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Accuracy of peak detector is critical to avoid false transitions. This generally uses a BPF filter , matched to the spectrum of the heartbeat with AGC and a comparator with optimal hysteresis.
Then for 3 digit resolution on rate from 0.8 to 3 Hz the time interval counter must be at least 1000x faster. And 4 digit resolution 10k faster.
Due to beat to beat variation, std deviation will reduce usually by 1/ root N samples averaged, so 9 seconds reduces variation in results by 3.