Stupid Questions About Dv

[youngcat]

A previous post inspired me to start this thread.

I think there are a lot of people (myself included) who are absolutely clueless about digital video in general, but are anxious to learn and trepidatious to ask for fear of looking stupid, or sounding like they don't know jack.

Fortunately, there are a lot of people who seem to be extremely educated on the subject that regularly visit the site.

So I was thinking....Lets start a Q&A thread. Those of us who are clueless about DV technology, and have questions, no matter how stupid they seem, can ask, and hopefully, someone who is educated (Spiro, Curtis, Mullaney, Terry, Nick, others...) can take a few minutes to reply with an answer, better educating all of us.


My first question is simple. I read in a previous post that DV is a "Compressed Format". What does that mean, exactly, in laymans terms?

[mullaney]

I'll take a stab at it.

Let's compare TIFs and JPGs. Both are formats for saving digitized pictures. A normal TIF is not compressed. For each pixel in the picture, a TIF file records a red value, a green value and a blue value (typically a number between 0 and 255 for each channel--it's RGB value). For even medium sized photographs, this can eat up a lot of computer space.

A JPG is a compressed format. It doesn't necessarily keep track of every RGB value for every pixel. Instead it recognizes that there can be a lot of redundant information. For instance a large chunk of the picture could be the same or nearly the same color. A JPG could compress the size of the file by recording the dimensions of the area with that color and what the color is. In reality, JPGs don't quite work like this. Instead they look for mathematical patterns in the data which may or may not be apparent to the naked eye. In any case a JPG allows you to save the same picture as a TIF but in a lot less space.

There one more thing to understand, a JPG can either be a high quality JPG in which only redundant information is lost and the image looks exactly like the equivalent TIF. (This is like zipping a file on a PC, the file becomes smaller but no actual information is lost). Or a JPG can be low quality, when it is compressed more and loses some of the information so that the end result can look very different to the eye and some of the information is actually lost. It's like having a list of 100 financial transactions and rounding all numbers to the nearest whole dollar. The overall picture looks the same, but the individual numbers are all slightly inaccurate.

So my question is which kind of compressed is DV? Is it high quality compressed, so that no actual information is lost in the compression? Or is it a low quality compression where information is lost in the compression process? My assumption is that it is a high quality compression (like zipping a file) where no information is lost, just redundant information is taken out of the file.

[fuzzy]

DV uses a (slightly) lossy compression scheme.

From http://www.dnai.com/~zane/dv/dv.html :

"DV compression is a hybrid -- it is compressed using a Discrete Cosine Transform (DCT), the same sort of compression used in motion-JPEG. However, DV's DCT allows for more optimization within the frame than do JPEG compressors, allowing for higher quality at the nominal 5:1 compression factor than motion-JPEG.

DV also uses intraframe compression: Each compressed frame depends entirely on itself, and not on any data from preceding or following frames. However, it also uses adaptive interfield compression; if the compressor detects little difference between the two interlaced fields of a frame, it will compress them together, freeing up some of the "bit budget" to allow for higher overall quality elsewhere in the image. In theory, this means that static areas of images will be more accurately represented than areas with a lot of motion; in practice, this can sometimes be observed as a slight degree of "blockiness" in the immediate vicinity of moving objects (an artifact)."

[Jeff Pacocha]

I couldn't agree with all of those compression descriptions more. Actually very complete; in laymans terms-DV (Mini DV) compresses it's recorded signal on to the video tape. Unlike it's counterpart the analog VHS tape. The "VHS" analog recording has no compression. For someone that is simply recording using a DV camera-the operation is mostly the same as a VHS camcorder except with a higher quality image. If you want to edit you project, DV allows you the luxury of using your home PC/MAC to do so. Importing and editing an analog (VHS) project into your computer can be tricky but possible. Your forum idea is a good one.