Down-converting HD to SD
© 2013 Telestream, Inc.
16:9 HD > 4:3 SD Letterbox Conversion
You perform letterbox (or 16:9) conversion of 16:9 format HD video to 4:3 for playout on SD
televisions to preserve the entire 16:9 frame in letterbox format. To perform this conversion, you
add bars (typically of equal size) to the top and bottom of the original 16:9 image.
Figure 15. Letterbox Conversion 16:9 HD > 4:3 SD
To set up a factory to convert HD 16:9 video into 4:3 SD letterbox video, perform these tasks:
1. Make a new SD factory and name it appropriately.
2. Create a product in your factory with an SD profile. For example, MPEG2 Program Stream.
3. Set your bit rate, GOP structure, and select a suitable video profile.
4. Make other settings in the codec editor panel as required.
5. Add a Mask filter to pad top and bottom, and select Letterbox in 16:9 or 14:9 format.
6. Set up monitors, destinations, and notifies as required.
The 16:9 ratio is calculated as 4/3 X 4/3. Therefore, 1/4 of the height of the SD output is excess
and 3/4 is available to receive the input active 16:9 region. Therefore, 1/4 of the height must be
masked; typically 1/8 at the top and 1/8 at the bottom.
In situations where the preset Letterbox settings in the Mask filter are not adequate, you can
calculate the dimensions based on the formula:
Total Bar Height = SD Profile Height - (SD Profile Height X 3/4)
1. Multiply the SD profile height by 3/4 to calculate the height of the encoded SD image in the
In the example of 480: 480 X 3/4 = 360.
2. Subtract the result from the SD profile height to calculate the total bar height.
Continuing the example: 480 - 360 = 120.
3. Divide the total bar height by 2 to determine the height of top and bottom bars.
In this example, the bars are 60 and 60, which total 120.
All NTSC (480 high) SD profiles require a total letterbox bar height of 120; top and bottom
even bars are 60 pixels each.
All PAL (576 high) SD profiles require a total letterbox bar height of 144; top and bottom even
bars are 77 pixels each.
down-convert to fill the SD
profile image dimensions.
Decode HD 16:9 input.
Fill the mask region and
encode the video.