diff --git a/doc/guide/signals.rst b/doc/guide/signals.rst
index c024513..4d6fa81 100644
--- a/doc/guide/signals.rst
+++ b/doc/guide/signals.rst
@@ -2,8 +2,8 @@
 Signals
 =======
 
-Signal Types
-------------
+Signal primitives
+-----------------
 
 MuzGen use different kind of signals for sound design.
 
@@ -49,3 +49,50 @@ The noise command generate uniform random frames.
 .. code-block::
 
    [noise]
+
+
+Signal operations
+-----------------
+
+The signal operation commands allows us to combine signals in
+different ways.
+
+Add
+^^^
+
+The ``sum`` command takes two signals as input and outputs their sum.
+
+.. code-block::
+
+   [sum [sine 220] [sine 220]]
+
+
+Sub
+^^^
+
+The ``sub`` command takes two signals as input and outputs their
+difference.
+
+.. code-block::
+
+   [sub [sine 460] [sine 80]]
+
+
+Mul
+^^^
+
+The ``mul`` command takes two signals as input and output their product.
+
+.. code-block::
+
+   [mul 0.7 [sine 220]]
+
+
+Div
+^^^
+
+The ``div`` command takes two signals as input and output their ratio.
+
+.. code-block::
+
+   [div [sine 220] 2]
diff --git a/lib/Add.cpp b/lib/Add.cpp
new file mode 100644
index 0000000..6114d26
--- /dev/null
+++ b/lib/Add.cpp
@@ -0,0 +1,33 @@
+#include "Add.hpp"
+
+namespace muz
+{
+  /*explicit*/ Add::Add(AudioConf const& conf,
+                        std::unique_ptr<Signal> lhs,
+                        std::unique_ptr<Signal> rhs)
+    : m_conf { conf }
+    , m_out_left { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_out_right { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_lhs { std::move(lhs) }
+    , m_rhs { std::move(rhs) }
+  {
+  }
+
+  /*virtual*/ Add::~Add()
+  {
+  }
+
+  void Add::next(std::vector<float>& out) /*override*/
+  {
+    m_lhs->next(m_out_left);
+    m_rhs->next(m_out_right);
+
+    assert(m_out_left.size() == m_out_right.size());
+    assert(m_out_left.size() == static_cast<size_t>(m_conf.channels()));
+
+    for (int i=0; i<m_conf.channels(); i++)
+      {
+        out[i] = m_out_left[i] + m_out_right[i];
+      }
+  }
+}
diff --git a/lib/Add.hpp b/lib/Add.hpp
new file mode 100644
index 0000000..29dabee
--- /dev/null
+++ b/lib/Add.hpp
@@ -0,0 +1,32 @@
+#ifndef muz_ADD_HPP
+#define muz_ADD_HPP
+
+#include "commons.hpp"
+#include "Signal.hpp"
+#include "AudioConf.hpp"
+
+namespace muz
+{
+  /**
+   * Sum of two input signals.
+   **/
+  class Add: public Signal
+  {
+  public:
+    explicit Add(AudioConf const& conf,
+                 std::unique_ptr<Signal> lhs,
+                 std::unique_ptr<Signal> rhs);
+
+    virtual ~Add();
+
+    void next(std::vector<float>& out) override;
+  private:
+    AudioConf m_conf;
+    std::vector<float> m_out_left;
+    std::vector<float> m_out_right;
+    std::unique_ptr<Signal> m_lhs;
+    std::unique_ptr<Signal> m_rhs;
+  };
+}
+
+#endif
diff --git a/lib/AudioEngine.cpp b/lib/AudioEngine.cpp
index 55bd698..bf93f00 100644
--- a/lib/AudioEngine.cpp
+++ b/lib/AudioEngine.cpp
@@ -5,6 +5,7 @@ namespace muz
 {
   /*explicit*/ AudioEngine::AudioEngine(AudioConf const& conf)
     : m_conf { conf }
+    , m_frame { std::vector<float>(m_conf.channels(), 0.0f) }
   {
     check_error(Pa_Initialize());
   }
@@ -80,15 +81,9 @@ namespace muz
         return {0.0f, 0.0f};
       }
 
-    auto frame = m_sig_queue.back()->next();
+    m_sig_queue.back()->next(m_frame);
 
