FFT and 60 Notch filter software

[loop123]

From the file, can you tell if the noise is external or in the circuit??

Are you only measuring one channel/electrode, or multiple channels/electrodes?  Your photo only shows one waveform, so I'm assuming you're only measuring a single channel/electrode.

If you measure multiple electrodes, you can mathematically remove the common noise.  The theory is that the skin potentials will be different at each electrode, but the noise (particularly 60Hz and other EMI) will be the same in all channels. 

EEGlab can do this multiple channel common noise reduction, and I'm pretty sure Audacity should be able to do it with two channel stereo.  The key is that you need multiple channels, not single.

Sorry. The following audio attachment (at bottom) is the file showing the distortions in the 10uV, 50Hz signal input, you have to zoom in the amplitude at Audacity or other editors to see them. I used single electrode pair connected to the Netech EEG simulator as shown in photo. My concern is the waveforms are distorted and even after I shielded the entire Netech and connector box. I still got distortions. So I just want to see the Fourier breakdowns of the waveforms to see what is the component frequencies causing the interference (if it is external or in the circuit). Please try loading it in Audicity and let me know. Your description about using multiples samples is a separate task. Remember at 30uV and higher, no distortions. only in the 10uV which happens to be the region I'd focus in.

[Andy Chee]

From the file, can you tell if the noise is external or in the circuit??

Are you only measuring one channel/electrode, or multiple channels/electrodes?  Your photo only shows one waveform, so I'm assuming you're only measuring a single channel/electrode.

If you measure multiple electrodes, you can mathematically remove the common noise.  The theory is that the skin potentials will be different at each electrode, but the noise (particularly 60Hz and other EMI) will be the same in all channels. 

EEGlab can do this multiple channel common noise reduction, and I'm pretty sure Audacity should be able to do it with two channel stereo.  The key is that you need multiple channels, not single.

I used single electrode pair connected to the Netech EEG simulator as shown in photo.

Your description about using multiples samples is a separate task.

It's actually not a separate task, all the channels can be done all together.  Noise removal is performed on all channels simultaneously, not individual channel one-by-one.

[loop123]

From the file, can you tell if the noise is external or in the circuit??

Are you only measuring one channel/electrode, or multiple channels/electrodes?  Your photo only shows one waveform, so I'm assuming you're only measuring a single channel/electrode.

If you measure multiple electrodes, you can mathematically remove the common noise.  The theory is that the skin potentials will be different at each electrode, but the noise (particularly 60Hz and other EMI) will be the same in all channels. 

EEGlab can do this multiple channel common noise reduction, and I'm pretty sure Audacity should be able to do it with two channel stereo.  The key is that you need multiple channels, not single.

I used single electrode pair connected to the Netech EEG simulator as shown in photo.

Your description about using multiples samples is a separate task.

It's actually not a separate task, all the channels can be done all together.  Noise removal is performed on all channels simultaneously, not individual channel one-by-one.

What I want to determine for now is whether the noise is caused by EMI or the amplifier not capable of 10uV as Ataranov remarked. Bec even if I spent $3000 on human size Faraday cage. The distortion would still be there if it is the amplifier that has problem. If thats the case, then have to dive and spent $5000 on USBamp software and actives if it is the only means of last resort. So how do you know if the distortion is really caused by EMI or the amplifier just not capable of handling 10uV even if the amp used is the 5nV/sqrt(Hz) noised AMP01?

I just want to know the frequencies involved in the distortions for now and not about eliminating them right away.

[BrianHG]

What I want to determine for now is whether the noise is caused by EMI or the amplifier not capable of 10uV as Ataranov remarked. Bec even if I spent $3000 on human size Faraday cage. The distortion would still be there if it is the amplifier that has problem. If thats the case, then have to dive and spent $5000 on USBamp software and actives if it is the only means of last resort. So how do you know if the distortion is really caused by EMI or the amplifier just not capable of handling 10uV even if the amp used is the 5nV/sqrt(Hz) noised AMP01?

