Digital Nerd

Friday, March 11, 2005

Questions Gathered from the Back of a DVD Case

Question 1

”I just bought a widescreen HDTV and I STILL have black bars on the top and bottom when I watch certain movies. What’s up with that?”

The answer is simple. Movies are often wider than HDTVs. HDTV’s 16:9 aspect ratio (a.k.a. 1.78) is simply a compromise between television’s 4:3 (1.33) and film’s 21:9 (2.35). If you’re willing to incur the wrath of film purists, many TV’s have stretch modes designed to compensate for these black bars. However, I wouldn’t recommend it.

Question 2
:

“Speaking of widescreen — What the heck does anamorphic mean? I often see it on DVD cases. I know that it’s good but I don’t know why.”

There are two main methods of producing a widescreen DVD: letterbox and anamorphic. Anamorphic discs simply conserve more of the original data. Here’s how:

Whether a movie is widescreen or pan-and-scan, the same number of pixels is being sent to the DVD player. In the case of NTSC DVDs that number is 345,600 (720 x 480). The important part is how you use those pixels.

Letterbox DVDs waste a lot of potential resolution by actually sending the black bars at the top and bottom of the picture. In fact, if you watch a letterbox DVD (1.78) the top 61 lines and the bottom 61 lines are nothing but black lines. That means that of the 480 potential lines of resolution 122 are wasted.

Anamorphic DVDs (1.78), on the other hand, don’t send the bars. Instead they treat the signal a little like a fun-house mirror. They pack data into all 480 lines of resolution. The result is that the raw image will looked stretched (i.e. the image will look too tall and objects will look too thin). See

The DVD player then compensates for this fun-house effect by stretching to fill the screen.

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