diff --git a/src/sfizz.h b/src/sfizz.h
index 42585a9d..69faa3ed 100644
--- a/src/sfizz.h
+++ b/src/sfizz.h
@@ -376,6 +376,24 @@ SFIZZ_EXPORTED_API void sfizz_set_logging_prefix(sfizz_synth_t* synth, const cha
  */
 SFIZZ_EXPORTED_API void sfizz_all_sound_off(sfizz_synth_t* synth);
 
+/**
+ * @brief Add external definitions prior to loading;
+ * Note that these do not get reset by loading or resetting the synth.
+ * You need to call sfizz_clear_external_definitions() to erase them.
+ *
+ * @param synth
+ * @param id
+ * @param value
+ */
+SFIZZ_EXPORTED_API void sfizz_add_external_definitions(sfizz_synth_t* synth, const char* id, const char* value);
+
+/**
+ * @brief Clears external definitions for the next file loading.
+ *
+ * @param synth
+ */
+SFIZZ_EXPORTED_API void sfizz_clear_external_definitions(sfizz_synth_t* synth);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/sfizz.hpp b/src/sfizz.hpp
index b2af98c1..49e9cd7a 100644
--- a/src/sfizz.hpp
+++ b/src/sfizz.hpp
@@ -296,6 +296,22 @@ public:
      */
     void allSoundOff() noexcept;
 
+    /**
+     * @brief Add external definitions prior to loading;
+     * Note that these do not get reset by loading or resetting the synth.
+     * You need to call clearExternalDefintions() to erase them.
+     *
+     * @param id
+     * @param value
+     */
+    void addExternalDefinition(const std::string& id, const std::string& value);
+
+    /**
+     * @brief Clears external definitions for the next file loading.
+     *
+     */
+    void clearExternalDefinitions();
+
 private:
     std::unique_ptr<sfz::Synth> synth;
 };
diff --git a/src/sfizz/Config.h b/src/sfizz/Config.h
index a71ad3db..727a125f 100644
--- a/src/sfizz/Config.h
+++ b/src/sfizz/Config.h
@@ -60,6 +60,7 @@ namespace config {
     constexpr int filtersInPool { maxVoices * 2 };
     constexpr int filtersPerVoice { 2 };
     constexpr int eqsPerVoice { 3 };
+    constexpr int oscillatorsPerVoice { 9 };
     constexpr float noiseVariance { 0.25f };
     /**
        Minimum interval in frames between recomputations of coefficients of the
diff --git a/src/sfizz/Defaults.h b/src/sfizz/Defaults.h
index 0e35ca49..158da597 100644
--- a/src/sfizz/Defaults.h
+++ b/src/sfizz/Defaults.h
@@ -63,6 +63,10 @@ namespace Default
     // Wavetable oscillator
     constexpr float oscillatorPhase { 0.0 };
     constexpr Range<float> oscillatorPhaseRange { -1.0, 360.0 };
+    constexpr int oscillatorMulti { 1 };
+    constexpr Range<int> oscillatorMultiRange { 1, config::oscillatorsPerVoice };
+    constexpr float oscillatorDetune { 0 };
+    constexpr Range<float> oscillatorDetuneRange { -9600, 9600 };
 
     // Instrument setting: voice lifecycle
 	constexpr uint32_t group { 0 };
diff --git a/src/sfizz/Range.h b/src/sfizz/Range.h
index b39b7c0d..9f339565 100644
--- a/src/sfizz/Range.h
+++ b/src/sfizz/Range.h
@@ -28,8 +28,8 @@ public:
     {
 
     }
-    Type getStart() const noexcept { return _start; }
-    Type getEnd() const noexcept { return _end; }
+    constexpr Type getStart() const noexcept { return _start; }
+    constexpr Type getEnd() const noexcept { return _end; }
     /**
      * @brief Get the range as an std::pair of the endpoints
      *
diff --git a/src/sfizz/Region.cpp b/src/sfizz/Region.cpp
index 76806b48..234878e9 100644
--- a/src/sfizz/Region.cpp
+++ b/src/sfizz/Region.cpp
@@ -102,6 +102,12 @@ bool sfz::Region::parseOpcode(const Opcode& opcode)
         if (auto value = readBooleanFromOpcode(opcode))
             oscillator = *value;
         break;
+    case hash("oscillator_multi"):
+        setValueFromOpcode(opcode, oscillatorMulti, Default::oscillatorMultiRange);
+        break;
+    case hash("oscillator_detune"):
+        setValueFromOpcode(opcode, oscillatorDetune, Default::oscillatorDetuneRange);
+        break;
 