I just want to know the frequencies involved in the distortions for now and not about eliminating them right away.

Oh, so you must be doing your tests somewhere along Salaberry Blvd in Dollard Des Ormeaux / or in Kirkland...
Good luck, you will never get rid of that interference in that area of town.

[loop123]

So in Audacity Frequency analysis, the frequency sides are being displayed. I want an app that can see the components (inside in illustration) in the following. What is the software called?

[oPossum]

Band-pass filter.

You really seem to be 'in over you head' on this.

[tggzzz]

Or he could look at p374 in https://www.worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1974-10.pdf and find an article "Mains Rejection Tracking Filter - using a tracking 'n-path' filter with wide dynamic range" to remove not only the fundamental but also the harmonics.

Not many people understand weird and wonderful n-path filters. I expect it would blow the OP's mind.

[BrianHG]

Or he could look at p374 in https://www.worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1974-10.pdf and find an article "Mains Rejection Tracking Filter - using a tracking 'n-path' filter with wide dynamic range" to remove not only the fundamental but also the harmonics.

Not many people understand weird and wonderful n-path filters. I expect it would blow the OP's mind.

I completely forgot about that concept.  Yes, done right, you can eliminate the mains interference completely.  However, if loop123 is using his amp to read human biometrics, you have the problem that every time you re-wire to a new person in a different place, you may too radically change your source interference characteristics.

If loop123 has enough input channels, just sacrifice 1 channel as you mains reference noise channel.  Then, subtract that channel's signal from all your channels of interest and you will null out your main's noise.

It's basically how a differential line signal works.

[loop123]

Or he could look at p374 in https://www.worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1974-10.pdf and find an article "Mains Rejection Tracking Filter - using a tracking 'n-path' filter with wide dynamic range" to remove not only the fundamental but also the harmonics.

Not many people understand weird and wonderful n-path filters. I expect it would blow the OP's mind.

I completely forgot about that concept.  Yes, done right, you can eliminate the mains interference completely.  However, if loop123 is using his amp to read human biometrics, you have the problem that every time you re-wire to a new person in a different place, you may too radically change your source interference characteristics.

If loop123 has enough input channels, just sacrifice 1 channel as you mains reference noise channel.  Then, subtract that channel's signal from all your channels of interest and you will null out your main's noise.

It's basically how a differential line signal works.

For now the input is a Netech sine wave simulator. In the following 10uV (microVolt), 50Hz source is used because the simulator only has 0.1Hz, 2Hz, 5hz, 50Hz, 60Hz setting. And I can't chose 60Hz because the Audacity notch filter can totally null all amplitude.

In the following, the BMA200 Amp has 1000Hz bandwidth selected. Enclose in it is the Frequency Analysis. How do you remove the noises? What noises do you call them? It is the same input as the original file but only now the Amp bandwidth has 1000Hz selected instead of just 100Hz. Attached is also the audio file. Please check using your FFT what noises are they comprised off. If I 60Hz notch filter it, the amplitude went down but the noises remain. Please try to find ways to remove the noises and share what you did. From the Frequency Analysis below, what frequencies are the noises? If they are harmonics, where are the frequencies of the harmonics located in the FFT. Thanks!

[BrianHG]

Oh man, that 6.7khz tone in the .wav is maddening.  And I see a main dominant 50hz spike. There was a weaker 60hz and 80hz tones as well.  Are you sure this was recorded in north America?  Or, did you attempt some sort of filter algorithm which injected some fundamentals?

[loop123]

Oh man, that 6.7khz tone in the .wav is maddening.  And I see a main dominant 50hz spike. There was a weaker 60hz and 80hz tones as well.  Are you sure this was recorded in north America?  Or, did you attempt some sort of filter algorithm which injected some fundamentals?

