DivX Converter: default video bit rate

[evilclive]

I've been here long enough to know that noone in Doom9 will ever recommend a DivX video bit rate on a one size fits all basis.

But DivX Converter (the utility program with DivX 6.5.1) always defaults to 1300kbps at 720x400 on the Home Theater Profile.

Any comments?

Did the DivX people choose this bit rate to optimise the quality-to-compression ratio, or because it makes an hour-long video recording fit nicely on a CDR?

I've done two test transcodes at 720x400, and the MPEG-4 artifacts are fairly prominent at 1000kbps video, but to my eyes at least, almost unnoticeable at 1300kbps. Should I continue to experiment with different video bit rates, or can I now save time and trust the default recommendation?

[writersblock29]

@evilclive

The defaults are kind of a compromise; they'll be fairly satisfactory for most viewers in most cases. If you're not concerned about the final file size, you can always download the Dr. Divx GUI or AutoGK and do a one-pass quality encode, setting the quality to 1. (The lower the number, the closer the copy will be to the original... in theory, at least.) Likewise, you can also use DVD Shrink to author one chapter of a given disk and run your own test using various bit rates before committing to doing full projects. (One chapter won't take as long to encode.) You can find AutoGK in plenty of places on this site, and the Dr. Divx frontend is availible for free download at the Divx.com site.

You're right that no one will give you a one-size-fits-all for all projects--nor should we: What looks great to me might look like crap to you. And my eyesight is slightly above 20/20! It's all subjective.

I remember reading a post on the Divx forums (at Divx.com, not here) where someone typically used 6K on everything. Overkill? Probably. Winding up with a copy larger than the original? I'd think almost definately. But that's what they were happy with. To each his own!

[DewAsmara]

But DivX Converter (the utility program with DivX 6.5.1) always defaults to 1300kbps at 720x400 on the Home Theater Profile.

Any comments?

I've done two test transcodes at 720x400, and the MPEG-4 artifacts are fairly prominent at 1000kbps video, but to my eyes at least, almost unnoticeable at 1300kbps.

Hi There,
Its all depend on your convenient and what player and display that you use. A normal CRT TV shows almost cannot see artefact or jagged. But when on cheap big LCD TV it shows big dissapoint result. I do always use bit rate above 4000 using HiDef profile with video size normal standard definition 720x400 so can be played with my Home Theater. But some files that I play from my Multimedia PC, just stay at maximum resolution at 1280x704. Not because I am crazy with bitrate or resolution but because my TV so big enough to show quality problem if using low bitrate.

[MarcioAB]

I'm experimenting to find the ideal "one-size-fits-all" using the concept of BPP (bits per pixel). BPP can be math expressed as:

Bitrate = BPP * Resolution * Framerate.

So far I'm converging to identify 0.15 BPP as the ideal "one-size-fits-all" magic number to be the limit of very acceptable image quality (that is subjective but should not vary much between person to person).

So far it's working well from resolutions from 512x272 up to 1280x720 and framerates from 24 fps to 60 fps.

So, back to initial question of 1300 Kbps at 720x400 at 30 fps, that also translates to 0.15 BPP.

0.15 * 720 * 400 * 30 = 1296 Kbps (or 1300 Kbps in short).


For a 1920x1080 at 24 fps it should be 7500 Kbps.
0.15 * 1920 * 1080 * 24 = 7465 Kbps (or 7500 Kbps in short).

Does it make sense ?

Regards
Marcio

[jggimi]

BPP is a key value used by both AutoGK and the original Gordian Knot. As to a single one-size-fits-all "visual quality" BPP, I don't think you will have terrific success using a single value.

The consensus of the members here over the last 5+ years with BPP has tended to indicate that results are usually content dependent.

[MarcioAB]

Thank you. That make sense (be content dependent).

Based on your an the team experience, could a table be constructed, for example:

0.13 BPP for very low action movies.
0.14 BPP for low action movies.
0.15 BPP for normal action movies.
0.16 BPP for high action movies.
0.17 BPP for very high action movies.

Could the values above be reasonable ?