-    if (frame.empty())
-      {
-        pop_signal();
-        return next();
-      }
-
-    return frame;
+    return m_frame;
   }
 
   /*static*/ int AudioEngine::callback(void const*, void* output,
@@ -103,7 +98,7 @@ namespace muz
 
     for (size_t i=0; i<frames_per_buffer; i++)
       {
-        auto frame = engine->next();
+        auto const& frame = engine->next();
 
         for (float val: frame)
           {
diff --git a/lib/AudioEngine.hpp b/lib/AudioEngine.hpp
index 2f79c17..cc8fa2f 100644
--- a/lib/AudioEngine.hpp
+++ b/lib/AudioEngine.hpp
@@ -47,6 +47,7 @@ namespace muz
     PaStream* m_stream = nullptr;
     std::vector<std::unique_ptr<Signal>> m_sig_queue;
     std::mutex m_sig_mtx;
+    std::vector<float> m_frame;
 
     /**
      * Throws an audio_error exception if err is an error.
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index a5a7da0..e49a811 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -18,6 +18,10 @@ add_library(muz-lib OBJECT
   Constant.cpp
   Sine.cpp
   Noise.cpp
+  Add.cpp
+  Sub.cpp
+  Mul.cpp
+  Div.cpp
 
   # Language
   # ========
diff --git a/lib/Compiler.cpp b/lib/Compiler.cpp
index f1ce4ea..165ff72 100644
--- a/lib/Compiler.cpp
+++ b/lib/Compiler.cpp
@@ -5,6 +5,10 @@
 #include "Constant.hpp"
 #include "Sine.hpp"
 #include "Noise.hpp"
+#include "Add.hpp"
+#include "Sub.hpp"
+#include "Mul.hpp"
+#include "Div.hpp"
 
 namespace muz
 {
@@ -56,10 +60,10 @@ namespace muz
           }
         else
           {
-            throw compile_error {
-              std::string()
-              + "cannot compile unknown directive '" + name + "'."
-            };
+            format_error<compile_error>(node->line(),
+                                        "cannot compile unknown directive '"
+                                        + name
+                                        + "'.");
           }
 
       } break;
@@ -92,12 +96,60 @@ namespace muz
         auto signal = std::make_unique<Noise>(m_conf);
         push(std::move(signal));
       }
+    else if (name == "add")
+      {
+        check_cmd_arity(*node, 2);
+
+        auto rhs = pop();
+        auto lhs = pop();
+
+        auto signal = std::make_unique<Add>(m_conf,
+                                            std::move(lhs),
+                                            std::move(rhs));
+        push(std::move(signal));
+      }
+    else if (name == "sub")
+      {
+        check_cmd_arity(*node, 2);
+
+        auto rhs = pop();
+        auto lhs = pop();
+
+        auto signal = std::make_unique<Sub>(m_conf,
+                                            std::move(lhs),
+                                            std::move(rhs));
+        push(std::move(signal));
+      }
+    else if (name == "mul")
+      {
+        check_cmd_arity(*node, 2);
+
+        auto rhs = pop();
+        auto lhs = pop();
+
+        auto signal = std::make_unique<Mul>(m_conf,
+                                            std::move(lhs),
+                                            std::move(rhs));
+        push(std::move(signal));
+      }
+    else if (name == "div")
+      {
+        check_cmd_arity(*node, 2);
+
+        auto rhs = pop();
+        auto lhs = pop();
+
+        auto signal = std::make_unique<Div>(m_conf,
+                                            std::move(lhs),
+                                            std::move(rhs));
+        push(std::move(signal));
+      }
     else
       {
-        throw compile_error {
-          std::string()
-          + "cannot compile unknown command '" + name + "'."
-        };
+        format_error<compile_error>(node->line(),
+                                    "cannot compile unknown command '"
+                                    + name
+                                    + "'.");
       }
   }
 
