Timestamps in LabVIEW SignalExpress

Timestamps in LabVIEW SignalExpress

Time-domain signals use a timestamp, which is a value that provides information about when the signal began. Depending on the source of the signal and the actual processing steps you added to the project, the timestamp of the signal can be an absolute or relative timestamp, or LabVIEW SignalExpress can ignore the timestamp.

An absolute timestamp value represents an actual date and time, such as 12:37 p.m., April 6. Some data acquisition boards, such as the E-Series acquisition board, can return absolute timestamps, which you can use for accurate datalogging.

A relative timestamp represents a time value that is relative to a known reference event, such as the start of an acquisition or the trigger point. For example, a continuous acquisition of a signal might have a timestamp value relative to the trigger point of the signal. Timestamps also can be relative to other timestamps. For example, you can extract a subset of a signal starting at a time that is relative to the beginning of that signal.

If you ignore the timestamp, you can perform operations like addition or subtraction on the two signals independently of the actual timestamp values. For example, you can add the signals as if they were aligned perfectly, which is useful if you want to compare a triggered signal to a model that originates from a file.

Several processing steps work on continuous signals and assume that the data are contiguous. For example, the Filter step resets the filter history when you call it the first time, but then in subsequent calls, the step filters the incoming time signal segments using the history information of the previous segment as long as the timestamps are contiguous. If the Filter step detects a time discontinuity, such as a missing segment or an inconsistency in timestamps, the Filter step resets itself and returns a warning. Using relative timestamps in a continuous acquisition does not conflict with the timestamp continuity requirements.

National Instruments recommends using relative timestamps as the default mode for all steps that can deliver a continuous signal, such as steps that acquire or create signals. Use absolute timestamps only when you require absolute date and clock information. Avoid ignoring timestamps when possible.