Beep on detection: fire-and-forget OutputStream, input keeps flowing

tests/02-wakeword-cs/Program.cs

@@ -11,6 +11,8 @@ const int Channels = 1;
 const uint BlockFrames = WakewordModel.FrameSamples; // 1280
 const float Threshold = 0.5f;
 const double CooldownSeconds = 1.0;
+const double BeepHz = 880.0;
+const double BeepSeconds = 0.2;
 
 // .NET-side mirror for any readers that go through Environment.GetEnvironmentVariable,
 // AND libc setenv so PortAudio / onnxruntime (which use getenv()) see the values.
@@ -79,6 +81,7 @@ try
     using var stream = new Stream(
         inParams, null, SampleRate, BlockFrames, StreamFlags.NoFlag, callback, IntPtr.Zero);
     stream.Start();
+    short[] beepBuffer = MakeBeep(BeepHz, BeepSeconds, SampleRate);
     Console.WriteLine("Listening. Say 'alexa'. Ctrl-C to exit.");
 
     var sw = Stopwatch.StartNew();
@@ -95,7 +98,7 @@ try
             {
                 Console.WriteLine($"DETECTED alexa score={score:0.000} t={now.TotalSeconds:0.0}s");
                 lastTrigger = now;
-                // Beep is added in Task 5.
+                FireAndForgetBeep(device, beepBuffer);
             }
         }
     }
@@ -130,6 +133,71 @@ static int FindUsbDevice()
     return -1; // unreachable
 }
 
+static short[] MakeBeep(double freqHz, double durationS, int sampleRate)
+{
+    int n = (int)(sampleRate * durationS);
+    var buf = new short[n];
+    for (int i = 0; i < n; i++)
+    {
+        double t = i / (double)sampleRate;
+        double v = 0.3 * Math.Sin(2.0 * Math.PI * freqHz * t);
+        buf[i] = (short)(v * short.MaxValue);
+    }
+    return buf;
+}
+
+static void FireAndForgetBeep(int device, short[] beepBuffer)
+{
+    int offset = 0;
+    var done = new ManualResetEventSlim(false);
+
+    var outParams = new StreamParameters
+    {
+        device = device,
+        channelCount = 1,
+        sampleFormat = SampleFormat.Int16,
+        suggestedLatency = PortAudio.GetDeviceInfo(device).defaultLowOutputLatency,
+        hostApiSpecificStreamInfo = IntPtr.Zero,
+    };
+
+    Stream.Callback playCb = (IntPtr _, IntPtr output, uint frameCount,
+        ref StreamCallbackTimeInfo _2, StreamCallbackFlags _3, IntPtr _4) =>
+    {
+        int remaining = beepBuffer.Length - offset;
+        int take = (int)Math.Min(frameCount, (uint)remaining);
+        int silence = (int)frameCount - take;
+        unsafe
+        {
+            short* dst = (short*)output.ToPointer();
+            if (take > 0)
+            {
+                fixed (short* src = &beepBuffer[offset])
+                    Buffer.MemoryCopy(src, dst, take * sizeof(short), take * sizeof(short));
+                offset += take;
+            }
+            for (int i = take; i < frameCount; i++) dst[i] = 0;
+        }
+        if (offset >= beepBuffer.Length)
+        {
+            done.Set();
+            return StreamCallbackResult.Complete;
+        }
+        return StreamCallbackResult.Continue;
+    };
+
+    var stream = new Stream(
+        null, outParams, WakewordModel.SampleRate, 1024, StreamFlags.NoFlag, playCb, IntPtr.Zero);
+    Task.Run(() =>
+    {
+        done.Wait();
+        Thread.Sleep(200);
+        stream.Stop();
+        stream.Dispose();
+        done.Dispose();
+    });
+    stream.Start();
+}
+
 static class Libc
 {
     [DllImport("libc", EntryPoint = "setenv")]