Nominal Bitrate vs Bitrate?

[jweaver]

I am a long time user of Hybrid and use it for one function only, which is to reduce the size of my movie collection to a standard video and audio bitrate.

Typically I encode everything at 384kbs AC3 audion, and have 3 video profiles (all 720p):

2200kbs for Low action movies
2500kbs for Medium action
2800kbs for High action.

And these profiles have served me well...

Typically after encoding my movies are of an expected size and Media Info shows:

Bit rate                                : 2 500 Kbps
Width                                    : 1 280 pixels
Height                                  : 696 pixels

But I am trying to encode a movie using my 2500kbs profile, and for a 90min film its just too big (around 2.7GB)... When I check it with MediaInfo I see this:

Bit rate                                : 3 364 Kbps
Nominal bit rate                        : 2 500 Kbps
Width                                    : 1 280 pixels
Height                                  : 534 pixels


Why does this movie have a higher Bitrate than normal but the correct "Nominal Bit Rate"... How can I get around this and force it to 2500kbs like every other movie? What am I overlooking here?

Can you help me understand what I am looking at?

Thanks in advance

Jon

[Selur]

To archive a target average bitrate you need to use 2pass encoding.

Cu Selur

[jweaver]

To archive a target average bitrate you need to use 2pass encoding.

Cu Selur

I am.. All of my profiles are 2 pass... Both of the examples I gave were from the same 'profile'. Sometimes (most) it results in a 2500kbps Bit Rate, but sometimes this "nominal bitrate" slips in and I don't know why.

[Selur]

Since I'm unclear about what 'norminal bitrate' for MediaInfo is what encoders/containers/etc. you use I can't really tell you.
Both x265&x264 during cfr encoding should properly hit the average bitrate specified. (assuming you use VBV, normal level restrictions and don't forcefully try to break it,..)
My guess is that you are encoding vfr content and with vfr the average bitrate during playback might differ and my guess is that MediaInfo somehow estimated a max bitrate that !might! occur with vfr content.
-> if you find a proper definition of what MediaInfo reports as nominal bitrate and how it comes up with that value there might be a way to take it into account.

Cu Selur

[jweaver]

Since I'm unclear about what 'norminal bitrate' for MediaInfo is what encoders/containers/etc. you use I can't really tell you.
Both x265&x264 during cfr encoding should properly hit the average bitrate specified. (assuming you use VBV, normal level restrictions and don't forcefully try to break it,..)
My guess is that you are encoding vfr content and with vfr the average bitrate during playback might differ and my guess is that MediaInfo somehow estimated a max bitrate that !might! occur with vfr content.
-> if you find a proper definition of what MediaInfo reports as nominal bitrate and how it comes up with that value there might be a way to take it into account.

Cu Selur

Understood.. But even if we ignore MediaInfo.. I have 2 movies which were encoded using the same profile (2500kbs)..

One is 99mins and ends up at 2.8GB

The other is 91mins and ends up at 1.8GB

I understand that the amount of motion can affect the ultimate size, but not 30% (and these are very similar).. But both of these movies are similar types and one has ended up 1GB smaller, for a movie which is 9 mins shorter.

I just know from experience that 2.8GB for a 90ish minute movie encoded at 2.5kbs is too big. .I wonder if I should just force it to be smaller, by encoding at something like 1800kbs, just to get the final size down?

[Selur]

If you are using x264/x265, 2pass encoding with the same bitrate the video size should in percent not differ more than the difference in frames.
In case this is the case, it's probably a bug in Hybrid.
Since you do not share enough details I can't help more.

I understand that the amount of motion can affect the ultimate size, but not 30%

Even 100% would not surprise me if you are not using 2pass encoding with a fixed file size.
It's not motion but complexity that is important.

As a side note:

• If you are not using 2pass encoding or two different movies with the same length (in frames) can result in huge different file size results.
  
• If you are not comparing the video but the overall file size and you are not aiming for a fixed file size the resulting file size can variate a lot.
  
• If you use transport streams as a target container just be the nature of the container and it's overhead huge fluctuations are possible.
  

-> share details or do not expect me to spend further time on this.

Cu Selur

[jweaver]

-> share details or do not expect me to spend further time on this.

Cu Selur

I will of course share more details... I just wanted to see if you had a quick answer or could explain what i am seeing before I start investigating myself

It all got a bit more complicated now..

Using my 2500kbs Profile I generated this:

Duration                                : 1h 39mn
Bit rate                                : 3 364 Kbps
Nominal bit rate                        : 2 500 Kbps

So last night, I simply changed the bitrate to 1800kbs, nothing else... And this is what was generated:

Duration                                : 1h 39mn
Bit rate                                : 1 800 Kbps


For some reason, when encoding this movie at 2500kbs, it ends up too big and has a higer bitrate than I wanted.. But if i lower it, it comes out a I would expect (although the bitrate is now far too low for a movie of this type).

So I now need to do some more testing to see if I can work out why this is happening.

Jon