Our approach in recording low level cardiac signals utilized a highly shielded room, multiple parallel inputs from closely spaced chest electrodes into an analog circuit for linear amplification followed by input into an equivalent number of logarithmic amplifiers. Further elimination of 60 hertz noise and its harmonics is achieved after which the signal enters a digital logic circuit which examines the instantaneous polarity from each of the original inputs and passes this information to gates. When identical polarities are present from all of the closely spaced inputs, the signal is regarded as true, and a coincidence condition fulfilled. When failure to meet such coincidence conditions obtains, the signal is rejected as probable amplifier noise. In the final step, a variable gain amplifier is employed so that the portion of the signal in which His-Purkinje activity is expected is accented.
