diff --git a/tests/01-record-play-cs/Probe.csproj b/tests/01-record-play-cs/Probe.csproj
index b92ac93..94d3d81 100644
--- a/tests/01-record-play-cs/Probe.csproj
+++ b/tests/01-record-play-cs/Probe.csproj
@@ -10,6 +10,7 @@
     <SelfContained>true</SelfContained>
     <PublishSingleFile>false</PublishSingleFile>
     <InvariantGlobalization>true</InvariantGlobalization>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="PortAudioSharp2" Version="1.0.6" />
diff --git a/tests/01-record-play-cs/Program.cs b/tests/01-record-play-cs/Program.cs
index b810bd1..ef50c72 100644
--- a/tests/01-record-play-cs/Program.cs
+++ b/tests/01-record-play-cs/Program.cs
@@ -1,22 +1,130 @@
+using System.Runtime.InteropServices;
 using PortAudioSharp;
+using RecordPlayProbe;
+using Stream = PortAudioSharp.Stream;
 
+const int SampleRate = 16_000;
+const int Channels = 1;
+const int DurationS = 5;
+const uint BlockFrames = 1024;
+const string OutWav = "out.wav";
+
+// Tell PortAudio's ALSA host API to wrap devices in `plughw:` so it does
+// rate/format conversion (USB Speaker Phone does not natively offer 16 kHz).
+// Environment.SetEnvironmentVariable updates the .NET table but the libc
+// getenv() view PortAudio actually reads can lag, so call setenv directly too.
 Environment.SetEnvironmentVariable("PA_ALSA_PLUGHW", "1");
+Libc.setenv("PA_ALSA_PLUGHW", "1", 1);
 
 PortAudio.Initialize();
 try
 {
-    int count = PortAudio.DeviceCount;
-    Console.WriteLine($"PortAudio version: {PortAudio.VersionInfo.versionText}");
-    Console.WriteLine($"Devices ({count}):");
-    for (int i = 0; i < count; i++)
+    int device = FindUsbDevice();
+    Console.WriteLine(
+        $"Recording {DurationS}s from device {device} " +
+        $"('{PortAudio.GetDeviceInfo(device).name}')");
+
+    short[] buf = new short[SampleRate * DurationS];
+    int writeOffset = 0;
+    object gate = new();
+
+    var inParams = new StreamParameters
     {
-        var info = PortAudio.GetDeviceInfo(i);
-        Console.WriteLine(
-            $"  [{i}] {info.name}  in={info.maxInputChannels} out={info.maxOutputChannels} " +
-            $"defaultSr={info.defaultSampleRate}");
+        device = device,
+        channelCount = Channels,
+        sampleFormat = SampleFormat.Int16,
+        suggestedLatency = PortAudio.GetDeviceInfo(device).defaultLowInputLatency,
+        hostApiSpecificStreamInfo = IntPtr.Zero,
+    };
+
+    Stream.Callback callback = (IntPtr input, IntPtr _, uint frameCount,
+        ref StreamCallbackTimeInfo _2, StreamCallbackFlags status, IntPtr _3) =>
+    {
+        if (status.HasFlag(StreamCallbackFlags.InputOverflow))
+            Console.Error.WriteLine("[status] input overflow");
+
+        lock (gate)
+        {
+            int remaining = buf.Length - writeOffset;
+            int take = (int)Math.Min(frameCount, (uint)remaining);
+            if (take > 0)
+            {
+                unsafe
+                {
+                    short* src = (short*)input.ToPointer();
+                    fixed (short* dst = &buf[writeOffset])
+                    {
+                        Buffer.MemoryCopy(src, dst, take * sizeof(short), take * sizeof(short));
+                    }
+                }
+                writeOffset += take;
+            }
+        }
+        return StreamCallbackResult.Continue;
+    };
+
+    using var stream = new Stream(
+        inParams, null, SampleRate, BlockFrames, StreamFlags.NoFlag, callback, IntPtr.Zero);
+    stream.Start();
+
+    var start = DateTime.UtcNow;
+    while (true)
+    {
+        int written;
+        lock (gate) { written = writeOffset; }
+        if (written >= buf.Length) break;
+
+        double level = Rms(buf, Math.Max(0, written - 1600), written);
+        int bars = (int)(level * 60);
+        double elapsed = (DateTime.UtcNow - start).TotalSeconds;
+        Console.Write($"\r{elapsed,4:0.0}s |{new string('#', bars).PadRight(60)}|");
+        Thread.Sleep(50);
     }
+    Console.WriteLine();
+    stream.Stop();
+
+    WavWriter.Write(OutWav, buf, SampleRate);
+    Console.WriteLine($"Wrote {OutWav} ({buf.Length} frames @ {SampleRate} Hz)");
 }
 finally
 {
     PortAudio.Terminate();
 }
+
+static int FindUsbDevice()
+{
+    int count = PortAudio.DeviceCount;
+    for (int i = 0; i < count; i++)
+    {
+        var info = PortAudio.GetDeviceInfo(i);
+        if (info.name.ToLowerInvariant().Contains("usb") && info.maxInputChannels >= 1)
+            return i;
+    }
+    Console.Error.WriteLine("USB audio device not found. Devices:");
+    for (int i = 0; i < count; i++)
+    {
+        var info = PortAudio.GetDeviceInfo(i);
+        Console.Error.WriteLine(
+            $"  [{i}] {info.name} in={info.maxInputChannels} out={info.maxOutputChannels}");
+    }
+    Environment.Exit(1);
+    return -1; // unreachable
+}
+
+static double Rms(short[] data, int from, int to)
+{
+    if (to <= from) return 0.0;
+    double sumSq = 0.0;
+    for (int i = from; i < to; i++)
+    {
+        double x = data[i] / 32768.0;
+        sumSq += x * x;
+    }
+    return Math.Sqrt(sumSq / (to - from) + 1e-12);
+}
+
+static class Libc
+{
+    [DllImport("libc", EntryPoint = "setenv")]
+    public static extern int setenv(string name, string value, int overwrite);
+}