Minor Question About Max Bitrate

[Sir Didymus]

Let me say it's just a curiousity, more than else...

And recently I started encoding assets in OPV mode...

Some months ago I noticed in every ecl segment it happens something similar:

vbr_brate_max=8552
opv_brate_max=6841

This lead to perform OPV encodings at max bitrate which is less than the same at VBR...

I assume there are valid reasons for the OPV max bitrate to be everytime much less than VBR, but these are still unclear to me...

Is it for taking care of some "singular behaviour" [i.e. not respecting of the upper bitrate boundary] of CCE ?

Cheers,
SD

[robot1]

sounds odd to me.
I think the max bitrate should be the same.

[Sir Didymus]

Hi robot1!!! :)

Strange, isn't it ?

OT: take the opportunity for a little statement of appreciation for your excellent work and improvements on ECLPRO... Started using Procoder on a standard basis for interlaced sources... And especially at low bitrates I am really happy about it...

[SkVid]

Why does DVD-RB use the maximum available bitrate for max_bitrate and not the one of the source?
Wouldn't that make sense?

[Sir Didymus]

Originally posted by SkVid
Why does DVD-RB use the maximum available bitrate for max_bitrate and not the one of the source?
Wouldn't that make sense?

Maximum bitrate of the source is simply unknown [I think] a priori. In any case it would be wrong [or better improper...] to apply the source value since this parameter is also depending on the number and the bitrate of the audio tracks to be remuxed, as indicated by the user in the "Audio to Keep" panel...

[jdobbs]

The maximum bitrate should be the same on both VBR and OPV. Anyone else see this?

[SkVid]

Originally posted by Sir Didymus
Maximum bitrate of the source is simply unknown [I think] a priori. In any case it would be wrong [or better improper...] to apply the source value since this parameter is also depending on the number and the bitrate of the audio tracks to be remuxed, as indicated by the user in the "Audio to Keep" panel...

If you use Prcoder2 and add a vob as source it shows you the maximum bitrate of this vob. Other programs like bbmpeg or bitrate viewer can ascertain the max bitrate.
And what I think even if you drop some audio tracks, this is good for the average bitrate not for the maximum bitrate. I think you can't be better than the source.
That's what I think. But maybe I am wrong.

[jdobbs]

Originally posted by SkVid
Why does DVD-RB use the maximum available bitrate for max_bitrate and not the one of the source?
Wouldn't that make sense? No. DVD-RB sets the maximum based upon the DVD standard and subtracts for audio that exists on the DVD. If you remove audio the maximum available could very possibly rise. But, remember, that maximum will almost never be reached because VBR has to keep the quality level fairly constant.

If you report the maximum bitrate on a VBR source, most viewers will only tell you the DVD max, not the actual composite that exists. You'd have to scan end-to-end to actual find our what it was.

[EDIT] Actually for max it would be the peak -- composite would be the average.

[SkVid]

O.K. I understand what you want to tell me.
But for a better ditribution of the available bitrate a lower bitrate could be better or not?

[robot1]

Spikes on max bitrate have virtually no effects on the average bitrate.

[jdobbs]

Right. You should always set the max to the highest it can be and let the encoder decide how much of it to use.

[jdobbs]

Okay... I should have thought about this before answering. I think you are comparing apples and oranges here. The reason the values differ is because of the one you have chosen to use.

DVD-RB normalizes the framerate at 23.976 as a part of it's mechanism for handling hybrid sources. The correct framerate (29.97) is reestablished after encoding (during rebuild). That's the reason playback of some M2Vs appears to be in slow motion.

The reason you see a lower bitrate is because this normalization distorts the true maximum bitrate when played back at 29.97. So the max has to be set at 80% of its true value (23.976/29.97). If you set it to the true maximum you run the risk of exceeding the maximum bitrate and causing playback problems.

The way DVD-RB works is very complex and a lot of thought went into ensuring it stayed compliant with DVD specs. I highly recommend you not change any of it's default settings.

As I keep repeating, my most frequently reported bug is the self inflicted wound of the well intentioned but unfamiliar.

[ADDITIONAL NOTE] This of course only applies to sources that are not progressive film (which is truly encoded at 23.976).

[jdobbs]

@Sir Didymus

Were you encoding a PAL source?

I think I just found a bug in the OPV max_bitrate code while looking at this. The normalization only happens on NTSC sources. The max_bitrate, therefore should stay at the higher level for PAL. I just noticed in researching this that I'd not excluded PAL when adjusting the max rate on OPV.

There's another fix for v0.73. Honestly it probably wouldn't have any effect, though, since it's unlikely you would ever hit the max anyway at the average bitrates used to fit most sources on DVD-5 and it only relates to OPV.

[Sir Didymus]

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Were you encoding a PAL source?
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Yes. Always...
One of the biggest troubles I have [for testing purposes not related to DVD-RB...] is the "natural" un-availability of NTSC titles here in PAL land...

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Honestly it probably wouldn't have any effect, though,
...
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Totally agree, it wouldn't have relevant effects...

Even now you are confirming the values should be the same, more than a little bug, as indicated in thread title, me think we talk about "A Minor Question About Max Bitrate"...

All the best,
SD

[Sir Didymus]

Just want to confirm 0.73 solved this minor issue. :)

Many thanks Jdobbs!!!

SD