Saturday, September 25, 2010

abVIEW subVI to detect the first start bit in the Caller ID message bitstream


  • Description: Detect the first start bit in the Caller ID message bitstream, and return only the remaining bits in the bitstream. The Caller ID message consists of three distinct regions: (1) channel seizure (alternating pattern of T and F values), (2) steady mark (constant T values), and (3) data block containing the message payload. This subVI detects the steady mark region and then identifies the array index (time point) at which the input bitstream first changes to F.
  • Category: Caller ID decoding ("cid" prefix)

Inputs (Controls)

  1. bitstream in -- 1-D Boolean array
Parentheses ( ) indicate default value; square brackets [ ] designate units.

Outputs (Indicators)

  1. datablock bitstream -- 1-D Boolean array
  2. start bit index -- I32

Required Behavior

  • The bitstream in input should contain a complete Caller ID message bitstream as generated by other demodulating and bit synchronization and sampling subsystems.
  • The datablock bitstream output contains only the data block portion of the input bitstream, beginning with the first start bit of the first character, i.e., the first frame. If no data block is detected then datablock bitstream will return empty.
  • start bit index is the index (location) of the data block detected in the input message. If no data block is detected then start bit index will return -1.

LabVIEW Coding Tips

View the screencast video in Create a SubVI in LabVIEW to learn the mechanics of subVIs.
Refer to the Figure 1 screencast video for LabVIEW coding tips and techniques specific to this subVI.
Figure 1: [video] LabVIEW coding tips and techniques for
Figure 1 (subVI_cid_DetectStartBit-vi.htm)

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