Detecting video volume

Question

I'm streaming few RTMP streams through nginx and I want to check every few seconds what stream has the highest volume. Specifically these streams are of talking heads and I assume that usually only one of them is speaking at a time, and I'm trying to find which one. Since nginx can output hls (Apple http live streaming) I decided to check every few seconds the last segment of each stream using ffmpeg. Example:

ffmpeg -f mp3 -i /my/path/camera67/123.ts -af "volumedetect" -f null /dev/null

For some reason the max_volume is always zero (max_volume: 0.0 dB) and mean_volume seems meaningless regarding the volume.

1. Do you have any idea why it's always zero?
2. Is there a helpful way to understand mean_volume?
3. Can you think of a different tool that may give me the volume (e.g. mediainfo or ffprobe)?

I also tried:

ffmpeg -f lavfi -i amovie=/my/path/camera67/123.ts,volumedetect

This time I got:

[mpegts @ 0x130bf40] start time for stream 1 is not set in estimate_timings_from_pts [mpegts @ 0x130bf40] Could not find codec parameters for stream 1 (Audio: aac ([15][0][0][0] / 0x000F), 0 channels, fltp): unspecified sample rate Consider increasing the value for the 'analyzeduration' and 'probesize' options [Parsed_amovie_0 @ 0x130bcc0] No audio stream with index '-1' found [lavfi @ 0x130abc0] Error initializing filter 'amovie' with args '/my/path/camera67/123.ts' amovie=/my/path/camera67/123.ts,volumedetect: Invalid argument

Any idea?

Thanks, T.

Answer 1

So that's what happened. I streamed MP3 to nginx that transcoded the input to HLS segments that doesn't support MP3. Listening to the RTMP output caused me thinking that the audio is working fine, but when I listened to the HLS output I heard nothing. I changed my original stream to AAC, then the HLS stream gave the right output and immediately I saw correlation between the music and the mean and max volumes.

Thank you all.