Seeking extrema

Why do you use the Value Index operator and Empty as second argument?
The result of GlobalMaximum is a data series with the indices of the maxima. In your case you need a scalar start index and a scalar end index. Use the Index operator. -1 is the index of the last value.
[code]
CH1[idxextrema[0], -1]
[/code]
The result is a signal starting from the first maximum.

If you only need the maximum y-value use the [b]Maximum[/b]-function.
[code]
Maximum(ch1) //is equivalent to CH1[idxextrema[0]]
[/code]
support@weisang.com