trying to understand encoding bitrate w/ GKnot

[Joshua Wood]

Hi all

Just trying to get my head around how the bitrate in Gordian Knot works....
This is on the resolution tab....I'm talking about the slider which affects the bits/(pixel*frame) setting...

My understanding is that you want this to be at least .17, and preferably around .25....

The way the slider seems to work, is the further left u move it, the higher this number increases....my understanding is this:
The video size is reduced, hence less total pixels. If you have a set file size (say 700mb CD), if you have less pixels you can afford to encode them at a higher rate....

The opposite works when moving the slider to the right....more pixels, so you have to reduce the bit rate.

.17 and .25 seem more to the right end of the scale. If you encode a movie with a bigger video size (and a low bit rate), it doesn't need to stretch it much when u make it full screen, so u don't lose too much quality....

On the other hand, if you have a small video size with a high bit rate, it will have to stretch it more making it full screen.....but you will have more info per pixel....so will this make it still look ok? Does the .17-.25 setting usually give you the best video size/bit rate ratio so movies look ok when makin them full screen?

Do I seem to be on the right track with my understanding here?

Also, sometimes the divxs seem to not have all the frames...like it's not running at say 25fps for a 25fps movie...u can notice this since it doesn't appear quite as smooth and fluid....does it infact lose fames/sec, and how does this relate to the bit rate/video size (if it does)? (ie, what do I need to do to make a divx more "fluid"?)

Or is this just the way the divx codec works. Has it not infact lost frames at all, but is rather simply the a by product of compressing video?

Cheers for all you help!!
Josh

[DJ Bobo]

huh?!
High bitrate -> you can choose a high resolution
Low bitrate -> you better choose a lower resolution until you get at least 0.2 bit/pixel

The more bit per pixel (=the higher the bitrate), the better the quality, no matter what resolution you take!

And a DivX have absolutely all frames, if you have not smooth motion, your machine is too weak to playback at full speed!

[jggimi]

Regarding b/p*f values: Use the initial number as an ESTIMATE of resolution to be used for your first compressibility test. After the first test, ignore that number and use the %LOAD returned by the test.

Regarding choppy playback: DivX 5 playback includes a great deal of post-processing options. You'll use less CPU if you set them all to off or to minimum. There are two windows worth to turn off or minimize, you'll find them with Start....Programs....DivX....DivX (Pro) Codec....Decoder Configuration.

I hope this helped.

[Joshua Wood]

Originally posted by DJ Bobo
[B]huh?!
High bitrate -> you can choose a high resolution
Low bitrate -> you better choose a lower resolution until you get at least 0.2 bit/pixel

The more bit per pixel (=the higher the bitrate), the better the quality, no matter what resolution you take!


But surely, you don't want to move the slider all the way to the left, having a massivley high bitrate, but very small video size?

What would happen then if you made it full screen? You make it sound like you should have the slider hard left...

Why do ppl recommend a bit rate of around .26 then?

[manono]

Hi-

You weren't listening (or reading, or understanding what you read). Of course you don't want the slider hard left. In that case, you can throw all the bits you want at it, but it will still look lousy.

Why do ppl recommend a bit rate of around .26 then?

That's not the bit rate-it's b/p*f. As jggimi said, run a compression test to help determine the best resolution for the given audio and file size.

As far as choppy playback goes, either you've done something stupid like Force Filmed in DVD2AVI when you shouldn't have (if it's a PAL DVD, then never use Force Film-if it's an NTSC DVD, then don't use it on pure interlaced content), or your CPU is borderline and the decoder is dropping frames. Personally, I use ffdshow (http://www.doom9.org/Soft21/Filters/ffdshow-20020923.exe) for decoding DivX5 content. It's less CPU intensive. And, as jggimi also said, turn off all post processing if you think the CPU speed might be the problem.

[Mac Sidewinder]

As Manono stated, what you are referring to is not called bitrate. If you want to see an estimate of the bitrate, go to the bitrate tab and look at the bitrate block. This rate is not affected by what resolution you use (normally).

To make it easier to understand. The bitrate is distributed according to the resolution you set. If you set a high resolution - then you have less b/p per frame to use so you get less quality. In contrast if you set a low resolution - you have more b/p per frame so you have a higher quality.

Now this only applies to a certain extent. If you go to one extreme or the other then unpredictable results will happen. That is why it is recommended to use the slider to get around .20 b/p THEN run a compression check. This compression check will tell you if the resulting video's resolution is appropriate. Try to achieve between 60 and 75% on the compression check. Remember that beauty is in the eye of the beholder though. Some people are content with a low compression check value (such as from 50 to 60%). Some people like me like to keep it up around 70 to 75%. Keep the following in mind though when you are setting your resolution:

1. Run a compression check each time you change the resolution. When you move the slider after a compression check, the resulting value in the load box is not entirely accurate untill you run another check.

2. Try to keep between 60 and 75% load value.

3. Try not to go below 572x horizontal resolution. IMO if you go below this, the resulting video looks blocky when you view it fullscreen.

4. If you can't get a high enough resolution then either drop the audio bitrate (but don't go below 128k mp3) OR go to a 2 cd rip.

5. Don't go above the input resolution (for dvd its 720x480) no matter what the compression check says. For example the check reports a load value of 101% at a resolution of 720x480. This is telling you that your source is very compressable. You probably won't reach the filesize you set (will be smaller). Either use a higher quality audio level or try encoding the source at 100% quality single pass.

6. Ensure that the recalculate bitrate block is check in the audio tab. This will re-adjust the bitrate after the audio is processed to try to maintain the filesize that you have set.

These are my recommendations (my opinion only) that I have gathered from reading other posts and testing. I'm sure I have forgotten a couple of pointers and invite the more knowledgable amongs us to add to this or give their recommendations. Good luck!

Mac