FEATURE ARTICLE (A00017):

On the Windows operating system, DV files are forms of AVI files - they come in two different flavors however: Type-1 and Type-2. To understand the difference, we must take a look at what a DV file contains and how this is stored in the AVI file format.

We describe how the DV data is stored in each of the two types and describe the benefits and problems associated with each type. The descriptions below are slightly simplified to make the descriptions easier to understand, however none of the important details have been removed.

DV Streams
When you use a video capture program to copy video from a camcorder and a FireWire card to your hard disk, you are actually doing very little with the video data (although you wouldn't believe it from the amount of programming we had to do!). The video data is stored on a DV camcorder as a stream of digital information which contains both the video picture and the sound.
When you send this to a FireWire capture card, the digital information remains unchanged and the video capture software receives this data stream unmodified.
When you send this to a FireWire capture card, the digital information remains unchanged and the video capture software receives this data stream unmodified.
So, what happens to this DV data stream when the capture software gets it?

AVI Files
In order to make the data stream understandable by other programs, Windows turns it into an AVI file. There are two formats that can be used: Type-1 and Type-2.

Type-1
This is the simplest format of DV AVI file, yet is generally the least compatible amongst applications. Type-1 DV AVI files simply add an AVI header to the data stream and then put the DV data stream into the AVI file without modification. Easy!
Well, not quite. The only part of the file which is actually in AVI format is the header (shown in dark blue) and this is generally the only part of the file that most programs can read. Most programs that read AVI files generally expect the AVI file to contain both video and audio streams in AVI format, whereas in a Type-1 format DV AVI file the video and audio are combined in a DV format stream.

A Type-1 DV AVI File

Type-2
To get around the problem of having all the useful data in a DV format stream of an AVI file, Type-2 AVI files extract the audio from the DV stream and add this as an additional AVI format stream to the AVI file.

A Type-2 DV AVI File

Now, programs that understand AVI files now see the file as a standard AVI file with separate audio and video streams. The audio stream is standard AVI and a video decoder can extract only the video from the DV stream. While Type-2 DV AVI files are more compatible than their Type-1 counterparts, they have a couple of disadvantages:

" As the audio data is stored twice in the file, once in the DV stream and once in the AVI stream, a Type-2 DV AVI file is around 5% larger than the Type-1 equivalent.
" Uploading and capturing Type-2 AVI files to and from a DV camcorder takes slightly more processor power, as the extra audio AVI stream needs to be added or removed.

Summary
Now we know the details of Type-1 and Type-2 DV AVI files, we can decide which one is best, right? Of course, it's not that simple. In cases where you want the minimum file size or minimum amount of processing required, Type-1 is preferable. Type-2 is the more compatible format, at the cost of a slightly larger file and it is also being used by most popular professional NLE software like Adobe Premiere and Vegas Video. Both types are identical in both picture and sound quality.