Public bug reported:
Hi. I've found something strange happening when I record the output of a
sound card through loopback device audio monitor.
If I load the snd-aloop module, then setup a player to play a chirp sound that
spans a sine wave between 10Hz and 22kHz through it, and then record the
virtual monitor device through Audacity it drops the frequencies something over
16kHz like it have a low pass filter on it.
The sampling rate is 44100 or 48000, it doesn't matters, the result is the
same.
Also in some cases I found that the signal path between channels doesn't
have correct syncing. A chirp sound is ideal for testing, because if
there is some delay between channels, it would produce a comb filter
like frequency response on the mono mix.
It also happens with real sound cards, not virtual ones.
The question is about how to narrow down the issue. I think it's not
ALSA related, because it happens both on the hardware device and the
virtual loop one.
Attached I put my chirp test sound.
I played it on the command line with:
$ ffplay -loop 1000 chirp_test.flac
Then I setup the recording path with pavucontrol, to target the Audacity
input
My versions:
pulseaudio 12.2-2
linux kernel 5.6.13
$ lsb_release -rd
Description: Ubuntu 18.04.4 LTS
Release: 18.04
pulseaudio:
Installed: 1:12.2-9~bionic1
Candidate: 1:12.2-9~bionic1
Version table:
*** 1:12.2-9~bionic1 500
500 http://ppa.launchpad.net/mikhailnov/pulseeffects/ubuntu bionic/main
amd64 Packages
100 /var/lib/dpkg/status
1:11.1-1ubuntu7.8 500
500 http://ar.archive.ubuntu.com/ubuntu bionic-updates/main amd64
Packages
1:11.1-1ubuntu7.7 500
500 http://security.ubuntu.com/ubuntu bionic-security/main amd64
Packages
1:11.1-1ubuntu7 500
500 http://ar.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
Attached on this there is a .tar.gz containing both .flac files
chirp_test.flac is the original file.
chirp_loopback_recorded.flac is the recorded file from the loopback device.
Thank You.
** Affects: pulseaudio (Ubuntu)
Importance: Undecided
Status: New
** Tags: chirp testcase
** Attachment added: "The files used for testing"
https://bugs.launchpad.net/bugs/1882587/+attachment/5381754/+files/sound_files.tar.gz
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https://bugs.launchpad.net/bugs/1882587
Title:
Truncated audio frequency response on monitor device
Status in pulseaudio package in Ubuntu:
New
Bug description:
Hi. I've found something strange happening when I record the output of
a sound card through loopback device audio monitor.
If I load the snd-aloop module, then setup a player to play a chirp sound
that spans a sine wave between 10Hz and 22kHz through it, and then record the
virtual monitor device through Audacity it drops the frequencies something over
16kHz like it have a low pass filter on it.
The sampling rate is 44100 or 48000, it doesn't matters, the result is the
same.
Also in some cases I found that the signal path between channels
doesn't have correct syncing. A chirp sound is ideal for testing,
because if there is some delay between channels, it would produce a
comb filter like frequency response on the mono mix.
It also happens with real sound cards, not virtual ones.
The question is about how to narrow down the issue. I think it's not
ALSA related, because it happens both on the hardware device and the
virtual loop one.
Attached I put my chirp test sound.
I played it on the command line with:
$ ffplay -loop 1000 chirp_test.flac
Then I setup the recording path with pavucontrol, to target the
Audacity input
My versions:
pulseaudio 12.2-2
linux kernel 5.6.13
$ lsb_release -rd
Description: Ubuntu 18.04.4 LTS
Release: 18.04
pulseaudio:
Installed: 1:12.2-9~bionic1
Candidate: 1:12.2-9~bionic1
Version table:
*** 1:12.2-9~bionic1 500
500 http://ppa.launchpad.net/mikhailnov/pulseeffects/ubuntu
bionic/main amd64 Packages
100 /var/lib/dpkg/status
1:11.1-1ubuntu7.8 500
500 http://ar.archive.ubuntu.com/ubuntu bionic-updates/main amd64
Packages
1:11.1-1ubuntu7.7 500
500 http://security.ubuntu.com/ubuntu bionic-security/main amd64
Packages
1:11.1-1ubuntu7 500
500 http://ar.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
Attached on this there is a .tar.gz containing both .flac files
chirp_test.flac is the original file.
chirp_loopback_recorded.flac is the recorded file from the loopback device.
Thank You.
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