There's a lot of confusion over how video encoding affects subsequent editing performance - a higher data rate has no direct relation to how much effort is required to decode and play the footage, in fact a higher data rate often makes playback easier - which is why transcoding to AVI can help with playback even though the files can be hundreds of times larger. Data rates with the same codec depend on the frame contents and recording quality, but for playback and editing what matters is the GOP (group-of-pictures) structure and the complexity of the encoding algorithm. Provided your disks are fast enough the data rate is immaterial to Premiere.
With long-GOP files (as typically created by consumer DSLRs and helmet cams) we have keyframes (I-frames) with a complete set of pixel data, then a series of intervening P- and B-frames which only store the difference between 'now' and 'one frame before' or 'one frame after'. Unless you're on an I-frame you have to calculate the chain of differences from the nearest I-frame, and when editing or applying an effect we must re-calculate the entire GOP, so Premiere has to hold a lot of data in the buffer. Remember all timelines in Premiere are internally transcoded to 32-bit floating point, so it's more work to 'play' the footage in the monitors than would be done by a simple playback-only application such as VLC, or in some other brands of NLE that only work in the footage's color space. Premiere's hardware acceleration (MPE) only kicks in once the buffer has been ingested, all the decoding of the original H.264 frames happens in your CPU.
With digital cinema cameras and some hacked DSLRs, we record "all-I" footage - so there's no GOP overhead and playback/editing is massively smoother. I can scrub all-I footage from a hacked 7D no problem on my relatively-ancient spare laptop, but the long-GOP files from a factory 7D struggle to play at all. Our all-I 7D footage can hit 300MBit/s compared to 35 for long-GOP. Helmet cams such as the GoPro do all they possibly can to squeeze data onto their storage cards, which means they have long-GOP codecs that have some of the most complicated compression algorithms out there. The Hero3's implementation of H.264 uses the Ambarella chip, whose files are very small but an utter nightmare to decode - hence far more of a demand on your CPU than an equivalent clip from a DSLR.
Jim Simon wrote: Might be the 35 Mbps that is causing the choppy playback. It shouldn't, assuming sufficient hardware. My GH2 clips often get up to over 90 Mb/s and play just fine on an older i7 920. |
45
Replies
AdChoices