Noise from Electricity – Part 1

See Noise from Electricity – Part 2

Curious, I ran a test where I took off the Mindwave headset and saved the results. The Brainwave Analyzer showed a large amount of Gamma “brainwaves”. Surprised, I googled around and it turns out that EEG readers are so sensitive that they pick up the electromagnetic waves from the alternating current (AC) flowing within the walls.

No Headset I ran a test where I took off the Mindwave, and saved the results: Mindwave512-No-Headset.csv 5.97 MB Download	AllNo-Headset No-Headset-01 No-Headset-02
Headset Slipped? KOB (adult male) baseline1-relaxing with eyes open Mindwave512-KOB-baseline1-relaxing with eyes open.csv 3.05 MB Download	AllKOB-baseline1-relaxing with eyes open Mindwave512-KOB-baseline1-relaxing with eyes open-Median Mindwave512-KOB-baseline1-relaxing with eyes open-Bands

According to wikipedia:

“The digital EEG signal is stored electronically and can be filtered for display …An additional notch filter is typically used to remove artifact caused by electrical power lines (60 Hz in the United States and 50 Hz in many other countries).”

https://en.wikipedia.org/wiki/Electroencephalography#Method

So I asked Nayuki if she could add an notch filter to take out the noise from the electrical power lines. She told me that she has had experience with audio recording software and is well aware of the AC issue. She pointed out that my “no headset” readings showed a small peak at 60 Hz (not surprising as I am in the USA) and a major peak at 119 Hz (due to harmonics), but that my other readings did not show such peaks. She said that this was probably due to the Mindwave reader having automatic gain control (ACG). When the headset is working but not on on anyone’s head, it would amplify any AC hum in the room. But when the Mindwave is attached to someone’s head, the brainwaves are much stronger than the AC hum, which is why the automatic gain control (ACG) on the Mindwave makes sure the AC hum doesn’t show up in the readings. Thus no Notch filter is needed for the Mindwave reader.

Here is one second from the brainwave readings of no headset attached to someone’s head (no brainwaves).  Showing a huge peak at the 120 Hz frequency (amplitude of about 60,000). And a smaller, but still impressive peak at the 60 Hz frequency (amplitude of about 15,000).

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And here is one second from the brainwave readings of someone reading. Showing no outsized 120 Hz frequency peak, nor an outsized 60 Hz peak.

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These readings provide strong evidence that Nayuki is absolutely correct that the Mindwave has automatic gain control (ACG) built in, thus negating any need for a notch filter. Note, clicking through “Frequency spectrum” readings of various brainwave reading, I have seen a number of smaller peaks at the 120 Hz frequency, so it could very well be that the automatic gain control (ACG) is not perfect and does allow some distortion due to noise from the electricity in the walls, but I do believe that the amount of this noise is pretty negligible.

I think this shows, that for something that is often marketed as a child’s educational toy, the Mindwave Headset is quite sophisticated.

More on the hum:

“Because the magnetic flux density is strongest twice every electrical cycle, the fundamental “hum” frequency will be twice the electrical frequency. Additional harmonics above 100 Hz or 120 Hz will be caused by the non-linear behavior of most common magnetic materials.”

https://en.wikipedia.org/wiki/Mains_hum