System audio recording have very low amplitude

System audio recording have very low amplitude

[Ariel Elkin]

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Hello,

I’m using sox to record the audio produced by the system (i.e. from the OS and other running apps), but the recordings sound very quiet.

I first create a virtual input/output device using Blackhole (https://github.com/ExistentialAudio/BlackHole/ <https://github.com/ExistentialAudio/BlackHole/>) or Loopback (https://rogueamoeba.com/loopback/ <https://rogueamoeba.com/loopback/>). I then start playback of a file using iTunes, and start recording thus:

rec --rate 44100 --channels 2 --no dither test-output.aiff

(Note that this spits out the following error messages:
rec WARN formats: can't set sample rate 44100; using 48000
rec WARN formats: can't set 2 channels; using 16

However, test-output.aiff ends up being a stereo file at 44.1 kHz)

test-output.aiff properly records system audio, but the mean amplitude is about half the mean amplitude of the original sounds I played back. 

Any thoughts? 

Thanks

Ariel

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Re: System audio recording have very low amplitude

[Jeremy Nicoll - ml sox users]

On 2020-07-29 10:39, Ariel Elkin wrote:
> Hello,
> 
> I’m using sox to record the audio produced by the system (i.e. from
> the OS and other running apps), but the recordings sound very quiet.
> 
> I first create a virtual input/output device using Blackhole
> (https://github.com/ExistentialAudio/BlackHole/
> <https://github.com/ExistentialAudio/BlackHole/>) or Loopback
> (https://rogueamoeba.com/loopback/
> <https://rogueamoeba.com/loopback/>). I then start playback of a file
> using iTunes, and start recording thus:
> 
> rec --rate 44100 --channels 2 --no dither test-output.aiff
> 
> (Note that this spits out the following error messages:
> rec WARN formats: can't set sample rate 44100; using 48000
> rec WARN formats: can't set 2 channels; using 16
> 
> However, test-output.aiff ends up being a stereo file at 44.1 kHz)
> 
> test-output.aiff properly records system audio, but the mean amplitude
> is about half the mean amplitude of the original sounds I played back.


What OS are you using?  (I see from those links that the apps are MacOS,
which I know nothing about.  Someone else might do; also what version?)

Does MacOS have an internal audio mixer that determines the levels of 
sound
from various apps and how they go to other devices?  There's certainly 
one
of those in Windows - so eg if I was going to record audio streamed via 
the
BBC's iPlayer and Firefox, I'd start by temporarily turning off the 
other
sources of sound that could get in the way (eg the various tones Windows
error-message boxes will play, eg when it insists on telling me that 
when
I rename a file it may not work properly in future...). I've found out 
that
the hard way, as it's ruined some recordings.


-- 
Jeremy Nicoll - my opinions are my own


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Re: System audio recording have very low amplitude

[Ariel Elkin]

> What OS are you using? (I see from those links that the apps are MacOS, which I know nothing about. Someone else might do; also what version?)

I’m using macOS 10.15

> Does MacOS have an internal audio mixer that determines the levels of sound from various apps and how they go to other devices?
No exactly, but there’s an app (Audio MIDI Setup) which will allow you to set the input/output volume and sample rate of your audio hardware. I’ve checked there already and ensured that the input/output volume of Loopback and BlackHole were at 1.

Can someone clarify if this command

rec test-output.aiff 

could do any implicit gain attenuation? It’s not clear from the documentation.

Thanks

Ariel

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Re: System audio recording have very low amplitude

[Måns Rullgård]

Ariel Elkin <ariel@arivibes.com> writes:

>> What OS are you using? (I see from those links that the apps are MacOS, which I know nothing about. Someone else might do; also what version?)
>
> I’m using macOS 10.15
>
>> Does MacOS have an internal audio mixer that determines the levels of sound from various apps and how they go to other devices?
> No exactly, but there’s an app (Audio MIDI Setup) which will allow you to set the input/output volume and sample rate of your audio hardware. I’ve checked there already and ensured that the input/output volume of Loopback and BlackHole were at 1.
>
> Can someone clarify if this command
>
> rec test-output.aiff 
>
> could do any implicit gain attenuation? It’s not clear from the documentation.

If you add the -V flag, the sox/play/rec commands will list the full
effect chain, including automatically added conversions.

-- 
Måns Rullgård


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