Noob question about packback delay

[MrBunny]

Hi everyone,

I've been playing around with XviD for a while now, and I think I have a decent understanding of how the settings work. However one question has been bugging me for a while now, and no one I've talked to seems to be able to explain it to me. Payback delay (I think, please correct me if I'm wrong) is a setting that controls how much leniency (time/frames) the codec is given to average out the bits being allocated. Ideally within the range, there would be some scenes/frames requiring less bits balancing off the scenes/frames requiring more.

However, in XviD payback seems to be defaulted to 250 frames, which is about 10 seconds (depending on the fps of the source of course). This seems to be very short to me as there are high motion scenes in many movies that are much longer than 10 seconds and thus there is nothing to offset those high bitrate frames with. In SBC encoding, payback delay had generally been set at 45-120 seconds. Is there a major difference between the general concept or implementation of payback delay in SBC and XviD? and secondly, assuming my conceptual view of packback is correct, why is the XviD default set at 250 frames?

Thanks in advance,

Mr. B

[Dali Lama]

Mr. B,

I think you got it straight. There isn't a difference between them as far as I know. I'd reccomend nandub settings of 45-120seconds. Xvid uses frames as you pointed out so, just multiply by frame rate. However, in the Divx 5 codec, it is reccomended to set this to half the # of frames of the movie. Give it a shot. I have used 120s payback delay in xvid with good results.

Bye,

Dali

[MrBunny]

Thanks for your reply Dami Lama. That's the information I was hoping for :) I'll try that half movie payback setting soon, it definately sounds interesting. It might be good for Koepi or Nic to change the payback default setting on their builds (if we're not both wrong), it might help the people that don't tweak their own settings too much and just use the defaults.

Mr. B

[Teegedeck]

Hi, MrBunny,

this sounds more as if you were talking about the 'averaging period' for CBR-encoding, could that be? At least the piece of advice for DivX5 mentioned, is about the averaging period.

The payback delay only determins how fast minor fluctuations in 2nd pass (and I mean minor, this has got nothing to do with redistributing bits over the whole curve!) get evened out; but as XviD calculates 2nd-pass-bitrate quite accurately, it shouldn't matter too much - better leave the value as it is.

[Koepi]

Yeah, setting that to "half the number of frames of the movie" is a very bad idea and will most likely mess up your encode (at least produce a worse result than a 10sec payback dealy).

Regards
Koepi

[MrBunny]

Thanks for your replies.
If I'm understanding Teegedeck properly, packback delay is basically local curve averaging for a short duration within the entire curve. This ensures that the short segment still has the appropriate amount of bits allocated to it by the main curve, but distributed with a bias towards the high or lower bit frames (as dictated by the other settings). Is this a more correct way of looking at it? Please do correct me if I'm wrong yet again.

@Koepi
If my new understanding of the packback setting is correct, I see how setting it to half a movie's length would be problematic. However, am I then right in assuming that the implementation of payback has changed from SBC since the recommended settings are so different? If you have the time, could you please give a brief explanation of the difference between the two as your guides seem to indicate that they play the same role and I would appreciate a better conceptual understanding of this setting.

TIA
Mr. B

[Koepi]

You used a payback delay of half the frames of a movie in nandub and your encodes came out nice? Can't believe that...

Well, I don't know what to write about the difference. It's just as the XOE/helpfile says: the delay for or period in which occuring overflow should be redsistributed. You would render curve treatment quite useless with huge values and since a huge stacked up overflow would be compensated in a region that's way off the origin you'd propably get strange results (ugly lomo scenes and nice himo scenes or vice versa).

I don't know how to explain that better, maybe someone else should step in here.

Regards
Koepi

[Didée]

Koepi,

regarding the fact that 2-pass-encoding with B-frames is a little strange in terms of overflow treatment:

Could increasing the payback delay be of any use here? (For the time being, of course, until 2nd pass behaviour is fixed.)

Or is the default delay of 250 already enough to compensate?
After all, even with this little flaw, XviD "hits the nail on its head" (German proverb) !

[MrBunny]

Hi Koepi, thanks again.

I believe you misunderstood me about the half movie payback, I just said that I could see how that could be a problem, not that I'd had any success with it. I believe I do understand packback now, so thanks a lot :) . I was hoping you could tell me what the XOE/helpfile is and where I can find it as it sounds like an informative read (I hope this doesn't count as a request).

Thanks,

Mr. B