convTime2ValidIndex

  convTime2ValidIndex converts time to linear indices into a waveform matrix

  Only indices for valid data epochs will be returned

  Example:
  idx=convTime2ValidIndex(channel, time)
  idx=convTime2ValidIndex(channel, start, stop)
    channel is a sigTOOL channel object
    time or start & stop are the times to convert

  idx contains the start and stop indices in columns 1 and 2 respectively

  When a single time is specified, idx is the index into adc for the sample
  at the specified time or the first sample afterwards
  When start and stop are given, idx are indices into the adc field such 
  that sampling occurred between the limits
                            start <= t < stop.

  With episodic or framed waveforms, idx will be a matrix of indices with
  one row for each period contained in the interval start to stop.
  e.g. if channels{1} contains episodes of length 16001,
                idx=convTime2ValidIndex(channels{1}, 0, 5);
  might return
                      idx =
                                 1       16001
                             16002       32002
                             32003       48003
                             48004       64004
                             64005       80005
  Access the first period with:
                      data=channels{1}.adc(idx(1,1):idx(1,2));


  convTime2ValidIndex differs from findVectorIndices in that:
        1. It returns indices only for valid epochs
        2. It returns indices that are linear indices into the adc matrix,
                not indices into the vector representing a specific epoch.

  See also findVectorIndices, ind2sub, sub2ind

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  Author: Malcolm Lidierth 09/06
  Copyright © The Author & King’s College London 2006-2007
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  Acknowledgements:
  Revisions: