Is oscilloscope trigger function something like audio compressor threshold ?

[komrad2236]

I was researching online about explanation to what oscilloscope trigger function is doing, to understand what is it, what can it do for me, how does it translate to real life. 

From what I gather, its something like a threshold of audio compressor.

Only when a signal goes over the threshold, does something happen.
In case of audio compressor, once signal is above threshold, compressor activates and affects it, aka compresses it.

I assume with oscilloscopes its, once the signal is above trigger(threshold), signal is displayed on screen ?

Is that the gist of it ?

[TomKatt]

That's the gist of it, but triggering is more nuanced where you can select 'above' a rising edge of a wave, or 'below' the falling edge or possibly in between where a runt pulse is higher than a low level but not quite great enough to be considered a high signal.  The idea is correct, but it can be targeted to different scenarios.

[pdenisowski]

I made this video showing how to configure different types of triggers on one of our basic oscilloscopes, but it also covers the fundamentals of triggering in general and the most common basic trigger types.

[Terry Bites]

Kinda
Basic scope triggering is either auto level threshold or manually defined threshold.
In "auto" the trigger circuit of the scope is tracking the envelope of the input.

What trigger does is to start the sweep from left to right across the screen.
It starts the X axis moving.
You might want to to that as a single shot or retrigger continuously.
You can even "free run" without a trigger.

[pdenisowski]

I assume with oscilloscopes its, once the signal is above trigger(threshold), signal is displayed on screen ?[/b]

Yes, that's pretty much the basic "rising edge" trigger that's the default on most oscilloscopes.  Note however that many oscilloscopes also default to an "auto" trigger setting where the scope will "trigger" and display a waveform every so often even if the signal does not rise about the trigger threshold.  A "norm" trigger setting means the scope will only trigger when the signal rises above the trigger level, so the screen may be blank if no valid trigger is seen.

[TimFox]

In an analog CRO, the trigger event (possibilities shown in a post above) starts the ramp waveform that scans the beam from left to right.
In "auto" mode, if there is no trigger event within a given time, the ramp starts automatically in order to keep something on screen.
In digital DSOs, usually the digitization is always running, and the trigger event determines which results are displayed.
An advantage of the latter is that stuff that happens before the trigger event can be displayed.

[komrad2236]

I assume with oscilloscopes its, once the signal is above trigger(threshold), signal is displayed on screen ?[/b]

Yes, that's pretty much the basic "rising edge" trigger that's the default on most oscilloscopes.  Note however that many oscilloscopes also default to an "auto" trigger setting where the scope will "trigger" and display a waveform every so often even if the signal does not rise about the trigger threshold.  A "norm" trigger setting means the scope will only trigger when the signal rises above the trigger level, so the screen may be blank if no valid trigger is seen.

And let me double check, if signal goes above trigger threshold, the signal that breached the threshold is shown in its entirety, its not just the part that went above the threshold.
Yes ?

[TimFox]

I assume with oscilloscopes its, once the signal is above trigger(threshold), signal is displayed on screen ?[/b]

Yes, that's pretty much the basic "rising edge" trigger that's the default on most oscilloscopes.  Note however that many oscilloscopes also default to an "auto" trigger setting where the scope will "trigger" and display a waveform every so often even if the signal does not rise about the trigger threshold.  A "norm" trigger setting means the scope will only trigger when the signal rises above the trigger level, so the screen may be blank if no valid trigger is seen.

And let me double check, if signal goes above trigger threshold, the signal that breached the threshold is shown in its entirety, its not just the part that went above the threshold.
Yes ?

On an analog CRO, the signal is delayed on purpose between where the trigger is sensed and the drive to the CRT, to allow time for the trigger circuit to work. 
The signal after the trigger event is displayed, within the vertical limits of the display, not clipped by the threshold.
On the DSO, signal can be displayed before the trigger event happened, and continuing thereafter.
Otherwise, the triggering system does not eliminate parts of the signal above or below the trigger threshold:  it's a timing system.

[donlisms]

No, it's similar but not the same as compression.  The thing they have in common is that there's a comparison between the signal and a reference.

In compression, that comparison is continuous; you might think if it as a "mode".  As long as the signal is high, you get compression.  Further, it's quite important how far above the threshold the signal is - that's kinda the whole point.

In a trigger, none of that applies, other than than that there is a comparison.  Other responders here have used the word "event."  It's a single point in time, meeting the specified conditions. The "normal" conditions are (traditionally) a specific signal level, and the slope of the signal when that level occurs.  Digital scopes can provide a much wider variety of conditions just by having the processor take a look at the record of the signal.  (Since it's in memory, we've got plenty of time to analyze it.)

In any case, the result of recognizing the event is to scream "NOW!"  (or in digital land, "HERE!"). A single point in real or virtual time.

Other elements of the scope are then responsible for displaying the signal starting at or around that single point. That stuff has no idea how the trigger event was generated.  It starts.  It also says "Busy drawing now, no new triggers for a while, please!"  (This is "holdoff.") Eventually it says "Okay, start looking for another trigger event."

Triggers are useful in devices other than oscilloscopes.  For example, I have a signal generator that can use triggers for several purposes, all based on "NOW!" - start a single wave, start a burst of them, synchronize the wave's starting point, etc. 

[pdenisowski]

And let me double check, if signal goes above trigger threshold, the signal that breached the threshold is shown in its entirety, its not just the part that went above the threshold.
Yes ?

Yes.  The trigger basically tells the scope "Acquire a waveform and display it on the screen" - it doesn't affect the waveform itself.