VCV Scope: External Trigger

Hi, just need some clarification of the external trigger function.

I am making an assumption that using the external trigger would result in a ‘one off’ trace, ie the trace would be captured/frozen/locked like the scope from Submarine.

But for me the trace is always free running no matter if the trigger is 0 or 10 volts or the level of the Trig value.

What am I not understanding?


I’ve always wondered how that worked exactly, but what i have noticed that if i want to see a vco’s waveform nicely, sometimes it helps to also put that same audio rate signal in the ext. So maybe it works like a clock input on how fast the visual is sampled for you to see? would be curious of the proper answer though…

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This is what I thought its function was also, not sure myself!?

The trigger sets the triggering threshold. When the input crosses the threshold, it goes. As you are observing. It it for sure not a one shot storage scope. Submarine makes a nice logic analyzer that does that (sort of).

Gee - didn’t you people have analog oscilloscopes in the 70’s? :wink:


I must have skipped that accounting class when they explained how to use them.

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There wasn’t one on the computer punch card machine, either!

At my school we had to use punch cards for CS1 - introduction to Fortan, then went through front panel toggle switches (PDP-11), teletype, paper tape, and finally terminals. yay!


Not my era!

VCV is a lot more fun.

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Aaaaah, the good old days! I love the smell of thermal paper in the morning!

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But it is always going. Trigger threshold = 10V, ext input = 5V.


Ah likewise during the 70s in my high school. How did I survive?!

Had one of these in the office:

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The trigger behavior is based on analog oscilloscopes:

  • In internal trigger mode, the horizontal scan will begin when channel X rises above the TRIG voltage, only if it is not already scanning.
  • In external trigger mode, the horizontal scan will begin when EXT IN receives a ~2V or higher pulse, only if it is not already scanning.
    • If no EXT IN is patched, the scan will begin as soon as it ends. This can be called “free mode”.

There is one additional rule, which I don’t think any analog oscilloscopes have.

  • If the scan is waiting for 1 second, a scan will begin, as if “timed out”.

I’m sorry to appear as thick as the Prince Regent from Blackadder, but I don’t understand. The horizontal scan is always running. I disconnect the X IN to get a flat trace then reconnect it only for the scan to start straightaway, I don’t get a chance to manually trip EXT IN.


Perhaps the “timed out” rule is what you’re experiencing?

Maybe, but it’s very much quicker than 1 second. And to be honest, and no disrespect intended, I don’t see the point of this “timed out” feature. If scope is waiting for an EXT IN but starts scanning anyway it defeats the purpose of EXT IN.

And if it’s not a one shot scan and capture as @Squinky alluded to then it’s not what I’m after anyway.

But I’m damned sure I would like to understand that EXT IN :smile:

From the looks of the code, it looks like it’s actually 0.5s. I kind of agree that this threshold should be longer (or even not implemented); if the user’s intent is to do a manual trigger and a scan is already happening when we didn’t call for it, it will not retrigger and the external trigger is ignored.

const float holdTime = 0.5f;

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This is how analog oscilloscope works. Some scopes had a “trig-blank” switch that would blank the trace if there’s no trigger lock, but that’s not the idea here. The idea of the trig input is to lock the pattern on the scope to get a stable image where the scan always starts at the same phase point of the input waveform. Try this out: Hookup the scope X IN to the SIN out of VCO-1 and switch the scope to EXT trig mode. If you adjust the frequency of the VCO the pattern on the scope will be running around. Now hookup a cable from the SQR out of VCO-1 to the scope EXT input. The pattern will lock and you’ll have a nice stable pattern.


And if you wanna have even more fun, have the SQR input connected to a delay plugin such as MSM Dual Delay. Connect the Delay out to the EXT trig input, dial the DRY/WET to full wet and the delay time to the minimum 1ms. Now when dialing the Time slowly up, you can get the sin wave start at different points of the cycle on the scope which really illustrates how this trig works. Sorry for getting carried away, but I’m really an electrical engineer at heart :smile: