I suggest you have a look at the following link which explains the EMA calculation in relation to stock market charts where the interval between samples is one day.
In our application the interval is variable in length but the initial assumption is that it is 200 ms
My terminology is very similar to stocksharts terminology.
http://stockcharts.com/school/doku.p...oving_averages

TC=multiplier
Period = Time Period
Drive=Close
Lasttime=Ema(previous day)

"Period" is really just a count of how many values to use for the calculation. When the interval between samples is fixed it ends up being a fixed amount of time as well, and so is a "Time period".