Tag Archives: biofeedback

Building the Mind Mirror filter bank

Drafting out the Mind Mirror analogue filters, scaling values to the nearest good combination of series resistors looks good at 1% tolerances of resistors and capacitances, according to LTspice

Monte Carlo simulation at 1% tolerance

but realistically the capacitance tolerances are 10% although resistors are 5%, and that’s makes a mess of some channels

Monte Carlo simulation with resistors at 5%, capacitors at 10%

in particular the 6,7.5, 9, 10.5, 12.5 and 19, 24, 30 and 38Hz channels. These are simulated using multiple-feedback bandpass filters (MFBP). I then simulated the same spread on the highest Q so most sensitive 9Hz band on the dual amplifier bandpass topology (DABP) and the state variable topology (SVBP) using three opamps per stage.

the DABP and SVBP simulated filters, SVBP on top. The funky values are so the Monte Carlo simulation can tinker with the values between runs

Both the latter are meant to have a lower sensitivity to tolerances, and both have the advantage of having a defined gain, whereas the MFBP gain varies quite dramatically with fractional bandwidth and Q.

sensitivity of the topologies, blue is the DABP, green is the MFBP and red is the SVBP

There’s not much to be won here with the different topologies regarding sensitivity to tolerances, which surprised me. Williams states that the DABP is less sensitive to component tolerances than the MFBP and the SVBP less still. Examining the SVBP I got a variation in level of 5dB, the MFBP of 2.4dB and the DABP of 3.07. I have the suspicion I will need to tune these, in which case the DABP is easier, since the MFBP has interaction between fc and Q in tuning, as well as a wider spread of resistor values with Q² as opposed to with Q in the DABP. However, that is 14ch × 2 stages × 2 sides = 56 pots or S.O.T. resistors 😉 As they said

The original analogue filters in Mind Mirrors 1 and 2 were precise but expensive to manufacture. The digital Band Pass filters parameters were modelled on the band pass characteristics of the analogue filters, and were able to more accurately guarantee the performance of the filters.

Lining up the analogue filters isn’t too hard – Williams 1 says set the input frequency to be the desired centre frequency, monitor the input and output of the filter on a scope set to XY mode and adjust filter centre frequency until the Lissajous figure closes to a straight line. The thought of doing this 56 times does not fill me with joy, however. So one last time, how about DSP? Continue reading


  1. Electronic Filter Design Handbook, A Williams, McGraw-Hill, 1981, p5-46

Mind Mirror reverse engineering

The are three functional blocks in the Mind Mirror – the electrode positioning and pickup, the filter banks and the display. As far as the electrode position goes I’d follow the original T5-O1 and T6-O2 placement.

There are few pictures of the Mind Mirror, because the first model was produced in June 1976, and presumably the computer version was developed in the mid 1980s. The Dragon Project Trust has some pictures from Paul Devereux’s 1970s monitoring project at the Rollright Stones. 1including a few photos of it in use.


Mind mirror in use with the Dragon Project

The display was each frequency band presented on a linear voltage scale via 16 LEDs in dot mode, presumably to save power. This was replicated 24 times, 12 for each frequency band and in two channels, which already tells me there is a difference between the original hardware Mind Mirror 1 and the software variants – the filter specs I got were for the MM3 developed in 1992. It appears the MM1 used red and green LEDs for the different bands.

