Bitrate for cropping 4:3 letterbox to 16:9

[abcedem]

I recently capped a movie off of cable which was widescreen
letterboxed in 4:3. I wanted to try cropping it and making
a 16:9 dvd.

I used TMPGENC to clip the bars off the top and bottom
(actually clipped 60 pixels off each) and set the output to 16:9.
So far so good. The resulting m2v file seemed to look fine.

The thing is, I used the bitrate calculation based on the video
duration as I normally do but the resulting m2v file size is much
smaller than a normal 4:3 encode would be. So instead of getting a
file around 4.xx GB, I get something around 3.75-ish GB instead.
I'm guessing that this is due to the cropping of the source image(?).

Is it correct that the file size is smaller than expected?
If so, is there any way I can calculate a higher bitrate to make
better use of my dvd space?

[guns1inger]

After you cropped, did you resize ? After croppping off 60 from the top and 60 from the bottom you have to resize the image back to 720 x 480 before encoding as 16:9.

A bitrate calculator (look in the tools section for a range of them) will tell you exactly what numbers to use. If you used a calulator and still got it wrong, you have miss-keyed somewhere along the way.

[abcedem]

I'm doing everything in one step. I frameserve my 720x480 AVI source from virtualdub to tmpgenc.

In Tmpgenc, I set the size to 720x480, aspect ratio to 16:9 on the Video tab. On the advanced tab, I believe I set the source aspect to 4:3 and video arrange method to center - keep aspect ratio. I use the clip frame filter to crop 60 off the top and bottom.

Your question implies that I ought to be breaking up the process into 2 (or more?) steps...true?

[guns1inger]

You need to crop in virtualdub, then resize in virtualdub, then frameserve the 720 x 480 image to tempgenc. From there you can simply encode as 16:9. You can then tell tmpgenc that you have 16:9 source material.

[abcedem]

I guess that the cropping and resizing are each a separate pass in virtualdub?

[guns1inger]

No. Add the null transform filter and do your cropping. Close the filters window. Open it, and add the resize filter. Adjust the settings and check out the results in the output preview.

[abcedem]

Thanks for the tip, guns1inger. Will try it out shortly.

[TJohns]

The thing is, I used the bitrate calculation based on the video
duration as I normally do but the resulting m2v file size is much
smaller than a normal 4:3 encode would be. So instead of getting a
file around 4.xx GB, I get something around 3.75-ish GB instead.
I'm guessing that this is due to the cropping of the source image(?).

Wrong! A 1 hr movie encoded at a certain bitrate will ALWAYS be a set size, independant of the frame size. Things that MAY be affecting the final size include:

* using VBR and having the encode decide it needs less that the average you've requested
* requesting a bitrate OVER the DVD specs maximum (say 12 Mbps) and having the encoder "cap" you to ~9Mbps
* encoding to elementary streams & forgetting about the audio portion

BTW, TMPGEnc has it's own "calculator" - on the SET OUTPUT stage just specify a required size (I usually bump it to 102% as it's built-in limit is a little low) and there you go. If you tweak the audio bitrate, you need to go back in & re-do the video again

Trev

[Abond]

*using CQ (or Automatic VBR) istead of 2pass VBR

[guns1inger]

A good encoder will always meet the designated average on a 2-pass VBR encode, whether it needs to or not. It will just pad scenes with extra bitrate to keep the numbers up. However a single pass encode, or a Constant Quality encode could produce under (or over) sized results.

However he wouldn't be the first person to mis-key a bitrate - I know I have on the odd occassion, and spent a good few minutes trying to work out why the hell an asset wouldn't fit.

[dphirschler]

Let's assume for a moment that the original video was realtime encoded (RTE as we used to call it at SA). RTE is very inefficient. It's also CBR (constant bitrate). So assuming you did the cropping and resizing correctly and then encoded using 2-pass VBR, your final size might just be smaller than the original just because the movie is short enough and because RTE is that inefficient. A short movie may not fill a DVD even if the bitrate is maxed out. What is the movie's length anyways?


Darryl

[abcedem]

Some of this discussion is a bit over my head, hence this being a newbie forum thread.

I definitely did not key the bitrate incorrectly. The movie is about 101 minutes. I remember that I set the bitrate at something like 5700 or 5800, based on the bitcalc applet available at this site. Max bitrate was 8000, min was 2100. It's entirely possible that everything else I did was wrong, but not the bitrate.

In fact, before trying this cropping exercise, I had already encoded the movie as a straight 4:3 with black bars intact and used the same bitrate settings. This encode turned out as expected.

The movie was capped to AVI via Virtualdub using Picvideo MJPEG codec set at 19.

By the way, so far I have not burned the full movie in any form to dvd. But when I was trying my cropping experiment I did do a short encode of a 5-ish minute segment to a dvd-rw. (This is before I started this thread.) It turned out fine when played on my tv...although embarrassingly I don't have a widescreen to test it on yet so I just played it on my regular 4:3 tv and player. As I said, I'm just experimenting for now.

[guns1inger]

5800 is about right. Was it 1 or 2 pass VBR ? 1 pass does not produce a predictable output size, as it is the second pass that distributes the bitrate to meet targets.

[abcedem]

It was 2 pass.

When I did the crop encode and I looked at the file size I was puzzled...so I checked the tmpgenc log and saw that it ended the encode with an average at 5098-ish. (I'm at work right now so don't have the exact figure at hand.) But TMPGENC definitely gave me a much lower average bitrate than I requested.

[abcedem]

Correction: the log indicated a much lower bitrate than 5098. I've thrown out the log now so I don't know what it turned out to be exactly. I'm thinking it could have been below 4000. And I'm certain that I set the bitrate as mentioned above (avg 5800, max 8000, min 2100).