@Aaronusername don't count that Fitbit will chime in 🙂 Readings coming from the accelerometer are usually very noisy and it requires applying a low-pass filter to remove unwanted high-frequency noise. It is required to do further signal processing, too. It is crucial to eliminate sensor noise. It may be that Fitbit devices are just not powerful enough to calculate and compute in real-time (Fitbit devices, in general, are rather on the lower end of hardware performance). Now, we know that devices with gyroscopes are able to count laps in real-time. Counting laps (and stroke recognition) may use data from as many useful sensors like magnetometers, barometers, etc. The more the better. It is possible to do it just with the accelerometer, though. The watch needs to detect turns and in general, it takes a bit of data to figure out that the swimmer turns. The turn may take 5-8 seconds (depends really on the swimmer, wall push-off takes some time). As much as it's possible to see the pattern in the acceleration, there is a lot of data to be analyzed and figure out what event has just happened. Now, with extra sensors like a gyroscope, it's possible to determine turn different way. It's because some events are easier to determine from one sensor while the other events are from another. Science says that the swimmer when turning reduces angular velocity and increases linear velocity. Since gyroscope data is less noisy (even without filtering) less computing power is needed. Also, it is easier to decide what stroke is used and what the lap count is using data from both sensors. Interestingly, even some swim strokes are easier to detect with the use of a gyroscope (breaststroke, front crawl, and butterfly - by observing a single axis of a gyroscope, it's actually a very regular pattern for each stroke). It is all possible just with the accelerometer but I think Fitbit tech isn't just performant enough to provide such data in real-time just using accelerometer. I see no other explanation (and Fitbit won't give any either) but all we can do here is just play a guessing game (well, backed with science) 😀.
