A Max/MSP external that performs real-time spectral freezing of audio buffer content, creating evolving drones and textural sounds through FFT-based granular synthesis.
Create a Max external that implements real-time spectral freezing of an audio buffer:
- Accept audio buffer name and position parameter (0.0-1.0)
- Extract 50-200ms windows for FFT analysis
- Maintain magnitude spectrum while randomizing phase spectrum
- Use overlap-add synthesis for continuous output
- Create evolving drones with spectral character of original audio
- Type: MSP Audio External (
t_pxobject
)
- FFT Implementation: Custom radix-2 Cooley-Tukey FFT/IFFT
- Processing: Real-time overlap-add granular synthesis
- Threading: Single-threaded with rate limiting for safety
- Spectrum Capture: Windowed FFT analysis of buffer content
- Spectral Freezing: Magnitude preservation with phase randomization
- Grain Synthesis: Overlap-add IFFT resynthesis
- Normalization: Energy-based magnitude control
- Configurable Sizes: 512, 1024, 2048, 4096, 8192 (powers of 2)
- Default Size: 2048 (good balance of quality vs performance)
- Hop Size: FFT_size/4 for 4:1 overlap
- Window: Hann window for analysis and synthesis
Buffer Audio → Windowed FFT → Magnitude Extraction → Phase Randomization → IFFT → Overlap-Add → Output
- Basic FFT/IFFT spectral processing
- Buffer~ integration and access
- Real-time grain generation
- Parameter control system
- Auto-freeze on position changes
- Rate limiting (500ms minimum between position changes)
- Concurrent capture protection
- Configurable FFT sizes at instantiation
- Comprehensive debug system (
bang
message)
- CRITICAL FIX: Spectrum magnitude normalization
- Amplitude variation implementation fix
- Universal binary support
Problem: Spectrum magnitudes growing exponentially (values reaching 334,635+)
Root Cause: FFT normalization feedback loop causing energy accumulation
Solution: Energy-based spectrum normalization in capture phase
double target_energy = x->fft_size * 0.1;
double normalization_factor = sqrt(target_energy / spectrum_energy);
Impact: Eliminated "incredibly noisy and loud" artifacts completely
Problem: Rapid position changes caused noise artifacts
Solution: 500ms minimum interval between position changes
if (current_time - x->last_position_change_time < 500.0) return;
Problem:
ampvar
parameter had no audible effect
Root Cause: Incorrect random distribution and formula
Solution: Fixed distribution range and calculation formula
cmake_minimum_required(VERSION 3.19)
project(chiller~)
include(../../source/max-sdk-base/script/max-pretarget.cmake)
cd projects/chiller
mkdir build && cd build
cmake -DCMAKE_OSX_ARCHITECTURES="x86_64;arm64" ..
cmake --build .
- Built external:
/Users/a1106632/Documents/Max 8/Packages/externals/chiller.mxo
- Universal binary (Intel + Apple Silicon)
set <buffername>
- Set buffer to analyzeposition <0.0-1.0>
- Set analysis position (auto-captures spectrum)freeze
- Manually capture spectrum at current positionbang
- Output comprehensive debug information
rate <0.1-4.0>
- Grain generation rate (default: 1.0)phaserand <0.0-1.0>
- Phase randomization amount (default: 0.1)ampvar <0.0-0.5>
- Amplitude variation amount (default: 0.1)overlap <1.0-8.0>
- Overlap factor for synthesis (default: 4.0)
- FFT Size:
chiller~ 2048
(512, 1024, 2048, 4096, 8192)
- Buffer Name:
chiller~ mybuffer
or chiller~ 2048 mybuffer
- 512: Very low CPU, basic frequency resolution
- 1024: Low CPU, good for multiple instances
- 2048: Moderate CPU, recommended default
- 4096: High CPU, detailed frequency analysis (caused >100% CPU)
- 8192: Very high CPU, maximum detail
- 6 vectors × FFT_size × 8 bytes per instance
- 2048 FFT: ~98KB per instance
- 4096 FFT: ~196KB per instance
Send
bang
to output detailed information:
- Configuration (FFT size, sample rate, buffer info)
- Analysis state (position, spectrum capture status)
- Timing (position change intervals - critical for noise diagnosis)
- Synthesis parameters
- Spectrum energy levels (watch for magnitude explosion)
- Overlap buffer states and sample values
Healthy State:
Spectrum Energy: 88633.919625
Max Magnitude: 94.018975
Noisy State (before fix):
Spectrum Energy: 871190700897.731689
Max Magnitude: 334635.621325
# Navigate to projects directory
cd "/Users/a1106632/Documents/Max 8/Packages/max-sdk-main/projects/chiller"
# Create and configure build
mkdir build && cd build
cmake -DCMAKE_OSX_ARCHITECTURES="x86_64;arm64" ..
# Build external
cmake --build .
# Verify output
ls "/Users/a1106632/Documents/Max 8/Packages/externals/chiller.mxo"
# Check architecture
file "/Users/a1106632/Documents/Max 8/Packages/externals/chiller.mxo/Contents/MacOS/chiller"
# Output should show: Mach-O universal binary with 2 architectures
- Drone Creation: Freeze percussive attacks for sustained textures
- Spectral Morphing: Sequence through different buffer positions
- Textural Layers: Multiple instances at different positions/parameters
- Real-time Performance: Map position to controllers for expressive control
- Percussive sounds (metallic textures)
- Vocal recordings (formant-rich drones)
- Field recordings (ambient soundscapes)
- Synthesized tones (harmonic content)
- README.md: Complete user documentation with installation, usage, parameters
- CLAUDE.md: Complete technical documentation for developers
- Max Help Patch: Interactive documentation with examples
- Code Comments: Comprehensive inline documentation
- Universal binary builds tested on M2 MacBook Air
- Multiple FFT sizes validated (512-8192)
- Rate limiting prevents noise artifacts
- Magnitude normalization eliminates spectrum explosion
- Debug system provides comprehensive state monitoring
- All core functionality verified working
Magnitude explosion causing extreme noise → Fixed with normalizationRapid position changes causing artifacts → Fixed with rate limitingAmplitude variation not working → Fixed implementationHigh CPU with large FFT sizes → User configurable, documented
- Single buffer support (no crossfading between buffers)
- No preset system for parameter combinations
- Position changes limited to 500ms intervals (by design)
- Multiple Buffer Support: Crossfade between different buffers
- Spectral Filtering: Frequency-selective freezing
- Preset System: Save/recall parameter combinations
- MIDI Integration: Direct MIDI control of parameters
- FFT Library Integration: Replace custom FFT with vDSP or FFTW
- Multi-threading: Separate capture and synthesis threads
- SIMD Optimization: Vector processing for FFT operations
This project demonstrates advanced Max SDK techniques including real-time FFT processing, buffer~ integration, parameter management, robust error handling, and comprehensive debugging systems.