     // Instrument settings: voice lifecycle
     case hash("group"): // fallthrough
diff --git a/src/sfizz/Region.h b/src/sfizz/Region.h
index a7a4e710..73f3bc53 100644
--- a/src/sfizz/Region.h
+++ b/src/sfizz/Region.h
@@ -60,6 +60,13 @@ struct Region {
      * @return false
      */
     bool isGenerator() const noexcept { return sample.size() > 0 ? sample[0] == '*' : false; }
+    /**
+     * @brief Is stereo (has stereo sample or is unison oscillator)?
+     *
+     * @return true
+     * @return false
+     */
+    bool isStereo() const noexcept { return hasStereoSample || ((oscillator || isGenerator()) && oscillatorMulti >= 3); }
     /**
      * @brief Is a looping region (at least potentially)?
      *
@@ -235,6 +242,8 @@ struct Region {
     // Wavetable oscillator
     float oscillatorPhase { Default::oscillatorPhase };
     bool oscillator = false;
+    int oscillatorMulti = Default::oscillatorMulti;
+    float oscillatorDetune = Default::oscillatorDetune;
 
     // Instrument settings: voice lifecycle
     uint32_t group { Default::group }; // group
@@ -318,7 +327,7 @@ struct Region {
     EGDescription pitchEG;
     EGDescription filterEG;
 
-    bool isStereo { false };
+    bool hasStereoSample { false };
 
     // Effects
     std::vector<float> gainToEffect;
diff --git a/src/sfizz/Synth.cpp b/src/sfizz/Synth.cpp
index 25393b4d..ec102de3 100644
--- a/src/sfizz/Synth.cpp
+++ b/src/sfizz/Synth.cpp
@@ -379,7 +379,7 @@ bool sfz::Synth::loadSfzFile(const fs::path& file)
             }
 
             if (fileInformation->numChannels == 2)
-                region->isStereo = true;
+                region->hasStereoSample = true;
 
             // TODO: adjust with LFO targets
             const auto maxOffset = region->offset + region->offsetRandom;
@@ -785,7 +785,7 @@ void sfz::Synth::pitchWheel(int delay, int pitch) noexcept
 {
     ASSERT(pitch <= 8192);
     ASSERT(pitch >= -8192);
-    const auto normalizedPitch = normalizeBend(pitch);
+    const auto normalizedPitch = normalizeBend(float(pitch));
 
     ScopedTiming logger { dispatchDuration, ScopedTiming::Operation::addToDuration };
     resources.midiState.pitchBendEvent(delay, normalizedPitch);
diff --git a/src/sfizz/Voice.cpp b/src/sfizz/Voice.cpp
index 4aa4e0bd..251a1167 100644
--- a/src/sfizz/Voice.cpp
+++ b/src/sfizz/Voice.cpp
@@ -21,7 +21,8 @@ sfz::Voice::Voice(sfz::Resources& resources)
     filters.reserve(config::filtersPerVoice);
     equalizers.reserve(config::eqsPerVoice);
 
-    waveOscillator.init(sampleRate);
+    for (WavetableOscillator& osc : waveOscillators)
+        osc.init(sampleRate);
 }
 
