Since googling up NASA’s online VLF receiver mp3 stream (seems to be defunct now, fortunately there are other streams available.. see http://abelian.org/vlf/) I wanted to try to receive these signals myself. So I came up with some schematics on the internet for a basic vlf receiver and built and tested a couple of these.
The sferics are crisp, loud and clear, the circuits work very well if the conditions are right. Small enough to fit a pocket, it’s interesting to walk aroud the area I live and listen in to the various weird signals eminating from all sorts of electronics and machinery. I’ve yet to record auroral activity, but that is next on the agenda.
I want to write a little bit more about my studies on waves and automata models. I wrote a vastly improved TLM code on MATLAB which now includes for example first order absorbing boundaries. It is important to distinct this approach from a mathematical model, this is a analogous physical system to wave propagation. You could think of it as using computer memory element grid as an discrete analogy to the vacuum.
This sort of physical modelling and computation was first used by a Hungary born mathematician and electrical engineer Dr. Gabriel Kron in 1943 while working for General Electric (see paper called ‘Equivalent Circuits to Represent the Electromagnetic Field Equations’ on Physical Review Vol.64 Numbers 3-4 1943.) The approach involved analog computing in the form of a RLC network. The approach was then picked up by P.B. Johns and R.L. Beurle (see paper ‘Numerical solution of 2-dimensional scattering problems using a transmission-line matrix’ on Proc. IEE, Vol. 118, No. 9, 1971, pp. 1203-1208) applied to ‘computors’ as they were then called.
The Johns and Beurle numerical method involves applying a simple scattering automata rule to a discrete node grid. This doesn’t exactly involve integration in the sense that a discretisized mathematical model would; only arithmetic needed is addition (floating or fixed-point) for summing up the node incidence and reflection time-step impulses involved in the scattering rule (which are directly derivable from normalized unitary impedance electrical node network.)
The simple TLM here is configured for the classic Young double slit experiment. Although this particular setup could be thought to just propagate the electric field, the corresponding magnetic field can be derived aswell together with different permittivity and permeability coefficients to model material properties.
Visited a small retro gaming event this weekend and came home with some cool new vintage computing junk. In particular I finally got hold of a Commodore Vic-20. This was my first computer back in the eighties so it holds a special place for me. It is in good condition, even has the first(?) ceramic revision of the VIC video chip. However it’s not the model that takes the C-64 type of power supply, instead it has a two prong connector which should feed the vic with 2 amps worth of 9 volt alternating current. I had some 9V ac wallwarts laying around so I decided to try it out if the thing is in working condition.
Turns out that the wallwart rated at 1A isn’t enough for the circuitry, or alternatively the VIC chip or something along the address bus decoding is busted. I’m guessing it has more to do with the power supply, since probing with a scope revealed that the TTL glue logic responsible for the VIC address decoding is stuck at around 2 volts.
The glitching causes interesting visual patterns though!
Got around breadboarding a voltage controlled filter for my analog synthesizer finally. The circuit is very simple, it is based on the SSM 2045 integrated circuit I salvaged from half working Siel polysynth. The circuit consists of the filter chip together with some opamp buffering and amplifying to get the signal levels matched with the rest of the modules and with the correct polarity. I still need to breadboard the two voltage controlled amplifier blocks in the circuit.
The operation is very stable and accurate, it’s the usual integrated circuit performance you’d except. The chip has outputs for 2-pole and 4-pole points from the integrator cascade. I really like the 2-pole sound of it. I think I’m going to wire both outputs to the panel with inverted phases so you also get some sort of bandpass output if you sum those together.