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DDisk Write PreStart
This control provides a "hedge" against reaction time when you are trying to record only certain events. You would typically set up by hitting DDisk or ALT+D to select a file name, then wait with the mouse over the Record button or your finger over the INS key while you monitor the signal or observe some test variable by other means. When the desired event happens, you immediately hit the button or key to capture it.
But no matter how fast your reflexes are, you could never capture the exact starting instant that you first detected, and you certainly couldn't capture precursor events that happened a moment before that event.
The PreStart control actually allows you to start the recording up to a half-second before you hit Record or INS. Although it sounds like some kind of psychic precognition, the trick is actually rather simple: During a recording, all incoming signals are sent to a large circulating buffer before they are written to disk. When the buffer is filled, writing just continues again back at the start in an endless cycle, always overwriting old data from the prior pass.
When you hit Record or INS normally (without PreStart), that just tells Daqarta to start copying to the hard drive from the current position in the buffer. All PreStart does is tell it to start copying from an earlier location in the buffer, and continue normally after that.
This method is obviously limited by the size of the circulating buffer, since it only holds a fixed amount of prior data. For this reason PreStart is limited to just over a half-second at 48000 Hz sample rate (24576 samples).
The default units for PreStart are Samples, but you can use the DDisk Units control in this same dialog to toggle the units among Bytes, Samples, Seconds, or time in HH:MM:SS format. (Local or UTC Time modes only apply when reading a file; HH:MM:SS will be substituted.)
Since you can toggle Record or INS to stop and restart a recording as often as you want (such as to exclude unwanted events), note that PreStart is applied every time you start or restart, not just at the initial start of the recording. You can thus capture infrequent activity without saving a lot of useless dead-time you would have to review manually later.
Note, however, that disk space is cheap; often it is better to let the recording run to insure you don't miss something important through a moment of distraction. When you later review the recording, you can usually set up the trigger to allow you to automatically advance past areas of inactivity.
Also, PreStart does not affect the overall length of the recording, only where it starts. If you have set a certain Size Preset, the recording will still be that size.
The DDiskPreStart macro uses the units (Bytes, Samples, or Seconds) set via the Units buttons in the DDisk Controls dialog. HH:MM:SS mode is treated as Seconds here. So depending on the Units, DDiskPreStart=10k will set PreStart to 10000 bytes, samples, or seconds. (Local or UTC Time modes only apply when reading a file; HH:MM:SS will be substituted and treated as Seconds.)
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