›

Inconsistent Sample Rate

biofeedbackuwabiofeedbackuwa
in iOS
Hi,

I can't seem to get expected sample rates from the accelerometer or gyroscope (CPro, latest firmware, latest Metawear framework) on iOS. Using timestamps, fps range from 33 into the 100s for a sample frequency of 50. Likewise, if I just use absolute system time to clock incoming events, it is also totally inconsistent. It's quite important for me to have reasonably reliable sample rates. Can you please advise?

Thanks.

--Alex

Comments

  • EricEric
    Please see my post from a similar thread regarding this issue:
  • biofeedbackuwabiofeedbackuwa
    Thanks, I read it now. But it doesn't really address my issue. I'm happy to hear that the appropriate number of samples are generated within a given time frame. What I need, however, is to know the time interval between successive samples. Particularly for the gyroscope data. Are the timestamps actually suitable for this?
  • EricEric
    As mentioned in my post, accelerometers, and similar sensors, have very accurate clocks, typically with only 1% error.  If you set the data rate to 50Hz, you will consistently receive data every 20ms.  Unless you need the absolute time of when the data was created, a sample count is sufficient enough to determine relative offset between two data points.  Otherwise, you will need to use the logger.
  • biofeedbackuwabiofeedbackuwa
    Hmm... So what do the timestamps on the streaming data objects represent?
  • EricEric
    They represent when the data was created, but are inaccurate for high frequency streams for the aforementioned reasons.
This discussion has been closed.