Composite algorithm description

Composite algorithms are a special class of algorithms which allow you to create complex algorithms using simpler ones. These can either be algorithms which are more complex than others, and want to use more atomic operations (e.g.: MFCC would use the MelBands and DCT algorithms), or can be entire extractors (e.g., the KeyExtractor would use the following: FrameCutter → Windowing → Spectrum → SpectralPeaks → HPCP → Key).

The AlgorithmComposite class inherit the base streaming::Algorithm, so you can use them everywhere you would use a normal algorithm otherwise. There is however one big advantage of composite algorithms over a normal encapsulation such as what you find in object-oriented programming, which is that the scheduler knows about the internals of composites, and as such you will keep all the parallelism and fine-grained execution you would have by connecting all the constituent components of a composite, all of this while giving you the nice API and ease-of-use of high-level signal processing blocks.

In order to achieve this, composites are not executed normally as the other algorithms, for which the scheduler just call the process() method. Composite algorithms declare the way they should be executed to the scheduler.

When defining an AlgorithmComposite you will have to implement the pure virtual declareProcessOrder() method.

In this method, you will specify a list of execution steps that need to be followed in sequential order, and which can be either one of two possible steps:

  • ChainFrom(algo): which will run the given algorithm and all algorithms connected to it (and contained in the composite) as if they were an entire network
  • SingleShot(algo): which will run a given algorithm once

Each step will depend upon the complete execution of the previous one, so if you have a small network that branches inside your composite, it will be better to run it using ChainFrom(root) rather than SingleShot(algo) on all algorithms, as this will keep any possible parallelization of the branches.


Here is the schema of a simplified KeyExtractor in the context of a larger network of algorithms, specified by a user.


You can see that although the user just connected the MonoLoader to the KeyExtractor, and the KeyExtractor to the pool, the underlying network is a bit more complex.

Other things we can note:

  • you can nest any number of times a composite into another one. This should be obvious due to the fact that a composite algorithm is itself an algorithm, so they can be used everywhere a “normal” algorithm can. For instance, here we have the Key algorithm (composite) used inside the KeyExtractor composite.

  • composite algorithms need to have Sinks and Sources of their own, but in fact they just act as relay to/from the source/sink of one of their inner algorithms.

    This is done using the SinkProxy / SourceProxy class. They work in the same way as normal sources and sinks do, and you also have to declare them when building your algorithm. However, you will then need to attach them to the actual source or sink of the inner algorithm.

    Note that the proxies do not need to have the same name as the connector they are attached to.

Here follows what would be an implementation for the previous example. (simplified version, irrelevant parts of code dropped)

class MonoLoader : public AlgorithmComposite {
  Algorithm* _audioLoader;
  Algorithm* _mixer;
  Algorithm* _resample;

  SourceProxy<AudioSample> _audio;

  MonoLoader() {
    // declare our output normally, except we're using a SourceProxy here
    declareOutput(_audio, "audio", "the audio signal");

    // create our inner network
    AlgorithmFactory& factory = AlgorithmFactory::instance();

    _audioLoader = factory.create("AudioLoader");
    _mixer       = factory.create("MonoMixer");
    _resample    = factory.create("Resample");

    _audioLoader->output("audio")           >>  _mixer->input("audio");
    _audioLoader->output("numberChannels")  >>  _mixer->input("numberChannels");
    _mixer->output("audio")                 >>  _resample->input("signal");

    // now attach the output of our last algorithm to the proxy of the composite
    // to allow data to be relayed outside of the composite
    _resample->output("signal")  >>  _audio;

  void declareProcessOrder() {
class KeyExtractor : public AlgorithmComposite {
  Algorithm *_frameCutter, *_windowing, *_spectrum, *_spectralPeaks, *_hpcp, *_key;

  SinkProxy<Real> _audio;
  SourceProxy<std::string> _keyKey;
  SourceProxy<std::string> _keyScale;
  SourceProxy<Real> _keyStrength;

  KeyExtractor() {
    // declare inputs/outputs
    declareInput(_audio, "audio", "the audio signal");
    declareOutput(_keyKey, "key", "see Key algorithm documentation");
    declareOutput(_keyScale, "scale", "see Key algorithm documentation");
    declareOutput(_keyStrength, "strength", "see Key algorithm documentation");

