I want to develop the EOG algorithms to do something useful, but having three electrodes dangling from your head and plugged into a box is quite uncomfortable and annoying. This is a frustration that I have seen echoed in papers, for example Bulling and Gellersen report the difficulties of capturing a labelled dataset in their paper Robust Recognition of Reading Activity in Transit.
This is a bit fragmented because I mostly described the differences between EOGee1 and EOGee2 from a circuit perspective in the previous article about DC coupling. Here I will give a quick overview of the intention behind EOGee2 as well as the physical differences
The main barrier to getting a DC coupled signal using EOGee1 was that the DC offset voltage of the EOGee signal was much larger than the actual signal itself. Because the signal itself is so small we need a large gain to amplify it, but this also amplifies the offset voltage which then saturates the amplifier.
My solution is to use a signal chain like this:
Previously we have focused on the mains interference component of noise in the EOG signal. This is because it has been the dominant source. However we have seen that this noise can be significantly reduced with appropriate shielding and we could reduce it further still with shorter leads.
Now that the 60Hz noise is reduced, there is a very clear spiking signal coming from somewhere. The signal does not have any obvious periodicity but it is significantly larger than the other noise and also generally affects only one sample. It is easier to see the noise if I remove R108 which means that the final gain stage is disconnected from previous gain stages so the ADC is driven to midrail and is not affected by 60Hz noise at the input.
A couple of posts ago I discussed the reason that the current circuit is AC coupled rather than DC coupled, and managed to get DC coupling briefly working. The advantage of this is that it enables us to measure the absolute voltage across the eyes, rather than just the changing voltage, which gives us more information. Now I am going to address how we can get to a fully DC coupled solution to work all the time.
In the last article I made a PCB that allowed me to easily inject signals from my signal generator into the EOG cables. This allows me to more easily simulate signals without wiring myself up, but also allows me to isolate the effects of the cables. The first thing I wanted to try was to shield the cables.
Now that I have an EOG system working to some extent, it is inconvenient to wire myself up each time I want to take a measurement. Not to mention that the silver chloride electrode pads are single use which is wasteful and expensive (I reuse them multiple times, but the connection and adhesion definitely degrade). On top of this, I don’t like being connected to the system while it is connected to any mains powered device due to risk of an electrical fault being directed straight to my head – I can unplug my laptop, but my oscilloscope isn’t battery powered. So I decided to make an adapter from my signal generator to the snap connectors so I can inject false EOG signals.