[x264-devel] question regading encoded keyframe size at 720p

[Henrik Gramner]

On Thu, Oct 15, 2015 at 2:51 PM, Carl Petersen <cpetersen at broadsoft.com> wrote:
> Hi Henrik,
>
> Yes. Vbv buffer size is 2000 and vbv max_rate is 1024. About a year ago
> buffer size was 350 and it was increased to 2000. I have a basic
> understanding of how vbv works but I still can't grasp on what values to
> choose for our application. 350 was too low. There were frequent artifacts
> that seemed to correspond to keyframes. Increasing it to 2000 alleviated the
> problem but I still cant fully comprehend the impact of that. I can sort of
> appreciate that it can increase keyframe size. What I don’t understand is
> why 350 was too low. Was that vbv overflow? And I don’t know what value to
> pick for vbv buffer size.

x264 will utilize the large buffer size to create big I-frames since
that is beneficial for quality, but it is not ideal for real-time
applications. Having a smaller buffer size solves it but will cause
I-frames to be bit rate starved resulting in poor quality, which is
why periodic intra refresh was created.

For zero-delay streaming with a fixed bandwidth you should use
periodic intra refresh and single-frame VBV (a VBV buffer size that is
equal to the VBV max rate divided by the frame rate) with zerolatency
tune.