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daviddoria
8th May 2008, 10:10 PM
I encoded my video at 800kbps, it looks pretty bad fullscreen. Then I did 2000 - looks decent. then i did 4000, looked only slightly better.

The question is, is there some kind of standard bit rate to use? Or do I just keep doing that until I find the best trade off for each particular video?

Thanks,

Dave

mndar
8th May 2008, 10:25 PM
There is in fact a formula for calculating bitrates for encoding. Its

Size = (VideoBitrate + AudioBitrate) * Length
Size is in kiloBytes . VideoBitrate and AudioBitrate are in kiloBytes/sec and Length is in secs

Here is a bitrate calculator http://www.videohelp.com/calc.htm
and http://mndar.phpnet.us/files/InfoCenter1.html#mencoder for a simplified explanation

daviddoria
8th May 2008, 11:31 PM
of course the size is the size per second * number of seconds....

this does not help with determining the quality...

woden
9th May 2008, 12:17 AM
The standart for MPEG-2 DVD-quality video is 6000kbps, dimensions - 720*480 (4:3).
The overall quality of the video depends not only on bitrate, but also on video size. You can check parameters of DVD-video by using GSpot under wine on one of the *.VOB files from DVD-Video disk.
There is also a Qf parameter (used often in DVD-backup ripping). It must be more than 0.25 (but you MUST check the overall quality, do not depend on this so much, it is just for you help). Here is how it can be calculated:
{YOUR BITRATE (in bits per second) / (WIDTH*HIGHT)} * FRAMES PER SECOND e.g.

{1500kbps / (640*480)} * 25fps = 0,122 bits/pixel
In this case I need whether raise the bitrate or resize the video. If the size of the output file is not critical, you should always concern raise your bitrate instead of resizing your video to the iPod size.

For the MPEG-2 DVD:
Qf = {6000 / (720*480)} * 25 = 0.434 bits/pixel

mndar
9th May 2008, 06:06 AM
This is something new for me. I always thought better bitrate=better quality.

stevea
9th May 2008, 02:06 PM
Not only is it new, but it's stated wrong here ! It should be obvious the the frame rate is NOT proportional to quality at a fixed bitrate and frame size. Frames belongs in the denominator.

Qf = B/(v*h*F)

where B is the bitrate and F is the frame rate. Qf units are "bits per pixel" and values below 0.2 are perceived as poor, and above 0.3 as decent.

Units:
v*h (pixels/frame)
F (frame/sec)
B (bits/sec)
==

For DVD, Qf = 6000000 /(480*720*25) = 0.69

The above examples
Qf = 800000 / (640*480 * 25) = 0.104 {poor quality at 800kbps}
Qf = 2000000 / (640*480 * 25) = 0.26 { OK at 2000kbps }
Qf = 4000000 / (640*480 * 25) = 0.52 { slightly better at 4000kbps }

Of course at 30hz the bitrate must be 20% higher to maintain Qf. Note that you gain in motion quality at 30hz vs 25, but 25hz may already be sufficient to get past the motion perception threshold.
==

Don't take Qf too seriously but is seems to be a reasonable gauge of perceived image quality.

Mndar is right - higher bitrate does produce better image quality, but there is a bound at abt 0.2 bit/pixel below which human visual perception finds unacceptable.

woden
10th May 2008, 05:01 AM
Not only is it new, but it's stated wrong here !
My bad. I've never calculate Qf by hands.
It is obvious enough that higher bitrate produce highter quality. BUT! output bitrate depends heavily on the quality of input video as well. So MPEG enconding from HDcam or VHS-rip makes huge difference in bitrate of output files.
So Qf is useful enough.
And if the video material has decent quality, I'd make a DVD-Video using DVD-authoring program (like DeVeDe, for example), which produce standart MPEG2 DVD-video compliant output material.

daviddoria
10th May 2008, 01:51 PM
stevea, is there a book on this topic (encoding, codecs, bitrate, resolution, framerate) that you can recommend? Or a good website at least? You seem like you have come across some good info in the past!

thanks to all for the input!

buddha
11th May 2008, 02:16 AM
daviddoria,

Start with the webpage below (from the mplayer documentation) if you really want to start learning the concepts involved with transcoding digital video. It's a complex topic.

14.1. Making a high quality MPEG-4 ("DivX") rip of a DVD movie
http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-dvd-mpeg4.html