Audio Tracks file size effects

[CatlessCat]

Hello.
Do unused audio tracks affect file size? See this example:

As you can see above I only want to record in-game audio with nothing else. When I do record and open them in the windows media player, it shows that the video has 4 active audio tracks as seen below.

My question can be split into a few questions:
1. In the example above (4 Audio Tracks selected with only 1 audio track active in the Advanced Audio Properties), do the other 3 unused audio tracks use any storage space?
2. If I have 4 Audio Sources, and I dedicate 1 Audio Track per source, but only one of the sources has any audio on it, will they still use storage space because it records silence?
3. How much do Audio Tracks actually affect file size? (320 Audio Bitrate per Audio Track)

 

[CatlessCat]

Edit:
Got my head together and realised I could test it.
1. They don't use storage space.
2. They use storage space.

 

[koala]

If you write 4 audio tracks to the video file (1st image), there will be 4 audio tracks in the video file, no matter how many sources are directed to each track. A track with no sources will contain silence, but occupy disk space to contain that silence.

Usually, audio tracks contain a negligible amount of disk space compared to video. At least if they are written with some audio codec that applies compression. Only audio in wav or pcm format is uncompressed, and this uses a non negligible amount of disk space, but still less than video. Compressed silence is as much as nothing.

The most easy way to see how much a track actually occupies is MediaInfo, which installs into the Windows Explorer context menu, so you can inspect each file.

A realistic ratio between video and audio for a 15 min long video is 560 MB video and 14 MB audio for 1 video and 1 audio track.

 

[CatlessCat]

If you write 4 audio tracks to the video file (1st image), there will be 4 audio tracks in the video file, no matter how many sources are directed to each track. A track with no sources will contain silence, but occupy disk space to contain that silence.

Usually, audio tracks contain a negligible amount of disk space compared to video. At least if they are written with some audio codec that applies compression. Only audio in wav or pcm format is uncompressed, and this uses a non negligible amount of disk space, but still less than video. Compressed silence is as much as nothing.

The most easy way to see how much a track actually occupies is MediaInfo, which installs into the Windows Explorer context menu, so you can inspect each file.

A realistic ratio between video and audio for a 15 min long video is 560 MB video and 14 MB audio for 1 video and 1 audio track.

Thank you for the fast reply!
The reason this question appeared in my mind was because I've recorded a 3 hour and 23 minute footage which ended up weighing 223GB.
Now my audio tracks for that video looked like this:

I didn't even think about changing them before recording. Now after doing a quick 5 second test with these tracks checked on a black screen (no video source), the file size was:

Afterwards, I've done another 5 second test with these audio tracks checked:

The file size:

Over twice the size difference despite there being no sound whatsoever.

 

[koala]

Don't judge from 5 second test videos. 5.0 seconds or 5.9 seconds? File overhead will also dilute your results. Only verify with files longer than a minute.

I made a mistake - silence is probably not compressed to zero but instead to the audio bitrate for the track.

As default, OBS records audio with the aac codec with a constant bitrate of 160 kbit/s, so we don't even need to measure. Just math.
If you recorded 3 hours 23 minutes, which is 12180 seconds, you got 12180 * 160 kbit of audio data. This is:
12180 s* 160 kbit/s = 1,948,800 kbit = 1,948,800 / 8 kbytes = 243,600 kbytes = 243,600 / 1024 mbytes = 237,9 mbytes

This 237,9 MB is probably what MediaInfo will tell you about the size of one audio track of that recording. If you recorded with a different bitrate, the audio size will change proportionally.

If your whole file is 223GB, 237.9MB for audio is about one thousandth - a negligible value compared to the whole file, as I said. So you don't need to be concerned about the size and number of audio tracks you record.

 

[CatlessCat]

Don't judge from 5 second test videos. 5.0 seconds or 5.9 seconds? File overhead will also dilute your results. Only verify with files longer than a minute.

I made a mistake - silence is probably not compressed to zero but instead to the audio bitrate for the track.

As default, OBS records audio with the aac codec with a constant bitrate of 160 kbit/s, so we don't even need to measure. Just math.
If you recorded 3 hours 23 minutes, which is 12180 seconds, you got 12180 * 160 kbit of audio data. This is:
12180 s* 160 kbit/s = 1,948,800 kbit = 1,948,800 / 8 kbytes = 243,600 kbytes = 243,600 / 1024 mbytes = 237,9 mbytes

This 237,9 MB is probably what MediaInfo will tell you about the size of one audio track of that recording. If you recorded with a different bitrate, the audio size will change proportionally.

If your whole file is 223GB, 237.9MB for audio is about one thousandth - a negligible value compared to the whole file, as I said. So you don't need to be concerned about the size and number of audio tracks you record.

I've downloaded MediaInfo to check yet I cannot seem to see audio track sizes.
View is set to Text. How can I actually view the size per track? Doesn't show video size either, only file size.

 

[koala]

If I use the MediaInfo context menu of a video file, I get this screen, where you clearly see one global section, then one video track section with its size ("Stream size"), then one audio track section with its size.

Well, after a bit of checking, it seems file size and stream size is not displayed for *.mkv files. Sorry, I didn't realize this until now. I remux all mkv files to mp4 after recording because of the better compatibility with postprocessing tools. Mkv is still not supported as widely as mp4.

 

[CatlessCat]

If I use the MediaInfo context menu of a video file, I get this screen, where you clearly see one global section, then one video track section with its size ("Stream size"), then one audio track section with its size.
View attachment 95920

Well, after a bit of checking, it seems file size and stream size is not displayed for *.mkv files. Sorry, I didn't realize this until now. I remux all mkv files to mp4 after recording because of the better compatibility with postprocessing tools. Mkv is still not supported as widely as mp4.

Thanks a lot for the information! Remuxed the video into an MP4.
Unused audio tracks are at 3.31MB stream size (the ones with no audio in them), and the two tracks that were used are at 459MB stream size due to me using a bitrate of 320 on OBS. Negligible.

Thanks a lot for your time in clearing this up and telling about MediaInfo. Have a wonderful week!