[x264-devel] Influence delay B-Frames on playout

[Davy De Winter]

Hi,

we've written a streamer using the x264-codec as encoder and ffmpeg as 
decoder. We wanted to test the influence on the playout-delay of the 
different parameters on encoder-side. We measured delay by continuously 
synchronizing the encoder & decoder (so the drift from our local 
NTP-server is below 1 ms). We took 2 timestamps: before grabbing the 
input-frame (from a framegrabber) and just after displaying the frame at 
the client. By subtracting these 2 values, we got in a 4 minute-sample 
(6000 frames) the following average values:
0-bframes: 86.6 ms
1-bframe: 136 ms
2-bframe: 181 ms
As can be seen, the average delay is always augmented by more or less 
the time of capturing one extra frame. However, 1 expected when using 
b-frames, the delay would augment by num_of_bframes * frame_readtime. 
Thus in the case of 1 b-frame: 80 ms extra delay instead of only 40 ms. 
As the encoder sends out the frames in playout-order and not (as I 
expected) in encoding-order when using b-frames, I wonder how the 
decoder "knows" how to decode a B-frame if het didn't receive the 
(future) P-or I-frame the B-frame encoder depends on. Or is this 
information included in the parameter-sets?

Can someone clarify this?

Thx!
Davy.

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