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<h1>ALSA + dmix + apulse Reproduction Notes</h1>
<p>This documents a working configuration that lets multiple applications share the
sound card via ALSA's <code>dmix</code> plugin, with <code>apulse</code> redirecting
PulseAudio-only apps to that same mixed device. No PulseAudio or PipeWire daemon
is involved.</p>

<h2>1. Required packages (Debian/Ubuntu)</h2>
<pre><code>apt install alsa-utils apulse</code></pre>
<p>Versions in use here: <code>alsa-utils 1.2.14-1</code>, <code>apulse 0.1.13-2+b3</code>.</p>

<h2>2. ALSA config — <span class="path">~/.asoundrc</span></h2>
<p>Sets the default PCM to a <code>plug</code> wrapper around a <code>dmix</code>
slave bound to <code>hw:0,0</code>. <code>plug</code> handles rate/format
conversion so apps that don't natively match 48 kHz / S16_LE still work.</p>
<pre><code>pcm.!default {
    type plug
    slave.pcm "dmixer"
}

pcm.dmixer {
    type dmix
    ipc_key 1024
    ipc_perm 0666
    slave {
        pcm "hw:0,0"
        period_time 0
        period_size 1024
        buffer_size 4096
        rate 48000
        channels 2
        format S16_LE
    }
    bindings {
        0 0
        1 1
    }
}

ctl.!default {
    type hw
    card 0
}</code></pre>
<div class="note">
  <strong>Notes:</strong>
  <ul>
    <li><code>ipc_perm 0666</code> lets multiple users share the dmix segment.</li>
    <li><code>ipc_key 1024</code> is arbitrary but must be consistent across users sharing the mix.</li>
    <li>If the card isn't index 0, change <code>hw:0,0</code> and <code>card 0</code>
        (check with <code>aplay -l</code>).</li>
    <li>For a system-wide version, drop the same content into
        <code>/etc/asound.conf</code> instead of <code>~/.asoundrc</code>.</li>
  </ul>
</div>

<h2>3. apulse global env var — <span class="path">/etc/zsh/zshrc</span></h2>
<p>Added to the system-wide zshrc so every zsh user inherits it. This tells
<code>apulse</code> to send output through the dmix-backed default rather than
straight to <code>hw:0,0</code> (which would block other apps).</p>
<pre><code>export APULSE_PLAYBACK_DEVICE=plug:dmixer</code></pre>
<p class="path">Currently sits at /etc/zsh/zshrc:65.</p>
<div class="note">
  For bash users, add the same line to <code>/etc/bash.bashrc</code> (or
  <code>/etc/profile.d/apulse.sh</code> for a shell-agnostic drop-in).
</div>

<h2>4. Usage</h2>
<pre><code># Native ALSA app — uses dmix automatically via the default PCM
aplay sound.wav

# PulseAudio-only app — wrap with apulse
apulse firefox
apulse some-electron-app</code></pre>

<h2>5. Verification</h2>
<pre><code># Confirm card index
aplay -l

# Test the dmix path directly
aplay -D plug:dmixer /usr/share/sounds/alsa/Front_Center.wav

# Confirm env var is exported in a fresh shell
echo $APULSE_PLAYBACK_DEVICE   # → plug:dmixer</code></pre>

<h2>6. Reproduction checklist</h2>
<ol>
  <li>Install <code>alsa-utils</code> and <code>apulse</code>.</li>
  <li>Write the asoundrc block above to <code>~/.asoundrc</code> (or
      <code>/etc/asound.conf</code> system-wide).</li>
  <li>Append <code>export APULSE_PLAYBACK_DEVICE=plug:dmixer</code> to
      <code>/etc/zsh/zshrc</code> (and/or <code>/etc/bash.bashrc</code>).</li>
  <li>Open a new shell; verify with the commands in §5.</li>
  <li>Make sure no PulseAudio/PipeWire daemon is running and grabbing the card.</li>
</ol>

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