Tektronix MTM400 - Manual (Page 148)

Triggered Recording
MTM400 MPEG Transport Stream Monitor User Manual 3-103
The proportion of pre-trigger data may be less than specified if the trigger event
occurred before enough packets were recorded in the circular buffer (particularly
after clearing the buffer).
For example, if the available memory will hold a 20 second recording, with 25%
pre-trigger specified and an event-triggered recording starting five seconds or more
into the run, the contents of the recorded file will be:
Run
10 20 30
Trigger
time in seconds 0
25% 75% File
If the Trigger event was detected earlier than five seconds into the run, the
20 second file will contain all of the data up to the Trigger. If recording was
triggered at three seconds only the first 15% of the file would hold pre-trigger data.
The file contents would be:
Run
10 20 30
Trigger
time in seconds 0
15% 85% File
Triggered Recording Operation
The Operation section of the Triggered Recordings dialog is used to arm the
trigger before recording and shows the status of the recording activity.