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Old 26th December 2018, 17:45   #1767  |  Link
Rocinante
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Join Date: Jul 2018
Posts: 9
Quote:
Originally Posted by LoRd_MuldeR View Post
If you use 2-Pass RC, there normally is no need to set VBV at all. And the same applies to CRF mode.

Just set the target average bitrate as needed (via "--bitrate" option) in order to hit the desired target file size, and let the encoder optimally distribute the available bits. If the encoder creates some bitrate "spikes" or bitrate "drops", then this is the perfectly normal and expected behavior of 2-Pass ABR (or CRF) mode – it happens for a reason! It's because the bits are "moved" to where they are actually needed

With VBV enabled, you are effectively forcing the encoder to create a more "uniform" distribution of the available bits. This avoids strong bitrate "spikes" or bitrate "drops", yes. But it also means that the distribution of the available bits is going to deviate from the "optimal" distribution that the encoder would have chosen without the VBV restrictions – sometimes more, sometimes less

In other words: VBV can only make the resulting video quality worse. In the best case, VBV won't affect video quality in a noticeable way. But, certainly, VBV restrictions do not improve video quality!


VBV is required for two scenarios:
  • Streaming: Here you necessarily need to avoid bitrate "spikes" that exceed the available network bandwidth in order to ensure "smooth" playback.
  • Hardware Players: These players read video data from the disc at a certain (maximum) speed, and they have limited buffering capabilities. So, again, strong bitrate "spikes" are to be avoided in order to ensure "smooth" playback.

(In both of these scenarios, we choose VBV as the lesser of two evils. And, in both scenarios, the required VBV parameters are obvious from the use case)
Yes, I understand vbv is hurting the encoding quality and I can get accurate average bitrate with 2pass RC, but not maximum bitrate.

I'm in a third case scenarios now that due to a website streaming policy, they request a limit for a "maximum bitrate" value (such as all video max bitrate needs to be below 15Mb/s and below 8Mb/s average bitrate) instead of matching buffering capabilities or like a certain fixed vbv value used in bluray.

Last edited by Rocinante; 26th December 2018 at 17:47.
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