Using OpenEEG’s Fiview to reproduce the Cade-Blundell filters

Now I have convinced myself that I can get a version of the OpenEEG hardware to run into EEGmir, I want to how see if I can reproduce one of the Cade-Blundell filters. I have an analogue simulation from earlier, and I want to see if I can reproduce this in EEGmir. The filter specification protocol in EEGmir is the same as in Fiview from Jim Peter’s site[ref]they use the same underlying library, fidlib[/ref], and since that displays the transfer function it looks like a good place to start.

a tale of linux graphical display woe…

The windows version doesn’t run, beats me why. So I try it on Linux. My most powerful Linux computer is an Intel NUC but because Debian is hair-shirt purist and therefore snippy about NDAs and proprietary drivers, I think it doesn’t like the graphics drivers. It was tough enough to get the network port working. Xserver and VNC is so deeply borked on that. If something is stuffed on Linux then it’s reload from CD and start again because I haven’t got enough life left to trawl through fifty pages of line noise telling me what went wrong. So I’m stuck with the command line. So I try fiview on the Pi, and this fellow sorts me out on tightVNC and the Pi which is a relief, trying to get a remote graphical display on a Linux box seems to be an endless world of hurt, and I only have a baseband video monitor on the Pi console.

Simulating the 9Hz Blundell filter

I already have SDL 1.2 on the Pi, so it goes. Let me try the 9Hz channel, which was the highest Q of the Cade-Blundell filters. If you munge the order and bandwidth specs you get fc=9Hz BW=1.51.

Converting that to Fiview-speak that is

fiview 256 -i BpBe2/8.22-9.72

which in plain English means simulate a sampling rate of 256Hz bandpass Bessel 2nd order IIR between 8.22 and 1.51. So let’s hit it.

fiview on the 9Hz Cade-Blundell filter. Shame about the linear amplitude axis…

Unfortunately the amplitude axis is linear, which is bizarre. Maybe mindful of their 10-bit (1024 level) resolution OpenEEG didn’t want to see the horror of the truncation noise and hash. I can go on Tony Fisher’s site (he wrote the base routines Jim Peters used in fiview) and have another bash

The same filter with a log amplitude scale, but a linear frequency scale Grr…

Running the analogue filter with the same linear frequency display I get

The analogue filter in LTspice on a linear frequency scale (256Hz × 0.15 = 38Hz, roughly the same endpoint as the digital chart)

which shows the same response[ref]It’s not strictly exactly the same because of the increasing effect of the frequency warping of the bilinear transformation as the frequency approaches fs/2. But in practice given the fractional bandwidth of the filters the warping only has an effect in giving the upper stopband a subtly different shape in the tails, I struggle to see it here.[/ref]. H/T to the bilinear transformation for that. I had reasonable confidence this would work, I did once cudgel my brain through this mapping of the imaginary axis of the s plane onto the unit circle when I did my MSc. Thirty summers have left their mark on the textbook and faded the exact details in my memory 😉 But I retained enough to know I’d get a win here.

My late 80s digital filtering textbook after many years

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