In the 1970s LEDs had only just come in and there were all sorts of display chips. I like the Telefunken U1096 which Charlieplexed 30 LEDs off 9 pins, but this and most of the 1970s chips are hopelessly obsolete. My choices now are either digitise and use an Arduino or a PIC, or use the LM3914. The LM3914 is only 10 output so it makes sense to cascade two, getting a 20-LED bargraph. I then rectify the output of the filters and feed that. A PIC would also do the job, perhaps better by controlling the meter dynamics digitally and multiplexing one 8-bit port across two banks of LEDs would give a 16dot display. It would also enable a hold command and be able to write out the digital value for a recording display. But it’ll be dearer…

Filter channels

Looking at the DPT machine, the original set of 12 frequencies on the Mind Mirror 1 can be seen. Let’s take a look

The overlaps are less even than they are in the new version, below

so it probably makes sense to make the display modular and provision 14 slots. I’ve now located a copy of Blundell’s Book

In which he has the technical specifications – the Dragon Project pic shows the MM1, but there was a Mind Mirror 2 which has the 14 more evenly spaced channels, which is shown on the cover of the book.

elsewhere it says the EMG channel displaying interference from the powerful neck muscles is showing 100-200 Hz. While the response of the bandpass filters is 40dB down an octave out, they response flattens out to the limiting case of 12dB/octave. However, a display resolution of 5% (if 20 LEDs are used) gives a minimum response of -26dB so that doesn’t matter.

Mind Mirror Filter sections

This is all low frequency stuff. I derived my simulation by calculating the staggered LC elements of a two-pole Bessel bandpass filter. For example, the 6Hz filter is this

and I’m immediately in trouble for the 7H inductors, and the 90µF capacitor isn’t that handy either, I’m not going to find these inductors at Digikey. I had been thinking along the lines of the LMF100 switched capacitor filter, but decided to compute the values for a standard multiple feedback bandpass filter(MFBP). These sweat a single stage and have the fewest components for a given shape, the downside is they can easily push the gain-bandwidth of the opamp, particularly as there is no independent control of the gain, which can end up quite high.

These are Bessel filters with low Q requirements, the highest I computed was <7. Williams 2 indicates the gain is 2Q^2 at resonance, so the gain of the amplifier needs to be a lot more than this. At such low frequencies this is doable, so choosing a value of C at 1µF and 0.47µF I can use normal MFBPs without resorting to switched capacitor filters. I was surprised but chuffed.


I was thinking of using something like OpenBCI’s Ganglion board which would be very good, but it is dear at $200 and I don’t need the digital whizzery, I will be using an analogue system. I will probably pinch their idea of using instrumentation amplifiers, which have come down a lot in price. I will wing this and assume the front end is soluble, after all it was in 1976 and things have got much better and cheaper since. Instrumentation amps are in the £5-£10 mark, they were much dearer way back then.

Next – deriving and simulating  the filter bank and the effect of tolerances.


  1. The Dragon Project was a fascinating 10-year attempt starting in 1977 to monitor physical characteristics around megalithic monuments, but details of that part of the work are tantalisingly scarce, Devereux seems to have come to the conclusion the physical monitoring delivered a null result.
  2. Electronic Filter Design Handbook, A Williams, McGraw-Hill, 1981, p5-43 Equn 5-70

Making a Mind Mirror EEG

Way back in the 1970s there was an EEG device called the Mind Mirror, which was a spectral display of the brain activity of the two sides of the user’s brain. This was in a world without desktop computers and smartphones, no DSP, and used analogue electronics to get the display of 14 frequency sub-bands in rows of 16 LEDs. Designed by Geoff Blundell in association with Max Cade, this was used to look at the brainwaves of people in meditative states.

the original 1970s device

Kindle version of this book

If you’re a materialist rationalist, you may as well stop reading now because there’s a fair amount of woo-woo in this. I personally like the combination of tech and woo-woo, but each to their own 🙂 The area of biofeedback has a lot of fantastic claims, but ranges from the sinple use of relaxation tapes through all sorts of werd and wonderful ideas of changing consciousness by feeding back signals from the body.

Although the development of the Mind Mirror was largely empirical, the studies leading to it’s development did at least use many subjects and try and control many of the variables.