 void sfz::Voice::startVoice(Region* region, int delay, int number, float value, sfz::Voice::TriggerType triggerType) noexcept
@@ -59,12 +60,18 @@ void sfz::Voice::startVoice(Region* region, int delay, int number, float value,
             wave = resources.wavePool.getWaveSaw();
             break;
         }
-        waveOscillator.setWavetable(wave);
-        waveOscillator.setPhase(region->getPhase());
+        for (WavetableOscillator& osc : waveOscillators) {
+            osc.setWavetable(wave);
+            osc.setPhase(region->getPhase());
+        }
+        setupOscillatorUnison();
     } else if (region->oscillator) {
         const WavetableMulti* wave = resources.wavePool.getFileWave(region->sample);
-        waveOscillator.setWavetable(wave);
-        waveOscillator.setPhase(region->getPhase());
+        for (WavetableOscillator& osc : waveOscillators) {
+            osc.setWavetable(wave);
+            osc.setPhase(region->getPhase());
+        }
+        setupOscillatorUnison();
     } else {
         currentPromise = resources.filePool.getFilePromise(region->sample);
         if (currentPromise == nullptr) {
@@ -83,7 +90,7 @@ void sfz::Voice::startVoice(Region* region, int delay, int number, float value,
     ASSERT((filters.capacity() - filters.size()) >= region->filters.size());
     ASSERT((equalizers.capacity() - equalizers.size()) >= region->equalizers.size());
 
-    const unsigned numChannels = region->isStereo ? 2 : 1;
+    const unsigned numChannels = region->isStereo() ? 2 : 1;
     for (auto& filter: region->filters) {
         auto newFilter = resources.filterPool.getFilter(filter, numChannels, number, value);
         if (newFilter)
@@ -187,7 +194,8 @@ void sfz::Voice::setSampleRate(float sampleRate) noexcept
 {
     this->sampleRate = sampleRate;
 
-    waveOscillator.init(sampleRate);
+    for (WavetableOscillator& osc : waveOscillators)
+        osc.init(sampleRate);
 }
 
 void sfz::Voice::setSamplesPerBlock(int samplesPerBlock) noexcept
@@ -218,7 +226,7 @@ void sfz::Voice::renderBlock(AudioSpan<float> buffer) noexcept
             fillWithData(delayed_buffer);
     }
 
-    if (region->isStereo) {
+    if (region->isStereo()) {
         ampStageStereo(buffer);
         panStageStereo(buffer);
         filterStageStereo(buffer);
@@ -518,9 +526,10 @@ void sfz::Voice::fillWithGenerator(AudioSpan<float> buffer) noexcept
         absl::c_generate(leftSpan, [&](){ return noiseDist(Random::randomGenerator); });
         absl::c_generate(rightSpan, [&](){ return noiseDist(Random::randomGenerator); });
     } else {
-        const auto numSamples = buffer.getNumFrames();
-        auto frequencies = resources.bufferPool.getBuffer(numSamples);
-        auto bends = resources.bufferPool.getBuffer(numSamples);
+        const auto numFrames = buffer.getNumFrames();
+
+        auto frequencies = resources.bufferPool.getBuffer(numFrames);
+        auto bends = resources.bufferPool.getBuffer(numFrames);
         if (!frequencies || !bends)
             return;
 
@@ -544,8 +553,25 @@ void sfz::Voice::fillWithGenerator(AudioSpan<float> buffer) noexcept
             applyGain<float>(*bends, *frequencies);
         }
 
-        waveOscillator.processModulated(frequencies->data(), leftSpan.data(), buffer.getNumFrames());
-        copy<float>(leftSpan, rightSpan);
+        if (waveUnisonSize == 1) {
+            WavetableOscillator& osc = waveOscillators[0];
+            osc.processModulated(frequencies->data(), 1.0, leftSpan.data(), buffer.getNumFrames());
+            copy<float>(leftSpan, rightSpan);
+        }
+        else {
+            buffer.fill(0.0f);
+
+            auto tempSpan = resources.bufferPool.getBuffer(numFrames);
+            if (!tempSpan)
+                return;
+
+            for (unsigned i = 0, n = waveUnisonSize; i < n; ++i) {
+                WavetableOscillator& osc = waveOscillators[i];
+                osc.processModulated(frequencies->data(), waveDetuneRatio[i], tempSpan->data(), numFrames);
+                sfz::multiplyAdd<float>(waveLeftGain[i], *tempSpan, leftSpan);
+                sfz::multiplyAdd<float>(waveRightGain[i], *tempSpan, rightSpan);
+            }
+        }
     }
 }
 
