Bit rate question

erock1 · Post by **erock1** » Mon Jul 18, 2005 2:42 pm

I've searched the archives and have found loads of info on lowering the bit rate to fit more material on a DVD disc but didn't seem to find an answer on lowering the bit rate when frist creating a video file (rendering). My question is, if you lower the bit rate before rendering will the resulting (rendered) .mpeg file be smaller? Will an .mpeg file for a 6000 VBR or CBR be smaller than that of an 8000 VBR or CBR?

TIA,
Erock

GeorgeW · Post by **GeorgeW** » Mon Jul 18, 2005 3:01 pm

In general, CBR bitrate (video + audio) x video-length will give you your resulting size. VBR throws in an added dimension (min/max/avg), but the average bitrate in a VBR encode times the video length should give you an idea of how large your file will be.

So, lowering the bitrate (CBR or Avg. VBR) should yield a smaller file.

erock1 · Post by **erock1** » Mon Jul 18, 2005 6:31 pm

Thanks GeorgeW, I appreciate your assistance and formula! Could I just impose again. I'm a bit (pun) slow on the math. My video length is 1 hr. 25 minutes and I'm encoding PCM audio to AC3 and the default setting (I think 56?). Do I multiply using total minutes: 1hr 25 min= 85 X 6000=51000? Will my file size be 5.1gb?

Regards,
Erock

GeorgeW · Post by **GeorgeW** » Mon Jul 18, 2005 6:50 pm

use a bitrate calculator

http://www.videohelp.com/calc.htm

1) Hit the Advanced button (top right in the link)

2) Plug in your Time (hours, then minutes)

3) Select your audio bitrate from pulldown options (PCM = 1536, or use one of the other values based on AC3 or mpeg audio bitrate)

4) in the Calculated Bitrate (green), enter your bitrate, and your custom size should show up in the Custom Size window (about middle near top)

p.s. in your equation, remember that the 6000 is kilobits per second -- so you either have to convert your time to seconds, or your kbps to kilobits per minute. Then the answer to that is in number of bits, so divide that by 8 to give you number of bytes (8 bits in a byte).

Also, don't forget to add in your audio bitrate -- so if your video bitrate is 6000kbps, and audio is 256kbps (one audio track), then use 6256 in your equation (instead of just the video bitrate of 6000kbps).

erock1 · Post by **erock1** » Tue Jul 19, 2005 3:08 am

Thanks again GeorgeW, I'm about ready to give it a try after using the bit rate calc. I'll let you know how it goes.

Best always,
Erock

Philander · Post by **Philander** » Tue Jul 19, 2005 9:57 am

Provided that the video file is a non-compliant DVD file, then it will be converted. If the video file is DVD compliant, be sure that the Do not convert compliant DVD file is UNCHECK so it will be converted to desired video bit rate.

erock1 · Post by **erock1** » Tue Jul 19, 2005 5:09 pm

GeorgeW, It worked like a charm using the Bit Rate Calc along with your super step by step. Thank you very much!

Thank you too Philander.

Erock