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What is "nominal bit rate"?
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Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 17, 2009 07:00 Posts Comp View users profile Send private message Reply with quote
I'm trying to re-encode an mkv using x264 in videodub.

I'm looking to produce around an 8gb file, and v calc says the required bit rate is c. 8446kbit/s,. So i enter 8446kbit/s in the x264vfw configuration dialogue box.

Trouble is, the file size produced is 3.6GB, and mediainfo tells me that the bit rate is 3949Kbps, although the 'nominal bit rate' is indeed 8446Kbps.

Could some kind person please explain the above, and how i can produce a file with an actual bit rate of 8446 and a size of 8GB?

Thanks in anticipation.

MOVIEGEEK
The Dude


Joined: 08 Mar 2002
Location: CA,USA
Post Posted: Oct 17, 2009 09:02 Posts Comp View users profile Send private message Reply with quote
Nominal bitrate is usually the average bitrate for VBR, some programs use the term nominal to mean maximum. If you want a fixed file size use CBR.
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Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 17, 2009 11:16 Posts Comp View users profile Send private message Reply with quote
Thanks for the reply but i really don't follow you at all.

Surely on a two pass it should be possible to produce a given file size using VBR? How do you produce a CBR video using X264 anyway?

And are you saying that in mediainfo nominal bitrate means maximum bitrate?

And therefore also that on the x264vfw configuration, the "target bitrate" actually means maximum bitrate? That doesn't seem to make a lot of sense.

In any event, if anyone can tell me how to make an 8gb file using x264, i'd be very grateful.

jagabo
Member


Joined: 09 Dec 2005
Location: none
Post Posted: Oct 17, 2009 11:21 Posts Comp View users profile Send private message Reply with quote
Yes, use 2-pass VBR with an average bitrate should deliver the average bitrate you request (and hence the specified file size). But other settings can keep the encoder from accurately hitting the specified average bitrate. Things like minimum and maximum quantizers, GOP size, etc.

Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 17, 2009 11:39 Posts Comp View users profile Send private message Reply with quote
So if i understand you, 'target bitrate' in x264 means average bitrate, not 'nominal bitrate' - whatever that means.

My settings from vcalc were therefore correct (a 2hr film allowing for 1 audio track at 448bitrate) but for some reason i ended up with a file half the size it should have been,

You say 'But other settings can keep the encoder from accurately hitting the specified average bitrate.'

But surely it's not a coincidence that mediainfo reports a 'nominal bitrate' (whatever that is! - maximum bitrate??) of exactly what i specified as an average bitrate in x264.

In other words, in a sense, i did accurately and exactly hit the specified bitrate. Just the wrong "type" of bitrate. In any event, all the other settings were default and surely could not cause such the inaccuracy of 50% out?

poisondeathray
Member


Joined: 07 Sep 2007
Location: Canada
Post Posted: Oct 17, 2009 11:43 Posts View users profile Send private message Reply with quote
Not recommended to use x264vfw. Buggy and you get sync issues with b-frames in avi container. No offical support from developers

What version are you using?

The default settings are very poor to use.

Use the cli encoder or a GUI for it. e.g. ripbot264, xvid4psp, megui

Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 17, 2009 12:10 Posts Comp View users profile Send private message Reply with quote
The dialogue box that opens in virtualdub/video/compression/x264...etc is headed up x264vfw configuration. No version number, but a build date of July 29 2009.

If i open windows/start/programs/x264/x264 command line interface encoder i get a dialogue box headed up MeGU! 0.2.3.2136.

Is the latter that which you are suggesting i should use? If so, do i integrate it with virtualdub? If so, how do i do this?

Sorry if these questions are a bit basic, but i'm not an expert as you'll have gathered from previous exchanges smile.gif

poisondeathray
Member


Joined: 07 Sep 2007
Location: Canada
Post Posted: Oct 17, 2009 12:17 Posts View users profile Send private message Reply with quote
You don't integrate it with vdub, unless you frameserve out. There are several guides for frameserving and vdub if you search, on this site and others.

Vdub only exports avi directly - that's a bad idea for h.264 and b-frames. Not to mention x264vfw is quite buggy as you can see

Unless you need something specific to vdub like a filter , It's easier, faster, better to use avisynth and most filters have an equivalent or better version in avisynth

If it's a straight encode, you might find ripbot264 or xvid4psp or handbrake easy to use.

jagabo
Member


Joined: 09 Dec 2005
Location: none
Post Posted: Oct 17, 2009 18:31 Posts Comp View users profile Send private message Reply with quote
My luck with h264vfw isn't as bad as poisondeathray's. I haven't seen any real problems with it. Yes, in order for VFW based applications to open and seek within the resulting AVI files you have to use the "Virtualdub Hack" option, but media players don't seem to have any problems with playing the resulting AVI files, with or without the hack.

Be sure you create a new stats file with each new project by running the Multipass 1st Pass option before running the Multipass Nth Pass option.

johns0
Puppet Master


Joined: 05 Jun 2002
Location: canada
Post Posted: Oct 17, 2009 19:22 Posts Comp View users profile Send private message Reply with quote
I use uncropmkv to re-encode mkv to different sizes
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Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 19, 2009 14:02 Posts Comp View users profile Send private message Reply with quote
Well, the original file was corruot which was clearly causing all sorts of problems. I tried more than one of the suggestions here, but without success.

Anyway, I'm grateful for all the responses - as usual, i've learnt a lot from those who took the trouble to reply.

Bully9
Member


Joined: 03 Aug 2005
Location: UK
Post Posted: Oct 29, 2009 17:24 Posts Comp View users profile Send private message Reply with quote
I finally managed successfully to reduce a large (uncorrupt!) mkv using megui (not the most intuitive of interfaces?). I could only get the x264 encoder to work if i set it to default and then chose just my mode (automated 2pass) and bitrate (8710). If i tried to do anything like increase the no. of reference frames, i got an error after the first pass (it took quite a while to work out what was going on!).

So, two questions:

1. Is it possible to encode in x264 using megui and to specify the no. of reference frames? Any other restrictions?

2. And (which takes me full circle to the original question) mediainfo showed the final video had a bit rate of 8530 (and a 'nominal' bit rate of 8710). So it was smaller than the 7.95 GB i was aiming for and that the specified 8710 would have given me.

Not a big problem, but can i find out how to specify a bitrate that will give me the exact size (DVD9) file i want?

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