How to write a simple extractor using the streaming mode of Essentia

The goal of this howto/tutorial is to show you how to write extractors in the streaming mode of Essentia. To this end, we will write an extractor that extracts the MFCCs of an audio file, computes their average, variance, min and max, and outputs that to a file.

This tutorial is the equivalent, and intend to achieve the same goal, as the How to write a simple extractor using the standard mode of Essentia tutorial.

Note: the source code for this example can be found in the git repository tree, in the src/examples/streaming_mfcc.cpp file.

First of all, let’s identify which algorithms we will need. We want to do the following processing:


The steps we will have to take are the following:

  • instantiate these Algorithms (from the streaming::AlgorithmFactory)

  • (possibly) configure them

  • connect them through their sinks/sources, make sure no sink/source is left unconnected

  • create a Network of those algorithms, and launch the whole processing with the run() method

You will notice that compared to the standard mode, here we don’t have to do anything in particular once everything is connected. The order of processing is automatically decided by the scheduler, and hence, this mode of operation looks a lot like functional programming.

Setting up our program

Let’s start again by examining the source code for the streaming_mfcc.cpp example:

using namespace essentia::streaming;

int main(int argc, char* argv[]) {

  if (argc != 3) {
    cout << "ERROR: incorrect number of arguments." << endl;
    cout << "Usage: " << argv[0] << " audio_input yaml_output" << endl;

  string audioFilename = argv[1];
  string outputFilename = argv[2];

  // register the algorithms in the factory(ies)

  Pool pool;

  /////// PARAMS //////////////
  Real sampleRate = 44100.0;
  int frameSize = 2048;
  int hopSize = 1024;

This is the same boilerplate code as in the standard example, so we won’t spend too much time on it. The namespace is essentia::streaming here, though.

Creating the required algorithms

// we want to compute the MFCC of a file: we need the create the following:
// audioloader -> framecutter -> windowing -> FFT -> MFCC -> PoolStorage

AlgorithmFactory& factory = streaming::AlgorithmFactory::instance();

Algorithm* audio = factory.create("MonoLoader",
                                  "filename", audioFilename,
                                  "sampleRate", sampleRate);

Algorithm* fc    = factory.create("FrameCutter",
                                  "frameSize", frameSize,
                                  "hopSize", hopSize);

Algorithm* w     = factory.create("Windowing",
                                  "type", "blackmanharris62");

Algorithm* spec  = factory.create("Spectrum");
Algorithm* mfcc  = factory.create("MFCC");

This is also very similar to the standard example, however note the differences:

  • the factory is now the essentia::streaming::AlgorithmFactory instead of the essentia::standard::AlgorithmFactory

  • the algorithm type is now essentia::streaming::Algorithm instead of essentia::standard::Algorithm

Connecting the algorithms

/////////// CONNECTING THE ALGORITHMS ////////////////
cout << "-------- connecting algos --------" << endl;

// Audio -> FrameCutter
audio->output("audio")    >>  fc->input("signal");

// FrameCutter -> Windowing -> Spectrum
fc->output("frame")       >>  w->input("frame");
w->output("frame")        >>  spec->input("frame");

// Spectrum -> MFCC -> Pool
spec->output("spectrum")  >>  mfcc->input("spectrum");

mfcc->output("bands")     >>  NOWHERE;                   // we don't want the mel bands
mfcc->output("mfcc")      >>  PC(pool, "lowlevel.mfcc"); // store only the mfcc coeffs

// Note: PC is a #define for PoolConnector

Here goes the connection of the algorithms. In streaming mode, you do not need an intermediate variable to connect the output of an algorithm and the input of another one on it, you simply connect the output of an algorithm directly to its corresponding input. You can either use the connect(input, output) function or the >> right-shift operator to connect an input to an output. In this example, we use the >> operator, because it looks nicer!

Note the special connector NOWHERE, which you need to specify. It is mandatory to connect all inputs/outputs, so if you want to discard one stream, you need to explicitly say it by connecting it to the NOWHERE connector. Failure to do so will result in an exception when you try to run the network.

You can also see another special connector on the next line, that allows you to store the output of an algorithm in a Pool, where you then specify the pool and descriptor name, and it will automatically get stored there as soon as it becomes available on the given output.

Processing the audio

/////////// STARTING THE ALGORITHMS //////////////////
cout << "-------- start processing " << audioFilename << " --------" << endl;

// create a network with our algorithms...
Network n(audio);
// ...and run it, easy as that!;

You need to create a network of algorithms by constructing it with the topmost algorithm in your processing tree, that is the audio loader (all algorithms are connected after it). The audio loader is referred to as the generator in this case.

And this is all you have to call to make all the processing happen. Basically, all the algorithms will do all the processing they can (that is, compute all the MFCCs for all the audio), and when the run() function returns, the Pool will be filled with the MFCC coefficients.

Aggregating the results and writing them to disk

// aggregate the results
Pool aggrPool; // the pool with the aggregated MFCC values
const char* stats[] = { "mean", "var", "min", "max" };

standard::Algorithm* aggr = standard::AlgorithmFactory::create("PoolAggregator",
                                                               "defaultStats", arrayToVector<string>(stats));


// write results to file
cout << "-------- writing results to file " << outputFilename << " --------" << endl;

standard::Algorithm* output = standard::AlgorithmFactory::create("YamlOutput",
                                                                 "filename", outputFilename);

At this point, the processing is the same as in the standard example: aggregate and output data to file. Note that the PoolAggregator and the YamlOutput still come from the non-streaming (standard) factory.

delete aggr;
delete output;

return 0;

And the cleanup part, which is also quite simplified with respect to the way it’s done in the standard way. As all the algorithms are connected in a network, you just need to call the Network::clear() method to delete all of them.

You also need to delete the PoolAggregator and YamlOutput which you allocated separately, call shutdown(), and you’re done!

Compiling extractor

Follow the same instructions as for standard extractors in order to compile your extractor.