XviD to MPEG-2 cropping/letterbox issue

[scott4862]

Here is my problem. I have some xvids that I would like to encode into mpeg2s so that i can stream them to my tivo. The resolution of the xvids is 608 x 336 and I would like the output to be 544 x 480 for compatibility with the tivo.

Here are the steps that I try to accomplish this.

I go to the video tab and select video codec as MPEG2 [.MPG] (ffmpeg)

For the video size i do the following 608 / 336 = 544 / x ... solve for x. In this case it is about 300. So i set my resolution to 544 x 300. The framerate must be NTSC.

Then I go to audio and select the .mp2 format.

Finally I go into filters. I need to make up the 480 - 300 = 180 pixels by letterboxing so I put 90 pixels in the top and 90 in the bottom.

I then click encode. The resulting file, when played in VLC, still looks like it is in something like a 16:9 aspect ratio. There are black bars at the top and bottom but it is off.

The assumption that I am working under here is that the the letterboxing is done after encoding. I believe this is the way that most enocoder progs work. I tried it the other way, assuming letterboxing before encoding and it doesn't work either.

Anybody have any idea what to do? Thanks.

[Case]

Your calculus is a bit off. You didn't take into account that the 544x480 will be displayed at 16:9 (assuming this tivo does anamorphic widescreen). As your source movie is also ~16:9, you won't need to letterbox at all.

Set the 16:9 aspect ratio before changing the Video size to 544x480, so that the mpeg stream will have the a.r. set to 16:9, regardless of frame size.

[scott4862]

Ahh, well thank you very much for figuring out what I was doing wrong. I guess I still would like to figure out how to do what I was intending to do. I wanted to change the aspect ratio to 4:3 with letterboxing and reduce the resolution to something that the TiVo supports.

From the tivo website, the supported resoltions are:

720 x 480, 704 x 480 (D1), 544 x 480 (3/4 D1), 480 x 480 (2/3 D1), and 352 x 480 (1/2 D1).

Given that the file must be encoded with mpeg2 video, either mp2 or ac3 audio, and NTSC framerate... how would you suggest going about doing this?

[Case]

Use the DVD preset, set Autosize to 'DVD' (=DVD 4:3), set framerate to 29.97. After doing that, you may optionally change the horizontal size to one of [720, 704, 544, 480, 352]. Lower numbers will lessen the perceived quality. Use the 'Best' button in the bitrate calculator to lower the bitrate accordingly.
If your XviD plays well in QuickTime Player, then don't use the manual letterboxing (in Image Filters), but use the auto letterbox feature from 'Decode with QT' and 'Letterbox' (Options tab). Much easier.

[scott4862]

Thank you very much... that worked great. Just out of curiosity, why was what I was doing wrong?

The other thing that I tried, unsuccessfully, was to crop the 16:9 to 4:3. To do this I solved x / 336 = 4 / 3... x = 448. So since 448 x 336 was 160 pixels narrower than 608 x 336, I figured I could just crop 80 pixels from the two sides.

So I set my desired video and audio codes... mpeg2 and ac3. Then I set the video size to 448 x 336, then I set the left and right crops to 80 pixels each. The resulting file was definately not 4:3 ratio... is there something wrong with this approach as well?

[Case]

If you want to letterbox 16:9 to 4:3, then you need a black bar of 12.5% of the height of the target format on top and 12.5% on the bottom: 12.5% x 480 = 60 pixels.
If you want to crop 16:9 to 4:3, then you need take off 12.5% of the width of the source format on each side. 12.5% x 608 = 76 pixels.
This does not change the video size of the target format.