resolution vs bitrate with same file size for better results?

[macanno]

hi there,

am new to AutoGK and encoded only the God Father series for viewing in my VGA PPC in 640x resolution in Xvid, i tend to use only around 220MB per hour of movie for best acceptable quality (video bitrate will be around 60-62kbps). A question arised and I hope anyone could advise about whether i should encode in higher res (640x) or lower res (320x) given the file size is the same for best display quality? given i have a PPC which could display VGA quality, shall I choose to encode in 320x so the bitrate is higher or shall i stay with my current settings? i understand this could be solved by trial and error but appreciate if anyone could share your experience (you know how many hours it needs for performing an encoding jsut for testing quality) :P

thanks very much for any advice.

cheers, macanno

[jggimi]

Hello, and welcome to the forum.

...video bitrate will be around 60-62kbps...

That's an extremely low bitrate for video -- consider that a typical stereo .mp3 soundtrack is 128 or 160 kbps.

There is a single numeric value that takes into account resolution, bitrate, and content length. This single value is used in GK and AGK to select an appropriate resolution for the bitrate (file size) you've selected, or, in GK only, to select an appropriate bitrate for the resolution you've selected. The value is:bits / pixels * framesBut this single value, in and of itself, is meaningless without taking into account the video's compressibility. In GK/AGK, a compressibility test is run to determine what the maximum possible bitrate can be at a chosen resolution. From that point, bitrate and resolution can be adjusted to produce an MPEG-4 video where the projected b/p*f value will be a percentage of the maximum. You have flexibility in GK, IIRC in AGK it is set to 75%.

To sum up, bitrate requirements for any particular resolution will vary. And smaller resolutions give you a higher b/p*f value. But regardless of the resolution, your chosen bitrate will likely not produce a result you like.

[tigerman8u]

I like to cut out app. 1-2 minutes of the video and use different settings on it to see what I like best.

[cyberVera]

...video bitrate will be around 60-62kbps...

That's an extremely low bitrate for video -- consider that a typical stereo .mp3 soundtrack is 128 or 160 kbps.


Bit/byte, maybe? Multiply it to 8. Right-clicking, properties of an avi file says that low number :-)

[jggimi]

220MB/hr ~= 60KB/sec ~= 488kbps. You were right, CyberVera!

[macanno]

thanks to all for the advice. i guess i'll need some time to digest the concepts. i double-checked my video info and found out it's really 62kbps not 62kB/s. here i attach the screen dump. (sorry but i'm using a chinese version of WinXP and i believe you could figure out the video data bitrate field) :)

p.s. image attached in the next message

[macanno]

this is a movie i encoded to Xvid using AutoGK.
Resolution 640 x 368
Audio bitrate 126Kbps
Video bitrate 62kbps (regarded as too low?)

File size is approx. 660MB

[cyberVera]

Just Windows likes to show it like that. Old codecs (Indeo, for instance) were using such type of measurement for bitrate.

Try GSpot 2.21
Doom9's Download page - expand to see the full software list - GSpot 2.21
or
http://files.divx-digest.com/software/codec/gspot221.exe

Please reply with what you get after that.

[macanno]

oh thanks a lot for the advice, i checked with GSpot and the correct bitrate should be 375kb/s

really appreciate your help. :)

[cyberVera]

Still I believe that video with 640 resolution @ 375kbps and audio @ 128kbps is not a good combination. You treat video part as a cinderella :-)

[macanno]

hi CyberVera,

do you mean i still have too low video bitrate with that resolution? or too high? could you advise any good combination you would recommend? i am still abit lost about how to manually configure bitrate in AGK, i only know how to set file size and resolution and audio bitrate...that was actually my question regarding which factor should i tweak to achieve best video quality as there's only Filesize and Resolution setting i can modify in AGK...

[cyberVera]

I'm not an expert in video encoding like most of the members here, and I could be wrong in this or that. But in my opinion,
There's one thing you continue to misunderstand - Bitrate does not depend on resolution

In your video with 640px you have 375kbps. You will be amased, but with 320px you will have again 375kbps! Why? Because bitrate is calculated from filesize and length. That's all. While increasing filesize and/or decreasing length you will see bitrate rising.

But bitrate is not that important in your case. So, you have 220Mb as target filesize and 55 minutes length. OK, nothing wrong with it. Now the only thing you can do is to choose the correct framesize. I would use 480px or 512px, not less and not more. Smaller width and height of a frame means smaller square in pixels (here, framesize = width x height). Every frame in your video sequence receives more or less fixed amount of bitrate to describe video, thus frame with 480px width will receive it more, and will look better. Also taking into account your hardware, it will be easier for your pc to show 480px video than 640px. Also, XviD codec is more resource consuming than DivX. And finally, I would use 96kbps bitrate for audio, not 128. This will increase video bitrate/quality as well.

All this issues a very basic. Please study all relevant guides of Doom9.

[homermaster]

I have been using AutoGK to make numerous encodes for use on my Palm. I have a screen resolution of 480x320 and encode everything at a fixed width of 480.
The simplest reason for this is the fact that if I do any kind of stretching from a smaller resolution to make it fit full screen our little handhelds, however incredible they are becoming, are not as versatile as a PC and show significant artifacting.
If you are looking at playing these movies on your PC and want them to look good there all of this advice is invaluable.
However, if you are doing these encodes primarily for your PPC my experience leads me to offer the advise of encoding at your screen resolution of 640.

I may be completely wrong about this and I would love for you to run an experiment with different resolutions and see if my experience with the Palm holds true for your player on the PPC.

Also curious what player you are using for movies on your PPC?

[macanno]

thanks again for all of you for the precious advice. now i'm clear what concept i got wrong, once the filesize is set as a constant, bitrate could be calculated from the file size and the video length, 220MB for 60 mins is basically 225280kB or 1802240kbits for 3600secs, so there will be 1802240kbits/3600sec data space available for storing each second of video/audio data, i.e. 500.6 kbps, and when audio set to 128kbps, remaining data for video is 500 minus 128kbps, approx. 373kbps.

so the logic here is whether the encoded quality with this bitrate is better with frame size set as 640x or 512x or 480x or 320x when viewed with a PPC with 640x480 display resolution. i'll do further testing base on this :)

btw, am using Betaplayer on a PPC running WinCE 2003SE.
homermaster- there's a palm port for Betaplayer, called TCPMP, not sure you've used it before. :)

[homermaster]

Oh yes .... have been using MMplayer for quite a while but changed over as soon as I tried TCPMP. Picard is absolutely amazing as well as lenox for creating THE bset products out there in their fields. I was hoping you were using this player.

Please let me know the results of your tests. I have some friends with PPCs that would like to know the results. Thanks!! :)