Output over Alsa Soundserver

[kinderschoko]

Hi,
first of all this is a great application! I studied the wiki and everything worked nice out of the box.
I use the raspberry 2 connected via hdmi to a soundbar. The problem here is, that my soundbar have a kind of "low noise detection" or something that turns the amp off on every track skipping and then on again. This is sometimes very loud.
I wasted a lot of time to fix it, but no solution found. I created a sound with very low frequency and very low amplitude, that cant be physically played, but when i play that the amp didnt turn off any more.
Now the goal is, to play that parallel to the raspotify output.
I found out that this should be possible with the Alsa soundserver, which can mix up several sources together. I'm new with raspberry and linux so this is not easy for me. I tried the following:

/etc/asound.conf
defaults.ctl.card 1
defaults.pcm.card 1
defaults.pcm.dmix.rate 48000
defaults.pcm.dmix.format IEC958_SUBFRAME_LE

Script
/etc/systemd/system/silentaudio.service
[Unit]
Description=Play silent Audio
After=raspotify.service

[Service]
ExecStart=/usr/bin/aplay -D default -t wav -r 44100 -c 2 -f IEC958_SUBFRAME_LE /dev/zero.wav

[Install]
WantedBy=multi-user.target

I can start the script with systemctl start silentadii. But the Problem is, the first come, the first served. I need it both together.
How should i configure the /etc/raspotify/conf?

[JasonLG1979]

If your device is "dmixable" you can configure a software mixed output so multiple applications can send sound to the soundbar at the same time.

The problem being though that dmix (ALSA's software mixer) does not support IEC958_SUBFRAME_LE, so software mixing in bare ALSA may not be possible unless your device is capable of accepting a linear format naively.

I'm afraid without software mixing of some kind 99.9% of devices can not play audio from multiple sources simultaneously. This is not a Linux or ALSA limitation it's a hardware limitation. Unless a device has a hardware mixer (again 99.9% don't) concurrent access of the raw device is not possible.

If the device as more than 2 channels a workaround may be to split the device into 2 virtual devices with your noise going to the channels not being used by librespot.

What is the output of aplay -L and aplay -l?

[JasonLG1979]

I'm sorry that's /dev/zero not null. /dev/zero give you an infinite amount of zeros which equals silence for an audio file.

[JasonLG1979]

aplay -r 44100 -f S16_LE /dev/zero

Should do it.

[kinderschoko]

You made my day! Its working now 💯

[JasonLG1979]

It's actually not an uncommon problem. I will document this in the wiki for anyone else that may have this issue.

And you're for sure welcome. I'm glad I could help and that the issue was not to difficult.

[JasonLG1979]

The problem comes from poorly designed hardware that does not mute the outputs during power on and off.

[kinderschoko]

Damn i run into a new problem using the sollution above. And its even more terrifying.
The volume setting of my soundbar is on 5 (max is 50), the spotify app and the raspberry are on 100%.
When i stop my service with systemctl stop silentaudio i have the loud clicks on every start and track skip
for some milliseconds much louder then the setting of 5.
When i start my service with systemctl start silentaudio i dont have the noisy clicks on every start and track skip. Thats cool!
But when i pause a playlist a have something like a bomb that explodes when i click on play again.
The volume goes up and stays on this high level while the soundbar shows still 5. When i set the volume + or - 1 it goes to normal level again.
How is this possible, when all sources are on 100%?

[kinderschoko]

Yes you are right. Never buy soundbars from Teufel again :D
I tried that. Unfortunately it has no effect on this problem.
I noticed an interesting correlation:
When i click on pause and on play again within under 5s, everything is nice. Pause over 5s an then play: The neighbor appears to knocking on my door.
The option with playing a .wav is not as good as i expected: The soundquality of softwaremixing is poor on my raspberry :(
But there are no other problems.

Is there a other possibility?
Get the play and pause state from libre and play a silent wave on pause and play this aplay -r 44100 -f S16_LE /dev/zero on regular playback?
I know this is really messy.

[JasonLG1979]

Sounds like overzealous power management in the sound bar itself maybe? Like it's turning itself off when it detects silence? There's not really anything that I know that you can do about that if there isn't a way to disable that behavior on the device itself. Sounds like you may have just bought a shitty soundbar. I'm sorry.

[JasonLG1979]

Although something you might try is using speaker-test to play a loop of very faint pink-noise. You'll have to inch the volume up so it reaches just above the threshold that the soundbar shuts off and see if that's acceptable. With luck the music will mask the pink-noise while it's playing.

So in your service you'd use:

speaker-test -c1 -l0 -FS16_LE -r44100 -S1 > /dev/null 2>&1

The -S1 part is the volume. S stands for scale (0-100). Inch the Scale up until your soundbar doesn't auto shutoff.

[kinderschoko]

this is the best solution for me so far. With -S1 the soundbar doesnt auto shutoff, but on louder volume the pink noise is clearly hearable.
Is it possible to change somewhere the datatype of S to float and then set on -S0.1?
I tried this:
speaker-test -c1 -l0 -FS16_LE -r44100 -S1 -t sine -X -f30000 (sinewave of 30kHz, 1%volume)
It works too, i cant hear any disturbing things. For some reason settings over 32kHz are hearable again. Limitations?

[JasonLG1979]

I would be careful about pushing ultrasound to your soundbar it can damage class D amps. Especially cheap ones that likely lack the proper filtering.

[JasonLG1979]

30kHz is well beyond human hearing. What you're maybe hearing at 32kHz is the amp becomeing unstable because you might be hitting some harmonic of the PWM's carrier freq? Ofc at that freq everything is all harmonics 44100 sampling rate can't produce anything over 22kHz.

[kinderschoko]

my final sollution is
ExecStart=speaker-test -c1 -l0 -FS16_LE -r44100 -S1 -t sine -X -f10
I tried 25Hz, this is the lowest setting where i can feel and see my sub moving. On 10Hz nothing happens. I think the amp should handle that. But the reslut is perfect for me.
Thank you @JasonLG1979!

[JasonLG1979]

Most amps unless they're DC coupled won't go down that low. I'd still check to make sure the amp isn't getting hot. You may not hear, see or feel it but it could cook your amp.

[JasonLG1979]

As far as the volume goes I'm pretty sure it's integer only.