@@ -117,15 +169,15 @@ namespace muz
   {
     if (node.size() - 1 != static_cast<size_t>(arity))
       {
-        throw compile_error {
-          std::string()
-          + "arity mismatch for '"
-          + node.child(0)->value()
-          + "': expected <"
-          + std::to_string(arity)
-          + ">, got <"
-          + std::to_string(node.size() - 1)
-          + ">."};
+        format_error<compile_error>(node.line(),
+                                    "arity mismatch for '"
+                                    + node.child(0)->value()
+                                    + "': expected <"
+                                    + std::to_string(arity)
+                                    + ">, got <"
+                                    + std::to_string(node.size() - 1)
+                                    + ">.");
+
       }
   }
 }
diff --git a/lib/Constant.cpp b/lib/Constant.cpp
index dd5b52f..fb3920c 100644
--- a/lib/Constant.cpp
+++ b/lib/Constant.cpp
@@ -12,15 +12,11 @@ namespace muz
   {
   }
 
-  std::vector<float> Constant::next() /*override*/
+  void Constant::next(std::vector<float>& out) /*override*/
   {
-    std::vector<float> out;
-
     for (int i=0; i<m_conf.channels(); i++)
       {
-        out.push_back(m_value);
+        out[i] = m_value;
       }
-
-    return out;
   }
 }
diff --git a/lib/Constant.hpp b/lib/Constant.hpp
index 8413826..90d5d48 100644
--- a/lib/Constant.hpp
+++ b/lib/Constant.hpp
@@ -15,10 +15,11 @@ namespace muz
     explicit Constant(AudioConf const& conf, float value=0.0f);
     virtual ~Constant();
 
-    std::vector<float> next() override;
+    void next(std::vector<float>& out) override;
   private:
     AudioConf m_conf;
     float m_value;
+
   };
 }
 
diff --git a/lib/Div.cpp b/lib/Div.cpp
new file mode 100644
index 0000000..6692e37
--- /dev/null
+++ b/lib/Div.cpp
@@ -0,0 +1,33 @@
+#include "Div.hpp"
+
+namespace muz
+{
+  /*explicit*/ Div::Div(AudioConf const& conf,
+                        std::unique_ptr<Signal> lhs,
+                        std::unique_ptr<Signal> rhs)
+    : m_conf { conf }
+    , m_out_left { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_out_right { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_lhs { std::move(lhs) }
+    , m_rhs { std::move(rhs) }
+  {
+  }
+
+  /*virtual*/ Div::~Div()
+  {
+  }
+
+  void Div::next(std::vector<float>& out) /*override*/
+  {
+    m_lhs->next(m_out_left);
+    m_rhs->next(m_out_right);
+
+    assert(m_out_left.size() == m_out_right.size());
+    assert(m_out_left.size() == static_cast<size_t>(m_conf.channels()));
+
+    for (int i=0; i<m_conf.channels(); i++)
+      {
+        out[i] = m_out_left[i] / m_out_right[i];
+      }
+  }
+}
diff --git a/lib/Div.hpp b/lib/Div.hpp
new file mode 100644
index 0000000..e5fe5f3
--- /dev/null
+++ b/lib/Div.hpp
@@ -0,0 +1,33 @@
+#ifndef muz_DIV_HPP
+#define muz_DIV_HPP
+
+#include "commons.hpp"
+#include "Signal.hpp"
+#include "AudioConf.hpp"
+
+namespace muz
+{
+  /**
+   * Division of two input signals.
+   **/
+  class Div: public Signal
+  {
+  public:
+    explicit Div(AudioConf const& conf,
+                 std::unique_ptr<Signal> lhs,
+                 std::unique_ptr<Signal> rhs);
+    virtual ~Div();
+
+    void next(std::vector<float>& out) override;
+
+  private:
+    AudioConf m_conf;
+    std::vector<float> m_out_left;
+    std::vector<float> m_out_right;
+
+    std::unique_ptr<Signal> m_lhs;
+    std::unique_ptr<Signal> m_rhs;
+  };
+}
+
+#endif
diff --git a/lib/Mul.cpp b/lib/Mul.cpp
new file mode 100644
index 0000000..3a88634
--- /dev/null
+++ b/lib/Mul.cpp
@@ -0,0 +1,33 @@
+#include "Mul.hpp"
+
+namespace muz
+{
+  /*explicit*/ Mul::Mul(AudioConf const& conf,
+                        std::unique_ptr<Signal> lhs,
+                        std::unique_ptr<Signal> rhs)
+    : m_conf { conf }
+    , m_out_left { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_out_right { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_lhs { std::move(lhs) }
+    , m_rhs { std::move(rhs) }
+  {
+  }
+
+  /*virtual*/ Mul::~Mul()
+  {
+  }
+
+  void Mul::next(std::vector<float>& out) /*override*/
+  {
+    m_lhs->next(m_out_left);
+    m_rhs->next(m_out_right);
+
+    assert(m_out_left.size() == m_out_right.size());
+    assert(m_out_left.size() == static_cast<size_t>(m_conf.channels()));
+
+    for (int i=0; i<m_conf.channels(); i++)
+      {
+        out[i] = m_out_left[i] * m_out_right[i];
+      }
+  }
+}
diff --git a/lib/Mul.hpp b/lib/Mul.hpp
new file mode 100644
index 0000000..dadf6b2
--- /dev/null
+++ b/lib/Mul.hpp
@@ -0,0 +1,32 @@
+#ifndef muz_MUL_HPP
+#define muz_MUL_HPP
+
+#include "commons.hpp"
+#include "Signal.hpp"
+#include "AudioConf.hpp"
+
+namespace muz
+{
+  /**
+   * Product of two input signals.
+   **/
+  class Mul: public Signal
+  {
+  public:
+    explicit Mul(AudioConf const& conf,
+                 std::unique_ptr<Signal> lhs,
+                 std::unique_ptr<Signal> rhs);
+    virtual ~Mul();
+
+    void next(std::vector<float>& out) override;
+
+  private:
+    AudioConf m_conf;
+    std::vector<float> m_out_left;
+    std::vector<float> m_out_right;
+    std::unique_ptr<Signal> m_lhs;
+    std::unique_ptr<Signal> m_rhs;
+  };
+}
+
+#endif
diff --git a/lib/Noise.cpp b/lib/Noise.cpp
index 5a31675..c8583fd 100644
--- a/lib/Noise.cpp
+++ b/lib/Noise.cpp
@@ -12,17 +12,13 @@ namespace muz
   {
   }
 
