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

# Maximum Sync Error

Controls: Gen Dlg >> Stream >> Tone Freq >> Err
Macro: SyncErr

This control is only relevant when Trigger is active and Gen Sync is selected in the Trigger dialog.

To get a stable display of an ongoing waveform, the trace must start on the same part of the wave each time the display is refreshed. Typically, with Main Phase set to 0, this sync point is where the wave first rises above zero.

Suppose we are generating a wave with a frequency that is an exact integer submultiple of the sample rate. For example, with a sample rate of 44100 Hz, we can generate a wave at 1/64 of that or 689.0625 Hz using the Line step option. Since the wave takes 64 samples per cycle, we can start a new trace at any multiple of 64 samples and it will appear stable. (In fact, if there is no other component present, it will appear absolutely stationary.)

But Daqarta allows waves of any arbitrary frequency, within the resolution of its math. Suppose the wave is instead 689 Hz exactly, with the same sample rate. Now there are 44100 / 689 = 64.00580552 samples per cycle. Where can we start the next display to get a stable trace? Since a waveform display holds 1024 samples, we might like to start the next trace right after the first. But 1024 samples is only 15.9985 cycles, so we are not quite at the same point we started. If we wait just one more sample, we jump to 16.0142 cycles, and we've missed the exact start. Thus, if we insist that the trace start on exactly the same relative point in the waveform, we may have to wait a l-o-o-o-ng time between display updates!

So we need to relax our standards a little, and accept a certain limited amount of jitter in the display. The Sync Err control allows you to specify how much that error can be. If the allowable error is very small, then we may have infrequent updates when syncing to certain wave frequencies. If we set it too large, then we may get excessive jitter. The default is 0.2 sample, which is probably a good setting for most work. To prevent really long waits, the minimum value you can set is 0.01 sample. The maximum is 0.50 sample.

If the wait for the next sync point exceeds one second, a "SYNC WAIT" message will appear superimposed on the trace area. It will be cleared when the next update arrives.

Even with moderate Sync Err settings, it is theoretically possible to encounter excessive waits when the samples per cycle are very close to, but not quite, an integer value. If Daqarta determines that any sync point would be 240000 or more samples away (5 seconds at a 48000 Hz sample rate), it ignores Sync Err for that trace update and accepts the current "best fit" sync point instead.

Macro Notes:

SyncErr=0.1 will set the maximum sync error to 0.1 sample. Note that this is a single global setting; channel and stream prefixes are not needed.

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