General > General Technical Chat
FFT and 60 Notch filter software
Andy Chee:
Assuming your equipment is portable and battery powered, how about going into the middle of a nature park reserve, which is a long distance away from any powerlines, TVs, power supplies, and any other electrical equipment.
You should be able to at least establish the noise floor performance of your equipment. My suggestion is at least 1000m distance in all directions from any electrical device, but I don't know whether you have a location that size.
This is for testing your equipment only, I'm not suggesting you perform your actual experiments in the middle of nowhere!
loop123:
--- Quote from: WatchfulEye on March 13, 2024, 03:50:12 pm ---Only the 60 Hz noise is powerline.
There is a mystery 78 Hz noise.
The main noise is broadband noise - which looks like noise floor, but by my calculations it is about 100-150 nV/sqrt Hz - much higher than specified noise of the AMP01. It could be noise from the waveform generator.
However, note that BMA-200 specifies 7uV noise - and your recording shows less noise than this.
You could try measuring the noise directly by using a 1 kohm resistor instead of the waveform generator. It would be interesting to have a series of recordings with different waveforms and 1 with just a resistor - all settings left exactly the same.
--- End quote ---
Bad news. I made mistake in my last test. I rechecked the amplitude and found out I used 30uV in 2nd BMA instead of 10uV that is why there was minimal noise compared to the 10uV waveform in the first BMA. So ignore my last message except the g.USBamp waveforms.
Anyway. I used 1k ohm resistor as you asked in the original BMA. This is the noise with 1kohm resistor alone connected to BMA with 50000 gain 1000Hz banwidth:
This is 10uV 50Hz from the Netech simulator connected to BMA with 50000 gain 1000Hz bandwidth.
Same setting for both only the resistor was replaced to the Netech as you asked.
computing for the resistor noise at 1k ohm and 1000 Hz
0.13*sqrt(R*f) nV (rms) noise = 0.13 sqrt (1000 x 1000 Hz) nv = 0.13 x 1000 = 130nV rms
the noise of the AMP01 is 5nv/sqrt (Hz) x sqrt (1000Hz bandwidth) = 158.11nV rms
combination is sqrt (130^2 + 158.11^2) = sqrt (16900 + 24999) = sqrt (41899) = 204nV rms
In the Audacity, the noise is not 204nV rms!
with 10uV x 50000 = 0.5V.. the noise is not far from 0.5V.. maybe 100 uV. What's going on? The 1kohm resistor test used the same scale as the 10uV and the noises are closed.
They are the noise floor, isn't it? and not interference.. because for interference, you are supposed to be able to filter it using 60Hz and harmonics. and not wide band noise, right?
With the AMP01 5nV/sqrt (Hz). Why are we getting more than say 1000nV/sqrt (Hz) in the BMA? Even if the BMA is so junk, why is the AMP01 also messed up as well. The 2nd BMA showed the same noise so it is not due to defective AMP01
loop123:
By using 2.5mV in the Netech Simulator and 2000 gain in the BMA. I get 5 volts in Audacity from 1 to -1.
Does it mean Audacity 1 to -1 is really 5 Volts? Or does it change depending on your setups? how?
So the 10uV x 50000 gain = 0.5V in last message is right scale at it is 0.1 instead of 1 or 5V/10 =0.5V.
tggzzz:
--- Quote from: loop123 on March 14, 2024, 10:37:43 am ---Does it mean Audacity 1 to -1 is really 5 Volts? Or does it change depending on your setups? how?
--- End quote ---
Sigh. You really don't understand.
Audacity processes numbers, either floating point or integer. Those numbers have absolutely no defined relationship to any input voltage.
Sometimes those numbers might originate in an ADC. Often they will have been "artificially generated" inside a computer program.
gf:
--- Quote from: tggzzz on March 14, 2024, 10:51:46 am ---
--- Quote from: loop123 on March 14, 2024, 10:37:43 am ---Does it mean Audacity 1 to -1 is really 5 Volts? Or does it change depending on your setups? how?
--- End quote ---
Sigh. You really don't understand.
Audacity processes numbers, either floating point or integer. Those numbers have absolutely no defined relationship to any input voltage.
Sometimes those numbers might originate in an ADC. Often they will have been "artificially generated" inside a computer program.
--- End quote ---
Exactly.
ffmpeg -i netech\ 10uv\ 50hz\ bma\ 1000hz\ 50000\ gain.wav
...
Input #0, wav, from 'netech 10uv 50hz bma 1000hz 50000 gain.wav':
Duration: 00:00:29.94, bitrate: 705 kb/s
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, 1 channels, s16, 705 kb/s
The file contains just a stream of signed 16-bit integer numbers whose full-scale range is mapped by Audacity to a -1...+1 range when it loads the file. Either the creator of the file can tell you the codes per Volt in the file, or you need to find out yourself.
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