-  std::vector<float> Noise::next() /*override*/
+  void Noise::next(std::vector<float>& out) /*override*/
   {
-    std::vector<float> out;
-
     float value = m_distribution(m_rand);
 
     for (int i=0; i<m_conf.channels(); i++)
       {
-        out.push_back(value);
+        out[i] = value;
       }
-
-    return out;
   }
 }
diff --git a/lib/Noise.hpp b/lib/Noise.hpp
index 4a7e24e..3e6906f 100644
--- a/lib/Noise.hpp
+++ b/lib/Noise.hpp
@@ -10,13 +10,17 @@
 
 namespace muz
 {
+  /**
+   * White noise signal.
+   * @see Signal
+   **/
   class Noise: public Signal
   {
   public:
     explicit Noise(AudioConf const& conf);
     virtual ~Noise();
 
-    std::vector<float> next() override;
+    void next(std::vector<float>& out) override;
   private:
     AudioConf m_conf;
     std::mt19937 m_rand;
diff --git a/lib/Signal.hpp b/lib/Signal.hpp
index 7832658..52842e4 100644
--- a/lib/Signal.hpp
+++ b/lib/Signal.hpp
@@ -19,11 +19,10 @@ namespace muz
     virtual ~Signal();
 
     /**
-     * Get the next sample.
-     * @return std::vector<float> of size N for a sample of N channels or an empty vector at end.
-     *
+     * Get the next frame.
+     * @param out the output buffer that will contain the next frame.
      **/
-    virtual std::vector<float> next() = 0;
+    virtual void next(std::vector<float>& out) = 0;
   private:
   };
 }
diff --git a/lib/Sine.cpp b/lib/Sine.cpp
index 9a89b5e..31e8a62 100644
--- a/lib/Sine.cpp
+++ b/lib/Sine.cpp
@@ -6,6 +6,8 @@ namespace muz
                           std::unique_ptr<Signal> freq,
                           std::unique_ptr<Signal> amplitude)
     : m_conf { conf }
+    , m_out_freq {std::vector<float> (m_conf.channels(), 0.0f)}
+    , m_out_amp {std::vector<float> (m_conf.channels(), 0.0f)}
     , m_freq { std::move(freq) }
     , m_amplitude { std::move(amplitude) }
   {
@@ -19,31 +21,28 @@ namespace muz
   {
   }
 