Thats bec the source is the Netech EEG simulator which produces 10uV, 50Hz output into the amplifier which is set at 1000Hz bandwidth. What is the origin of the noises at each waveform? No I didnt inject any filter algorithm. It is directly from the BMA-200 amplifier to the E1DA ADC (the world highest performing 32 bit ADC) and direct to Audacity. The ambient AC capacitive coupling in the location is 60Hz. Are the noises harmonics?  Pls break it down into the fourier component FFT style and recover the 10uV, 50Hz input. I want to see examples how you fix it.

[Karel]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

[loop123]

First, please tell me what exactly are these noises? In the 2nd image, they don't look like harmonics:





If they are harmonics. They should be resultant in the waveform. But my audacity waveforms don't show any resultant. The noises are riding in each part of the sine wave. How do you differentiate between EMI, RFI, amplifier noise, and quantization ADC noises? How sure are you they are powerline noises or harmonics? Please tell me what exactly those are first.

[loop123]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

I talked to the designer of E1DA. Ivan said:



Well. Ivan is a Russian electronics wizard. If he could say that. How can I even counter it. Ok. Someone please give an actual example how to filter the waveforms in the file I sent 2 messages prior. Please do it directly using your Apps, then I'll filter hundreds of other inputs myself after you actually showed the filter output by actually running it in your software. Thanks.

[Karel]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

I talked to the designer of E1DA. Ivan said:

(Attachment Link)

Well. Ivan is a Russian electronics wizard. If he could say that. How can I even counter it. Ok. Someone please give an actual example how to filter the waveforms in the file I sent 2 messages prior. Please do it directly using your Apps, then I'll filter hundreds of other inputs myself after you actually showed the filter output by actually running it in your software. Thanks.

That's because your samplerate is not an integer multiple of the powerline frequency.
I thought we were talking about EEG and/or EKG here and you could select the samplerate.

If your application is about recording audio, then it's much easier to get rid of powerline interference (compared to EEG/EKG)
but you need to pay attention to grounding.

[loop123]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

I talked to the designer of E1DA. Ivan said:

(Attachment Link)

Well. Ivan is a Russian electronics wizard. If he could say that. How can I even counter it. Ok. Someone please give an actual example how to filter the waveforms in the file I sent 2 messages prior. Please do it directly using your Apps, then I'll filter hundreds of other inputs myself after you actually showed the filter output by actually running it in your software. Thanks.

That's because your samplerate is not an integer multiple of the powerline frequency.
I thought we were talking about EEG and/or EKG here and you could select the samplerate.

If your application is about recording audio, then it's much easier to get rid of powerline interference (compared to EEG/EKG)
but you need to pay attention to grounding.

My amplifier target input is 10uV, 1000Hz with gain of 50000.  The output bandwidth is set at 1000Hz. So the output is 10uVx50000= 0.5V, 1000Hz.  The ADC is to record the 0.5V, 1000Hz.  What samplerate must I adjust in Audacity so I can filter the powerline frequency and harmonics? 

[Karel]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

I talked to the designer of E1DA. Ivan said:

(Attachment Link)

Well. Ivan is a Russian electronics wizard. If he could say that. How can I even counter it. Ok. Someone please give an actual example how to filter the waveforms in the file I sent 2 messages prior. Please do it directly using your Apps, then I'll filter hundreds of other inputs myself after you actually showed the filter output by actually running it in your software. Thanks.

That's because your samplerate is not an integer multiple of the powerline frequency.
I thought we were talking about EEG and/or EKG here and you could select the samplerate.

If your application is about recording audio, then it's much easier to get rid of powerline interference (compared to EEG/EKG)
but you need to pay attention to grounding.

My amplifier target input is 10uV, 1000Hz with gain of 50000.  The output bandwidth is set at 1000Hz. So the output is 10uVx50000= 0.5V, 1000Hz.  The ADC is to record the 0.5V, 1000Hz.  What samplerate must I adjust in Audacity so I can filter the powerline frequency and harmonics?

Change the recording samplerate of your device from 44.1 KHz to 48 KHz which is an integer multiple of 60 (and 50) Hz.
Design a PLL controlled VCO that creates the clock for your ADC chip and synchronize it with the powerline frequency.
That gives you much more options to use sophisticated algorithms in order to remove the powerline interference.

