Transcoding how-to

This page contains useful information about how to do proper transcoding both for Nokia 770 and N800.

Video Player limitations

It can only decode videos where horizontal and vertical dimensions are multiples of 16
It cannot decode videos with high bitrates and/or high frame sizes. The limits are at about 800 kbps or:
- 352x288 for 770
- 640x480 for N800
It cannot decode both a high frame rate and a high frame size, so if you want to keep the natural frame rate of your video (25 or 30 fps) you have to keep the frame size below:
- 288x160 for 770
- 352x288 for N800
Audio must be at 48kHz or lower
External subtitles cannot be read so they have to be written inside the video frames
For a list of the supported formats you can check here.

Things to keep in mind

Don't exceed the max bandwidth
See Max Bandwidth
Try to keep the frame-rate exactly as the original
So the resulting clip looks as smooth as possible
Try to keep the aspect ratio of the original clip as much as possible
So the resulting clip doesn't look distorted

Try to keep the original clip's fame size
So no scaling glitches are introduced

Natural frame-sizes

Nokia 770

240 x 144
352 x 208
352 x 288
176 x 144
320 x 240

N800

The same as Nokia 770 plus:

400 x 240
640 x 480

Max bandwidth

There's a limit to how many macroblocks per second each device can handle properly:

770: 352 * 288 * 15 = 1520640
N800: 640 * 480 * 15 = 4608000

Let's consider these examples on a N800:

400 x 240 @ 25: 2400000: Less than 4608000, so it plays OK
640 x 480 @ 25: 7680000: More than 4608000, it won't play fine

Script

This script tries to intelligently find the best video frame size that would look good in the desired device (N770 or N800) while trying to keep the same aspect ratio as the original clip, as well as trying to fit the aspect ratio of the device.

You'll need Ruby and mplayer to execute the script. It will generate the recommended mencoder command to run.

You can find it here

-i, --input: Input file
-o, --output: Output file
-q, --quality: The quality of the output clip (1..6)
-d, --device: The device for which the output shall be compatible {770,N800}
-v, --verbose: Run verbosely

Example:

./transcode -v -i phantom-limbs.divx

Generates:

 * Input: 640x352 [20:11], 23.976 fps, 1297760 kbps.
 * Output: 400x240 [5:3], 23.976 fps, 400 kbps.
 mencoder /data/shared/videos/phantom-limbs.divx -o phantom-limbs.avi \
 -srate 44100 -oac mp3lame -lameopts vbr=0:br=128 -af volnorm \
 -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=400 -ofps 23.976000 \
 -vf-add scale=400:240 -noidx

Please note how the algorithm finds an output size that follows the principles mentioned before:

Don't exceed the max bandwidth
400 * 240 * 23.975 = 2301696 < 4608000
Try to keep the frame-rate exactly as the original
Original: 23.976, New: 23.976
Try to keep the aspect ratio of the original clip as much as possible
Original: 1.81, New: 1.66
Try to keep the aspect ratio of the device's screen
Device: 1.66, New: 1.66
Try to keep the original clip's fame size.
Original 640x352, New: 400x240 (Any bigger frame-size would result in a slow video)

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