Dear Mr Luo,
The 4a softmixer is a sound server. It is a more secured, more
CPU-friendly server than PulseAudio.
It is mandatory to have a sound server to share a sound card between
several applications, that may write to the sound card at different
sound rates and formats.
It is the responsibility of the 4a-softmixer to instantiate the dmix
alsa plugin, based on the 4A config files.
snd-aloop, which has recently been replaced by a more flexible
implementation (called avirt), proxifies the access from playback
application to sound cards.
The 4a-softmixer reads from snd-aloop (or avirt) captures, and
will perform the rate conversion, soft volume control for hardware that
does not implement it, routing to configured audio channels,
and mixing at the end.
A security label (SMACK) can be put on the snd-aloop PCM playback
devices, and since each these devices matches an audio role, that
applications have to claim to the 4A high level API, this makes a
gatekeeper for audio apps.
I agree that snd-aloop introduces one more buffer copy;
but the measured performances and CPU load on the tested
boards do not have revealed any unacceptable levels, up to now.
The CPU load (which could be even better with some unimplemented
optimizations today) is typically less than 5% on the Renesas RCAR
board, and similar numbers have been seen on Intel Atom boards.
Finally, some words about avirt.
Introduced recently on the master branch of AGL, it provides
to 4A high level a common interface for both softmixing,
and DSP-accelerated mixing. I also sets user-readable
names to the dynamically-configured playback devices, that
match the 4A audio roles (aka streams).
Hope this helps !
Best regards,
Thierry Bultel
On 04/01/2019 03:14 AM, luop wrote:
Hello,
My name is Frank Luo, and I am an engineer of Neusoft.
I just started studying the AGL audio system.
After Reading the code of softmixer, I realized that this module
based on snd_aloop driver. However, I have a questions about this.
What's the purpose of using this aloop as a sound card ?
Audio data is passthrough the loop sound card(card 0), than
written into the real sound card(card 1) by softmixer. The alsa plugins,
including dmix plugin, are based on the real sound card.
Audio data can be directly written into the real card(with the
dmix plugin). So, I am wondering if you could send me the intent of
appending this snd_aloop driver.
Thank you, and I look forward to hearing from you.
Best Regards
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Frank Luo(罗鹏)
Neusoft Automotive Solution Business Unit
Neusoft Corporation
901 Huangpu Road High-Tech Zone Dalian 116085, PRC
Tel: (86 411) 8446 6663
Fax: ((86 411) 8446 6742
Mobile:15840620310
Email:_luo.p__@neusoft.com_ <mailto:xxxxxxxxxx@xxxxxxxxxxx>
_www.neusoft.com <http://www.neusoft.com/>
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