Editing: MPEG2 vs DV AVI

[daamon]

Hi All,

My Dad recently asked if he could record his footage (from a camcorder) to a DVD recorder and send me the disc(s) for editing. I said it's possible, but not preferable. I was pretty pleased with my explanation, so I thought I'd share it here:

Note: I particularly refer to MPEG2 vs DV AVI. The editing of MPEG2 can be substituted with any video format that is compressed to the point where it's not entirely composed of "I" (or full) frames. Examples would be MPEG1, Divx / Xvid etc.

Likewise, DV AVI can be replaced with any video format that only has "I" frames. Examples being huffyuv, uncompressed etc.

There are loads of different formats for video - i.e. ways of storing all that information. The best quality is uncompressed, that is all the information is retained. Video is very complex, and so there's a lot of information.

The challenge for DVDs is to retain enough information so that the quality still looks good - to the human eye - whilst not having huge amounts of data (and so huge file sizes) because discs can only hold so much.

This has been achieved by compressing the source video to a format known as MPEG2. A very clever algorithm is used to "throw away" what the algorithm thinks won't make any difference to the human viewing experience - and so achieving the goal of wathcable quality vs filesize on a disc.

The problem is that by "throwing away" some information, only one in every 15 / 18 frames (PAL / NTSC) is a complete picture - all the ones in between are just partial pictures. PAL video runs at 25 frames per second, with NTSC at 29.97.

The complete frames are called "I" frames - the "I" standing for "Intra".
The partial frames are either "P" or "B" frames. "P" = "Predicted" and "B" = "Bi-directionally predicted". In essence, the "P" and "B" frames are guessed at by the algorithm (MPEG2 encoder) based on the frames around them.

When it comes to editing, the editor (person) ideally needs to be able to work at a frame-specific level. Clearly, if only one in every 15 or 18 frames is complete then editing with MPEG2 isn't ideal.

This is why editors typically work with a format known as DV AVI - it's the format that miniDV camcorders record in. Although it's lightly compressed, each frame is still a complete frame and so can be easily edited.

DV AVI isn't suitable for DVDs though, as the filesizes are way too big to get any decent length (in terms of time) on to discs with their current capacities. It's approximately 13.5Gb per hour of footage and single layer DVDs can only hold around 4.3Gb (I'm using "computer" gigabyte as opposed to a "marketing" gigabyte, which makes the discs look like they hold more than they actually do).

And here endeth the lesson...

Comments / inclusions / corrections are welcome. Post below and I'll amend the original post so it's all in one place.

[SaSi]

Your point about I-Frames vs. B/P-Frames (I'd rather call them Delta frames for general refrence) is almost correct and it is why DV video files are in general "more editable" than MPEG-2 or MPEG-4 files.

However, there are some exceptions and some fine-tuning to the general statement.

It is very possible to generate a DivX AVI file with only Key frames. Just select the K-Frame interval as = 1 in the codec settings. In this way, the DivX AVI file will be as editable as any DV file (and almost as large). In this way, the MPEG-4 video will be diminished to something very close (but not identical) to MJPEG.

A second exception to the general perception that MPEG-2 files are not very editable is VirtualDUB-MPEG. This nice free video editor can open and edit MPEG-2 video files with frame accurate trimming. However, since the output file must be re-encoded with any codec you choose, the issue of quality degratation reopens.

Having said that, a DV-AVI editor is able to perform frame accurate trimming and save the resulting video to DV-AVI without any re-encoding. Besides avoiding quality degradation, the whole process is way faster - as no re-encoding is done.

[daamon]

Hi SaSi,

Thanks for the refinement. Good points, well made.

[mikiem]

Perhaps Adding a bit of confusion here...

Editing &/or encoding video, equal sources give equal results. This includes decoding/generating a picture from a DV, mpg2, mjpeg, just about any format. Of course in real life these sources aren't often equal , but it is possible to compare an mpg2 with bitrate ~= to DV & DV. Adam Wilt I think had something on this at his site (adamwilt.com)

Similarly, editing a complete frame may or may not be necessary. Cutting/joining takes place at complete frames, but when the video is in an editor, the decoded/generated picture is used, as with sampling during mpg2 encoding. Won't usually work as well as DV because the editors have limited decoding abilities, but in tests I've run, don't see an improvement using all I frame mpg2 versus standard. IN theory perhaps, a capture to std mpg2 (vs all I frame), using maxed bitrate, might even be a bit better as less info written to disc should allow less compression at same bitrate.

One very real prob. however is as mentioned, having to re-encode an entire file after a small edit, but I think this is much more of a software problem then a format one -- perhaps something that will be cured if or as recording to mpg2, mpg4 etc. becomes more popular. Some prog. can edit mpg2 the same way DV is edited, with only changes re-compressed. Microsoft also says this is possible with wmv, but even their use of this feature is rather limited. In moviemaker you have to use just the supplied profiles I think, and in wmencoder you're limited to encoding the same profile inputting a wmv file (haven't figured out what good that is yet).

That all said, your dad really should transfer the DV to his PC & edit before going to mpg2 (DVD), especially encoding with a DVDR. Bitrate roughly = quality, the more the better, and going to disc from DV, bitrate drops from around 25 to somewhere around 8 or 9! If mpg2 & DV are anywhere equal at the same bitrates as proposed by Adam Wilt, that would be a huge drop, before editing & recompression. Perhaps more ammo, assuming you don't mention more efficient codecs then mpg2.

[SaSi]

Two additional points. (My turn to stir chaos).

Transfering the DV video from the camer to the PC in DV format (using the firewire or USB cable) is as obvious as ripping a CDAudio rather than recording it via the sound card. No point in doing anything else. And since the video will be in DV format, no point in changing it to anything else prior to final encoding.

As for the point that I made and mikiem expanded (the issue of re-encoding a complete source MPEG2 video even for minor edits), I think we need to be fair and point out Tmpgenc MPEG Editor. A not very succesful product, judging by the times it gets mentioned here. However, this MPEG2 editor will allow you to trim out whatever you want and save the resulting file re-encoding ONLY the frames around the cut points if they need re-encoding. And since it allows the encoding bitrate to be selected, one can select a rather high bitrate and loose nothing really in terms of quality. And as it re-encodes only a few frames, saving the edited file is a matter of copying it to another filename - in terms of speed. Cannot get much faster than that.

[Dasun]

Use Ulead VideoStudio (I assume there are others!) it only reencodes the edit points. I set VS to reencode the edit points at 8Mbps and the resulting MPEG2 edited file with the edit points reencoded looks just like the original DV. The source DV is MPEG2 encoded at 6.5Mbps at variable bit rate and is indistinguishable from the original DV on a 68CM TV. The only drama I have is an occasional project with a loss of audio synch. I never found a good cure for this, and I have to start the project from scratch if it happens - I must stress this is rare but very frustrating!

Frame Accurate edit MPEG2 with fancy transitions and all and retain quality, it is doable and not the devil!

[mod_a]

Thank you for these detailed descriptions and recommendations, they have been very helpful for me. Thanks!