diff --git a/src/nodes.jl b/src/nodes.jl
index 7706dd7..d6bedbd 100644
--- a/src/nodes.jl
+++ b/src/nodes.jl
@@ -17,7 +17,7 @@ function render(node::SinOsc, device_input::AudioBuf, info::DeviceInfo)
     phase = AudioSample[1:info.buf_size] * 2pi * node.freq / info.sample_rate
     phase += node.phase
     node.phase = phase[end]
-    return sin(phase)
+    return sin(phase), true
 end
 
 #### AudioMixer ####
@@ -41,9 +41,10 @@ function render(node::AudioMixer, device_input::AudioBuf, info::DeviceInfo)
     # calls
     mix_buffer = zeros(AudioSample, info.buf_size)
     for in_node in node.mix_inputs
-        mix_buffer += render(in_node, device_input, info)
+        in_buffer, active = render(in_node, device_input, info)
+        mix_buffer += in_buffer
     end
-    return mix_buffer
+    return mix_buffer, true
 end
 
 #### Array Player ####
@@ -69,7 +70,7 @@ function render(node::ArrayPlayer, device_input::AudioBuf, info::DeviceInfo)
         output = vcat(output, zeros(AudioSample, info.buf_size - length(output)))
     end
     node.arr_index = range_end + 1
-    return output
+    return output, true
 end
 
 #### AudioInput ####
@@ -82,5 +83,5 @@ end
 
 function render(node::AudioInput, device_input::AudioBuf, info::DeviceInfo)
     @assert size(device_input, 1) == info.buf_size
-    return device_input[:, node.channel]
+    return device_input[:, node.channel], true
 end
diff --git a/src/portaudio.jl b/src/portaudio.jl
index 1aae5f4..22f0c8d 100644
--- a/src/portaudio.jl
+++ b/src/portaudio.jl
@@ -88,7 +88,8 @@ function portaudio_task(jl_filedesc::Integer, stream::PortAudioStream)
     jl_stream = fdio(jl_filedesc)
     jl_rawfd = RawFD(jl_filedesc)
     while true
-        out_array = render(stream.mixer, in_array, stream.info)::AudioBuf
+        # assume the root mixer is always active
+        out_array, _ = render(stream.mixer, in_array, stream.info)::AudioBuf
         # wake the C code so it knows we've given it some more data
         wake_callback_thread(out_array)
         # wait for new data to be available from the sound card (and for it to
diff --git a/test/test_AudioIO.jl b/test/test_AudioIO.jl
index 0819508..894f232 100644
--- a/test/test_AudioIO.jl
+++ b/test/test_AudioIO.jl
@@ -18,7 +18,8 @@ end
 # to simulate the audio callback that's normally called by the device.
 function process(stream::TestAudioStream)
     in_array = zeros(AudioIO.AudioSample, stream.info.buf_size)
-    return AudioIO.render(stream.mixer, in_array, stream.info)
+    out_array, _ = AudioIO.render(stream.mixer, in_array, stream.info)
+    return out_array
 end
 
 
diff --git a/test/test_nodes.jl b/test/test_nodes.jl
index 4b0e295..d7f705f 100644
--- a/test/test_nodes.jl
+++ b/test/test_nodes.jl
@@ -11,7 +11,7 @@ end
 function AudioIO.render(node::TestNode,
                 device_input::AudioIO.AudioBuf,
                 info::AudioIO.DeviceInfo)
-    return AudioIO.AudioSample[1:info.buf_size]
+    return AudioIO.AudioSample[1:info.buf_size], true
 end
 
 #### AudioMixer Tests ####
@@ -21,18 +21,22 @@ end
 
 info("Testing AudioMixer...")
 mix = AudioMixer()
+render_output, active = AudioIO.render(mix, dev_input, test_info)
 @test mix.mix_inputs == AudioIO.AudioNode[]
-@test AudioIO.render(mix, dev_input, test_info) == zeros(AudioIO.AudioSample, test_info.buf_size)
+@test render_output == zeros(AudioIO.AudioSample, test_info.buf_size)
+@test active
 
 testnode = TestNode()
 mix = AudioMixer([testnode])
+render_output, active = AudioIO.render(mix, dev_input, test_info)
 @test mix.mix_inputs == AudioIO.AudioNode[testnode]
-@test AudioIO.render(mix, dev_input, test_info) == AudioIO.AudioSample[1:test_info.buf_size]
+@test render_output == AudioIO.AudioSample[1:test_info.buf_size]
+@test active
 
-test1 = TestNode()
-test2 = TestNode()
-mix = AudioMixer([test1, test2])
-@test AudioIO.render(mix, dev_input, test_info) == 2 * AudioIO.AudioSample[1:test_info.buf_size]
+mix = AudioMixer([TestNode(), TestNode()])
+render_output, active = AudioIO.render(mix, dev_input, test_info)
+@test render_output == 2 * AudioIO.AudioSample[1:test_info.buf_size]
+@test active
 
 info("Testing SinOSC...")
 freq = 440
@@ -41,5 +45,6 @@ t = linspace(1 / test_info.sample_rate,
              test_info.buf_size)
 test_vect = convert(AudioIO.AudioBuf, sin(2pi * t * freq))
 osc = SinOsc(freq)
-rendered = AudioIO.render(osc, dev_input, test_info)
-@test_approx_eq(rendered, test_vect)
+render_output, active = AudioIO.render(osc, dev_input, test_info)
+@test_approx_eq(render_output, test_vect)
+@test active