[Question]: Multiple Shairport Instances - how to split a DAC into separate instances?

[DerPicknicker]

What happened?

Hey @mikebrady ,

what a great piece of Software. I currently testing multiple instances (which are working fine - even with Airplay 2) see here.

The Problem for me now is:
I can't mix two Instances of Shairport into one Stereo Signal which is sent to the DAC (Hifiberry HAT).
Here is the Alsaconfig:

# virtual device 1 (routed to left schannel)
pcm.virtual1 {
    type plug
    slave.pcm "dmix_left"
}

# virtual device 2 (routed to right channel)
pcm.virtual2 {
    type plug
    slave.pcm "dmix_right"
}

# Dmix for left channel
pcm.dmix_left {
    type dmix
    ipc_key 1024
    slave {
        pcm "hw:0,0"
        channels 2
    }
    bindings {
        0 0  # left channel
    }
}

# Dmix for right channel
pcm.dmix_right {
    type dmix
    ipc_key 1025
    slave {
        pcm "hw:0,0"
        channels 2
    }
    bindings {
        1 1  # Right Channel
    }
}

Relevant log output

ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.000982343 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.012397657 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.001334895 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.022532240 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.001219740 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.001140156 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.021944635 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.001030365 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance
         0.000668906 "player.c:2713" Delay error -22 when checking running latency.
ALSA lib pcm_direct.c:2194:(_snd_pcm_direct_new) unable to create IPC shm instance

System Information.

Raspberry PI OS Lite 64Bit Linux raspberry 6.6.74+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.6.74-1+rpt1 (2025-01-27) aarch64 GNU/Linux

Configuration Information.

Every default except for output device and port (increased by one for the second instance) and the Airplay Offset ID (also increased by one). 

Build with Alsa and Airplay 2 from your documentation.

PulseAudio or PipeWire installed?

• Check if your system uses a Sound Server.

How did you install Shairport Sync?

Built from source

Check previous issues

• Confirm

[mikebrady]

Thanks for the post. It's not completely clear to me what you are trying to do. Do you wish to send output from one instance to the left channel and the output from another instance to the right channel?

Certainly, other contributors have managed to split multichannel (e.g. eight-channel) DACs into separate pairs of stereo outputs.

[DerPicknicker]

Thanks for the post. It's not completely clear to me what you are trying to do. Do you wish to send output from one instance to the left channel and the output from another instance to the right channel?

I want to split a stereo dac into two separate Instances. So I can send from to IOS Devices Music to a single Stereo DAC via I2S (HifiBerry DAC).

[mikebrady]

Thanks. Maybe some other contributors could advise -- I don't have experience in this. Also, it might even be worth asking an AI agent about it. (BTW, I've changed the title to make it more specific, if that's okay.)

[DerPicknicker]

Yep, I already tried chatgpt o1 but without luck. Maybe a Pipe is easier. Let's wait for some feedback.

[DerPicknicker]

@mikebrady If you have any other idea let me know.

[DerPicknicker]

Hey again,

I asked in a different forum and found a Solution... I can now get 2 Instances running via Shairportsync using the same DAC. It's in German but I think it's easy to understand (if you use a proper translation tool.) See here

I have only one Problem remaining. I am now syncing two instances (to test how well it works) (from the same device). If I now de-select one instance the other also stops playing. I can only get it running again to disconnect and reconnect. Which is not really convenient.

In the Log is nothing shown up (-v flag). I cannot see that a device is dropped in the log. Should I open a new Issue for that or is it already known?

[jgillies]

I have only one Problem remaining. I am now syncing two instances (to test how well it works) (from the same device). If I now de-select one instance the other also stops playing. I can only get it running again to disconnect and reconnect. Which is not really convenient.

In the Log is nothing shown up (-v flag). I cannot see that a device is dropped in the log. Should I open a new Issue for that or is it already known?

I'm running a very similar setup (HiFiBerry amp with two output devices set up in asound.conf). I'm seeing the same behavior as you're describing, including no information in the logs. I'd be happy to help in providing any additional information that might prove useful.

[mikebrady]

Thanks for the posts.

This problem is already pretty well known.

The main problem is that AirPlay devices seem to get confused when different AirPlay sinks, e.g. multiple Shairport Sync instances, share the same IP addresses.

The mechanism of the issue you are reporting is that although there might be two Shairport Sync instances on the system, there is only be one instance of NQPTP. When one of the Shairport Sync instances stops, it stops NQPTP, and this in turn (sometimes) stops the other Shairport Sync instance.

This immediate problem could be fixed by making NQPTP work with more than one Shairport Sync client, and indeed it was like that for quite some time. Unfortunately, due to the main problem, there seems no point in giving NQPTP back this ability.

[DerPicknicker]

@mikebrady why is that not possible to make nqptp work again with multiple instances?

I think that would be great - at some point this must be possible because JukeAudio is also running multiple AirPlay 2 instances on one host. Maybe we're thinking too complex.

Would it help to create multiple Docker container with individual MAC addresses?

[mikebrady]

It's a lot of work, and in my experience it is pointless. I don't know anything about JukeAudio, I'm afraid.

Some people do seem to be using Docker. If you search through the issues and discussion, you'll find some references.

[DerPicknicker]

With independent docker containers I don't have the issue with NQPTP, right?

Out of interest: was there a specific reason to drop multiple instances support of NQTP?