Help understanding file formats and codecs

[rbd]

Hi, new to the forum and to digital video, although I have a digital video camera for some time (not near me right now, I think it’s a Canon MVX350i).

This forum seems to have so much info, probably too much for me right now. I already transferred some videos to PC but used a very high quality format wich almost filled up my disk – avi with about 173MB/minute. Two 8GB files are waiting for me to do something. I didn’t want to edit the video right now but I have more video to transfer and don’t have enough disk space.

My problems are basic ones and are related to file formats and codecs. Although there are plenty of guides for transforming a file format into another with specific tools, I need more basic info, like:

- What is each file format suited to, like avi and mpeg? What are the basic differences? What defines a video file, its type (avi, mpeg, ..) and bitrate or anything more?
- Wich are used to create a DVD to watch in any DVD player? With what bit rate for a quality level similar to commercial DVD’s?
- I wanted to store un-edited video to free disk space and edit the video later. Is it the same to compress it first and edit later or should I edit first? In each case what formats should I use?

I tried a few searches for this but with no good results, only those codecX-to-codecY guides, but that would be for after my questions above.

Hope you can help me…

[jagabo]

AVI is a container. It can contain audio and video with any number of different compression/decompress (codec) schemes.

Since you have a digital camcorder you should be capturing as DV AVI. This is basically a copy of the DV audio/video stream from your camcorder packaged in an AVI container.

The next step is to convert to MPEG with DVD compatible settings. For best quality with camcorder sources you need to use a video bitrate of 8000+. Camcorder video tends to be noisy, and if you didn't use a tripod, shakey. Both those characteristics make it much harder for MPEG to encode.

For storage your best bet is to break your DV AVI into <4GB segments and burn as data DVDs (and keep the DV tapes). It's not recommended that you convert to MPEG first, as editing MPEG is more problematic than editing DV AVI. If you have to recompress the MPEG data you will lose quality.

[rbd]

Thanks. It's a start. I now have questions on your answers:

Why should I keep the tapes, for backup or will I need them anyway?
Editing MPEG means I have to compress again or do you mean that it's not necessary but if I have to it will become worse than if I did it from AVI?
And about the AVI, if it is a "container", what does it contain?

[guns1inger]

Why should I keep the tapes, for backup or will I need them anyway?

You don't have too, but these are the best quality versions of your footage as they are the originals. Technology and techniques are constantly evolving and improving. You may find that while you are happy with what you can do now, in two years time you might want to try again with HiDef as the target. In which case you will want to start with your originals, not some compressed intermediate.

Editing MPEG means I have to compress again or do you mean that it's not necessary but if I have to it will become worse than if I did it from AVI?

It depends on the editor, and what you do to it. Most decent mpeg capable editors will only re-encode things that have changed. So if you add a transition between two scenes, then the transition will be re-encoded, but the rest will be left alone. However, if you find that you want to colour correct a whole scene, then the whole scene will be re-encoded. Mpeg suffers from re-encoding much moreso than DV, which is one of the reasons DV is a better editing and processing format, and mpeg2 is best left for the final cut.

about the AVI, if it is a "container", what does it contain?

Primarily video and audio data, although it can be extended to include other things. It is called a container format because it doesn't have a proscribed format for the video and audio, just a format for the container. How the video and audio is encoded/compressed/stored in the avi is determined by the codec you choose. These can be highly compressed formats, such as Xvid or Divx, or losslessly compressed or even uncompressed formats.

[jagabo]

Since you have a digital camcorder the issue of better quality in the future is moot. What's in the AVI will be an exact copy of what's on the camcorder tape.

For long term storage of precious video I recommend keeping the DV tapes as a second backup. If you don't want to keep the tapes then make two backups on DVD and monitor them. If one ever goes bad make a copy of the other to replace it -- so you always have two copies.

Regarding MPEG editing, you may or may not need to reencode depending on the editor and what you do to the video.

Some editors have cut/paste without reencoding. But with this type of editor you can only cut on keyframes. (MPEG consists of a whole frame encoded like a JPEG picture {a keyframe} then several frames which only encode the differences between frames. If you try to start a scene on a non-keyframe the decoder wouldn't know what the whole picture looks like.) There's a keyframe about ever about half second. Other editors only reencode when necessary. If you start a scene on a non-keyframe they will only encode from that frame to the next keyframe. Then there are the dumb encoders which just reencode the whole thing every time.

But, as guns1inger pointed out, if you need to color correct, or do anything that changes the picture, the video will have to be reencoded, incurring a second round of MPEG compression artifacts. If you add transition effects or titles the video during those transitions/titles have to be reencoded.

Another problematic scenario: suppose you decide to store all you footage as MPEG at 9000 kbps for good quality. Then you want to put 90 minutes on a single layer DVD. You will have to reencode to a lower bitrate to do it.

Regarding containers: Digital audio and video is just a bunch of numbers. You need some form of organization so that different programs can identify exactly what those numbers are and how to deal with them. Think of the AVI container as an envelope. The envelope has a place where you can log the characteristics of what's inside (the frame rate, the frame size, the codecs that were used to compress the audio and video, etc.). Inside the envelope the data is organized in a specific way so that programs can access it. For example "chunks" audio and video have to be identified so a program can tell which is which.

There are many different containers for audio and video, AVI, MKV, MOV, VOB, RM... Each is arranged differently so programs have to handle them differently.

[rbd]

thanks! I'll dig deeper into this later but seems great information to work with.