Why is MPEG-compressed video not stored in AVI file format?

[fourthquark]

OK, I have a basic undertanding of video formats but there is one question that has been bugging me for awhile that I have not yet been able to find a satisfactory explanation for.

The question is this: Why is MPEG-compressed video stored in its own "stand-alone" file format, while video compressed by any other method gets stored as AVI format?

We have MJPEG compressed AVI.
We have DivX compressed AVI.
We have HuffYUV compressed AVI.
We have DV compressed AVI.
We have INDEO compressed AVI.

So... why don't we have MPEG-compressed AVI?

I am familiar with the IBP-structure of MPEG files and I am aware that MPEG compression involves both spatial and temporal compression. Is there something about the AVI format spec that can't deal with temporal compression, thereby preventing MPEG-compressed video from being stored in an AVI wrapper?

[fingernailX]

fyi - there is mpeg in avi wrapper see: http://www.etymonix.com/products/VideoCodec/VideoCodec.htm

[fourthquark]

Thanks for your reply fingernailX, but my main question still stands.

MPEG-compressed files are typically saved as something like filename.MPG .

But DV-compressed (or MJPEG-compressed or divX compressed, etc.) are stored as filename.AVI .

You never see filename.DV or filename.MJPEG or filename.DIVX .

So my question is still: Why is MPEG-compressed video stored in its own "stand-alone" file format, while video compressed by any other method gets stored as AVI format?

[bugster]

You never see filename.DV or filename.MJPEG or filename.DIVX .

You do actually, I have certainly seen .divx anyway, though rarely, and it is usually an avi.

Avi is simply a wrapper format and can contain many different types of audio and video. I believe the original file spec was a Microsoft thing. Mpeg has been developed independantly of Home computers, so its file format was developed specifically to support it and is not reliant on any other companies work. I don't know if MS has any sort of IP claim over the avi format or if it is public domain, however the fact remains that mpeg was developed by a seperate group with its own specific file format.


As fingernailX said, you can actually use mpeg encoded video in an avi wrapper (avi IS flexible) but this format is NOT accepted by appliactions specifically written for handling mpeg targeted at consumer playback devices.

[BJ_M]

Quicktime is also a wrapper - with .mov or other extensions ..

[Xesdeeni]

Yes, and when you capture DV on a Mac, you get it in .MOV format. The DV format is not AVI natively, it's just wrapped when you transfer it. MJPEG isn't a standard format at all (although there is a defacto standard). So putting it in AVI is as good a choice as any. Indeo and HuffYUV were specifically designed for AVI. DivX is actually MPEG-4, and as bugster points out, it can be stand-alone, in the ISO-specified format, which is actually based on Quicktime (http://www.apple.com/mpeg4/).

Also, the MPEG-1/2 formats have forward and backward frame differences. This is not supported by VFW, for which AVI was created.

Be glad Microsoft didn't abscond with MPEG-1 and MPEG-2 the way they are trying to do with MPEG-4 (wrapping it in WMV and/or ASF and then adding proprietary additions to it).

Xesdeeni

[fourthquark]

Thanks for the info everyone. I have a better understanding now.

Xesdeeni, your post implies that MPEG4 does not have forward and backward frame differences (i.e., no temporal compression) like MPEG1 and MPEG2. Can you confirm this or am I misinterpreting?

[Cornucopia]

Coupla things:

1. DV on a Mac is usually wrapped inside a QT .mov, but can be stripped of the wrapper and be just a DV stream (as *.dv or *.dif). DV on a PC is mainly wrapped inside an AVI (both Type 1 and 2), but on occasion is wrapped in a QT .mov (e.g. AVID XpressDV works with it) and also can be a raw stream (*.dv or *.dif as before). This usually only happens when transporting/converting files between systems and applications.

2. Mpeg 1 and 2 came into use mainly for universal, cross-platform PC and consumer (and professional/broadcast) electronics applications, especially with fairly simplistic decoder chips. QT and AVI have until very recently required a whole OS and Multimedia API support to be able to decode as play back correctly and smoothly. When VCD players and then DVD players came out, Apple and Microsoft were Not yet Ready for Prime-Time (Thank God!). With DivX/MPEG4 players and supposedly Corona players available, things are starting to change.

3. Mpeg 1, 2, and 4 all support bi-directional frame differencing/temporal compression, although they don't have to use it. Of course, QT and AVI have supported some forms of temporal compression from the beginning (e.g. Cinepak and Indeo) they just weren't as advanced--weren't bi-directional.

Scott

[Xesdeeni]

Xesdeeni, your post implies that MPEG4 does not have forward and backward frame differences (i.e., no temporal compression) like MPEG1 and MPEG2. Can you confirm this or am I misinterpreting?

No, I probably wasn't clear. You'll notice that the MPEG-4 derivatives still aren't wrapped by AVI. Microsoft created new formats (ASF and WMV) for that, because (like MPEG-1 and MPEG-2), MPEG-4 also has bi-directional delta coding. What's really annoying is that this becomes QT format inside WMV...ech!

Xesdeeni