8–26
Triggers and When to Use Them
Notes for TV Trigger
Ø Because most TV systems have more than two fields, the enhanced field-counting
capability (FIELDLOCK) allows the oscilloscope to trigger consistently on a chosen
line within a chosen field of the signal. The field-numbering system is relative, in that
the oscilloscope cannot distinguish between lines 1, 3, 5, and 7 (or 2, 4, 6, and 8) in an
absolute way.
Ø For each of the characteristics the following remarks apply:
Ø 625/50/2:1 (PAL and SECAM systems)
This setting should be used for most of the standard 50-field signals. The lines
may be selected in the range 1 to 626 where line 626 is identical to line 1.
Number of fields: eight is most useful for color PAL signals; four for SECAM
signals.
Ø 525/60/2:1 (NTSC systems)
This setting should be used for standard 60-field NTSC signals. The lines can
be selected in the range 1 to 1051, where line 1051 is identical to line 1.
Number of fields: four is most useful for US-type NTSC systems.
Ø ?/50/?, ?/60/?
In order to allow maximum flexibility, no line-counting convention is used. The
line count should be thought of as a line-synchronizing pulse count. It
includes the transitions of the equalizing pulses. For certain extreme cases,
the field transition recognition will no longer work, and only the “any line”
mode will be available.
Ø The enhanced field-counting capability cannot be used for RIS acquisitions.
Ø Composite video signals must have negative-going synch to be decoded correctly.
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