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Is dynamic GOP the same as closed GOP. GOP of half the frame rate?
Dynamic GOP means the GOP size varies depending on the content. Usually a new GOP starts at each shot change.
A closed GOP is a GOP in which b-frames only reference other frames within the GOP. An open GOP is a GOP where b-frames at the end of the GOP can reference the i-frame of the next GOP.
Half frame rate GOP is to speed up decoding - digital receiver switched from channel to channel need to wait until new GOP arrive then it can start decoding - with half frame rate this is literally 0.5 second delay - after all, with newer codecs this requirement was somehow relaxed and seem it is commonly accepted that 1 - 2 second is fine so GOP can be between frame rate and 2x frame rate - this should improve compressibility. In case where stream is stored locally so decoder can quickly navigate over stream (seek) GOP can be even 10 seconds i.e. GOP is 10x framerate (but usually no more than 250 frames).