feat(host/windows): client→host mic passthrough via a virtual audio device

The host received the client's mic uplink (0xCB Opus) but dropped it on Windows ("requires
Linux"). Windows has no user-mode way to CREATE a capture endpoint, so target an existing
virtual audio device and write the decoded mic PCM into its RENDER endpoint — the device's
CAPTURE endpoint then surfaces as a microphone host apps record from (the inverse of a
virtual cable). New audio::wasapi_mic::WasapiVirtualMic: finds the device by friendly-name
(Steam Streaming Microphone / VB-Audio CABLE Input / VoiceMeeter / "virtual", override with
PUNKTFUNK_MIC_DEVICE), opens a WASAPI shared event-driven RENDER client (48 kHz stereo f32,
autoconvert), and a dedicated COM thread writes a bounded (~80 ms drop-oldest) inject queue
with silence-fill. open_virtual_mic() gets a Windows arm; mic_service_thread (Opus decode →
push) now compiles for windows too (opus is already a windows dep). Clear error + install
guidance when no virtual device is present.

Linux/cross-platform side cargo-checks; the Windows path is built/validated when the box is
back (the wasapi render API was cross-checked against the docs + the existing capture path).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/audio.rs

@@ -67,18 +67,27 @@ pub trait VirtualMic: Send {
     }
 }
 
-/// Open a virtual microphone PipeWire source with `channels` interleaved channels (1 or 2).
+/// Open a virtual microphone with `channels` interleaved channels (1 or 2). Linux: a PipeWire
+/// `Audio/Source`. Windows: writes into an existing virtual audio device's render endpoint (whose
+/// capture endpoint apps see as a mic) — see [`wasapi_mic`].
 #[cfg(target_os = "linux")]
 pub fn open_virtual_mic(channels: u32) -> Result<Box<dyn VirtualMic>> {
     linux::PwMicSource::open(channels).map(|m| Box::new(m) as Box<dyn VirtualMic>)
 }
 
-#[cfg(not(target_os = "linux"))]
+#[cfg(target_os = "windows")]
+pub fn open_virtual_mic(channels: u32) -> Result<Box<dyn VirtualMic>> {
+    wasapi_mic::WasapiVirtualMic::open(channels).map(|m| Box::new(m) as Box<dyn VirtualMic>)
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
 pub fn open_virtual_mic(_channels: u32) -> Result<Box<dyn VirtualMic>> {
-    anyhow::bail!("virtual mic requires Linux + PipeWire")
+    anyhow::bail!("virtual mic requires Linux + PipeWire or Windows + a virtual audio device")
 }
 
 #[cfg(target_os = "linux")]
 mod linux;
 #[cfg(target_os = "windows")]
 mod wasapi_cap;
+#[cfg(target_os = "windows")]
+mod wasapi_mic;

