FTA Movie Timing
There are three separate concepts of time in the FTA system:
- the time stamp noting when the Movie was started: when Run or Snap Shot were clicked
- the time of each image after the Movie was started
- the time axis of the Graph plot
The following image shows a typical situation.
click to enlarge and show detail
The time stamp for the start of the Movie is always shown in the title caption above the image. In this case this Movie was started August 5, 2003 at 6 hours, 49 minutes, and 53 seconds PM. The title caption, medium gray in this image, shows the file name also. The caption reads
Movie C:\Source\Fta32\Data\USB\LongTermWater.mdb 8/5/2003 6:49:53 PM
You may expand the image to see this more clearly.
The time of each image after the start is shown in the middle of the lower status bar for the image. For this image it is 11.7000 seconds. We are looking at image #3 in a Movie containing 101 images. The absolute time for each image is the sum of the start time stamp and the image time. For this particular image
53 + 11.7000 seconds = 1 minute + 4.7000 seconds
and the sum is 6 hours, 50 minutes, 4.7000 seconds PM on August 5, 2003.
Note there is no particular coding for when the Trigger occured. However you can deduce that from how the Capture tab was setup for the Movie. If, for example, you asked for 2 images before the Trigger, and this is image 3 in the total Movie, then the Trigger occured between the preceeding image (i.e., image #2) and this image. What is important is that the absolute time for each image is available by the above sum.
The Graph defaults to a new time axis: t=0 is set at the first image bearing analysis. You can change this, if you wish, by providing a different "offset" on the Graph | Options tab.
How is all of this accomplished? All images coming from the camera have a time stamp in them. Each camera or frame grabber has its own way of imprinting this time stamp. The time stamps are, of course, in increasing order. When you click Run or SnapShot, the next available image is captured and its time stamp recorded. Think of this as the origin time. Simultaneously, the current date and time is grabbed from Windows. This is the local clock-on-the-wall time. This Windows clock-on-the-wall time is what is written in the upper title caption. In the above example, it is some time in August 2003.
The FTA software assigns a local time to each camera image that comes into the capture routine. The local time is difference between the camera's imprint time and the origin time. For the first image that came in, the one that triggered the clock-on-the-wall time, this difference is, of course, zero. Each successive image will have a positive local time as its camera imprint time will be greater than the origin time. These times are assigned irrespective of whether the particular image actually appears in the final Movie. That depends on when the Trigger occurs and how the Capture parameters were setup.
After enough images are collected, the Movie is assembled by selecting images from the "before" and "after" Trigger buffers. Each image selected will have its own local time stamp. This time stamp is not perturbed in any way during the Movie assembly process. This local time (measured relative to the origin time) is what is displayed at the bottom center of each image in the Movie (11.700 seconds above). To summarize:
The clock-on-the-wall time of the first image from the camera after Run or SnapShot is recorded and is assigned to the Movie as the time stamp. This image may or may not appear in the final Movie. The time stamp of this first image is recorded locally and serves as the origin, or reference, for a local time clock.
Subsequent images have local times relative to the origin image camera time stamp.
Local times are shown in the bottom status bar in its middle.
Each image placed in the temporary buffers during capture has its own local time. It is immaterial to the time keeping whether any particular image is included in the final Movie.
When the Trigger occurs, it stops accumulation of the "before" buffer and starts accumulation in the "after" image buffer. It does nothing per se to the time keeping.
If you want to identify which image of your Movie was the "Trigger" image, you must look at your Capture tab settings and count from either end of the Movie, whichever is more convenient. Note that you could have possibly truncated the "before" buffer by having a Trigger too early after Run and you could have truncated the "after" buffer by clicking Abort before all "after" images had been collected.
With some cameras, their mode is such that a SnapShot will have a local time of zero and with others it will have a local time of one image period later. This is unimportant and does not occur in Run Movies as Runs and SnapShots are acquired differently.
The above scheme is typical of transient data recorders.