Thank you
Marcio

[jggimi]

The key is compressibility testing, using BPP only as a starting point.

That is what GK and AGK do. Well, AGK does it, with GK, it is semiautomated.

[DewAsmara]

for example:

0.13 BPP for very low action movies.
0.14 BPP for low action movies.
0.15 BPP for normal action movies.
0.16 BPP for high action movies.
0.17 BPP for very high action movies.

Could the values above be reasonable ?

Yes this make sense,
Now I understand why DVD only achieved maximum arround 4000 bitrate something even if I boost to use HD setting at 6000. As they have limitation on size and quality.

[MarcioAB]

DewAsmara, I also have a reasonable 40" LCD TV with 1366x768 resolution and I also look for best possible quality for that screen.
My DVD-player is DivX HT certified, so max 720x480@30fps or 720x576@25fps far from 1366x768. So I gave up the DVD-player and connected the TV directly to PC and I am able to send 1280x768@60fps to TV. That is nice.
So the problem WAS resolution and not bitrate.

But the question is what is the BEST bitrate to use in favor of image quality and not being "insane" ?

That is a good question but BITRATE is not the variable to be considered. The variable to consider is BPP (bits per pixel), that at first seems to means nothing. But I do believe 0.15 BPP is a magical number for DivX.

Of course 0.16 is better than 0.15 ...
0.17 is better than 0.16 ... and so on.

But try for example the following:

Compress at 0.15 and check image quality. It may be acceptable or not.
If it's not acceptable then 0.16 will be the number with acceptable quality (not necessary to check 0.17).
If 0.15 is acceptable, then try 0.14. It may be acceptable or not.
It it's not acceptable then 0.15 is the number.
If it is acceptable then 0.14 is the number (not necessary to check 0.13).

The vast majority of my encodes felt into this rule ...

For example to encode 1280x768@30fps assuming 0.15 is the number, the bitrate that must be used in DivX with Unconstraind profile, Quality-based or Multi-Pass with Insane Quality and etc is: 0.15 x 1280 x 768 x 30 = 4500 Kbps

When moving to full HD at 1920x1080x30fps and 0.15 BPP, the necessary bitrate will be 9400 Kbps.

But who cares for bitrate ... keep focus in BPP.

Regards
Marcio

[weaver4]

I always thought that the Converter was a little simple. I put on the DivX forum a message about that saying:

"The Converter knows the resolution of the final movie that it is making (640x272) and the framerate. So how hard is it to calculate .24 bits per pixel. Let me see that is 640*272*23.97*0.24 = 1020 kbs. Which is a whole lot better than some some wild-ass guess of 1300kbs for everything."

[MarcioAB]

Well, 0.24 BPP is great ... but it's too much (maybe a waste).

Using the same bitrate you proposed and keeping the same 2.35:1 AR, I will prefer to encode at 828x352 than 640x272.

a) 828*352*23.97*0.15 = 1050 Kbps
b) 640*272*23.97*0.24 = 1020 kbps

Regards
Marcio

[weaver4]

Well, 0.24 BPP is great ... but it's too much (maybe a waste).

Using the same bitrate you proposed and keeping the same 2.35:1 AR, I will prefer to encode at 828x352 than 640x272.

a) 828*352*23.97*0.15 = 1050 Kbps
b) 640*272*23.97*0.24 = 1020 kbps

Regards
Marcio

Generally I do a Constant Quality with a quantizer of 4.0 and forget completely about the bit rate. [If the movie is really a keeper I use 3.5] The reason I do that is because sometimes you need more bits and sometimes you don't so a quantizer figures that out for you. How many bits you need varies considerably depending on movie. Some parameters are very unpredictable like how much texture shows up in movie, how much black is in a movie, how busy are the backgrounds in a movie, etc. I don't feel that a person can do a very good job of estimating those parameters, so I just let a quantizer figure it out. For example the movie Firewall gives you great quality movie at 700kbs but Casablanca need 1500kbs, figure that out.

So I use tools that allow me to set up the quantizer, like avi.net, autogk or Dr Divx, all free, all better than the converter.

But when I do a movie by BPP I generally use .20.