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Detect a Change in Duty Cycle

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ForestCat

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Hi,

Is there a way using discrete components and/or IC's like the 555, comparators, etc., to isolate/extract the "blip" in the following scope trace?

**broken link removed**

I'm pretty stupid when it comes to raw digital waveforms, but I think I'm seeing the duty cycle go from ~80% to 100%? This signal is from a piece of gear I need to modify w/ no docs/schematics, etc. That "blip" is a 1/4 note "click" which I need to extract for synchronization w/ other gear.

I really appreciate any thoughts/insight.

Thanks.
 

Watching your video I believe the sweep occasionally triggers early. Do you have it set on Auto? Experiment with the direction (downgoing vs upgoing trigger). This might reduce false triggers.

Or instead it might help if you take it off Auto. Adjust the up/down trigger level to some position midway. This might reduce false triggers.

Sometimes it's interesting to set it on free run. Adjust speed until you get stationary waveforms.
 

Hi,

555 can be used as a missing pulse detector, only need to add a BJT, page 13, 9.2.1 of the attached pdf, the description reads:

"The circuit shown in Figure 16 can be used to detect a missing pulse or abnormally long spacing between consecutive pulses in a train of pulses. The timing interval of the monostable circuit is re-triggered continuously by the input pulse train as long as the pulse spacing is less than the timing interval. A longer pulse spacing, missing pulse, or terminated pulse train permits the timing interval to be completed, thereby generating an output pulse as shown in Figure 17."

Frequency is usually not too high with a 555, 100kHz for the SE and SA and NA types, etc., but it doesn't cope so well at so high a frequency, and the LMC555 says 3MHz, I hate to say anything negative about the 555 but without having tested an LMC at that frequency I would take that with a pinch of salt or assume "datasheet optimism."
 

Attachments

  • Timer 555 sa and se na and ne TI.pdf
    1.5 MB · Views: 70
Brad,

Although it might not be apparent in the cellphone video, that blip where the first division goes from about 2v to 5v for an instant is regular, in this case, every quarter note at around 60bpm. It is definitely a part of the signal, it is only there when the the device is clocked externally via MIDI. What I want to do is to be able to grab that event, hopefully using discrete components, ttl, cmos, etc. Any idea how I could do that?

Thanks for the reply






BradtheRad said:
Watching your video I believe the sweep occasionally triggers early. Do you have it set on Auto? Experiment with the direction (downgoing vs upgoing trigger). This might reduce false triggers.

Or instead it might help if you take it off Auto. Adjust the up/down trigger level to some position midway. This might reduce false triggers.

Sometimes it's interesting to set it on free run. Adjust speed until you get stationary waveforms.
Click to expand...
 

I had seen some of the "missing pulse" 555 app notes, etc., but do you think it is applicable here? This ~2ms pulse I'm trying to isolate is regular, i.e. not missing or rogue, it's on every quarter note for a given bpm. How can I isolate it from the "background" clock, or whatever, and use it to trigger other stuff at that quarter-note rate?

re: 555, I think the freq of this signal is low enough, I'm not worried about that. I just don't know how to use a 555 to grab that ~2ms pulse...

Thanks for your reply



d123 said:
Hi,

555 can be used as a missing pulse detector, only need to add a BJT, page 13, 9.2.1 of the attached pdf, the description reads:

"The circuit shown in Figure 16 can be used to detect a missing pulse or abnormally long spacing between consecutive pulses in a train of pulses. The timing interval of the monostable circuit is re-triggered continuously by the input pulse train as long as the pulse spacing is less than the timing interval. A longer pulse spacing, missing pulse, or terminated pulse train permits the timing interval to be completed, thereby generating an output pulse as shown in Figure 17."

Frequency is usually not too high with a 555, 100kHz for the SE and SA and NA types, etc., but it doesn't cope so well at so high a frequency, and the LMC555 says 3MHz, I hate to say anything negative about the 555 but without having tested an LMC at that frequency I would take that with a pinch of salt or assume "datasheet optimism."
Click to expand...
 

Set the sweep to free run. From your description the event occurs every 4 cycles, therefore adjust sweep speed so that 4 waveforms appear onscreen. This is the easiest way you can observe the event in the same position, each time it occurs.

goes from about 2v to 5v for an instant
Click to expand...

The peak of a waveform can be used as the trigger to begin a sweep. Did you try playing with the trigger settings? A delicate touch might find the right spot.

You would need to build a sophisticated circuit, for it to (a) detect the event, and (b) wait for a delayed time period which you adjust, and (c) send a trigger signal to the scope.

My scope is a Tektronix around 60 years old. It has sufficient knobs and switches to allow the tricks I've mentioned. It would be surprising if your Tektronix doesn't have them.

Mine is not a storage scope. You might need a storage scope to isolate an event in the manner you wish.
 

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