Bitrate in regard to quality

[Shatylle]

Hi, I was wondering if I keep on cranking up my bitrate in AS @ L5 profile will I gain a sharper image. I am encoding sections of a DVD with Xvid but in certain scenes it goes all fuzzy and blocky, but in all scenes I can still tell the difference between the original and the encoded. I encoded at 780 kbps which is what I guess is the average DVD bitrate.

I want to acheive near identical quality for my encoding regardless of size (but I really don't want to maintain the MPEG-2 files) or compatablity. Is there things I could try to improve my quality or am I being overly ambitious.

Saw in one of the guides that I should not generally use GMC or will this make little difference to quality. (Q pixels and GMC normally enabled)

[Soulhunter]

Hi, I was wondering if I keep on cranking up my bitrate in AS @ L5 profile will I gain a sharper image. I am encoding sections of a DVD with Xvid but in certain scenes it goes all fuzzy and blocky, but in all scenes I can still tell the difference between the original and the encoded. I encoded at 780 kbps which is what I guess is the average DVD bitrate...

Erm, 780kbps is far-far-far away from the average DVD bitrate... O.o

And to get "DVD quality" the bitrate should be ~4x higher than this !!!


Bye

[Shatylle]

Ok gonna try encoding at 4000 will see what I think

[Shatylle]

Ok so in reply the general rule is I will continue to gain quality as long as I crank my bitrate up. I see now that virtualdubmod file info tells me the Kbps and that I should really be at 4915 to match my source.

Next sort of question would be are any of the settings really gonna make a difference at this bitrate (Q pixel's, GMC, VHQ mode .... ) or can I cut them out to save on encoding time. Also has anyone got a rough guide to what bitrate I could use to get my desired result, ie there should be a point long before what I am using where i still would not notice the difference. Thanks for the help Soulhunter just assumed 780 was the correct rate as both Divx and Xvid defaulted to it as I selected target bitrate.

[dvd_maniac]

Why don't you just take a part of a dvd and encode using different bitrates and check quality yourself. It Depends on "YOUR" setup and "YOUR" level of what is acceptible.

[Soulhunter]

Next sort of question would be are any of the settings really gonna make a difference at this bitrate (Q pixel's, GMC, VHQ mode .... ) or can I cut them out to save on encoding time...

B-VOPs, Trellis and VHQ4 will always gain you compression/quality !!!

But the usefulness of QPel and GMC is imo questionable...


Bye

[Soulhunter]

Also has anyone got a rough guide to what bitrate I could use to get my desired result, ie there should be a point long before what I am using where i still would not notice the difference...

Impossible to answer as it depends on the source characteristics! If the source is grainy/noisy/sharp and has much fast motion scenes, it needs more bitrate to look good. But if the source is smooth/blurry/dark and has mainly low motion scenes, it needs less bitrate. If you dont want to match a certain filesize, better encode at fixed quantizer... This way you get "constant" quality, no matter what the source is! Well, as dvd_maniac suggested, simply encode a short part of the DVD at different quantizers and check which result matches you quality needs!


Bye

[Teegedeck]

I absolutely agree with Soulhunter on this, but can I say it all again? :): I'd say that you'd be very happy with a constant quantizer encode. Because I'd really be surprised if you could distinguish an encode with, say, the SixOfNine custom matrix at a constant quantizer=3 from the original DVD. Yet, the final file will still be about 60% smaller than the DVD. It is also a no-brainer; the quality will remain constantly good, the filesize will just come out as big as it has to.

VHQ-as-high-as-possible is indeed a quality-bringer. Personally I also love the sharp look that Quarterpel gives, but preferably on high-quality sources. B-VOPs and Trellis are good - smaller filesize, same quality. Why unnecessarily bloat the filesize after all? :) Adaptive quantization saves some more percent. GMC at least theoretically gives a 'little bit extra' precison, in practice a minimally higher efficiency.

[Shatylle]

Thank you all for your help, my questions have been answered. I expected that I would have to just encode at different bitrates to see how i feel. I just didn't know if there was a limit to how much data the Xvid codec could code off a specific sized source.

I will try encoding at a target quantizer, I will turn adaptive quantization on as I don't normaly run with it but I think I will chuck GMC. But I gather in general no matter what you are doing you can benefit from running all of the options (if you have the time).

Thanks again for all the help I am sure I will need it agian in the future.

[Teegedeck]

You're very welcome. :) Everybody loves it to be asked questions politely and even get a 'thanks' afterwards.
I just didn't know if there was a limit to how much data the Xvid codec could code off a specific sized source.That is adjustable. You can always find a custom quantization matrix (CQM) which produces even bigger files. But for me it stops making sense when I can't see a difference anymore.

[johnhamler1]

my DVD 4.7 movies are between 2500bps and 5000.!

[Backflip]

Sorry to bring this thead up again. I've been meaning to reply to it, but either got sidetracked by other threads or completely forgot about it.

Personally I also love the sharp look that Quarterpel gives, but preferably on high-quality sources.

What would be considered a high enough quality source to consider using QPel? Any relatively modern DVD? I do like sharpness, but is it much more CPU intensive is it decoding QPel?

GMC at least theoretically gives a 'little bit extra' precison, in practice a minimally higher efficiency.

Is GMC a definite no-no? Is there any situation where it would be wise to use GMC?

Thanks :)

[JoeShrubbery]

Qpel does kick up the CPU usage on playback quite a bit, and if you're thinking about playing it back on a standalone DVD player with mpeg4 support you have to keep in mind that many such units can't handle Qpel, or Xvid's implementation of GMC for that matter. Personally I've never seen much benefit in using Qpel, but others do. You're just gonna have to encode a clip both with and without Qpel and compare for yourself. As for GMC, it's really just not worth it.