crates/punktfunk-host/src/audio/wasapi_mic.rs

@@ -0,0 +1,235 @@
+//! WASAPI virtual microphone (Windows) — the inverse of [`super::wasapi_cap`]. Windows has no
+//! user-mode way to *create* a capture (microphone) endpoint, so we target an EXISTING virtual audio
+//! device and write the client's decoded mic PCM into that device's **render** endpoint; the device's
+//! **capture** endpoint then surfaces as a microphone that host apps can record from.
+//!
+//! Target device, by friendly-name substring (first match wins; override with `PUNKTFUNK_MIC_DEVICE`):
+//! "Steam Streaming Microphone" (ships with Steam Remote Play — exactly this purpose), VB-Audio
+//! "CABLE Input", VoiceMeeter, or anything with "virtual" in the name. If none is present we return an
+//! error with install guidance and the host runs without mic passthrough.
+//!
+//! `push` enqueues decoded interleaved-f32 PCM into a bounded ring (drop-oldest beyond ~80 ms so mic
+//! latency stays bounded); a dedicated COM-apartment thread renders it event-driven, filling silence
+//! when the client isn't talking. WASAPI objects are `!Send`, so they live entirely on that thread
+//! (mirrors `WasapiLoopbackCapturer`).
+
+use super::{VirtualMic, SAMPLE_RATE};
+use anyhow::{anyhow, Context, Result};
+use std::collections::VecDeque;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::mpsc::{sync_channel, SyncSender};
+use std::sync::{Arc, Mutex};
+use std::thread::{self, JoinHandle};
+use std::time::Duration;
+use wasapi::{Direction, SampleType, StreamMode, WaveFormat};
+
+const CHANNELS: u32 = 2;
+/// 48 kHz stereo f32: 2 channels * 4 bytes.
+const BLOCK_ALIGN: usize = 2 * 4;
+/// Bound the inject queue at ~80 ms so the passed-through mic stays low-latency (drop oldest beyond).
+const MAX_QUEUE_BYTES: usize = (SAMPLE_RATE as usize * 80 / 1000) * BLOCK_ALIGN;
+
+/// Render-endpoint friendly-name substrings (lowercased) we can write into so the device's capture
+/// endpoint becomes a host mic. Ordered by preference.
+const CANDIDATES: &[&str] = &[
+    "steam streaming microphone",
+    "cable input",
+    "voicemeeter input",
+    "voicemeeter aux input",
+    "virtual",
+];
+
+pub struct WasapiVirtualMic {
+    queue: Arc<Mutex<VecDeque<u8>>>,
+    stop: Arc<AtomicBool>,
+    join: Option<JoinHandle<()>>,
+}
+
+impl WasapiVirtualMic {
+    pub fn open(channels: u32) -> Result<Self> {
+        anyhow::ensure!(
+            channels == CHANNELS,
+            "virtual mic is stereo-only (got {channels})"
+        );
+        let queue = Arc::new(Mutex::new(VecDeque::<u8>::new()));
+        let stop = Arc::new(AtomicBool::new(false));
+        // Bring-up handshake: report the resolved device (or the error) before returning, so a missing
+        // virtual-mic device surfaces as Err (the caller retries with backoff) not a silent dead thread.
+        let (ready_tx, ready_rx) = sync_channel::<Result<String>>(1);
+        let (q, st) = (queue.clone(), stop.clone());
+        let join = thread::Builder::new()
+            .name("punktfunk-wasapi-mic".into())
+            .spawn(move || {
+                if let Err(e) = render_thread(q, st, ready_tx) {
+                    tracing::error!(error = %format!("{e:#}"), "wasapi virtual-mic thread failed");
+                }
+            })
+            .context("spawn wasapi mic thread")?;
+        match ready_rx.recv_timeout(Duration::from_secs(3)) {
+            Ok(Ok(name)) => {
+                tracing::info!(device = %name,
+                    "WASAPI virtual mic ready (client mic → this device's render endpoint)");
+                Ok(WasapiVirtualMic {
+                    queue,
+                    stop,
+                    join: Some(join),
+                })
+            }
+            Ok(Err(e)) => Err(e),
+            Err(_) => Err(anyhow!("wasapi virtual-mic init timed out")),
+        }
+    }
+}
+
+impl Drop for WasapiVirtualMic {
+    fn drop(&mut self) {
+        self.stop.store(true, Ordering::SeqCst);
+        if let Some(j) = self.join.take() {
+            let _ = j.join();
+        }
+    }
+}
+
+impl VirtualMic for WasapiVirtualMic {
+    fn push(&self, pcm: &[f32]) {
+        let Ok(mut q) = self.queue.lock() else {
+            return;
+        };
+        q.reserve(pcm.len() * 4);
+        for &s in pcm {
+            q.extend(s.to_le_bytes());
+        }
+        // Drop-oldest to keep latency bounded (mic is real-time; stale audio is worse than dropped).
+        if q.len() > MAX_QUEUE_BYTES {
+            let excess = q.len() - MAX_QUEUE_BYTES;
+            q.drain(..excess);
+        }
+    }
+    fn channels(&self) -> u32 {
+        CHANNELS
+    }
+}
+
+/// Resolve the virtual-mic target among render endpoints by friendly-name. Logs all candidates so a
+/// missing device is diagnosable.
