Letterbox

Picture Size Conversion

All DVD players are required to have built-in vertical filters which scale a 16:9 coded video image to fit within a traditional 4:3 display. This player feature is needed since it is anticipated that a majority of movies will be coded for the 16:9 aspect ratio, while at the same time most TV displays (in the early years) will be 4:3. In the same vien as multilingual audio, a single coded aspect ratio in market distribution reduces confusion and bolsters economy of scale.

525/60 (NTSC-rate display):

(Note: 480*(4/3)/(16/9) = 480*0.75 = 360)

   _____________________
  |  60                 |
  |---------------------|
  |                     |
  |   360               |  480 lines total
  |                     |
  |---------------------|
  |   60                |
  -----------------------

625/50 (PAL-rate display):

(Note: 576*(4/3)/(16/9) = 576*0.75 = 432)

   _____________________
  |  72                 |
  |---------------------|
  |                     |
  |  432                |  576 lines total
  |                     |
  |---------------------|
  |  72                 |
  -----------------------

A simple bi-linear vertical filter can be applied, yielding good visual results. Here, two source samples (s[n],s[n+1]) are weighted by simple complementary factors and added together to form the destination sample value (d[m]). These weights are easily implemented with shifters. For interlaced displays, vertical filtering occurs only within the same field parity.

     d[0] = (3/4)*s[0] + (1/4)*s[1]
     d[1] = (1/2)*s[1] + (1/2)*s[2]
     d[2] = (1/4)*s[2] + (3/4)*s[3]

A decoder can determine whether inter or intra-parity vertical filtering is applied by testing the progressive_frame flag of the MPEG-2 video stream. (MPEG-1 frames are always progressive by definition). This flag indicates that a picture contains interlaced or progressive vertically correlated information. Almost all MPEG-2 coded movies consist exclusively of progressive frames. In a sense, MPEG-2's interlaced prediction modes are underutilized by DVD.