But we can help you better if you tell us what you want to achieve. What is it what you want to measure? Audio? Something else?

[WatchfulEye]

First, please tell me what exactly are these noises? In the 2nd image, they don't look like harmonics:

If they are harmonics. They should be resultant in the waveform. But my audacity waveforms don't show any resultant. The noises are riding in each part of the sine wave. How do you differentiate between EMI, RFI, amplifier noise, and quantization ADC noises? How sure are you they are powerline noises or harmonics? Please tell me what exactly those are first.

You seem to be looking for an individiual frequency as the source of your noise/distortion. It actually appears that this is broadband noise/noise floor that you are seeing. Also, a hint when using the audacity spectrum viewer - you need to amplify the signal first, as the viewer has a fixed minimum on the Y axis. You can then magnify as required in the spectrum viewer.

This is the spectrum of the last waveform you posted. I've also calculated normalised amplitudes for narrow band noise, on the basis that the signal is 10 uV rms.

You can see the 50 Hz signal, and small 2nd and 3rd harmonics. You can also see a small amount of 60 Hz noise (1 uV), but no harmonics.
However, there are 2 unknown and more important noise sources - 78 Hz (1.7 uV) as well as harmonics of 78 Hz, and broadband noise (approx 100 nV/sqrt Hz).

The 60 Hz is likely coming from power line noise, and could be mitigated with better shielding, or the use of a notch filter (depending on the frequencies of interest in your signal), or differential measurement.
The 78 Hz is a mystery, and may be noise generated by the simulator.
The broadband noise may be the noise floor of your setup, and because of it's 1 kHz bandwidth, it has a total contribution of approx 3 uV.

I suggest repeating the measurements with other waveforms and amplitudes, to see what happens to the unknown noise sources. It may be that some of the unknown noise is from your waveform generator.

[loop123]

Synchronize the sampleclock of the ADC to the powerlinefrequency e.g. using a PLL.
That makes it way easier to remove the powerline interference (including harrmonics).

https://www.mainsfrequency.com/

I talked to the designer of E1DA. Ivan said:

(Attachment Link)

Well. Ivan is a Russian electronics wizard. If he could say that. How can I even counter it. Ok. Someone please give an actual example how to filter the waveforms in the file I sent 2 messages prior. Please do it directly using your Apps, then I'll filter hundreds of other inputs myself after you actually showed the filter output by actually running it in your software. Thanks.

That's because your samplerate is not an integer multiple of the powerline frequency.
I thought we were talking about EEG and/or EKG here and you could select the samplerate.

If your application is about recording audio, then it's much easier to get rid of powerline interference (compared to EEG/EKG)
but you need to pay attention to grounding.

My amplifier target input is 10uV, 1000Hz with gain of 50000.  The output bandwidth is set at 1000Hz. So the output is 10uVx50000= 0.5V, 1000Hz.  The ADC is to record the 0.5V, 1000Hz.  What samplerate must I adjust in Audacity so I can filter the powerline frequency and harmonics?

Change the recording samplerate of your device from 44.1 KHz to 48 KHz which is an integer multiple of 60 (and 50) Hz.
Design a PLL controlled VCO that creates the clock for your ADC chip and synchronize it with the powerline frequency.
That gives you much more options to use sophisticated algorithms in order to remove the powerline interference.

But we can help you better if you tell us what you want to achieve. What is it what you want to measure? Audio? Something else?

Surface biopotentials of 10uV, 1000Hz. Experimenting on bionics implants. It will be difficult to "Design a PLL controlled VCO that creates the clock for your ADC chip and synchronize it with the powerline frequency.". Who has actually built them?  can you guys just do it on existing software? are you saying no software can even filter them? but pls answer 2 messages prior what those noises are in the first place bec they seemed not to be harmonics owing to lack the resultant as shown in the figure.