In the 1970s Max Cade was studying biofeedback using skin resistance, then in 1973 using a single channel EEG, with a single channel display where the filters were switchable to present a choice of frequency bands, one at a time. He ran this with a bunch of people chosen for experience with meditation, the long-form description is in the book “The Awakened Mind” by Nona Coxhead. Basically they observed similarities in the mix of brain activity between different people in similar states of consciousness.

The trouble with using an EEG is that it’s like trying to get information about a crowd by recording the amplitude of the sound picked up a distance away, but since there’s no mind-jack in the side of people’s heads it’s the best to be had. Nowadays you can get spatial detail of what’s going on in the brain using fMRI but this is still a macro observation, in that case of changes in blood flow as a result of brain activity. The EEG is picking up the electrical signals from the brain, but averaged over many neurons.

There was also a more specific book on the Mind Mirror called The Meaning of EEG by Geoff Blundell which I gather was the instruction manual, but there’s not much on that to be found, apart from a cover picture.

Why the Mind Mirror – forty years of better tech has overtaken it surely?

Getting an EEG is a lot easier now. Get yourself onto OpenBCI and you’ll have no end of fascinating stuff to play with, or review some more approaches here. Looks to me like the tech has been sorted.

But at the end of the day, it’s all just sensor data. We are taking the faint signals averaged across a load of wetware and insulating material and displaying them on the screen. Woo-hoo, but so what?  It’s all just numbers on a screen, there is no meaning to it. What Cade and Blundell did was actually trial their machine on real people –

Maxwell Cade and Geoff Blundell calibrated the first prototype Mind Mirrors on people with known advanced training in mind states and were able to bridge the gap between internal descriptions and measurable EEG states on the brain.

The limitations of their hardware led them to focus on two channels, near the occipital lobe, and they experimented to try and get some reproducibility and correlation with different states of consciousness/relaxation/meditation. It’s this part of the puzzle that’s missing from the geeky big data stuff out there, and without that it’s just data, not information. As lifehacker says

Of course, self-awareness is a big part of both therapy and philosophy. It’s also the basis of the quantified self movement , which assumes that if you collect data about yourself you can make improvements based on that data.

The trouble with quantification is that data is not knowledge and knowledge is not wisdom. Where Cade and Blundell scored versus a lot of quantified self data is they looked at the quantified data across many people, trying to correlate it with characteristics of self-awareness, or at least chilled-outness.

The advantages of the Mind Mirror is partly due to the simplicity of the rig, picking up signals from two channels and displaying them. It meant that the machine was portable, but it also makes correlation of the display with other people’s states of mind a lot easier than trying to parse the welter of data from, say, a 16 channel EEG display. The value of the Mind Mirror to my eyes is the combination of work of Cade and his successors with this particular methodology and filter bank, and the fact that it isn’t limited to a particular place.

1970s image of Mind Mirror used in the field

Reverse engineering the Mind Mirror

There’s a lot of good information about the machine on Mind Mirror EEG.

Mind Mirror EEG are good enough to give us the filter frequency specifications indirectly, and more directly here.

I converted these to a staggered tuned second order bandpass filter and simulated this.

And you can immediately see that they adjusted the centre frequencies unevenly, presumably to get more resolution in the alpha and beta wave regions. This is a log frequency display, and the obvious way is to spread the channels evenly keeping a constant fractional bandwidth.

Now the obvious way to do this nowadays is with a PC and a FFT, and you can buy a Mind Mirror from Vilistius  that’s probably how the latest incarnation of this works. But it is £1.5k, and it uses a computer for the display, which is not a thing of beauty.

the software Mind Mirror display. It’s a little bit gonzo DOS 1984 style for me, but the £1500 is the killer, although to be honest it’s not an unreasonable price for such a device made in small numbers given how dear the OpenBCI boards are.

I don’t find computers and smartphones conducive to relaxation and meditation. They are good at what they do, but relaxation not one of them. Whereas the original Mind Mirror was self-contained and used LEDs for a display.

In the next part I will look at what can be gleaned about the Mind Mirror hardware.