Martino FerrariandClaude Opus 4.6
1627abcd4a
Converts every --buffer-mb / buffer_mb mention to --buffer-samples / buffer_samples, records the unit change as a deviation (README, SPECS, CLAUDE.md), documents the buffer: and planner: dump lines and the new Preferences row, and updates the help pages and the help-shot script. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
66 lines
2.9 KiB
Plaintext
66 lines
2.9 KiB
Plaintext
<page xmlns="http://projectmallard.org/1.0/"
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type="topic" style="task"
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id="window">
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<info>
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<link type="guide" xref="index#viewing"/>
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<link type="seealso" xref="navigate"/>
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<desc>The time window control: how to set it, and why a long window is decimated rather
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than refused.</desc>
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<credit type="author copyright">
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<name>Martino Ferrari</name>
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</credit>
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</info>
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<title>Time window</title>
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<p>The window is how much time the plot shows. Every row shares one window: the figure has
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a single x link group, so the rows cannot drift apart. The trigger's event span follows it,
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and the pre-trigger share is a share of <em>this</em> window.</p>
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<section id="reach">
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<title>How to reach it</title>
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<steps>
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<item>
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<p>The header bar's window button opens a popover: type a number, pick a unit under
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it, or step a decade with the arrows.</p>
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</item>
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<item>
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<p>The button's own label is a reading of the current window in a natural unit,
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width-locked so a change of window cannot widen the header bar.</p>
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</item>
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</steps>
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</section>
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<section id="knowing">
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<title>Worth knowing</title>
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<list>
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<item><p>A window too long to store at the wire rate is <em>decimated, not refused</em>.
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The shared clock takes a power-of-two stride, and each group of samples is
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<em>averaged</em> rather than strided, so a tone above the stored Nyquist is attenuated
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instead of aliasing to a false low frequency.</p></item>
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<item><p>The stamp stored for a group is the group's <em>mean</em>, which is where the
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averaging puts the value. Stamping at the end of a group would shift every trace half a
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stored sample late.</p></item>
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<item><p>Moving the window across a decimation boundary rebuilds the buffer and drops
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the history. Walking it inside one decade is free.</p></item>
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<item><p><em>This control is the only thing that changes the window.</em> A zoom, a pan
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or a fit moves the view <em>inside</em> it and leaves it alone — which is the point of
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the previous bullet: a wheel notch that redefined the window could re-plan the buffer
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and throw your samples away.</p></item>
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<item><p>The trigger sees the <em>stored</em> (filtered) signal, not the wire signal,
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because an edge found on a sample that is not in the buffer could not be cut out of
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it.</p></item>
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</list>
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</section>
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<section id="example">
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<title>Example</title>
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<p>1 MS/s with the default buffer of 8 388 608 samples per signal (8.39 s). A 2 s window
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stores every sample (<code>time x1</code>). Widen it to 600 s and the stride becomes
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<code>x128</code>: 7.8 kS/s stored, anti-aliased at 3.9 kHz. A 5 kHz sine seen through that
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window is attenuated by the filter, not folded down to a full-amplitude low-frequency one.</p>
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</section>
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</page>
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