<b>Trigger</b>

The trigger watches one signal for an edge through a level and captures a window around it.
The captured window is frozen on screen until you re-arm.

<b>How to reach it</b>

The trigger button in the toolbar (<tt>t</tt>) reveals the trigger panel: the signal, the level,
the edge (rising, falling, either), the mode (Normal or Single) and the pre-trigger share.
<b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard.

<b>Worth knowing</b>

• Arming <i>enables</i> the signal if it was off, because a trigger on a signal nobody acquires
  can never fire.
• The pre-trigger share is a share of the <i>display window</i>, not of the ring. A 2 s window
  at 50% captures 1 s before the edge and 1 s after.
• There are four display states, and the status bar names each: <tt>armed</tt> (watching,
  nothing drawn yet), <tt>capturing</tt> (the edge was accepted and the post window is filling
  <i>in view</i>, with the marker at x = 0), <tt>event held</tt> (the frozen capture) and
  <tt>disarmed</tt>. A held capture is never quietly replaced by live data.
• The first edge after arming is only accepted once the history reaches back a full pre-trigger
  window, so a capture may legitimately be shorter than you asked for. The axis is pinned to
  what the capture actually got.
• Disarming <i>during</i> a capture publishes what the capture got to, so the picture on screen
  at that moment stays on screen. No event fired, so the event count does not move.
• The trigger reads the <b>stored</b> signal — decimated and averaged — not the wire signal.
  A spike narrower than one decimation group is filtered away rather than triggering on data
  the buffer does not contain.

<b>Example</b>

A 1 Hz sine, level 0, rising, 2 s window at 50%, Single. The capture spans
<tt>[-1.000 .. 1.000]</tt> with <tt>n=20001</tt> samples at 10 kS/s — the span times the real
rate, plus one — and the marker sits at x = 0 throughout.
