For ALL-I case the only way to compress a frame is to apply DCT and entropy encoding (JPEG-like compression), which requires quite a bit of bitrate for a decent quality. The higher is the compression rate here, the more prominent the macro blocks are. The two obvious benefits of this compression are the ability to capture very quick movements (those do not require any extra bandwidth/processing compared to more static ones) and lower computational complexity during video editing in post (no need to recreate frames using information from the neighboring frames, like it is done for IPB encoding). For IPB case, the same approach is used for "I" frames, but those are emitted only once in a while (typically - half the framerate, every 12-th frame for 24p). Another approach is used for the remaining 11 frames of each 12-frame period (for 24p example). The encoder divides the frame into macroblocks and for each of the macroblocks tries to find its previous location in the preceding frame. If it succeeds, it records only the information about how the macroblock has moved (plus some bitmap error correction data as moving objects ca slightly change their shape/color/luminosity). If it fails, it encodes the macroblock as bitmap (just like for "I" frame case). Thus, for the cases, when there is no rapid movement in the frame (that causes the encoder not to find previous locations for macroblocks and thus requiring more bandwidth due to the emitted bitmaps), IPB encoding gives better image quality for the same bitrate. It has much higher computational complexity in both - encoding and decoding. Raising the quality for IPB encoding would mean dropping less bits in DCT (not adding much for processing) plus looking for moving macroblocks in wider areas (adds a lot and is memory bandwidth bound). To summarize, ALL-I encoding handles fast movements much better, but requires higher bitrates for equivalent image quality (compared to IPB), while IPB encoding handles less dynamic streams much better compression rate wise, allowing higher image quality for the same bitrate (compared to ALL-I). What my petition is about is giving both tools to people and letting them choosing, whether they want higher IQ at the expense of inability to keep the image quality (IQ) at that level in case of rapid movements, or they want to have constant IQ that is lower, but handles rapid movements with no IQ degradation. IPB at 150 Mbps gives IQ equvalent to 400 Mbps ALL-I (if we have no rapid movement in the frame). By implementing 400 Mbps IPB Panasonic would give us an ability to get maximum IQ out of the camera (for some subjects).