-  std::vector<float> Sine::next() /*override*/
+  void Sine::next(std::vector<float>& out) /*override*/
   {
     assert(m_freq);
     assert(m_amplitude);
 
-    std::vector<float> out;
-    auto freqs = m_freq->next();
-    auto amps = m_amplitude->next();
+    m_freq->next(m_out_freq);
+    m_amplitude->next(m_out_amp);
 
-    if (freqs.size() != amps.size()
-        || freqs.size() != m_phases.size())
+    if (m_out_freq.size() != m_out_amp.size()
+        || m_out_freq.size() != m_phases.size())
       {
         throw signal_error {"cannot generate sine: channel number mismatch"};
       }
 
     for (size_t i=0; i<static_cast<size_t>(m_conf.channels()); i++)
       {
-        float const value = amps[i] * std::sin(m_phases[i]);
+        float const value = m_out_amp[i] * std::sin(m_phases[i]);
 
-        m_phases[i] += 2 * M_PI * freqs[i]
+        m_phases[i] += 2 * M_PI * m_out_freq[i]
           / static_cast<float>(m_conf.samplerate());
 
-        out.push_back(value);
+        out[i] = value;
       }
-
-    return out;
   }
 }
diff --git a/lib/Sine.hpp b/lib/Sine.hpp
index ae71f13..9416076 100644
--- a/lib/Sine.hpp
+++ b/lib/Sine.hpp
@@ -18,9 +18,12 @@ namespace muz
                   std::unique_ptr<Signal> amplitude);
     virtual ~Sine();
 
-    std::vector<float> next() override;
+    void next(std::vector<float>& out) override;
   private:
     AudioConf const& m_conf;
+    std::vector<float> m_out_freq;
+    std::vector<float> m_out_amp;
+
     std::unique_ptr<Signal> m_freq;
     std::unique_ptr<Signal> m_amplitude;
     std::vector<float> m_phases;
diff --git a/lib/Sub.cpp b/lib/Sub.cpp
new file mode 100644
index 0000000..4c0113a
--- /dev/null
+++ b/lib/Sub.cpp
@@ -0,0 +1,33 @@
+#include "Sub.hpp"
+
+namespace muz
+{
+  /*explicit*/ Sub::Sub(AudioConf const& conf,
+                        std::unique_ptr<Signal> lhs,
+                        std::unique_ptr<Signal> rhs)
+    : m_conf { conf }
+    , m_out_left { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_out_right { std::vector<float>(m_conf.channels(), 0.0f) }
+    , m_lhs { std::move(lhs) }
+    , m_rhs { std::move(rhs) }
+  {
+  }
+
+  /*virtual*/ Sub::~Sub()
+  {
+  }
+
+  void Sub::next(std::vector<float>& out) /*override*/
+  {
+    m_lhs->next(m_out_left);
+    m_rhs->next(m_out_right);
+
+    assert(m_out_left.size() == m_out_right.size());
+    assert(m_out_left.size() == static_cast<size_t>(m_conf.channels()));
+
+    for (int i=0; i<m_conf.channels(); i++)
+      {
+        out[i] = m_out_left[i] - m_out_right[i];
+      }
+  }
+}
diff --git a/lib/Sub.hpp b/lib/Sub.hpp
new file mode 100644
index 0000000..87a9927
--- /dev/null
+++ b/lib/Sub.hpp
@@ -0,0 +1,32 @@
+#ifndef muz_SUB_HPP
+#define muz_SUB_HPP
+
+#include "commons.hpp"
+#include "Signal.hpp"
+#include "AudioConf.hpp"
+
+namespace muz
+{
+  /**
+   * Substraction of two input signals.
+   **/
+  class Sub: public Signal
+  {
+  public:
+    explicit Sub(AudioConf const& conf,
+                 std::unique_ptr<Signal> lhs,
+                 std::unique_ptr<Signal> rhs);
+    virtual ~Sub();
+
+    void next(std::vector<float>& out) override;
+
+  private:
+    AudioConf m_conf;
+    std::vector<float> m_out_left;
+    std::vector<float> m_out_right;
+    std::unique_ptr<Signal> m_lhs;
+    std::unique_ptr<Signal> m_rhs;
+  };
+}
+
+#endif