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5.3 Multiplexed Signal Files

Multiplexed signal files may be identified by examining the group fields of the array of WFDB_Siginfo objects returned by isigopen or wfdbinit. Signals belonging to the same group are multiplexed together in the same file. If all signals in a given signal file have been sampled at the same frequency, and there are n signals in the file, then each group of n successive samples in that file contains a sample from each signal, always in the same order (but see section Multi-Frequency Records).

Multiplexed signal files can be useful if the storage device is sequential-access only (e.g., 9-track tape), if the storage device has lengthy seek times (e.g., optical disk), if many signals must be recorded and Unix’s per-process limit on open files would otherwise be exceeded, or if very high speed is required while the file is being created (because of sampling constraints). CDROM signal files, and those available from PhysioNet, are multiplexed unless the record contains only one signal.

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