    // instantiate all required algorithms
    _frameCutter   = factory.create("FrameCutter");
    _windowing     = factory.create("Windowing", "type", "blackmanharris62");
    _spectrum      = factory.create("Spectrum");
    _spectralPeaks = factory.create("SpectralPeaks",
                                    "orderBy", "magnitude", "magnitudeThreshold", 1e-05,
                                    "minFrequency", 40, "maxFrequency", 5000, "maxPeaks", 10000);
    _hpcpKey = factory.create("HPCP");
    _key     = factory.create("Key");

    // attach input proxy(ies)
    _audio  >> _frameCutter->input("signal");

    // connect inner algorithms
    _frameCutter->output("frame")          >>  _windowing->input("frame");
    _windowing->output("frame")            >>  _spectrum->input("frame");
    _spectrum->output("spectrum")          >>  _spectralPeaks->input("spectrum");
    _spectralPeaks->output("magnitudes")   >>  _hpcpKey->input("magnitudes");
    _spectralPeaks->output("frequencies")  >>  _hpcpKey->input("frequencies");
    _hpcpKey->output("hpcp")               >>  _key->input("pcp");

    // attach output proxy(ies)
    _key->output("key")       >>  _keyKey;
    _key->output("scale")     >>  _keyScale;
    _key->output("strength")  >>  _keyStrength;

  void declareProcessOrder() {

And here is the code for the Key algorithm. As you will see, this one is a bit different than the previous ones. Let’s have a look at it first:

class Key : public AlgorithmComposite {
  SinkProxy<std::vector<Real> > _pcp;

  Source<std::string> _key;
  Source<std::string> _scale;
  Source<Real> _strength;

  Pool _pool;
  Algorithm* _poolStorage;
  standard::Algorithm* _keyAlgo;

  Key() {
    declareInput(_pcp, "pcp", "the input pitch class profile");
    declareOutput(_key, 0, "key", "the estimated key, from A to G");
    declareOutput(_scale, 0, "scale", "the scale of the key (major or minor)");
    declareOutput(_strength, 0, "strength", "the strength of the estimated key");

    _keyAlgo = standard::AlgorithmFactory::create("Key");
    _poolStorage = new PoolStorage<std::vector<Real> >(&_pool, "internal.hpcp");

    _pcp  >>  _poolStorage->input("data");

  void declareProcessOrder() {

  bool process() {
    // we only want to output a Key estimate at the end of our stream
    if (endOfStream()) {
      // here, we want to call the std algo on the mean of the pcp frames
      vector<Real> hpcpMean = meanFrames(_pool.value<vector<vector<Real> > >("internal.hpcp"));
      string key, scale;
      Real strength;

      // now we have our values, push them out of the streaming algorithm

So, what can we see here:

  • the sources are actually Sources, not SourceProxies
  • the declareProcessOrder() method declares a process step on the algorithm itself
  • the process() method is actually defined here, alongside the declareProcessOrder()

What happens here? Why are both the process() and the declareProcessOrder() methods defined?

What is actually happening is that the part calling the std version of the key algorithm is quite small, and wouldn’t warrant the creation of a new algorithm just for this purpose. So we decided to keep this inside the Key algorithm, as if it were not a composite. This is also the reason why the sources are actually Sources, as we need them for pushing the data through. If we had SourceProxies here, we wouldn’t know where to attach them.

The following happens when the scheduler tries to run the Key algorithm:

  • the scheduler wants to run the Key algorithm, it is a composite
  • it looks at Key::declareProcessOrder(); this contains 2 steps:
    1. SingleShot(_poolStorage) : fine, this is a normal call to _poolStorage.process(), which it executes
    2. SingleShot(this) : this is a recursive call of the Key algo, so the scheduler knows it shouldn’t look at declareProcessOrder() now, but rather execute it normally, that is call the Key::process() method.
  • the scheduler then goes on with the following algorithms

This way, there is no infinite recursion and everything is well-behaved. This might look a hack at first sight, but actually it is a rather powerful mechanism that allows composite algorithms to be more than just “chains” of other algorithms. Instead, they can be a mix of those chains and specific code, without requiring this specific code to be artificially encapsulated in some proxy algorithm.

Scheduling algorithm

For more details on the inner workings of the scheduler, please have a look at the Execution network algorithm page.