@@ -620,3 +646,60 @@ void sfz::Voice::setMaxEQsPerVoice(size_t numFilters)
     ASSERT(equalizers.size() == 0);
     equalizers.reserve(numFilters);
 }
+
+void sfz::Voice::setupOscillatorUnison()
+{
+    int m = region->oscillatorMulti;
+    float d = region->oscillatorDetune;
+
+    // 3-9: unison mode, 1: normal/RM, 2: PM/FM
+    // TODO(jpc) RM/FM/PM synthesis
+    if (m < 3) {
+        waveUnisonSize = 1;
+        waveDetuneRatio[0] = 1.0;
+        waveLeftGain[0] = 1.0;
+        waveRightGain[0] = 1.0;
+        return;
+    }
+
+    // oscillator count, aka. unison size
+    waveUnisonSize = m;
+
+    // detune (cents)
+    float detunes[config::oscillatorsPerVoice];
+    detunes[0] = 0.0;
+    detunes[1] = -d;
+    detunes[2] = +d;
+    for (int i = 3; i < m; ++i) {
+        int n = (i - 1) / 2;
+        detunes[i] = d * ((i & 1) ? -0.25f : +0.25f) * float(n);
+    }
+
+    // detune (ratio)
+    for (int i = 0; i < m; ++i)
+        waveDetuneRatio[i] = std::exp2(detunes[i] * (0.01f / 12.0f));
+
+    // gains
+    waveLeftGain[0] = 0.0;
+    waveRightGain[m - 1] = 0.0;
+    for (int i = 0; i < m - 1; ++i) {
+        float g = 1.0f - float(i) / float(m - 1);
+        waveLeftGain[m - 1 - i] = g;
+        waveRightGain[i] = g;
+    }
+
+#if 0
+    fprintf(stderr, "\n");
+    fprintf(stderr, "# Left:\n");
+    for (int i = m - 1; i >= 0; --i) {
+        if (waveLeftGain[i] != 0)
+            fprintf(stderr, "[%d] %10g cents, %10g dB\n", i, detunes[i], 20.0f * std::log10(waveLeftGain[i]));
+    }
+    fprintf(stderr, "\n");
+    fprintf(stderr, "# Right:\n");
+    for (int i = 0; i < m; ++i) {
+        if (waveRightGain[i] != 0)
+            fprintf(stderr, "[%d] %10g cents, %10g dB\n", i, detunes[i], 20.0f * std::log10(waveRightGain[i]));
+    }
+#endif
+}
diff --git a/src/sfizz/Voice.h b/src/sfizz/Voice.h
index f0a70af9..1723c416 100644
--- a/src/sfizz/Voice.h
+++ b/src/sfizz/Voice.h
@@ -244,6 +244,10 @@ private:
     void panStageStereo(AudioSpan<float> buffer) noexcept;
     void filterStageMono(AudioSpan<float> buffer) noexcept;
     void filterStageStereo(AudioSpan<float> buffer) noexcept;
+    /**
+     * @brief Initialize frequency and gain coefficients for the oscillators.
+     */
+    void setupOscillatorUnison();
 
     Region* region { nullptr };
 
@@ -284,7 +288,13 @@ private:
     ADSREnvelope<float> egEnvelope;
     float bendStepFactor { centsFactor(1) };
 
-    WavetableOscillator waveOscillator;
+    WavetableOscillator waveOscillators[config::oscillatorsPerVoice];
+
+    // unison of oscillators
+    unsigned waveUnisonSize { 0 };
+    float waveDetuneRatio[config::oscillatorsPerVoice] { };
+    float waveLeftGain[config::oscillatorsPerVoice] { };
+    float waveRightGain[config::oscillatorsPerVoice] { };
 
     Duration dataDuration;
     Duration amplitudeDuration;
diff --git a/src/sfizz/Wavetables.cpp b/src/sfizz/Wavetables.cpp
index c65b244a..b04c0a8c 100644
--- a/src/sfizz/Wavetables.cpp
+++ b/src/sfizz/Wavetables.cpp
@@ -34,10 +34,10 @@ void WavetableOscillator::setPhase(float phase)
     _phase = phase;
 }
 