+fn find_device() -> Result<wasapi::Device> {
+    let enumerator = wasapi::DeviceEnumerator::new().context("DeviceEnumerator")?;
+    let collection = enumerator
+        .get_device_collection(&Direction::Render)
+        .context("render device collection")?;
+    let n = collection.get_nbr_devices().context("device count")?;
+    let want = std::env::var("PUNKTFUNK_MIC_DEVICE")
+        .ok()
+        .map(|s| s.to_lowercase());
+    let mut names = Vec::new();
+    let mut found = None;
+    for i in 0..n {
+        let Ok(dev) = collection.get_device_at_index(i) else {
+            continue;
+        };
+        let name = dev.get_friendlyname().unwrap_or_default();
+        let lname = name.to_lowercase();
+        let hit = match &want {
+            Some(w) => lname.contains(w),
+            None => CANDIDATES.iter().any(|c| lname.contains(c)),
+        };
+        if hit && found.is_none() {
+            found = Some(dev);
+        }
+        names.push(name);
+    }
+    found.ok_or_else(|| {
+        anyhow!(
+            "no virtual-mic device among render endpoints {names:?}. Install VB-Audio Virtual Cable \
+             or enable Steam Remote Play's microphone (Steam Streaming Microphone), or set \
+             PUNKTFUNK_MIC_DEVICE=<friendly-name substring>."
+        )
+    })
+}
+
+fn render_thread(
+    queue: Arc<Mutex<VecDeque<u8>>>,
+    stop: Arc<AtomicBool>,
+    ready: SyncSender<Result<String>>,
+) -> Result<()> {
+    if let Err(e) = wasapi::initialize_mta()
+        .ok()
+        .context("CoInitializeEx (MTA)")
+    {
+        let _ = ready.send(Err(e));
+        return Ok(());
+    }
+    // Open + start the render stream. The WASAPI objects must outlive the loop, so build them here and
+    // keep them (a closure that *returned* them would drop them); on any failure report Err and exit.
+    let setup = (|| -> Result<(wasapi::AudioClient, wasapi::AudioRenderClient, wasapi::Handle, String)> {
+        let device = find_device()?;
+        let name = device.get_friendlyname().unwrap_or_else(|_| "virtual mic".into());
+        let mut audio_client = device.get_iaudioclient().context("IAudioClient")?;
+        // 48 kHz stereo f32; autoconvert lets WASAPI shared-mode SRC match the device mix format.
+        let desired = WaveFormat::new(
+            32,
+            32,
+            &SampleType::Float,
+            SAMPLE_RATE as usize,
+            CHANNELS as usize,
+            None,
+        );
+        let (default_period, _min) = audio_client.get_device_period().context("device period")?;
+        let mode = StreamMode::EventsShared {
+            autoconvert: true,
+            buffer_duration_hns: default_period,
+        };
+        audio_client
+            .initialize_client(&desired, &Direction::Render, &mode)
+            .context("initialize render client")?;
+        let h_event = audio_client.set_get_eventhandle().context("event handle")?;
+        let render_client = audio_client
+            .get_audiorenderclient()
+            .context("IAudioRenderClient")?;
+        // Pre-fill the whole buffer with silence so the stream starts cleanly (no startup glitch).
+        let buf_frames = audio_client.get_buffer_size().context("buffer size")? as usize;
+        let _ = render_client.write_to_device(buf_frames, &vec![0u8; buf_frames * BLOCK_ALIGN], None);
+        audio_client.start_stream().context("start render stream")?;
+        Ok((audio_client, render_client, h_event, name))
+    })();
+    let (audio_client, render_client, h_event, name) = match setup {
+        Ok(t) => t,
+        Err(e) => {
+            let _ = ready.send(Err(anyhow!("{e:#}")));
+            return Ok(());
+        }
+    };
+    let _ = ready.send(Ok(name));
+
+    let mut buf: Vec<u8> = Vec::new();
+    while !stop.load(Ordering::Relaxed) {
+        // The device signals when it wants more data; finite timeout keeps `stop` responsive.
+        if h_event.wait_for_event(100).is_err() {
+            continue;
+        }
+        let space = audio_client
+            .get_available_space_in_frames()
+            .context("available space")? as usize;
+        if space == 0 {
+            continue;
+        }
+        let need = space * BLOCK_ALIGN;
+        if buf.len() < need {
+            buf.resize(need, 0);
+        }
+        // Silence base; overwrite with queued mic PCM (zero-pad the tail when the client is quiet).
+        buf[..need].fill(0);
+        {
+            let mut q = queue.lock().unwrap();
+            let n = q.len().min(need);
+            for (i, b) in q.drain(..n).enumerate() {
+                buf[i] = b;
+            }
+        }
+        render_client
+            .write_to_device(space, &buf[..need], None)
+            .context("write_to_device")?;
+    }
+    audio_client.stop_stream().ok();
+    Ok(())
+}

