Often sound for ‘voice over slides’ videos gets recorded badly (or at least needs some fixing up), so here are three approaches to enhancing the sound quality (and some reasonable settings for video).

Method 1 : Direct in-place volume normalisation

From this helpful Q&A :

ffmpeg -i video.avi -af "volumedetect" -vn -sn -dn -f null /dev/null

# systematise a little ...

ffmpeg -i ${stub}.mp4 -af "volumedetect" -vn -sn -dn -f null /dev/null

[Parsed_volumedetect_0 @ 0x5642c8652c40] n_samples: 3407232
[Parsed_volumedetect_0 @ 0x5642c8652c40] mean_volume: -34.2 dB
[Parsed_volumedetect_0 @ 0x5642c8652c40] max_volume: -16.1 dB
[Parsed_volumedetect_0 @ 0x5642c8652c40] histogram_16db: 168
[Parsed_volumedetect_0 @ 0x5642c8652c40] histogram_17db: 1039
[Parsed_volumedetect_0 @ 0x5642c8652c40] histogram_18db: 3538

# Seems best
ffmpeg -i ${stub}.mp4 -af "volume=16dB" -c:v copy -c:a aac -b:a 64k ${stub}_volfix.mp4 

# This resamples to 192kHz, which seems extreme
#ffmpeg -i ${stub} -af "loudnorm" -c:v copy -c:a aac -b:a 64k ${stub}_volfix-loudnorm.mp4 

Method 2 : More complex ffmpeg filters

It would be nice if there was a ‘Noise Reduction’ option for ffmpeg - though perhaps there is good news :

FFmpeg now have 2 native filters to deal with noise background: afftdn and anlmdn. Also, for some time one has been able to use ladspa (look for noise-supressor) and/or lv2 (look for speech denoiser) filters with FFmpeg.

Ideas :

# Fix volume

# Dynamic volume normalization
#   https://ffmpeg.org/ffmpeg-filters.html#dynaudnorm

# Half-a-second fade-in

# FFT-wise eliminate white noise (needs ffmpeg >= 4.1, not in Fedora 29...)

so (without the denoising, for now) :

ffmpeg -i ${stub}.mp4 -c:v copy -c:a aac -b:a 64k -af "dynaudnorm, afade=t=in:ss=0:d=0.5" ${stub}_fixed.mp4 

Method 3 : Treating the audio separately

  • Take the audio from the combined file
    • https://www.bugcodemaster.com/article/extract-audio-video-using-ffmpeg
  • Process the audio (keep length the same)

  • Recombine the audio into the video file
    • https://superuser.com/a/1137613

Check the formats in the file

ffmpeg -i vokoscreen-2019-01-10_15-00-45.mkv 

Input #0, matroska,webm, from 'vokoscreen-2019-01-10_15-00-45.mkv':
    ENCODER         : Lavf58.12.100
  Duration: 01:08:23.50, start: 0.000000, bitrate: 265 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 1366x768, 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
      ENCODER         : Lavc58.18.100 libx264
      DURATION        : 01:08:23.503000000
    Stream #0:1: Audio: mp3, 48000 Hz, stereo, fltp, 128 kb/s (default)
      ENCODER         : Lavc58.18.100 libmp3lame
      DURATION        : 01:08:22.709000000

HD 720p 25fps?

ffmpeg -i Seer\ 01\ 2019\ 01\ 09.mp4 

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Seer 01 2019 01 09.mp4':
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.12.100
  Duration: 00:51:21.65, start: 0.041016, bitrate: 3575 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 3379 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc (default)
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 192 kb/s (default)
      handler_name    : SoundHandler

Take the audio from the combined file

# This is the default, apparently:
ffmpeg -i SEER-01_2019-01-09.mp4 -vn -acodec copy SEER-01_audio_raw.aac

# Get a list of all supported formats
ffmpeg -codecs

# Best choice for 'regular WAV'
ffmpeg -i SEER-01_2019-01-09.mp4 -vn -acodec pcm_s16be SEER-01_audio_raw.wav

Process the audio (keep length the same)

Load SEER-01_audio_raw.wav manually into Audacity to do the following processing :

  • Effect - Fade-in / out to smooth out the recording
  • Effect - Noise Reduction (need to find a section of ‘silence’ for it to fingerprint first)
  • Effect - Amplify (so that maxiumum amplitude is 0dB)
  • File - Export as OGG (Quality=5 seems fine) &r_arr; SEER-01_audio_clean.ogg

Recombine the audio into the video file

Two versions of the same command (illustrating combinations tried) :

ffmpeg -i SEER-01_2019-01-09_high-xvid_HD720p.25.mp4 -i SEER-01_audio_clean.ogg \
  -c:v copy -map 0:v:0 -map 1:a:0 SEER-01_2019-01-09_clean.mp4

ffmpeg -i SEER-01_2019-01-09_Med-VimeoWide_1Mb.mp4 -i SEER-01_audio_clean.ogg \
  -c:v copy -map 0:v:0 -map 1:a:0 SEER-01_2019-01-09_small-clean.mp4

Choices, choices…

Will be much easier to go with Method 2 (but need to wait for newer ffmpeg to arrive in the repos to get the denoising filter).

Martin Andrews

{Finance, Software, AI} entrepreneur, living in Singapore with my family.

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