My kids figured out pretty early how to capture videos and pictures on our tablets. I ran across a video recently that was taking up almost 2 GB of space in my library. I played it back to find that my toddler (at the time) had captured an hour-long video, but had left the tablet on the ground the entire time, camera facing down.
The entire video was black, but the audio was pretty adorable. I wanted to keep the audio, but hated that it was taking up so much space. Fortunately, I’ve used ffmpeg enough to know that I can strip streams out of a media file like this.
In this post, I’ll walk you through how I accomplished this. I’ll show you how I was able to:
- Remove the video, since I know it was all black.
- Keep the audio.
- Keep all the metadata from the original file (e.g.
creation_time).
Let’s start by figuring out what streams are in the file. We’ll use ffprobe, which gathers information from multimedia streams and prints it in human- and machine-readable fashion1.
ffprobe 20140716_071935.mp4
I won’t include the entire output, because it’s fairly verbose. Instead, I’ll skip down to the section that describes the input.
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '20140716_071935.mp4':
Metadata:
major_brand : isom
minor_version : 0
compatible_brands: isom3gp4
creation_time : 2014-07-16T12:17:10.000000Z
Duration: 00:57:32.52, start: 0.000000, bitrate: 4743 kb/s
Stream #0:0[0x1](eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(progressive), 1280x720,
4615 kb/s, 29.91 fps, 29.92 tbr, 90k tbn (default)
Metadata:
creation_time : 2014-07-16T12:17:10.000000Z
handler_name : VideoHandle
vendor_id : [0][0][0][0]
encoder :
Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 123 kb/s (
default)
Metadata:
creation_time : 2014-07-16T12:17:10.000000Z
handler_name : SoundHandle
vendor_id : [0][0][0][0]
We can see that the file has one audio stream (Stream #0:1[0x2](eng): Audio) and one video stream (Stream #0:0[0x1](eng): Video).
I want to remove the video stream since I know it’s all black. This will result in a much smaller file. I want to keep the audio stream as well as all the metadata (e.g. creation_time). The command below will do exactly that.
ffmpeg \
# Path to the input file.
-i 20140716_071935.mp4 \
# Copy all streams without re-encoding.
-codec copy \
# Copy all the streams.
-map 0 \
# Omit the video stream.
-map -0:v:0 \
# Copy the global metadata.
-map_metadata 0 \
# Copy the audio stream's metadata.
-map_metadata:s:a 0:s:a \
# Add some custom metadata.
-metadata comment="Removed video stream because it was all black." \
# Path to the output file.
20140716_071935-audio-only.mp4
Let’s use ffprobe to confirm the metadata and audio stream were copied to the output file.
ffprobe 20140716_071935-audio-only.mp4
Again, I’m omitting some of the verbose output at the beginning.
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '20140716_071935-audio-only.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2mp41
creation_time : 2014-07-16T12:17:10.000000Z
encoder : Lavf59.27.100
comment : Removed video stream because it was all black.
Duration: 00:57:32.52, start: 0.000000, bitrate: 125 kb/s
Stream #0:0[0x1](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 123 kb/s (
default)
Metadata:
creation_time : 2014-07-16T12:17:10.000000Z
handler_name : SoundHandle
vendor_id : [0][0][0][0]
We can see that we retained the original creation_time metadata, both globally and in the stream. There’s only one stream, an audio stream, and we added our comment metadata.
Now, let’s check the file size.
-rw-r--r-- 1 blachniet staff 51M Jan 16 15:03 20140716_071935-audio-only.mp4
-rw-r--r--@ 1 blachniet staff 1.9G Dec 18 07:30 20140716_071935.mp4
Wow! Our new file is around 97% smaller! 🎉
All that’s left to do is play the output file in an audio player to confirm everything sounds good! If you want to dig into the ffmpeg options further, check out the documentation.