* "index + length" must not exeed the length of the data array set in the constructor.
* @param start the start index of the subarray of the data array.
* @param length the number of samples to use starting from index
* "index + length" must not exeed the length of the data array set in the constructor.
* @param start the start index of the subarray of the data array.
* @param length the number of samples to use starting from index