Acquisition Formats and Re encoding

_ Popular Acquisition Formats

Interframe Formats: AVCHD-AVCCAM-NXCAM-HDV-XDCAM-Canon DSLR
Intraframe Formats: HD CAM-DVCProHD -AVCIntra

Although there are many different formats, video compression comes in two basic flavours,intraframe and interframe.

Intraframe compression is the type of frame-based editing codec used by Avid (DNxHD) and Apple Final Cut Pro (ProRes 422). It is also used in certain acquisition formats such as DVCProD and HDCam. Here each frame is separately Jpeg compressed and contains full image information. This produces a high quality, very useable format but takes requires a lot of data and storage. As the overall is calculated on individual frames of video.

With interframe formats MPEG compression is used in order to retain the quality of the image but reduce the amount of data that needs to be stored. These formats are also known as Long Gop formats because a Group Of Frames is constructed that is started and ended by an I-frame. The I-frame is a regular Jpeg compressed frame and is created every 15 frames (12 for dvd mpeg2), and MPEG compression works by removing redundant visual information the frames that lie in between I-frames, so they carry a, much smaller, amount of difference data only.

And thus, editing Long-GOP MPEG is like editing spaghetti. Firstly, unless you get lucky and pause the clip on an I-frame, the system will have to load an entire GOP and decode it before it’s able to display that frame of video to you. This takes both time and significant processing power, so jogging through the video and selecting your edit point is not a quick or slick experience you have come to expect from your editing system of choice.

Chances are the edits you make are likely to be on one of the frames within the GOP, and the result of this is that if you stay in the native format, eventually the whole sequence will have to be decoded and re-encoded again to reconstruct the GOP structure uniformly throughout. The same happens when any effect is added.
Lengthy render alert.
Mind you, not so bad if you are heavy smoker, have a weak bladder, or both...

And this is the reason why it's always recommended to convert your footage to Pro Res or DnxHD frame based (intraframe) format before editing. Much more drive space is required, but drives are still cheaper than editors (praise be...) and you really want your editor to be cutting and not swearing/smoking rendering.
Addendum: the above refers to software in the 32 bit world. i.e. Avid Media Composer up to V.5.5 and Final Cut up to V7.
The recent arrival of 64 bit software (MC6, FCPX, and Premiere Pro from CS5) have made the editing of footage in it native format much more palatable, and when these become more common place I shall report further on the new workflows.