Wednesday, November 7, 2012

Nyquist Rate

The Omnivore, from Ocean Systems, is an essential part of my field kit. I've used it with great success on many cases. An interesting question came in related to it, it's Optimization process, and the accompanying instructions. Here's my answer.

While it's not explained or referenced, it appears that the software samples the playback rate, then uses a variation on the Nyquist Rate theme (The Nyquist-Shannon sampling theorem establishes that "when sampling a signal (e.g., converting from an analog signal to digital), the sampling frequency must be greater than twice the Band Width of the input signal in order to be able to reconstruct the original perfectly from the sampled version") in performing it's work.

It seems that the Omnivore is sampling (or capturing) at twice the playback rate, then discarding the duplicates to best re-create the proprietary file in a usable format.

So, from a workflow standpoint, you open the player, load the file, launch Omnivore .... and capture the video into Omnivore's proprietary format. With me so far.

What do you do when you have multiple clips from a single location - same player, many camera views? The Omnivore help files says this, The “Reset” button located in the Video Setting Tab (Optimization section) should only be used when you are changing from one player to another. If you are performing multiple captures of video from different camera views within the same player, you do not need to Re-Optimize each time or “Reset” the optimization process.

So in the case that generated the question, each of the individual views was playing back at a different rate - a rate that was variable. The only way to get an accurate grab was to Reset after playing back each view. Then Re-Optimize with the new camera view.

So, in this case, RTFM wasn't the best solution. Who knew?


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