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Elder Geek
Asked: 2017-02-10 12:24:43 +0800 CST

Does the -an switch speed up the first pass of a 2 pass ffmpeg encode?

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I've seen a number of 2-pass encode examples and they often use the -an switch on the first pass. This seems to be because on the first pass only video data is sampled for the log file used by the second pass to create a more efficient encoding. I noticed this switch again when answering this question and I'm wondering if there is any effect whatsoever in using this switch on the first pass of a 2 pass encode.

command-line

1 Answers

  1. Best Answer

    Any effect at all?

    Yes. Using the -an switch causes ffmpeg to skip processing the audio stream.

    Any positive benefit?

    Yes there is a measurable decrease in processing time.. I took a 60 second mp4 source clip to use for testing and issued the command

    time ffmpeg -i input.mp4 -y -an -c:v libvpx -quality best -b:v 225k -qmin 10 -qmax 42 -maxrate 300k -bufsize 1000k -threads 4 -pass 1 -f webm /dev/null

    I obtained the following output:

    frame= 1799 fps=174 q=0.0 Lsize=       0kB time=00:00:00.00 bitrate=N/A    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

    Output file is empty, nothing was encoded 

    real    0m10.477s
user    0m12.972s
sys 0m0.188s

    and then renamed the resulting log file an.log for comparison later.

    I ran the command again without the -an switch as in

    time ffmpeg -i input.mp4 -y -c:v libvpx -quality best -b:v 225k -qmin 10 -qmax 42 -maxrate 300k -bufsize 1000k -threads 4 -pass 1 -f webm /dev/null

    The resulting output was:

    frame= 1799 fps=165 q=0.0 Lsize=     505kB time=00:01:00.01 bitrate=  68.9kbits/s    
video:0kB audio:483kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 4.367473%

    real    0m11.088s
    user    0m13.792s
    sys 0m0.156s

    So it looks like it's a little bit faster to run the first pass with the -an

    Any change to the 1st pass log file? No. As expected the log files are identical as running diff on them proves.

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