crates/punktfunk-host/src/m3.rs

@@ -201,7 +201,8 @@ pub(crate) async fn serve(opts: M3Options, np: Arc<NativePairing>) -> Result<()>
     // wedged KWin's EIS setup ("EIS setup timed out"). Gamepads stay per-session (uinput).
     let injector = InjectorService::start();
     // One virtual microphone for the whole host lifetime (see MicService): the client's mic uplink
-    // (0xCB) is Opus-decoded and fed into a persistent PipeWire Audio/Source host apps record from.
+    // (0xCB) is Opus-decoded and fed into a persistent virtual mic host apps record from (Linux
+    // PipeWire Audio/Source; Windows a virtual audio device's render endpoint).
     let mic_service = MicService::start();
     // Host-lifetime worker that fires debounced TV-session restores (the managed gamescope path
     // restores the box's autologin gaming session on idle, not per-disconnect — see
@@ -1094,22 +1095,20 @@ impl MicService {
     }
 }
 
-/// Stub — mic passthrough needs Linux (PipeWire source + libopus); non-Linux dev builds
-/// drain and drop the frames (sessions still count the datagrams), same as when the
-/// source fails to open.
-#[cfg(not(target_os = "linux"))]
+/// Stub — mic passthrough needs a virtual-mic backend (Linux PipeWire source / Windows virtual audio
+/// device); other platforms drain and drop the frames (sessions still count the datagrams).
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
 fn mic_service_thread(rx: std::sync::mpsc::Receiver<Vec<u8>>) {
-    tracing::warn!(
-        "punktfunk/1 mic passthrough requires Linux (PipeWire + libopus) — frames dropped"
-    );
+    tracing::warn!("punktfunk/1 mic passthrough unsupported on this platform — frames dropped");
     for _ in rx {}
 }
 
 /// The host-lifetime mic worker: lazily open the virtual mic + decoder, then Opus-decode each
 /// forwarded frame and push the PCM into the source. Reopen (after [`INJECTOR_REOPEN_BACKOFF`])
 /// on open failure or a decode error. Exits when every session sender and the service's own
-/// sender drop (host shutdown), tearing the PipeWire source down.
-#[cfg(target_os = "linux")]
+/// sender drop (host shutdown), tearing the virtual mic down. Linux = PipeWire `Audio/Source`;
+/// Windows = a virtual audio device's render endpoint (see `audio::wasapi_mic`).
+#[cfg(any(target_os = "linux", target_os = "windows"))]
 fn mic_service_thread(rx: std::sync::mpsc::Receiver<Vec<u8>>) {
     let mut mic: Option<Box<dyn crate::audio::VirtualMic>> = None;
     let mut decoder: Option<opus::Decoder> = None;