Real-time Vamp plugin SDK for C++20

Vamp is an C/C++ plugin API for audio analysis / feature extraction plugins: https://www.vamp-plugins.org

This SDK for plugins and hosts targets performance-critical applications by:

• reducing memory allocations, no memory allocation during processing
• simplifying and restricting the plugin API
• constexpr evaluation for compile time errors instead of runtime errors

The SDK aims to be well tested, cross-platform and use modern C++. The plugin SDK is available as a single-header library (download as asset from latest release page).

Compiler support: GCC >= 10, Clang >= 11, MSVC >= 19.30

‍Note: Python bindings for the hostsdk are available via PyPI. Please check out the documentation.

Links

• API documentation
• Examples
• Vamp ecosystem:
  • Great collection of plugins
  • Sonic Visualiser: Open-source software to visualize, analyze and annotate audio
  • Sonic Annotator: Batch tool for feature extraction
  • Audacity supports Vamp plugins

Performance

Following benchmarks compare the performance/overhead of the plugin SDKs based on a simple RMS plugin. The performance is measured as throughput (number of processed samples per second).

Results with an i7-9850H CPU (12 cores):

Throughput vs. block size	Multithreading

Results with an ARMv7 CPU: Throughput vs block size, Multithreading

Why another SDK?

The official SDK offers a convenient C++ plugin interface. But there are some drawbacks for real-time processing:

• Huge amount of memory allocations due to the use of C++ containers like vectors and lists passed by value.

  Let's have a look at the process method of the Vamp::Plugin class which does the main work:

  FeatureSet process(const float *const *inputBuffers, RealTime timestamp)

  FeatureSet is returned by value and is a std::map<int, FeatureList>. FeatureList is a std::vector<Feature> and Feature is struct containing the actual feature values as a std::vector<float>. So in total, those are three nested containers, which are all heap allocated.

• The C++ API is a wrapper of the C API:

  On the plugin side, the PluginAdapter class converts the C++ containers to C level (code). Therefore the C++ containers are temporary objects and will be deallocated shortly after creation.

  On the host side, the PluginHostAdapter converts again from the C to the C++ representation (code).

Solution approach

The rt-vamp-plugin-sdk aims to to keep the overhead minimal but still provide an easy and safe to use API:

1. Static plugin informations are provided as static constexpr variables to generate the C plugin descriptor at compile time.
2. The computed features are returned by reference (as a std::span) to prevent heap allocations during processing.
3. The input buffer is provided either as a TimeDomainBuffer (std::span<const float>) or a FrequencyDomainBuffer (std::span<const std::complex<float>>). The process method takes a std::variant<TimeDomainBuffer, FrequencyDomainBuffer>. A wrong input buffer type will result in an exception. The sized spans enable easy iteration over the input buffer data.

Plugin restrictions

Following features of the Vamp API Vamp::Plugin are restricted within the rt-vamp-plugin-sdk:

• OutputDescriptor::hasFixedBinCount == true for every output. The number of values is constant for each feature during processing. This has the advantage, that memory for the feature vector can be preallocated.

• OutputDescriptor::SampleType == OneSamplePerStep for every output. The plugin will generate one feature set for each input block.

  Following parameters are therefore negitable:

  • OutputDescriptor::sampleRate
  • OutputDescriptor::hasDuration
  • Feature::hasTimestamp & Feature::timestamp
  • Feature::hasDuration & Feature::duration
• Only one input channel allowed: getMinChannelCount() == 1

Minimal example

More examples can be found here: https://github.com/lukasberbuer/rt-vamp-plugin-sdk/tree/master/examples.

Plugin

class ZeroCrossing : public rtvamp::pluginsdk::Plugin<1 /* one output */> {
public:
    using Plugin::Plugin;  // inherit constructor
 
    static constexpr Meta meta{
        .identifier    = "zerocrossing",
        .name          = "Zero crossings",
        .description   = "Detect and count zero crossings",
        .maker         = "LB",
        .copyright     = "MIT",
        .pluginVersion = 1,
        .inputDomain   = InputDomain::Time,
    };
 
    OutputList getOutputDescriptors() const override {
        return {
            OutputDescriptor{
                .identifier  = "counts",
                .name        = "Zero crossing counts",
                .description = "The number of zero crossing points per processing block",
                .unit        = "",
                .binCount    = 1,
            },
        };
    }
 
    bool initialise(uint32_t stepSize, uint32_t blockSize) override {
        initialiseFeatureSet();  // automatically resizes feature set to number of outputs and bins
        return true;
    }
 
    void reset() override {
        previousSample_ = 0.0f;
    }
 
    const FeatureSet& process(InputBuffer buffer, uint64_t nsec) override {
        const auto signal = std::get<TimeDomainBuffer>(buffer);
 
        size_t crossings   = 0;
        bool   wasPositive = (previousSample_ >= 0.0f);
 
        for (const auto& sample : signal) {
            const bool isPositive = (sample >= 0.0f);
            crossings += int(isPositive != wasPositive);
            wasPositive = isPositive;
        }
 
        previousSample_ = signal.back();
 
        auto& result = getFeatureSet();
        result[0][0] = crossings;  // first and only output, first and only bin
        return result;             // return span/view of the results
    }
 
private:
    float previousSample_ = 0.0f;
};
 
RTVAMP_ENTRY_POINT(ZeroCrossing)

Host

// list all plugins keys (library:plugin)
for (auto&& key : rtvamp::hostsdk::listPlugins()) {
    std::cout << key.get() << std::endl;
}
 
auto plugin = rtvamp::hostsdk::loadPlugin("minimal-plugin:zerocrossing", 48000 /* samplerate */);
plugin->initialise(4096 /* step size */, 4096 /* block size */);
 
std::vector<float> buffer(4096);
// fill buffer with data from audio file, sound card, ...
 
auto features = plugin->process(buffer, 0 /* timestamp nanoseconds */);
std::cout << "Zero crossings: " << features[0][0] << std::endl;