Download ADVANCED CODAS USER`S MANUAL

ADVANCED CODAS
130 mmHg
Mean Arterial BP
60 mmHg
FIGURE 6A.5
This figure demonstrates the ability of Advanced CODAS's moving average filter to generate a
mean waveform from a periodic input waveform.
When using Advanced CODAS's moving average program to display the mean value of an input signal, the
smoothing factor should be selected so that the number of points contained in the moving average window is
proportional to the number of sampled points contained in one typical signal cycle. A good first order
approximation for the frequency response of the mean signal is as follows:
Frequency mean = (1.6 x Sample Rate) ÷ n
Where: n = The number of points contained in the moving average filter.
Sample Rate = The per channel sample rate at which the input signal was originally acquired in Hertz.
Frequency mean = The bandwidth of the mean signal in Hertz.
This formula may be algebraically rearranged as follows to solve for n:
n = (1.6 x Sample Rate) ÷ Frequency mean
The use of this formula can best be described by example: Assume that an arterial blood pressure waveform was
sampled at 250Hz, and that a mean ABP signal with a frequency response of 1Hz needs to be generated. Plugging
these numbers into the above equation solves for the moving average span (n).
n = (1.6 x 250) ÷ 1 = 400 samples
Using 400 as the moving average smoothing constant is a good first order approximation for the desired result.
The waveform moving average utility may also be used to simulate a high pass filter by specifying smoothing
factors less than zero. When a moving average factor less than zero is specified, the resulting output is the input
signal minus the moving averaged result.
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