Example: Talk Encoding

Example 2: Talk Encoding

The whole video is about somebody talking, nothing but talking. Here is its details:

$ ffprobe Talk-A.MP4 
	Stream #0:0(eng): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuvj420p(pc, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 15250 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
	Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 255 kb/s (default)

The whole video is 4G in size, and it is only the first half. So before the re-encode, I need to make some compromises:

• A -preset fast is good enough for me, because I'm not too keen about spending 4x time yet not able to enhance the video quality much.
• The whole video is about him talking, so I don't mind his face not being crystal clear 100% of time. Thus I choose crf=30 to maximize file size saving.
• I also don't need video to have such gigantic resolution of 1920x1080, as it is only a fixed camera shooting a fixed scene. Half the size (1920/2 = 960) would be good enough for me.
• For human voices, opus bandwidth as low as 16k can already do a pretty good job, but I don't want any compromise in audio, so I'll keep the 64k opus bandwidth. Furthermore, as the recorded sound is a bit low, I need to increase the sound volume, to 200% of the original volume, which means the -af volume=2 parameter to ffmpeg.

The final command is:

 $ nice -n 19 ffcvt -f Talk-A.MP4 -debug 1 -force -crf=30 -o '-preset fast -vf scale=960:-1 -af volume=2' 

What's actually being invoked:

ffmpeg -i Talk-A.MP4 -c:a libopus -b:a 64k -c:v libx265 -x265-params crf=30 -y -preset fast -vf scale=960:-1 -af volume=2 Talk-A_.mkv

And the final result --

4273831936  Talk-A.MP4*
  44232824  Talk-A_.mkv*

I.e., the converted file came down from the original 4.0G to only 43M in size, only about one percent of the original size!

Ref:

• Scaling (resizing) with ffmpeg
• How to change audio volume up-down with FFmpeg