Which crf settings for streaming?

[DoomNlNE]

Hi,

I will use x264 to encode videos for streaming on my website. It will be better over youtube because I can have 60fps (this is mandatory for my goals).

My question is, which crf is recommended for streaming videos at 60fps? The issue is I don't want people with low bandwidth to get screwed. The source resolution will be 320x240.

So just wondering what will be a good starting point in terms of crf?

Also, is there a way to use x264 and allow ios devices to play them (streamed in safari)?

[LoRd_MuldeR]

CRF mode may not be a good choice for streaming, because the resulting bitrate is source-dependent and therefore unpredictable.

In 2-Pass mode the average bitrate is known beforehand. But even then there may be bitrate "pikes" that exceed the networks bandwidth.

This can be resolved by using VBV (also possible with CRF!). Just set the VBV parameters (VBV Maxrate and VBV BuffSize) according to your requirements.

A general recommendation for CRF values can not be given, because it totally depends on what level of quality you expect/need...

(Simply choose the highest possible CRF value that meets your quality requirements)

[benwaggoner]

CRF in streaming can be thought of as a more advanced "min QP" where the codec won't waste bits improving quality beyond a certain threshold. You'd then set the peak bitrate and VBV to whatever the max you want for your stream to be. Peak is really what matters in streaming for compatibility and support.

[DoomNlNE]

I see.

Do you know bitrate Youtube uses then?

[stic]

For what is worth, for my application in which I'm trying to investigate the overall QoE of video quality at the end user side, I've been investigating the proper settings for video streaming for quite some time. I do not know, what bitrates YouTube is using,however depending on my user study I think 2 Mbits/s average bitrate with vbv-buffer and vbv-maxrate set to 4 Mbits/s for HD (720p) with 25fps is more than enough.
Some of the video materials, which users have been evaluating, is in 1800 kbit/s and 2400 kbit/s, and the corresponding MOS scales don't differ to much. Both of them are averaging around ~4.2 value .This corresponds to average bit per pixel information of 0.078 and 0.104. In all cases the 2-pass encoding with x264 slow preset have been used.(the YouTube settings are very similar)
For your purpose you can check these setting with 400kbit/s (0.09 bpp).