4 ms·

How precise is this, do we have data? I'm guessing it depends on the refresh rate of your webcam.

pokpokpok 14y ago
it should still be very accurate though right? and so decently precise over a period of time?
nl 14y ago
It's precise enough for the application. Webcams typically have a 15-30 fps framerate. Heartrates are typically 30-200 bpm (~= 0.5 to 3.3 bps).
- vacri 14y ago
  The duration of the pulse is much shorter than the beat, though. Should still be visible at that framerate, just noting that pulses aren't sinewaves :)
- afhof 14y ago
  It seems that the Nyquist-Shannon sampling rate would be too low for the results to be accurate.
  - Qantourisc 14y ago
    No ? The frequency needs to be at-least double this seems the be the case. However webcam noise and the algorithm might increase the need.
    - afhof 14y ago
      The webcam framerate should be at least double the heart rate to get an accurate measurement, not the other way around.
      - nl 14y ago
        And isn't this the case? 15 fps (worst case framerate) vs 3 bps (worst case heart rate)
        afhof 14y ago
        I dont believe so. Since the heart rate could be much higher, it would alias, and the camera wouldn't be able to tell the difference between normal heart rate and low heart rate. This is what I suspect could happen, with two possible heart rates matching the same set of samples: http://upload.wikimedia.org/wikipedia/commons/2/28/AliasingSines.svg http://upload.wikimedia.org/wikipedia/commons/2/28/AliasingS...
        vacri 14y ago
        If your heart rate is higher than 3bps, you won't be sitting patiently in front of a webcam waiting for lock-on. Sedentary (calm, sitting) heartrates go from ~40bpm for extremely fit people to ~90-100 for particularly unfit people. Higher than 100 is unusual for sedentary people, and works out to ~ 1.5bps. The waveforms in the pulse itself (note pulse, not ECG, which has much faster elements) speed up with the pulse, but I would estimate (could be wrong) that your highest frequency significant elements would be around 5hz for a 1.5bps heartrate. Your example from wikimedia has a signal that is ten times faster than the sample rate - and the nyquist limit says this would be aliased. My back-of-the-envelope calcs above suggest a 5Hz signal at the high end against a 15Hz sampling at the low end, which is a ratio of 3:1, which is enough to satisfy nyquist. Disclaimer: I am an ex-neuro tech and ex-sleep tech. I am used to sticking electrodes onto people and studying them. The exact form of the pulse wasn't big in my area, so maybe a cardio tech can chime in and correct me.
        nl 14y ago
        Since the heart rate could be much higher Err - why do you say that? If your heart rate is much higher then either you aren't human or your are probably dying. Did you read my original comment? Human heart rates range roughly between 30 to 200 beats per minute. That maxes out at a bit over 3 beats per second.
shabble 14y ago
when I have some time and can dig out my pulse oximeter, I'll see if I can hack it in for comparison. Based on trying to take my pulse manually it certainly seemed in the ballpark, and got even better/stronger locked results from using my thumb ~5-10cm from the camera. It seems quite sensitive to image saturation for losing lock though.

it should still be very accurate though right? and so decently precise over a period of time?

nl 14y ago

It's precise enough for the application. Webcams typically have a 15-30 fps framerate. Heartrates are typically 30-200 bpm (~= 0.5 to 3.3 bps).

vacri 14y ago
The duration of the pulse is much shorter than the beat, though. Should still be visible at that framerate, just noting that pulses aren't sinewaves :)
afhof 14y ago
It seems that the Nyquist-Shannon sampling rate would be too low for the results to be accurate.
- Qantourisc 14y ago
  No ? The frequency needs to be at-least double this seems the be the case. However webcam noise and the algorithm might increase the need.
  - afhof 14y ago
    The webcam framerate should be at least double the heart rate to get an accurate measurement, not the other way around.
    - nl 14y ago
      And isn't this the case? 15 fps (worst case framerate) vs 3 bps (worst case heart rate)
      - afhof 14y ago
        I dont believe so. Since the heart rate could be much higher, it would alias, and the camera wouldn't be able to tell the difference between normal heart rate and low heart rate. This is what I suspect could happen, with two possible heart rates matching the same set of samples: http://upload.wikimedia.org/wikipedia/commons/2/28/AliasingSines.svg http://upload.wikimedia.org/wikipedia/commons/2/28/AliasingS...
        vacri 14y ago
        If your heart rate is higher than 3bps, you won't be sitting patiently in front of a webcam waiting for lock-on. Sedentary (calm, sitting) heartrates go from ~40bpm for extremely fit people to ~90-100 for particularly unfit people. Higher than 100 is unusual for sedentary people, and works out to ~ 1.5bps. The waveforms in the pulse itself (note pulse, not ECG, which has much faster elements) speed up with the pulse, but I would estimate (could be wrong) that your highest frequency significant elements would be around 5hz for a 1.5bps heartrate. Your example from wikimedia has a signal that is ten times faster than the sample rate - and the nyquist limit says this would be aliased. My back-of-the-envelope calcs above suggest a 5Hz signal at the high end against a 15Hz sampling at the low end, which is a ratio of 3:1, which is enough to satisfy nyquist. Disclaimer: I am an ex-neuro tech and ex-sleep tech. I am used to sticking electrodes onto people and studying them. The exact form of the pulse wasn't big in my area, so maybe a cardio tech can chime in and correct me.
        nl 14y ago
        Since the heart rate could be much higher Err - why do you say that? If your heart rate is much higher then either you aren't human or your are probably dying. Did you read my original comment? Human heart rates range roughly between 30 to 200 beats per minute. That maxes out at a bit over 3 beats per second.

shabble 14y ago

when I have some time and can dig out my pulse oximeter, I'll see if I can hack it in for comparison. Based on trying to take my pulse manually it certainly seemed in the ballpark, and got even better/stronger locked results from using my thumb ~5-10cm from the camera. It seems quite sensitive to image saturation for losing lock though.