-void WavetableOscillator::process(float frequency, float* output, unsigned nframes)
+void WavetableOscillator::process(float frequency, float detuneRatio, float* output, unsigned nframes)
 {
     float phase = _phase;
-    float phaseInc = frequency * _sampleInterval;
+    float phaseInc = frequency * (detuneRatio * _sampleInterval);
 
     const WavetableMulti& multi = *_multi;
     unsigned tableSize = multi.tableSize();
@@ -56,7 +56,7 @@ void WavetableOscillator::process(float frequency, float* output, unsigned nfram
     _phase = phase;
 }
 
-void WavetableOscillator::processModulated(const float* frequencies, float* output, unsigned nframes)
+void WavetableOscillator::processModulated(const float* frequencies, float detuneRatio, float* output, unsigned nframes)
 {
     float phase = _phase;
     float sampleInterval = _sampleInterval;
@@ -66,7 +66,7 @@ void WavetableOscillator::processModulated(const float* frequencies, float* outp
 
     for (unsigned i = 0; i < nframes; ++i) {
         float frequency = frequencies[i];
-        float phaseInc = frequency * sampleInterval;
+        float phaseInc = frequency * (detuneRatio * sampleInterval);
         absl::Span<const float> table = multi.getTableForFrequency(frequency);
 
         float position = phase * tableSize;
diff --git a/src/sfizz/Wavetables.h b/src/sfizz/Wavetables.h
index 6290f01f..3d0c9039 100644
--- a/src/sfizz/Wavetables.h
+++ b/src/sfizz/Wavetables.h
@@ -47,12 +47,12 @@ public:
     /**
        Compute a cycle of the oscillator, with constant frequency.
      */
-    void process(float frequency, float* output, unsigned nframes);
+    void process(float frequency, float detuneRatio, float* output, unsigned nframes);
 
     /**
        Compute a cycle of the oscillator, with varying frequency.
      */
-    void processModulated(const float* frequencies, float* output, unsigned nframes);
+    void processModulated(const float* frequencies, float detuneRatio, float* output, unsigned nframes);
 
 private:
     /**
diff --git a/src/sfizz/sfizz.cpp b/src/sfizz/sfizz.cpp
index 08f88c1a..3e17af01 100644
--- a/src/sfizz/sfizz.cpp
+++ b/src/sfizz/sfizz.cpp
@@ -210,3 +210,13 @@ void sfz::Sfizz::allSoundOff() noexcept
 {
     synth->allSoundOff();
 }
+
+void sfz::Sfizz::addExternalDefinition(const std::string& id, const std::string& value)
+{
+    synth->getParser().addExternalDefinition(id, value);
+}
+
+void sfz::Sfizz::clearExternalDefinitions()
+{
+    synth->getParser().clearExternalDefinitions();
+}
diff --git a/src/sfizz/sfizz_wrapper.cpp b/src/sfizz/sfizz_wrapper.cpp
index bc62c4c7..3c9d88f0 100644
--- a/src/sfizz/sfizz_wrapper.cpp
+++ b/src/sfizz/sfizz_wrapper.cpp
@@ -256,6 +256,18 @@ void sfizz_all_sound_off(sfizz_synth_t* synth)
     return self->allSoundOff();
 }
 
+void sfizz_add_external_definitions(sfizz_synth_t* synth, const char* id, const char* value)
+{
+    auto self = reinterpret_cast<sfz::Synth*>(synth);
+    self->getParser().addExternalDefinition(id, value);
+}
+
+void sfizz_clear_external_definitions(sfizz_synth_t* synth)
+{
+    auto self = reinterpret_cast<sfz::Synth*>(synth);
+    self->getParser().clearExternalDefinitions();
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/tests/DemoWavetables.cpp b/tests/DemoWavetables.cpp
index d5b9ba5a..e573260b 100644
--- a/tests/DemoWavetables.cpp
+++ b/tests/DemoWavetables.cpp
@@ -172,7 +172,7 @@ int DemoApp::processAudio(jack_nframes_t nframes, void* cbdata)
     self->fSweepCurrent = sweepCurrent;
 