[tggzzz]

Surface biopotentials of 10uV, 1000Hz. Experimenting on bionics implants. It will be difficult to "Design a PLL controlled VCO that creates the clock for your ADC chip and synchronize it with the powerline frequency.". Who has actually built them?  can you guys just do it on existing software? are you saying no software can even filter them? but pls answer 2 messages prior what those noises are in the first place bec they seemed not to be harmonics owing to lack the resultant as shown in the figure.

For medical electronics, especially implants, it is essential that you use hardware designed for such purposes in a way specified by the manufacturer.

Any homebrew hardware or hardware operated in a system not designed for medical purposes may be a safety hazard. The tone of your repeated questions makes me doubt that you are qualified to design and/or operate such hardware. Hence be very careful of the consequences of your actions and inactions. Consult the terms and conditions of relevant professional insurance policies, looking to see whether you might be personally liable.

If the digitiser has adequate dynamic range (see the ENOB specification) and higher than necessary sampling rate, then downconverting to any lower rate is a standard DSP operation with well known consequence. Consult any DSP textbook, and use DSP software. Do not expect standard audio hardware and software to be sufficient.

Wrtng sntncs like this mks u lk unprofessional.

[gf]

Is the .wav file supposed to contain any useful information?
Besides the spurs, everything looks like random noise to me

[Karel]

Surface biopotentials of 10uV, 1000Hz. Experimenting on bionics implants. It will be difficult to "Design a PLL controlled VCO that creates the clock for your ADC chip and synchronize it with the powerline frequency.". Who has actually built them?  can you guys just do it on existing software? are you saying no software can even filter them? but pls answer 2 messages prior what those noises are in the first place bec they seemed not to be harmonics owing to lack the resultant as shown in the figure.

For medical electronics, especially implants, it is essential that you use hardware designed for such purposes in a way specified by the manufacturer.

Any homebrew hardware or hardware operated in a system not designed for medical purposes may be a safety hazard. The tone of your repeated questions makes me doubt that you are qualified to design and/or operate such hardware. Hence be very careful of the consequences of your actions and inactions. Consult the terms and conditions of relevant professional insurance policies, looking to see whether you might be personally liable.

If the digitiser has adequate dynamic range (see the ENOB specification) and higher than necessary sampling rate, then downconverting to any lower rate is a standard DSP operation with well known consequence. Consult any DSP textbook, and use DSP software. Do not expect standard audio hardware and software to be sufficient.

Wrtng sntncs like this mks u lk unprofessional.

If the device is not advertized as a medical device, for example when it's intended use is for research only, you can do almost whatever you want
(prohibited the approval of an ethical commision).
As soon when it's going to be used for patient treatment and/or monitoring, you'll need to comply with EN/CEI 60601 and all the applicable/resulting other norms...

[loop123]

Is the .wav file supposed to contain any useful information?
Besides the spurs, everything looks like random noise to me

It is just sine wave generator output of 10uV, 50Hz sine wave. Simple only but why is the output at Audacity has so much noise? Goal is simply to remove the noise and regain the pure 10uV, 50Hz sine wave. pls help do that.

Btw the amplifier is for use on mice so dont worry guys. see specs at

https://cwe-inc.com/products/bma-200-acdc-bioamplifier

[gf]

Is the .wav file supposed to contain any useful information?
Besides the spurs, everything looks like random noise to me

It is just sine wave generator output of 10uV, 50Hz sine wave. Simple only but why is the output at Audacity has so much noise? Goal is simply to remove the noise and regain the pure 10uV, 50Hz sine wave. pls help do that.

My understanding was that the 50 Hz component (and the other spurs) are interfering signals which you want to suppress. If, OTOH, the 50 Hz signal is the useful signal you want to keep (while all the other spectral components should be suppressed), then this can easily be done with a narrow bandpass filter. However, I think this is not what you really want. A real EEG signal is not a pure 50 Hz sine wave, therefore such a bandpass filter would suppress most of the useful components in a real EEG signal. Is the EEG simulator not able to simulate a more realistic EEG signal?

[Karel]

I opened your wav file in EDFbrowser and it doesn't look noisy.