General > General Technical Chat
FFT and 60 Notch filter software
tggzzz:
--- Quote from: Karel on March 11, 2024, 11:20:02 am ---
--- Quote from: tggzzz on March 11, 2024, 11:02:13 am ---Once the OP has managed to notch out 60Hz, he'll probably realise it is 60Hz+-allowed variability.
Then he might want to notch out the harmonics, since modern mains waveforms are not sinusoidal.
The first time I looked at that this millennium, I thought something was wrong with the scope, transformer, etc. Nope: the problem was with the waveform.
--- End quote ---
And then he might probably realize that notch filters can create ringing with particular waveforms, for example with EKG.
Usually, the best approach is to avoid powerline interference from being recorded.
--- End quote ---
Definitely!
He could always try to notch out the ringing >:D
Siwastaja:
--- Quote from: tggzzz on March 11, 2024, 11:02:13 am ---Then he might want to notch out the harmonics, since modern mains waveforms are not sinusoidal.
--- End quote ---
Yeah. Anyone who ever worked with audio and tried to filter out mains hum by using a band-stop filter, realized a long time ago it's a futile task. Heck, you can hear that the hum is much more than the 50 or 60Hz component. Harmonics go to kHz range.
The only way really is to design audio (or other measurement) circuits such that they don't pick up the interference in the first place, or interference is cancelled e.g. by differential design, where cancellation happens with wide BW.
loop123:
--- Quote from: ataradov on March 11, 2024, 05:35:39 am ---Both things come with Audacity by default.
The notch filter even defaults to 60 Hz, since they anticipate it as the most common use.
--- End quote ---
I created a 2.5mV 60Hz output using a Netech simulator. It displayed in Audacity. When I used the notch filter at 60Hz. The waveform disappeared so it is not on at default.
When I tried Frequency Analysis. Why don't I get just a peak at 60Hz (or 50Hz), instead, it shows many frequencies up to 9kHz like the following. I also attached the audio file. I need a frequency analyzer that will only show the 60Hz sine wave (or other frequency) in the display. I want one that will show the frequency in the plots. Attached is the 60Hz audio file so other can also check if the frequency is only 60HZ or a range of frequencies up to 9kHz not showed at Audacity?
Netech 2point5mV 60Hz output.wav (1652.6 kB - downloaded 39 times.)
ataradov:
--- Quote from: loop123 on March 11, 2024, 11:11:31 pm ---The waveform disappeared so it is not on at default.
--- End quote ---
I did not say it was on by default. This is not even how Audacity works, nothing is on by default. The workflow if such that you apply plugins. I said that the default value when you apply the filter is 60 Hz.
--- Quote from: loop123 on March 11, 2024, 11:11:31 pm ---When I tried Frequency Analysis. Why don't I get just a peak at 60Hz (or 50Hz), instead, it shows many frequencies up to 9kHz like the following.
--- End quote ---
Enable logarithmic scale for the frequency axis and increase the size of the FFT.
This is basic FFT stuff. If you can't figure that out at this point, you are not likely to get anywhere.
tggzzz:
--- Quote from: loop123 on March 11, 2024, 11:11:31 pm ---I created a 2.5mV 60Hz output using a Netech simulator. It displayed in Audacity. When I used the notch filter at 60Hz. The waveform disappeared so it is not on at default.
When I tried Frequency Analysis. Why don't I get just a peak at 60Hz (or 50Hz), instead, it shows many frequencies up to 9kHz like the following.
--- End quote ---
Welcome to the world of windowing functions, e.g. Hanning, Hamming, flat-top, Blackman, and many others. Do you want to emphasise amplitude precision, frequency precision, a balance?
Thought experiment: if you have a 999Hz sinewave that is turned on/off at 10Hz, what do you think the frequency domain equivalent is - or should be?
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