Patent GB-1410939-A (GB)
Title: Signal processors for physiological signals
IPC: A61B5/344
Filed: 1972-12-29
Abstract: 1410939 Electro-medical measurement BRATTLE INSTRUMENT CORP 29 Dec 1972 [30 Dec 1971] 60120/72 Heading G1N A system for selecting weak physiological signals from a mixture of time-spaced strong and weak signals comprises means responsive to the strong signals to remove at least a major part of the strong signals and means, responsive to past signals, for providing signals representing missing weak signals. The invention is described as applied to selecting the fetal heartbeats from a mixture of maternal and fetal beats but could be applied to detecting T waves in a ECG signal by eliminating QRS waves and detecting P waves by successive removal of QRS and T waves, for synchronizing x-rays in diostole or systole. The combination of maternal and fetal heartbeats are received in one channel and predominantly maternal beats in a second channel. The maternal beats in the second channel are selected on an amplitude basis and used to block the maternal beats in the combined channel. The resulting fetal beats are used to control the spacing and period of window pulses used to select the fetal beats, to centre the window thereon. In addition missing the position of fetal beats e.g. where fetal beats are coincident with maternal beats, is estimated in providing the window pulses and means are provided for preventing false operation by spurious pulses and excessive missing pulses. The window pulses represent the complete fetal pulse train and their repetition period  indicates the fetal heartbeat period. Fig. 1 shows a threshold and gating circuit for eliminating maternal pulses. The mixed maternal and fetal pulses pass through a pre-amplifier Fig. 2a, an active filter 30 covering the expected range of fetal beat frequency and an isolator comprising an oscillator 31 modulated in frequency by the beats, a transformer and a phase locked loop 35 reproducing at its output the original pulses. These pass through a further beat rate filter 32 and a 60Hertz filter 38 to a rectifier detector 42 followed by a higher frequency noise filter 60. The predominantly maternal pulses picked up from the chest are received at 10 and pass through a similar channel tuned to the maternal beat frequency band but this channel includes an automatic amplitude thresholder comprising a comparator 45 and a peak rectifier 52 which provides a threshold level equal to 90% of the peak signal amplitude so that only the maternal beats pass. The comparator drives a monostable circuit 56 which closes gate 70 to block all but the leading edges of the maternal pulses from the first channel. The resulting predominantly fetal pulses pass through a further automatic thresholder 76, 78 having upper and lower threshold levels which may be caused to vary in accordance with the previous signal value. The residual leading edge of the maternal beat is eliminated by means of a shift register delay circuit 90 zeroed by the maternal beat from (56) Fig. 2b (not shown). The input circuits of Fig. 2a have an isolated power supply. A digital phase-locked loop Fig. 9 (not shown), generates the window pulses in response to fetal pulses and these pulses also represent the fetal beats. It ignores pulses outside the window as well as double pulses occurring within the window. The circuit normally is responsive to the time between signal pulses in adjacent windows. When however one pulse is missing it responds to the time between the centre of one window and the next pulse and when pulses are missing from two successive windows it responds to the time between the centres of the two windows. Output is halted if too many windows do not include pulses or too many pulses occur outside windows. To this end a counter is incremented one for each "outside" pulse and decremented ¢ for each inside pulse, the value ¢ being chosen to prevent operation in an harmonic mode. A count is halted on reaching 5 and restarted at less than 5. A count of 8 missing pulses zeroes the counter and operation is restarted, the next pulse being assumed to be fetal. If a further pulse occurs too soon (within 250msec) it is ignored. If too late (after 800msec) a count is reset and the process restarted. The upper limit is chosen to prevent operation at half rate. If a pulse occurs between these times the average window period is reset and the count is set to 6. If the counter decrements to 4, read-out is resumed whereas if it returns to 8 the restart routine is recommenced. When multiple pulses occur in a window or no pulses occur the frequency (or period) is unchanged and the interval between pulses remains at half the average period. When a pulse in one window only is missing the period is unchanged but the phase is adjusted by adjusting the interval to centre the window on the expected pulse by taking ¥ of the average period and subtracting the time from the next pulse to the end of the window. If pulses are present in both adjacent windows then the new period is regarded as that between the pulses but the average period (which is used to effect the control of the gate) is adjusted to the average between this and the last period, which latter is stored in a "new period store". The interval is adjusted to be ¥ of the average period less the time from the pulse to the end of the window. A stored value of the new period Fig. 10 (not shown), is converted to an analogue value and recorded. It also provides a frequency reading. Digital circuits for the interval generator, new period measurement, average period data generation and store, clock generation and window generation are shown in outline.