What is CFM5330 simulation?
Digital simulation means reproducing in a computer program the signal processing sequence of the analogue circuitry of the CFM format, such that the output is virtually indistinguishable from the original. For CFM studies, digital CFM format monitors are required to reproduce the behaviour of the Lectromed CFM5330 monitor. The 5330 was the last of a series of monitors derived from an original design in 1967/68. It is worth remembering that the purpose of the original monitor was to determine, over a period of days, if there was any measurable EEG in patients in coma in ITU, after a 20 minute multi-channel EEG recording had established that normal EEG was non-existent or nearly so. Consequently the amplitude trace was weighted to give most attention to low voltages, compressing those above 10uV on a near logarithmic scale
The CFM amplifies the EEG signal, applies it first to a band-pass filter that has most gain at higher frequencies, then to an amplitude compression network, then rectifies it and low pass filters it.
The signal in the 5330 is then applied to a compression circuit comprising a feed-back system of diodes and resistors. This gives a linear response to 10uV peak to peak and then compresses the signal logarithmically. At much higher amplitudes the log law fails but this does not matter as the signal is clipped before this becomes significant. A mathematical formula can reproduce the behaviour of this circuit.
Subsequently the signal is rectified by diodes feeding into holding capacitors, then low pass filtered. Because the holding capacitors are charged quickly but only discharge slowly, the 5330 response to a step amplitude change is faster for an increase than it is for a decrease. Software can readily simulate the charge, discharge and filter behaviour. ( In contrast the CFAM format operates in a different way and its rise and fall times are the same, hence it is better at indicating short EEG suppressions).
Finally, because the analogue CFMs use a hot galvanometer pen to write on thermal paper, the convention has been that, on input overloads, the pen was moved to below the 0 microvolt baseline. This behaviour is copied in the simulation.
Some things have been changed. In the CFM, impedance and muscle activity were combined. In the simulation this has been replaced by the CFAM format that shows muscle and impedance separately. Also copied is the CFAM method of indicating the amount of mains interference as this is invaluable when recording in environments of high electrical interference that may overload the input amplifiers.
Analogue band-pass filters (made of amplifiers, resistors and capacitors) produce substantial phase shifts in the signals, these shifts being different at different frequencies. In contrast digital filters (in software) are usually designed to produce zero phase shifts at all frequencies. RDM have developed a digital filter design algorithm that could produce filters of arbitrary amplitude and phase responses. This was used to emulate the CFM filter’s amplitude and phase response.
Although, in the original CFM, the system could respond to large voltages, in later commercial models this was reduced. Consequently a signal that is equivalent to more than approximately 155 microvolts peak to peak at 10Hz is clipped by the 5330. This prevents the system responding appropriately to large voltages. Seizure activity can reach in excess of 800 microvolts peak to peak. ( In contrast the CFAM
