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

[Davy De Winter]

Loren Merritt wrote:

> On Wed, 4 Jan 2006, Davy De Winter wrote:
>
>> 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.
>
>
> You saw (num_of_bframes * frame_readtime), I would expect 
> ((num_of_bframes+1) * frame_readtime). Which, yes, is +80ms for the 
> 1st B-frame.
>
>> 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?
>
>
> x264 returns frames in encoding order, and ffmpeg requires that the 
> frames be recieved in encoding order.
>
Dear Loren, thx for the answers! I made some mistakes in my 
counter-order at the encoder-side, but now I have the correct ciphers. 
Now I see indeed they are sent out in encoding order, and decoding 
starts immediatly after reception of the first 2 non-Bframes @ decoder 
side.....

> --Loren Merritt
>


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