For most prosumers, their first experiences with interframe acquisition, positive and negative, came with HDV. The arrival of HD, with its elevated bitrates, brought interframe encoding into the acquisition and production stage. An hour of SD video – 13gigs at DV datarates – could easily compress down to 500 megabytes with a decent interframe codec.
#SCOPEBOX 3 CODE#
Once you’d finished shooting and editing your video using one of the popular intraframe codecs, you’d compress it with an interframe code to deliver on DVD (MPEG-2), or the web (H.264, VP6, etc). Interframe compression has been around for decades, but until a few years ago, it was considered a delivery technology. This sequence is called a Group Of Pictures, or GOP.
Finally, another I-Frame closes the sequence. A third type of frame, called a B-Frame is based not only on the frames that have already been passed, but also frames that will occur in the future (more on this later).
#SCOPEBOX 3 SERIES#
In doing so, the codec avoids wasting bits sending the same data twice.Ī simplified interframe stream would consist of an initial complete image (called an “I-Frame”) followed by a series of frames consisting primarily of motion vectors and partially changed pixels (“P-Frame”). A motion vector tells the codec to take some pixels from one frame, and move them a little bit for the next frame.
As we pan across a scene, some parts of the scene enter one side of the frame, and some parts leave the other side, but the section of the image in the middle just moves laterally. Motion vectors allow the encoder to describe movement within a frame. For example, most interframe formats use something called motion estimation and motion prediction. Interframe compression uses a variety of techniques to encode the changes from one frame to the next. Why would we go to all the trouble of sending a complete image each time when all we need to do is describe what changed? A panning shot primarily consists of the same image, just moving sideways. A talking head video primarily consists of a moving mouth, and perhaps some blinking eyes. Interframe compression is based on the assumption that any two sequential frames of video are unlikely to be all that different. And you need to hope that those miniDV tapes won’t spontaneously combust.
Even worse, you need to engineer motors, write heads, and lots of other bits that work reliably at four times the speed. Suddenly your 60 minute tape is only good for about 12 minutes. Why not just put HD on those? Well, HD is more than four times the number of pixels, so we need to run the tape four times faster. The world already knows and loves cheap and cheerful miniDV tapes. Let’s say you want to implement a tape-based HD acquisition format. In our last post, we covered intraframe compression – easy to edit, high quality, and inexpensive to implement.
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