Electronics > Metrology

Triggering a scope/frequency counter off of audio tones

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I'm trying to do some experiments with measuring audio (frequency, harmonics, etc.) on my test gear and checking the interfaces between things, and I've run into a small issue that makes the equipment much less versatile than I expected: it's very difficult to find a good trigger level that will work on more than one sound or one sound at different volumes.

Basically, I'm measuring tones from an instrument (periodic but non-sinusoidal), and while some are near sine waves and are very easy to trigger off of, many have a peak and then several smaller peaks in between and it's difficult to get an accurate frequency measurement of the fundamental tone - the highest peak in the pattern.  This is further complicated by playing at a different volume level, and because certain notes have such a pronounced set of harmonics that the fundamental tone is only slightly higher amplitude.

I'm wondering if there's a simple way to find this fundamental and dynamically adjust the trigger level (or dynamically adjust the gain of the signal) so that a scope or a frequency counter can trigger off of the signal easily, or if there's some alternative scheme that would do the job I'm looking for - finding that highest periodic peak even in signals that are softer (lower amplitude) or those with prominent upper harmonics.  I've seen techniques that just increase the gain to the point of clipping most of the signal and then measuring the edge of the clipped waveforms, but I'm not certain this will do a good job on some high-harmonic notes.

Have you tried AC coupling the signal and setting the trigger at 0V/div? (+/-100mV if you need to add some noise immunity.)

Failing that, you could try triggering on the falling edge (DC coupled) at a level of the lowest expected signal.

Exactly what are you trying to measure - and why? In other words, what's your problem to which you think the measurement is a solution?

Try a paper experiment: draw a graph of  a1sin(\$\omega\$t)+a2sin(2\$\omega\$t)+a9sin(9\$\omega\$t) for different amplitudes, and draw on it where you want the trigger to be; you may find that to be impossible.

If you are interested in the amplitude of various frequencies, then a scope is the wrong tool: use a spectrum analyser.


--- Quote from: tggzzz on September 27, 2016, 06:58:19 am ---If you are interested in the amplitude of various frequencies, then a scope is the wrong tool: use a spectrum analyser.

--- End quote ---
Or you could use the FFT function of your oscilloscope. In that case, run the scope in single-shot mode and don't worry too much about the trigger. So long as you trigger somewhere on the waveform, and capture the start and end points of the time period you are interested in, the FFT display will show peaks at the dominant frequencies in the signal. You can use the scope's cursors to measure their frequencies and amplitudes. The details of this operation depend on the exact scope you are using.
One tip: To get a good FFT you will need to set the scope horizontal controls to a much slower time/div than you would normally use for looking at the shape of the waveform.

I will try to take some video to show the waveforms and the problem I'm having, but at the moment the sun is backlighting the window in the room and making the screens very hard to read, so it will be a few hours.

Basically, I am trying to measure the frequency of the fundamental tone, the lowest frequency component of the signal and the highest amplitude of the signal, rejecting the harmonic content above that in such a way that I can get a frequency counter to trigger off of it properly.  The end goal is making a high-accuracy tuner - a normal tuner can measure cents, 1/100ths of a half step (1200 divisions per octave), so around three decimal places on the low end of the hearing spectrum and one on the high end, and I want to get at least 3 orders of magnitude better than that using a frequency counter, a precision timebase (GPSDO), and maybe some custom software to give output in note letter and cents as opposed to just a frequency readout.  I do want to do some harmonic analysis stuff, but I'm not expecting the same precision, since my scope won't accept the reference frequency and I don't have the cash to shell out for a proper low frequency spectrum analyzer - so I'd be doing it through an audio interface and a DAW plugin and just relying on whatever oscillator the interface uses.

The best triggering results I've had have been about 350mV (though that doesn't matter since you can just adjust the volume on the line out) AC coupled (lowest fundamental on the horn is just below 60Hz) and matching the level on the frequency counter, but while I can get this to trigger properly for a handful of notes - some notes don't trigger at all without adjustment and some notes have to be played notably louder or quieter to trigger on only the fundamental - not a useful feature for a tuner.


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