avi vs mp4

In addition to what Steve and John have offered, the two main determinants of AV file size are:

Duration

Bit-Rate

The CODEC used has a little effect, but mostly they allow for the data to be compressed, yet still leave acceptable quality. The lower the Bit-Rate, the lower the quality, but also the smaller the file.

Where CODEC's differ is by the allowable compression of the data, coupled with the acceptable quality. Some are better at the compression, than are others. The H.264 CODEC, usually found in MP4 wrappers, is very good at giving one acceptable quality, but still compressing with lower Bit-Rates.

Many of the CODEC's, commonly found in the AVI wrapper, are not quite so good, so to get the higher quality (read higher Bit-Rate), the files' sizes WILL be larger. One exception is the DivX CODEC, which does yield good quality, and lower Bit-Rates, and can compress the data very well. However, this CODEC is designed for delivery of streaming media, and is NOT meant to be edited later.

Editability is a big plus for the H.264 CODEC, and that is why it is used in so many HD formats, like AVCHD (a sub-set of H.264), today.

However, and with all of that said, there are major variations in H.264, so there are few universal specs. With DV-AVI, a particular CODEC, in the AVI wrapper, things are much simpler - the MicroSoft DV CODEC is standard, and uses low-compression, and higher Bit-Rates, to keep quality high, but at the sacrifice of file size.

There ARE a few CODEC's, usually wrapped as an AVI, like Lagarith Lossless and UT Lossless, that maintain lossless qualtiy, while still doing a good job at compression, but no where near as good, as most versions of H.264 - however, and just as their names imply, they are "visually" lossless - great quality, though at higher Bit-Rates and with larger file sizes, than most versions of H.264. For intermediate files, that will be edited later, I rely heavily on both of those lossless CODEC's. With the proper CODEC's installed, they both edit very, very well too. Also, one can do many generations of Encoding, without a noticeable loss in quality, where that is not true with H.264, where the compression will show an accumulation of a loss in quality, because of the high compression.

Hope that helps,

Hunt

Some reading...

Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037?tstart=0

What is a CODEC... a Primer http://forums.adobe.com/thread/546811?tstart=0

What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037?tstart=0

.

Report back with the codec details of your file, use the programs below

.

For PC http://www.headbands.com/gspot/ or http://mediainfo.sourceforge.net/en

For Mac http://mediainfo.massanti.com/

Well, there is no one definition of an avi or an mp4, Koen.

An AVI is just a package, an envelope. What's inside it can be made up of any of thousands of codecs (compression systems), from DV to Divx and anything in between. So some AVIs are large and others are very small.

Likewise, MP4s are usually created using the H.264 codec -- but often are not. Sometimes product makers create their own codecs in order to, for instance, squeeze lots of video onto a very small memory card. In addition, MP4s can be created at a variety of quality levels. An MP4 you view online, for instance, can be very different (and a very different size) than one you view on your iPad.

In other words, comparing MP4s to AVIs is not only like comparing apples to oranges -- it's like comparing any of thousands of varieties of apples to any of hundreds of varieties of oranges.