update affects is how fast the spectrogram appears.

Pitch Track behaves just like a conventional spectrogram in this respect. If you are using Pitch-to-MIDI on file data, you can use Trace Update to control the tempo of the result, independent of the original tempo.

Note that Trace Update values are maintained separately for live and file operation. This allows slow updates for DDisk files run in triggered decimated mode that might otherwise zip past too fast to study, without affecting the setting for normal live runs.

Macro Notes:

TraceUpdate=10 sets Trace Update directly to 10 msec. TraceUpdate=>1 increments the current value by 1 msec, and TraceUpdate=>-1 decrements by 1 msec.

See also X-Axis Control Dialog