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MEGA electrical measurements
5 years 9 months ago #1452
by Ray
Replied by Ray on topic MEGA electrical measurements
Hi,
These are the answers for your questions:
We did not try a noise generator, people use smartphones with signal generator apps.
These are the answers for your questions:
On the MEGA, the best point to measure FX_in may be on top of the input connector (the are is big and you can attach an oscilloscope probe easily):- Where is best location to measure input signal to board. Is this FX_In on schematic?
as far as the levels are sensible you can do it without problems, the shield is designed for guitar signals (around 200mVpp) but you can go around line levels (3Vpp) without problems (but you would need to adjust the input stage level for that with the VR1 trimmer.- Rather than guitar input, can you hook up a noise generator at input jack? Has anyone done this? I am thinking that noise would be easier than transient guitar signals for forming transfer functions
We did not try a noise generator, people use smartphones with signal generator apps.
We never try to measure the transfer function of it but it should not be difficult. I can do it if you need it. The signal is heavily attenuated over 5KHz to avoid noise (many digital guitar pedals do this "trick", there is not much info in the guitar signal over 5K and the Arduino sampling rate is not hi-fi)- Has anyone made these types of measurements before? What advice/guidance would you give? Did you use oscilloscope, FFT box,...??
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5 years 8 months ago #1465
by Ray
This is a clean signal going through the pedalSHIELD MEGA in bypass mode:
Nothing special up there, this image is just for reference, my measurement system is band-limited as you can see the signal rolling off over 20K and below 10Hz.
Input Stage Measurements:
Measuring from the input jack to the ADC (R6-C5 junction):
Adjusting with the trimmer to have minimum gain:
Adjusting with the trimmer to have more gain:
I wrote more and not "maximum" because if you apply maximum gain with this trimmer you will get the signal clipped and measurement will go wrong. Guitar signals are pretty small (100-200mVpp) so they won't clip.
You can see how the signal is low-pass filtered to improve the ADC aliasing.
Complete System Measurements:
Measuring from the input jack to the output jack:
Minimum Gain:
More Gain:
You can see how the signal is low-pass filtered even more. Many digital effects remove all content over 5KHz (have a look to the Boss CE2 Analysis: www.electrosmash.com/boss-ce-2-analysis the antialiasing and reconstruction filters are around 5/6KHz)
Replied by Ray on topic MEGA electrical measurements
I did some measurements today. They are limited because if I apply maximum gain to the input signal I end up having clipping and the TF gets messed up.1) I would appreciate you making some transfer function measurements. It would provide a baseline for me to compare students' measurements against. Consider making the input stage TF measurement to see op-amp gain and effects of (low pass) filtering
This is a clean signal going through the pedalSHIELD MEGA in bypass mode:
Nothing special up there, this image is just for reference, my measurement system is band-limited as you can see the signal rolling off over 20K and below 10Hz.
Input Stage Measurements:
Measuring from the input jack to the ADC (R6-C5 junction):
Adjusting with the trimmer to have minimum gain:
Adjusting with the trimmer to have more gain:
I wrote more and not "maximum" because if you apply maximum gain with this trimmer you will get the signal clipped and measurement will go wrong. Guitar signals are pretty small (100-200mVpp) so they won't clip.
You can see how the signal is low-pass filtered to improve the ADC aliasing.
Complete System Measurements:
Measuring from the input jack to the output jack:
Minimum Gain:
More Gain:
You can see how the signal is low-pass filtered even more. Many digital effects remove all content over 5KHz (have a look to the Boss CE2 Analysis: www.electrosmash.com/boss-ce-2-analysis the antialiasing and reconstruction filters are around 5/6KHz)
I use ARTA and a good external sound card (EMU 0404), I think that you can use cheaper sound cards for this. There is plenty of info online or in the DIYaudio forum. Sometimes I use signal generators apps from my phone to generate the input signal, there is plenty of them. Right now I have this one: play.google.com/store/apps/details?id=radonsoft.net.signalgen but again, there is plenty of alternatives.2) Can you provide any further information on (recommended?) smart phone apps for noise generation in the context of making TF measurements?
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