External data (resampled)
Definition
EthoVision XT only analyzes external data after resampling. That is, it does not analyze the original data, but a Dependent variable representing the imported signal, resampled to the same sample rate as the EthoVision XT sample rate. This is accomplished by combining upsampling and downsampling (see below).
Downsampling
If the sample rate of the external data signal is higher than the EthoVision XT sample rate, the signal is downsampled. For each sample k, a new value is calculated from the values of the original signal in the interval (k-1; k] (or in multiple preceding sample intervals, depending on the Averaging interval chosen). The interval is half-closed, that is, all values of the signal between k-1, (not included) and k are considered. The new value gets the time stamp of the sample k.
Five methods are available to downsample the signal.
Below: Effect of downsampling of an external data signal. Left: original signal imported in EthoVision XT. Vertical hatched lines represent the EthoVision XT sample intervals. Right: Signal upsampled in EthoVision XT using the following five methods (for simplicity, only the values for sample k are shown). 1 Last value, 2 Maximum, 3 Mean, 4 Minimum, 5 Total value.
If the sample rate of the external data signal is lower than the EthoVision XT sample rate, the signal is upsampled.
For each sample k, a new value is calculated from the values present in the sample interval (k-1; k] (or in multiple preceding sample intervals, depending on the Averaging interval chosen). If no value is found, A new value is interpolated using one of the methods available, or replaced by “missing sample” or a zero value.
Below: Effect of upsampling of an external data signal. Left: original signal imported in EthoVision XT. Vertical hatched lines represent the EthoVision XT sample intervals. Right: Signal upsampled in EthoVision XT in the following four methods: 1 Last value, 2 Linear interpolation, 3 Missing value, 4 Zero value.
With Averaging interval you can smooth the values of the converted signal. This has an effect on both resampled and state variables.
▪With Averaging interval 1, the new value for sample k only depends on the original values of the signal in the interval (k-1, k].
example Downsampling a EEG signal with the Mean value method and the Averaging interval of 1. See 3 in the figure under Downsampling.
▪With Averaging interval x (where x=2, 3,...), the new value for sample k depends on the original values of the signal in the interval (k-x, k].
example Downsampling a EEG signal with the Mean value method and the Averaging interval of 3.
Left: Original EEG signal. Right: Signal downsampled with Mean value and Averaging interval 3 (for clarity only sample k is shown). Green dots: averages in the 3 sample intervals. Red dot: the average of the three averages.
If the sample rate of the external data signal is the same as the EthoVision XT sample rate, but with different time stamps, the signal is re-sampled to align its values to the EthoVision XT samples. Values are shifted to the right.
How to specify an external data variable (resampled)
1.Click the Add button next to the [data set name].
2.Under Select, choose the Downsampling method (default: Mean value) and the Upsampling method (default Last value).
3.Under Outlier filter, select the Averaging interval (default: 1).
4.Complete the procedure to add the variable. See Calculate statistics: procedure.
5.Plot Integrated Data or calculate the statistics.
Notes
▪If you upsample a signal with a sample rate much lower than the EthoVision sample rate, and you choose Missing value as the Upsampling method, it may be difficult to see the data points. In Integrated Visualization, choose Show/Hide > Show Graph Data Points and zoom in the plot.
▪See also Averaging interval