     // compute oscillator
-    osc.processModulated(frequency, left, nframes);
+    osc.processModulated(frequency, 1.0, left, nframes);
     std::memcpy(right, left, nframes * sizeof(float));
 
     return 0;
diff --git a/tests/FilesT.cpp b/tests/FilesT.cpp
index f4bdc3c4..51f5e40f 100644
--- a/tests/FilesT.cpp
+++ b/tests/FilesT.cpp
@@ -254,9 +254,46 @@ TEST_CASE("[Files] Channels (channels.sfz)")
     synth.loadSfzFile(fs::current_path() / "tests/TestFiles/channels.sfz");
     REQUIRE(synth.getNumRegions() == 2);
     REQUIRE(synth.getRegionView(0)->sample == "mono_sample.wav");
-    REQUIRE(!synth.getRegionView(0)->isStereo);
+    REQUIRE(!synth.getRegionView(0)->isStereo());
     REQUIRE(synth.getRegionView(1)->sample == "stereo_sample.wav");
-    REQUIRE(synth.getRegionView(1)->isStereo);
+    REQUIRE(synth.getRegionView(1)->isStereo());
+}
+
+TEST_CASE("[Files] Channels (channels_multi.sfz)")
+{
+    sfz::Synth synth;
+    synth.loadSfzFile(fs::current_path() / "tests/TestFiles/channels_multi.sfz");
+    REQUIRE(synth.getNumRegions() == 6);
+
+    REQUIRE(synth.getRegionView(0)->sample == "*sine");
+    REQUIRE(!synth.getRegionView(0)->isStereo());
+    REQUIRE(synth.getRegionView(0)->isGenerator());
+    REQUIRE(!synth.getRegionView(0)->oscillator);
+
+    REQUIRE(synth.getRegionView(1)->sample == "*sine");
+    REQUIRE(synth.getRegionView(1)->isStereo());
+    REQUIRE(synth.getRegionView(1)->isGenerator());
+    REQUIRE(!synth.getRegionView(1)->oscillator);
+
+    REQUIRE(synth.getRegionView(2)->sample == "ramp_wave.wav");
+    REQUIRE(!synth.getRegionView(2)->isStereo());
+    REQUIRE(!synth.getRegionView(2)->isGenerator());
+    REQUIRE(synth.getRegionView(2)->oscillator);
+
+    REQUIRE(synth.getRegionView(3)->sample == "ramp_wave.wav");
+    REQUIRE(synth.getRegionView(3)->isStereo());
+    REQUIRE(!synth.getRegionView(3)->isGenerator());
+    REQUIRE(synth.getRegionView(3)->oscillator);
+
+    REQUIRE(synth.getRegionView(4)->sample == "*sine");
+    REQUIRE(!synth.getRegionView(4)->isStereo());
+    REQUIRE(synth.getRegionView(4)->isGenerator());
+    REQUIRE(!synth.getRegionView(4)->oscillator);
+
+    REQUIRE(synth.getRegionView(5)->sample == "*sine");
+    REQUIRE(!synth.getRegionView(5)->isStereo());
+    REQUIRE(synth.getRegionView(5)->isGenerator());
+    REQUIRE(!synth.getRegionView(5)->oscillator);
 }
 
 TEST_CASE("[Files] sw_default")
diff --git a/tests/TestFiles/channels_multi.sfz b/tests/TestFiles/channels_multi.sfz
new file mode 100644
index 00000000..8146ba45
--- /dev/null
+++ b/tests/TestFiles/channels_multi.sfz
@@ -0,0 +1,6 @@
+<region> sample=*sine
+<region> sample=*sine oscillator_multi=3
+<region> sample=ramp_wave.wav oscillator=on
+<region> sample=ramp_wave.wav oscillator=on oscillator_multi=3
+<region> sample=*sine oscillator_multi=1
+<region> sample=*sine oscillator_multi=2
diff --git a/tests/TestFiles/ramp_wave.wav b/tests/TestFiles/ramp_wave.wav
new file mode 100644
index 00000000..838d8556
Binary files /dev/null and b/tests/TestFiles/ramp_wave.wav differ